Article: 151737 of rec.autos.tech From: B.Hamilton@irl.cri.nz (Bruce Hamilton) Subject: Gasoline FAQ - Part 1 of 4 Date: Wed, 18 Sep 1996 09:34:34 GMT Expires: 18 October 1996 00.00.01 GMT Organization: Industrial Research Limited Lines: 1028 Archive-name: autos/gasoline-faq/part1 Posting-Frequency: monthly Last-modified: 17 September 1996 Version: 1.11 FAQ: Automotive Gasoline Bruce Hamilton B.Hamilton@irl.cri.nz This FAQ is posted monthly to the Usenet groups news.answers, rec.answers, and rec.autos.tech. The latest copy should be available by FTP from rtfm.mit.edu in the pub/usenet-by-hierarchy/rec/autos/tech directory, and also available on the WWW from sites that automatically convert those FAQs. Changes: - updated energy use data, added some more references. - corrected another couple of typos. ------------------------------ Subject: 1. Introduction, Intent, Acknowledgements, and Abbreviations 1.1 Introduction and Intent. The intent of this FAQ is to provide some basic information on gasolines and other fuels for spark ignition engines used in automobiles. The toxicity and environmental reasons for recent and planned future changes to gasoline are discussed, along with recent and proposed changes in composition of gasoline. This FAQ is intended to help readers choose the most appropriate fuel for vehicles, assist with the diagnosis of fuel-related problems, and to understand the significance of most gasoline properties listed in fuel specifications. I make no apologies for the fairly heavy emphasis on chemistry; it is the only sensible way to describe the oxidation of hydrocarbon fuels to produce energy, water, and carbon dioxide. 1.2 Acknowledgements. Thanks go to all the posters in sci.energy and rec.autos.tech who spend valuable time responding to questions. I would also like to acknowledge the considerable effort of L.M.Gibbs of Chevron, who has twice spent his valuable time courteously detailing errors and providing references for his corrections. All remaining errors and omissions are mine. 1.3 Abbreviations. AKI = Antiknock Index of Gasoline ( (RON+MON)/2 ) CI = Compression Ignition ( Diesel ) Gasoline = Petrol ( Yes, complaints were received :-) ) IC = Internal Combustion MON = Motor Octane Rating Octane = The Octane Rating of the Gasoline RFG = Reformulated Gasoline ( as defined by US Clean Air Act ) RON = Research Octane Rating SI = Spark Ignition ( Gasoline ) ------------------------------ Subject: 2. Table of Contents 1. Introduction, Intent, Acknowledgements, and Abbreviations 1.1 Introduction and Intent. 1.2 Acknowledgements. 1.3 Abbreviations. 2. Table of Contents 3. What Advantage will I gain from reading this FAQ? 4. What is Gasoline? 4.1 Where does crude oil come from?. 4.2 When will we run out of crude oil?. 4.3 What is the history of gasoline? 4.4 What are the hydrocarbons in gasoline? 4.5 What are oxygenates? 4.6 Why were alkyl lead compounds added? 4.7 Why not use other organometallic compounds? 4.8 What do the refining processes do? 4.9 What energy is released when gasoline is burned? 4.10 What are the gasoline specifications? 4.11 What are the effects of the specified fuel properties? 4.12 Are brands different? 4.13 What is a typical composition? 4.14 Is gasoline toxic or carcinogenic? 4.15 Is unleaded gasoline more toxic than leaded? 4.16 Is reformulated gasoline more toxic than unleaded? 4.17 Are all oxygenated gasolines also reformulated gasolines? 5. Why is Gasoline Composition Changing? 5.1 Why pick on cars and gasoline? 5.2 Why are there seasonal changes? 5.3 Why were alkyl lead compounds removed? 5.4 Why are evaporative emissions a problem? 5.5 Why control tailpipe emissions? 5.6 Why do exhaust catalysts influence fuel composition? 5.7 Why are "cold start" emissions so important? 5.8 When will the emissions be "clean enough"? 5.9 Why are only some gasoline compounds restricted? 5.10 What does "renewable" fuel or oxygenate mean? 5.11 Will oxygenated gasoline damage my vehicle? 5.12 What does "reactivity" of emissions mean? 5.13 What are "carbonyl" compounds? 5.14 What are "gross polluters"? 6. What do Fuel Octane ratings really indicate? 6.1 Who invented Octane Ratings? 6.2 Why do we need Octane Ratings? 6.3 What fuel property does the Octane Rating measure? 6.4 Why are two ratings used to obtain the pump rating? 6.5 What does the Motor Octane rating measure? 6.6 What does the Research Octane rating measure? 6.7 Why is the difference called "sensitivity"? 6.8 What sort of engine is used to rate fuels? 6.9 How is the Octane rating determined? 6.10 What is the Octane Distribution of the fuel? 6.11 What is a "delta Research Octane number"? 6.12 How do other fuel properties affect octane? 6.13 Can higher octane fuels give me more power? 6.14 Does low octane fuel increase engine wear? 6.15 Can I mix different octane fuel grades? 6.16 What happens if I use the wrong octane fuel? 6.17 Can I tune the engine to use another octane fuel? 6.18 How can I increase the fuel octane? 6.19 Are aviation gasoline octane numbers comparable? 6.20 Can mothballs increase octane? 7. What parameters determine octane requirement? 7.1 What is the Octane Number Requirement of a Vehicle? 7.2 What is the effect of Compression ratio? 7.3 What is the effect of changing the air-fuel ratio? 7.4 What is the effect of changing the ignition timing 7.5 What is the effect of engine management systems? 7.6 What is the effect of temperature and Load? 7.7 What is the effect of engine speed? 7.8 What is the effect of engine deposits? 7.9 What is the Road Octane Number of a Fuel? 7.10 What is the effect of air temperature?. 7.11 What is the effect of altitude?. 7.12 What is the effect of humidity?. 7.13 What does water injection achieve?. 8. How can I identify and cure other fuel-related problems? 8.1 What causes an empty fuel tank? 8.2 Is knock the only abnormal combustion problem? 8.3 Can I prevent carburetter icing? 8.4 Should I store fuel to avoid the oxygenate season? 8.5 Can I improve fuel economy by using quality gasolines? 8.6 What is "stale" fuel, and should I use it? 8.7 How can I remove water in the fuel tank? 8.8 Can I use unleaded on older vehicles? 8.9 How serious is valve seat recession on older vehicles? 9. Alternative Fuels and Additives 9.1 Do fuel additives work? 9.2 Can a quality fuel help a sick engine? 9.3 What are the advantages of alcohols and ethers? 9.4 Why are CNG and LPG considered "cleaner" fuels. 9.5 Why are hydrogen-powered cars not available? 9.6 What are "fuel cells" ? 9.7 What is a "hybrid" vehicle? 9.8 What about other alternative fuels? 9.9 What about alternative oxidants? 10. Historical Legends 10.1 The myth of Triptane 10.2 From Honda Civic to Formula 1 winner. 11. References 11.1 Books and Research Papers 11.2 Suggested Further Reading ------------------------------ Subject: 3. What Advantage will I gain from reading this FAQ? This FAQ is intended to provide a fairly technical description of what gasoline contains, how it is specified, and how the properties affect the performance of your vehicle. The regulations governing gasoline have changed, and are continuing to change. These changes have made much of the traditional lore about gasoline obsolete. Motorists may wish to understand a little more about gasoline to ensure they obtain the best value, and the most appropriate fuel for their vehicle. There is no point in prematurely destroying your second most expensive purchase by using unsuitable fuel, just as there is no point in wasting hard-earned money on higher octane fuel that your automobile can not utilize. Note that this FAQ does not discuss the relative advantages of specific brands of gasolines, it is only intended to discuss the generic properties of gasolines. ------------------------------ Subject: 4. What is Gasoline? 4.1 Where does crude oil come from?. The generally-accepted origin of crude oil is from plant life up to 3 billion years ago, but predominantly from 100 to 600 million years ago [1]. "Dead vegetarian dino dinner" is more correct than "dead dinos". The molecular structure of the hydrocarbons and other compounds present in fossil fuels can be linked to the leaf waxes and other plant molecules of marine and terrestrial plants believed to exist during that era. There are various biogenic marker chemicals ( such as isoprenoids from terpenes, porphyrins and aromatics from natural pigments, pristane and phytane from the hydrolysis of chlorophyll, and normal alkanes from waxes ), whose size and shape can not be explained by known geological processes [2]. The presence of optical activity and the carbon isotopic ratios also indicate a biological origin [3]. There is another hypothesis that suggests crude oil is derived from methane from the earth's interior. The current main proponent of this abiotic theory is Thomas Gold, however abiotic and extraterrestrial origins for fossil fuels were also considered at the turn of the century, and were discarded then. A large amount of additional evidence for the biological origin of crude oil has accumulated since then. 4.2 When will we run out of crude oil? It has been estimated that the planet contains over 6.4 x 10^15 tonnes of organic carbon that is cycled through two major cycles, but only about 18% of that contributes to petroleum production. The primary cycle ( turnover of 2.7-3.0 x 10^12 tonnes of organic carbon ) has a half-life of days to decades, whereas the large secondary cycle ( turnover 6.4 x 10^15 tonnes of organic carbon ) has a half-life of several million years [4]. Much of this organic carbon is too dilute or inaccessible for current technology to recover, however the estimates represent centuries to millenia of fossil fuels, even with continued consumption at current or increased rates [5]. The concern about "running out of oil" arises from misunderstanding the significance of a petroleum industry measure called the Reserves/Production ratio (R/P). This monitors the production and exploration interactions. The R/P is based on the concept of "proved" reserves of fossil fuels. Proved reserves are those quantities of fossil fuels that geological and engineering information indicate with reasonable certainty can be recovered in the future from known reservoirs under existing economic and operating conditions. The Reserves/Production ratio is the proved reserves quantity divided by the production in the last year, and the result will be the length of time that those remaining proved reserves would last if production were to continue at the current level [6]. It is important to note the economic and technology component of the definitions, as the price of oil increases ( or new technology becomes available ), marginal fields become "proved reserves". We are unlikely to "run out" of oil, as more fields become economic. Note that investment in exploration is also linked to the R/P ratio, and the world crude oil R/P ratio typically moves between 20-40 years, however specific national incentives to discover oil can extend that range upward. Concerned people often refer to the " Hubbert curves" that predict fossil fuel discovery rates would peak and decline rapidly. M. King Hubbert calculated in 1982 that the ultimate resource base of the lower 48 states of the USA was 163+-2 billion barrels of oil, and the ultimate production of natural gas to be 24.6+-0.8 trillion cubic metres, with some additional qualifiers. As production and proved resources were 147 billion barrels of oil and 22.5 trillion cubic metres of gas, Hubbert was implying that volumes yet to be developed could only be 16-49 billion barrels of oil and 2.1-4.5 trillion cubic metres. Technology has confounded those predictions [6a]. The US Geological Survey has also just increased their assessment of US ( not just the lower 48 states ),inferred reserves crude oil by 60 billion barrels, and doubled the size of gas reserves to 9.1 trillion cubic metres. When combined with the estimate of undiscovered oil and gas, the totals reach 110 billion barrels of oil and 30 trillion cubic metres of gas [7]. The current price for Brent Crude is approx. $20/bbl. The world R/P ratio has increased from 27 years (1979) to 43.1 years (1993). The 1995 BP Statistical Review of World Energy provides the following data [6,7]. Crude Oil Proved Reserves R/P Ratio Middle East 89.4 billion tonnes 93.4 year USA 3.8 9.8 years USA - 1995 USGS data 10.9 33.0 years Total World 137.3 43.0 years Coal Proved Reserves R/P Ratio USA 240.56 billion tonnes 247 years Total World 1,043.864 235 years Natural Gas Proved Reserves R/P Ratio USA 4.6 trillion cubic metres 8.6 years USA - 1995 USGS data 9.1 17.0 years Total World 141.0 66.4 years. One billion = 1 x 10^9. One trillion = 1 x 10^12. One barrel of Arabian Light crude oil = 0.158987 m3 and 0.136 tonnes. If the crude oil price exceeds $30/bbl then alternative fuels may become competitive, and at $50-60/bbl coal-derived liquid fuels are economic, as are many biomass-derived fuels and other energy sources [8]. 4.3 What is the history of gasoline? In the late 19th Century the most suitable fuels for the automobile were coal tar distillates and the lighter fractions from the distillation of crude oil. During the early 20th Century the oil companies were producing gasoline as a simple distillate from petroleum, but the automotive engines were rapidly being improved and required a more suitable fuel. During the 1910s, laws prohibited the storage of gasolines on residential properties, so Charles F. Kettering ( yes - he of ignition system fame ) modified an IC engine to run on kerosine. However the kerosine-fuelled engine would "knock" and crack the cylinder head and pistons. He assigned Thomas Midgley Jr. to confirm that the cause was >from the kerosine droplets vaporising on combustion as they presumed. Midgley demonstrated that the knock was caused by a rapid rise in pressure after ignition, not during preignition as believed [9]. This then lead to the long search for antiknock agents, culminating in tetra ethyl lead [10]. Typical mid-1920s gasolines were 40 - 60 Octane [11]. Because sulfur in gasoline inhibited the octane-enhancing effect of the alkyl lead, the sulfur content of the thermally-cracked refinery streams for gasolines was restricted. By the 1930s, the petroleum industry had determined that the larger hydrocarbon molecules (kerosine) had major adverse effects on the octane of gasoline, and were developing consistent specifications for desired properties. By the 1940s catalytic cracking was introduced, and gasoline compositions became fairly consistent between brands during the various seasons. The 1950s saw the start of the increase of the compression ratio, requiring higher octane fuels. Octane ratings, lead levels, and vapour pressure increased, whereas sulfur content and olefins decreased. Some new refining processes ( such as hydrocracking ), specifically designed to provide hydrocarbons components with good lead response and octane, were introduced. Minor improvements were made to gasoline formulations to improve yields and octane until the 1970s - when unleaded fuels were introduced to protect the exhaust catalysts that were also being introduced for environmental reasons. From 1970 until 1990 gasolines were slowly changed as lead was phased out, lead levels plummetted, octanes initially decreased, and then remained 2-5 numbers lower, vapour pressures continued to increase, and sulfur and olefins remained constant, while aromatics increased. In 1990, the US Clean Air Act started forcing major compositional changes on gasoline, resulting in plummeting vapour pressure and increaing oxygenate levels. These changes will continue into the 21st Century, because gasoline use in SI engines is a major pollution source. Comprehensive descriptions of the changes to gasolines this century have been provided by L.M.Gibbs [12,13]. The move to unleaded fuels continues worldwide, however several countries have increased the aromatics content ( up to 50% ) to replace the alkyl lead octane enhancers. These highly aromatic gasolines can result in in damage to elastomers and increased levels of toxic aromatic emissions if used without exhaust catalysts. 4.4 What are the hydrocarbons in gasoline? Hydrocarbons ( HCs ) are any molecules that just contain hydrogen and carbon, both of which are fuel molecules that can be burnt ( oxidised ) to form water ( H2O ) or carbon dioxide ( CO2 ). If the combustion is not complete, carbon monoxide ( CO ) may be formed. As CO can be burnt to produce CO2, it is also a fuel. The way the hydrogen and carbons hold hands determines which hydrocarbon family they belong to. If they only hold one hand they are called "saturated hydrocarbons" because they can not absorb additional hydrogen. If the carbons hold two hands they are called "unsaturated hydrocarbons" because they can be converted into "saturated hydrocarbons" by the addition of hydrogen to the double bond. Hydrogens are omitted from the following, but if you remember C = 4 hands, H = 1 hand, and O = 2 hands, you can draw the full structures of most HCs. Gasoline contains over 500 hydrocarbons that may have between 3 to 12 carbons, and gasoline used to have a boiling range from 30C to 220C at atmospheric pressure. The boiling range is narrowing as the initial boiling point is increasing, and the final boiling point is decreasing, both changes are for environmental reasons. Detailed descriptions of structures can be found in any chemical or petroleum text discussing gasolines [14]. 4.4.1 Saturated hydrocarbons ( aka paraffins, alkanes ) - stable, the major component of leaded gasolines. - tend to burn in air with a clean flame. - octane ratings depend on branching and number of carbon atoms. alkanes normal = continuous chain of carbons ( Cn H2n+2 ) - low octane ratings, decreasing with carbon chain length. normal heptane C-C-C-C-C-C-C C7H16 iso = branched chain of carbons ( Cn H2n+2 ) - higher octane ratings, increasing with carbon chain branching. iso octane = C C ( aka 2,2,4-trimethylpentane ) | | C-C-C-C-C C8H18 | C cyclic = circle of carbons ( Cn H2n ) ( aka Naphthenes ) - high octane ratings. cyclohexane = C / \ C C | | C6H12 C C \ / C 4.4.2 Unsaturated Hydrocarbons - Unstable, are the remaining component of gasoline. - Tend to burn in air with a smoky flame. Alkenes ( aka olefins, have carbon=carbon double bonds ) - These are unstable, and are usually limited to a few %. - tend to be reactive and toxic, but have desirable octane ratings. C | C5H10 2-methyl-2-butene C-C=C-C Alkynes ( aka acetylenes, have carbon-carbon triple bonds ) - These are even more unstable, are only present in trace amounts, and only in some poorly-refined gasolines. _ Acetylene C=C C2H2 Arenes ( aka aromatics ) - Used to be up to 40%, gradually being reduced to <20% in the US. - tend to be more toxic, but have desirable octane ratings. - Some countries are increasing the aromatic content ( up to 50% in some super unleaded fuels ) to replace the alkyl lead octane enhancers. C C // \ // \ C C C-C C Benzene | || Toluene | || C C C C \\ / \\ / C C C6H6 C7H8 Polynuclear Aromatics ( aka PNAs or PAHs ) - These are high boiling, and are only present in small amounts in gasoline. They contain benzene rings joined together. The simplest, and least toxic, is Naphthalene, which is only present in trace amounts in traditional gasolines, and even lower levels are found in reformulated gasolines. The larger multi-ringed PNAs are highly toxic, and are not present in gasoline. C C // \ / \\ C C C Naphthalene | || | C10H8 C C C \\ / \ // C C 4.5 What are oxygenates? Oxygenates are just preused hydrocarbons :-). They contain oxygen, which can not provide energy, but their structure provides a reasonable antiknock value, thus they are good substitutes for aromatics, and they may also reduce the smog-forming tendencies of the exhaust gases [15]. Most oxygenates used in gasolines are either alcohols ( Cx-O-H ) or ethers (Cx-O-Cy), and contain 1 to 6 carbons. Alcohols have been used in gasolines since the 1930s, and MTBE was first used in commercial gasolines in Italy in 1973, and was first used in the US by ARCO in 1979. The relative advantages of aromatics and oxygenates as environmentally-friendly and low toxicity octane-enhancers are still being researched. Ethanol C-C-O-H C2H5OH C | Methyl tertiary butyl ether C-C-O-C C4H9OCH3 (aka tertiary butyl methyl ether ) | C They can be produced from fossil fuels eg methanol (MeOH), methyl tertiary butyl ether (MTBE), tertiary amyl methyl ether (TAME), or from biomass, eg ethanol(EtOH), ethyl tertiary butyl ether (ETBE)). MTBE is produced by reacting methanol ( from natural gas ) with isobutylene in the liquid phase over an acidic ion-exchange resin catalyst at 100C. The isobutylene was initially from refinery catalytic crackers or petrochemical olefin plants, but these days larger plants produce it from butanes. MTBE production has increased at the rate of 10 to 20% per year, and the spot market price in June 1993 was around $270/tonne [15]. The "ether" starting fluids for vehicles are usually diethyl ether (liquid) or dimethyl ether (aerosol). Note that " petroleum ethers " are volatile alkane hydrocarbon fractions, they are not a Cx-O-Cy compound. Oxygenates are added to gasolines to reduce the reactivity of emissions, but they are only effective if the hydrocarbon fractions are carefully modified to utilise the octane and volatility properties of the oxygenates. If the hydrocarbon fraction is not correctly modified, oxygenates can increase the undesirable smog-forming and toxic emissions. Oxygenates do not necessarily reduce all exhaust toxins, nor are they intended to. Oxygenates have significantly different physical properties to hydrocarbons, and the levels that can be added to gasolines are controlled by the 1977 Clean Air Act amendments in the US, with the laws prohibiting the increase or introduction of a fuel or fuel additive that is not substantially similar to any fuel or fuel additive used to certify 1975 or subsequent years vehicles. Waivers can granted if the product does not cause or contribute to emission device failures, and if the EPA does not specifically decline the application after 180 days, it is taken as granted. In 1978 the EPA granted 10% by volume of ethanol a waiver, and have subsequently issued waivers for <10 vol% ethanol (1982), 7 vol% tertiary butyl alcohol (1979), 5.5 vol% 1:1 MeOH/TBA (1979), 3.5 mass% oxygen derived from 1:1 MeOH/TBA = ~9.5 vol% of the alcohols (1981), 3.7 mass% oxygen derived from methanol and cosolvents = 5 vol% max MeOH and 2.5 vol% min cosolvent - with some cosolvents requiring additional corrosion inhibitor (1985,1988), 7.0 vol% MTBE (1979), and 15.0 vol% MTBE (1988). Only the ethanol waiver was exempted >from the requirement to still meet ASTM volatility requirements [16]. In 1981 the EPA ruled that fuels could contain aliphatic alcohols ( except MeOH ) and/or ethers at concentrations until the oxygen content is 2.0 mass%. It also permitted 5.5 vol% of 1:1 MeOH/TBA. In 1991 the maximum oxygen content was increased to 2.7 mass%. To ensure sufficient gasoline base was available for ethanol blending, the EPA also ruled that gasoline containing up to 2 vol% of MTBE could subsequently be blended with 10 vol% of ethanol [16]. Initially, the oxygenates were added to hydrocarbon fractions that were slightly-modified unleaded gasoline fractions, and these were known as "oxygenated" gasolines. In 1995, the hydrocarbon fraction was significantly modified, and these gasolines are called "reformulated gasolines" ( RFGs ), and there are differing specifications for California ( Phase 2 ) and Federal ( simple model ) RFGs, however both require oxygenates to provide Octane. The California RFG requires the hydrocarbon composition of the RFG to be significantly more modified than the existing oxygenated gasolines to reduce unsaturates, volatility, benzene, and the reactivity of emissions. Federal regulations only reduce vapour pressure and benzene directly, however aromatics are also reduced to meet emissions criteria [16]. Oxygenates that are added to gasoline function in two ways. Firstly they have high blending octane, and so can replace high octane aromatics in the fuel. These aromatics are responsible for disproportionate amounts of CO and HC exhaust emissions. This is called the "aromatic substitution effect". Oxygenates also cause engines without sophisticated engine management systems to move to the lean side of stoichiometry, thus reducing emissions of CO ( 2% oxygen can reduce CO by 16% ) and HC ( 2% oxygen can reduce HC by 10%) [17], and other researchers have observed similar reductions also occur when oxygenates are added to reformulated gasolines on older and newer vehicles, but have also shown that NOx levels may increase, as also may some regulated toxins [18,19,20]. However, on vehicles with engine management systems, the fuel volume will be increased to bring the stoichiometry back to the preferred optimum setting. Oxygen in the fuel can not contribute energy, consequently the fuel has less energy content. For the same efficiency and power output, more fuel has to be burnt, and the slight improvements in combustion efficiency that oxygenates provide on some engines usually do not completely compensate for the oxygen. There are huge number of chemical mechanisms involved in the pre-flame reactions of gasoline combustion. Although both alkyl leads and oxygenates are effective at suppressing knock, the chemical modes through which they act are entirely different. MTBE works by retarding the progress of the low temperature or cool-flame reactions, consuming radical species, particularly OH radicals and producing isobutene. The isobutene in turn consumes additional OH radicals and produces unreactive, resonantly stabilised radicals such as allyl and methyl allyl, as well as stable species such as allene, which resist further oxidation [21,22]. 4.6 Why were alkyl lead compounds added? The efficiency of a spark-ignited gasoline engine can be related to the compression ratio up to at least compression ratio 17:1 [23]. However any "knock" caused by the fuel will rapidly mechanically destroy an engine, and General Motors was having major problems trying to improve engines without inducing knock. The problem was to identify economic additives that could be added to gasoline or kerosine to prevent knock, as it was apparent that engine development was being hindered. The kerosine for home fuels soon became a secondary issue, as the magnitude of the automotive knock problem increased throughout the 1910s, and so more resources were poured into the quest for an effective "antiknock". A higher octane aviation gasoline was required urgently once the US entered WWI, and almost every possible chemical ( including melted butter ) was tested for antiknock ability [24]. Originally, iodine was the best antiknock available, but was not a practical gasoline additive, and was used as the benchmark. In 1919 aniline was found to have superior antiknock ability to iodine, but also was not a practical additive, however aniline became the benchmark antiknock, and various compounds were compared to it. The discovery of tetra ethyl lead, and the scavengers required to remove it from the engine were made by teams lead by Thomas Midgley Jr. in 1922 [9,10,24]. They tried selenium oxychloride which was an excellent antiknock, however it reacted with iron and "dissolved" the engine. Midgley was able to predict that other organometallics would work, and slowly focused on organoleads. They then had to remove the lead, which would otherwise accumulate and coat the engine and exhaust system with lead. They discovered and developed the halogenated lead scavengers that are still used in leaded fuels. The scavengers, ( ethylene dibromide and ethylene dichloride ), function by providing halogen atoms that react with the lead to form volatile lead halide salts that can escape out the exhaust. The quantity of scavengers added to the alkyl lead concentrate is calculated according to the amount of lead present. If sufficient scavenger is added to theoretically react with all the lead present, the amount is called one "theory". Typically, 1.0 to 1.5 theories are used, but aviation gasolines must only use one theory. This ensures there is no excess bromine that could react with the engine. The alkyl leads rapidly became the most cost-effective method of enhancing octane. The introduction was not universally acclaimed, as the toxicity of TEL soon became apparent, and several eminent public health officials campaigned against the widespread introduction of alkyl leads [25]. Their cause was assisted by some major disasters at TEL manufacturing plants, and although these incidents were mainly attributable to a failure of management and/or staff to follow instructions, they resulted in a protracted dispute in the chemical and public health literature that even involved Midgley [25,26]. We should be careful retrospectively applying judgement to the 1920s, as the increased octane of leaded gasoline provided major gains in engine efficiency and lower gasoline prices. The development of the alkyl leads ( tetra methyl lead, tetra ethyl lead ) and the toxic halogenated scavengers meant that petroleum refiners could then configure refineries to produce hydrocarbon streams that would increase octane with small quantities of alkyl lead. If you keep adding alkyl lead compounds, the lead response of the gasoline decreases, and so there are economic limits to how much lead should be added. Up until the late 1960s, alkyl leads were added to gasolines in increasing concentrations to obtain octane. The limit was 1.14g Pb/l, which is well above the diminishing returns part of the lead response curve for most refinery streams, thus it is unlikely that much fuel was ever made at that level. I believe 1.05 was about the maximum, and articles suggest that 1970 100 RON premiums were about 0.7-0.8 g Pb/l and 94 RON regulars 0.6-0.7 g Pb/l, which matches published lead response data [27,28] eg. For Catalytic Reformate Straight Run Naphtha. Lead g/l Research Octane Number 0 96 72 0.1 98 79 0.2 99 83 0.3 100 85 0.4 101 87 0.5 101.5 88 0.6 102 89 0.7 102.5 89.5 0.8 102.75 90 The alkyl lead antiknocks work in a different stage of the pre-combustion reaction to oxygenates. In contrast to oxygenates, the alkyl lead interferes with hydrocarbon chain branching in the intermediate temperature range where HO2 is the most important radical species. Lead oxide, either as solid particles, or in the gas phase, reacts with HO2 and removes it from the available radical pool, thereby deactivating the major chain branching reaction sequence that results in undesirable, easily-autoignitable hydrocarbons [21,22]. By the 1960s, the nature the toxicity of the emissions from gasoline-powered engines was becoming of increasing concern and extensive comparisons of the costs and benefits were being performed. By the 1970s, the failure to find durable, lead-tolerant exhaust catalysts would hasten the departure of lead, as the proposed regulated emissions levels could not be economically achieved without exhaust catalysts [29]. A survey in 1995 indicated that over 50 countries ( 20 in Africa ) still permit leaded fuels containing 0.8g Pb/l, whereas the European maximum is 0.15 g Pb/l [29a]. 4.7 Why not use other organometallic compounds? As the toxicity of the alkyl lead and the halogenated scavengers became of concern, alternatives were considered. The most famous of these is methylcyclopentadienyl manganese tricarbonyl (MMT), which was used in the USA until banned by the EPA from 27 Oct 1978 [30], but is approved for use in Canada and Australia. Recently the EPA ban was overturned, and MMT can be used up to 0.031gMn/US Gal in all states except California ( where it remains banned ). The EPA has stated it intends to review the whole MMT siuation and , if evidence supports removing MMT, they will revisit banning MMT. Automobile manufacturers believe MMT reduces the effectiveness of the latest emission control systems [31]. Canada also contemplated banning MMT because of the same concerns, as well as achieving fuel supply uniformity with the lower 48 states of the USA [31]. MMT is more expensive than alkyl leads and has been reported to increase unburned hydrocarbon emissions and block exhaust catalysts [32]. Other compounds that enhance octane have been suggested, but usually have significant problems such as toxicity, cost, increased engine wear etc.. Examples include dicyclopentadienyl iron and nickel carbonyl. Germany used iron pentacarbonyl (Fe(CO)5) at levels of 0.5% or less in gasoline during the 1930s. While its cost was low, one of its major drawbacks was that the carbonyl decomposed rapidly when the gasoline was exposed to light. Iron oxide (Fe3O4) also deposited on the spark plug insulator causing short circuits, and the precipitation of iron oxides in the lubricating oil also led to excessive wear rates [33]. 4.8 What do the refining processes do? Crude oil contains a wide range of hydrocarbons, organometallics and other compounds containing sulfur, nitrogen etc. The HCs contain between 1 and 60 carbon atoms. Gasoline contains hydrocarbons with carbon atoms between 3 and 12, arranged in specific ways to provide the desirable properties. Obviously, a refinery has to either sell the remainder as marketable products, or convert the larger molecules into smaller gasoline molecules. A refinery will distill crude oil into various fractions and, depending on the desired final products, will further process and blend those fractions. Typical final products could be:- gases for chemical synthesis and fuel (CNG), liquified gases (LPG), butane, aviation and automotive gasolines, aviation and lighting kerosines, diesels, distillate and residual fuel oils, lubricating oil base grades, paraffin oils and waxes. Many of the common processes are intended to increase the yield of blending feedstocks for gasolines. Typical modern refinery processes for gasoline components include * Catalytic cracking - breaks larger, higher-boiling, hydrocarbons into gasoline range product that contains 30% aromatics and 20-30% olefins. * Hydrocracking - cracks and adds hydrogen to molecules, producing a more saturated, stable, gasoline fraction. * Isomerisation - raises gasoline fraction octane by converting straight chain hydrocarbons into branched isomers. * Reforming - converts saturated, low octane, hydrocarbons into higher octane product containing about 60% aromatics. * Alkylation - reacts gaseous olefin streams with isobutane to produce liquid high octane iso-alkanes. The changes to the US Clean Air Act and other legislation ensures that the refineries will continue to modify their processes to produce a less volatile gasoline with fewer toxins and toxic emissions. Options include:- * Reducing the "severity" of reforming to reduce aromatic production. * Distilling the C5/C6 fraction ( containing benzene and benzene precusers ) from reformer feeds and treating that stream to produce non-aromatic high octane components. * Distilling the higher boiling fraction ( which contains 80-100% of aromatics that can be hydrocracked ) from catalytic cracker product [34]. * Convert butane to isobutane or isobutylene for alkylation or MTBE feed. Some other countries are removing the alkyl lead compounds for health reasons, and replacing them with aromatics and oxygenates. If the vehicle fleet does not have exhaust catalysts, the emissions of some toxic aromatic hydrocarbons can increase. If maximum environmental and health gains are to be achieved, the removal of lead from gasoline should be accompanied by the immediate introduction of exhaust catalysts and sophisticated engine management systems, 4.9 What energy is released when gasoline is burned? It is important to note that the theoretical energy content of gasoline when burned in air is only related to the hydrogen and carbon contents. The energy is released when the hydrogen and carbon are oxidised (burnt), to form water and carbon dioxide. Octane rating is not fundamentally related to the energy content, and the actual hydrocarbon and oxygenate components used in the gasoline will determine both the energy release and the antiknock rating. Two important reactions are:- C + O2 = CO2 H + O2 = H2O The mass or volume of air required to provide sufficient oxygen to achieve this complete combustion is the "stoichiometric" mass or volume of air. Insufficient air = "rich", and excess air = "lean", and the stoichiometric mass of air is related to the carbon:hydrogen ratio of the fuel. The procedures for calculation of stoichiometric air-fuel ratios are fully documented in an SAE standard [35]. Atomic masses used are:- Hydrogen = 1.00794, Carbon = 12.011, Oxygen = 15.994, Nitrogen = 14.0067, and Sulfur = 32.066. The composition of sea level air ( 1976 data, hence low CO2 value ) is Gas Fractional Molecular Weight Relative Species Volume kg/mole Mass N2 0.78084 28.0134 21.873983 O2 0.209476 31.9988 6.702981 Ar 0.00934 39.948 0.373114 CO2 0.000314 44.0098 0.013919 Ne 0.00001818 20.179 0.000365 He 0.00000524 4.002602 0.000021 Kr 0.00000114 83.80 0.000092 Xe 0.000000087 131.29 0.000011 CH4 0.000002 16.04276 0.000032 H2 0.0000005 2.01588 0.000001 --------- Air 28.964419 For normal heptane C7H16 with a molecular weight = 100.204 C7H16 + 11O2 = 7CO2 + 8H2O thus 1.000 kg of C7H16 requires 3.513 kg of O2 = 15.179 kg of air. The chemical stoichiometric combustion of hydrocarbons with oxygen can be written as:- CxHy + (x + (y/4))O2 -> xCO2 + (y/2)H2O Often, for simplicity, the remainder of air is assumed to be nitrogen, which can be added to the equation when exhaust compositions are required. As a general rule, maximum power is achieved at slightly rich, whereas maximum fuel economy is achieved at slightly lean. The energy content of the gasoline is measured by burning all the fuel inside a bomb calorimeter and measuring the temperature increase. The energy available depends on what happens to the water produced from the combustion of the hydrogen. If the water remains as a gas, then it cannot release the heat of vaporisation, thus producing the Nett Calorific Value. If the water were condensed back to the original fuel temperature, then Gross Calorific Value of the fuel, which will be larger, is obtained. The calorific values are fairly constant for families of HCs, which is not surprising, given their fairly consistent carbon:hydrogen ratios. For liquid ( l ) or gaseous ( g ) fuel converted to gaseous products - except for the 2-methylbutene-2, where only gaseous is reported. * = Blending Octane Number as reported by API Project 45 using 60 octane base fuel, and the numbers in brackets are Blending Octane Numbers currently used for modern fuels. Typical Heats of Combustion are [36]:- Fuel State Heat of Combustion Research Motor MJ/kg Octane Octane n-heptane l 44.592 0 0 g 44.955 i-octane l 44.374 100 100 g 44.682 toluene l 40.554 124* (111) 112* (94) g 40.967 2-methylbutene-2 44.720 176* (113) 141* (81) Because all the data is available, the calorific value of fuels can be estimated quite accurately from hydrocarbon fuel properties such as the density, sulfur content, and aniline point ( which indicates the aromatics content ). It should be noted that because oxygenates contain oxygen that can not provide energy, they will have significantly lower energy contents. They are added to provide octane, not energy. For an engine that can be optimised for oxygenates, more fuel is required to obtain the same power, but they can burn slightly more efficiently, thus the power ratio is not identical to the energy content ratio. They also require more energy to vaporise. Energy Content Heat of Vaporisation Oxygen Content Nett MJ/kg MJ/kg wt% Methanol 19.95 1.154 49.9 Ethanol 26.68 0.913 34.7 MTBE 35.18 0.322 18.2 ETBE 36.29 0.310 15.7 TAME 36.28 0.323 15.7 Gasoline 42 - 44 0.297 0.0 Typical values for commercial fuels in megajoules/kilogram are [37]:- Gross Nett Hydrogen 141.9 120.0 Carbon to Carbon monoxide 10.2 - Carbon to Carbon dioxide 32.8 - Sulfur to sulfur dioxide 9.16 - Natural Gas 53.1 48.0 Liquified petroleum gas 49.8 46.1 Aviation gasoline 46.0 44.0 Automotive gasoline 45.8 43.8 Kerosine 46.3 43.3 Diesel 45.3 42.5 Obviously, for automobiles, the nett calorific value is appropriate, as the water is emitted as vapour. The engine can not utilise the additional energy available when the steam is condensed back to water. The calorific value is the maximum energy that can be obtained from the fuel by combustion, but the reality of modern SI engines is that thermal efficiencies of only 20-40% may be obtained, this limit being due to engineering and material constraints that prevent optimum thermal conditions being used. CI engines can achieve higher thermal efficiencies, usually over a wider operating range as well. Note that combustion efficiencies are high, it is the thermal efficiency of the engine is low due to losses. For a water-cooled SI engine with 25% useful work at the crankshaft, the losses may consist of 35% (coolant), 33% (exhaust), and 12% (surroundings). 4.10 What are the gasoline specifications? Gasolines are usually defined by government regulation, where properties and test methods are clearly defined. In the US, several government and state bodies can specify gasoline properties, and they may choose to use or modify consensus minimum quality standards, such as American Society for Testing Materials (ASTM). The US gasoline specifications and test methods are listed in several readily available publications, including the Society of Automotive Engineers (SAE) [38], and the Annual Book of ASTM Standards [39]. The 1995 ASTM edition includes:- D4814-94d Specification for Automotive Spark-Ignition Engine Fuel. This specification lists various properties that all fuels have to comply with, and may be updated throughout the year. Typical properties are:- 4.10.1 Vapour Pressure and Distillation Classes. 6 different classes according to location and/or season. As gasoline is distilled, the temperatures at which various fractions are evaporated are calculated. Specifications define the temperatures at which various percentages of the fuel are evaporated. Distillation limits include maximum temperatures that 10% is evaporated (50-70C), 50% is evaporated (110-121C), 90% is evaporated (185-190C), and the final boiling point (225C). A minimum temperature for 50% evaporated (77C), and a maximum amount of Residue (2%) after distillation. Vapour pressure limits for each class ( 54, 62, 69, 79, 93, 103 kPa ) are also specified. Note that the EPA has issued a waiver that does not require gasoline with 9-10% ethanol to meet the required specifications between 1st May - 15 September. 4.10.2 Vapour Lock Protection Classes 5 classes for vapour lock protection, according to location and/or season. The limit for each class is a maximum Vapour-Liquid ratio of 20 at one of the specified testing temperatures of 41, 47, 51, 56, 60C. 4.10.3 Antiknock Index ( aka (RON+MON)/2, "Pump Octane" ) The ( Research Octane Number + Motor Octane Number ) divided by two. Limits are not specified, but changes in engine requirements according season and location are discussed. Fuels with an Antiknock index of 87, 89, 91 ( Unleaded), and 88 ( Leaded ) are listed as typical for the US at sea level, however higher altitudes will specify lower octane numbers. 4.10.4 Lead Content Leaded = 1.1 g Pb / L maximum, and Unleaded = 0.013 g Pb / L maximum. 4.10.5 Copper strip corrosion Ability to tarnish clean copper, indicating the presence of any corrosive sulfur compounds 4.10.6 Maximum Sulfur content Sulfur adversely affects exhaust catalysts and fuel hydrocarbon lead response, and also may be emitted as polluting sulfur oxides. Leaded = 0.15 %mass maximum, and Unleaded = 0.10 %mass maximum. Typical US gasoline levels are 0.03 %mass. 4.10.7 Maximum Solvent Washed Gum ( aka Existent Gum ) Limits the amount of gums present in fuel at the time of testing to 5 mg/100mls. The results do not correlate well with actual engine deposits caused by fuel vaporisation [40]. 4.10.8 Minimum Oxidation Stability This ensures the fuel remains chemically stable, and does not form additional gums during periods in distribution systems, which can be up to 3-6 months. The sample is heated with oxygen inside a pressure vessel, and the delay until significant oxygen uptake is measured. 4.10.9 Water Tolerance Highest temperature that causes phase separation of oxygenated fuels. The limits vary according to location and month. For Alaska - North of 62 latitude, it changes from -41C in Dec-Jan to 9C in July, but remains 10C all year in Hawaii. Because phosphorus adversely affects exhaust catalysts, the EPA limits phosphorus in all gasolines to 0.0013g P/L. As well as the above, there are various restrictions introduced by the Clean Air Act and state bodies such as California's Air Resources Board (CARB) that often have more stringent limits for the above properties, as well as additional limits. More detailed descriptions of the complex regulations can be found elsewhere [16,41,42] - I've just included some of the major changes, as some properties are determined by levels of permitted emissions, eg the toxics reduction required for fuel that has the maximum permitted benzene (1.0%), means total aromatics are limited to around 27%. There have been some changes in early 1996 to the implementation timetable, and the following timetable has not yet been changed. The Clean Air Act also specifies some regions that exceed air quality standards have to use reformulated gasolines (RFGs) all year, starting January 1995. Other regions are required to use oxygenated gasolines for four winter months, beginning November 1992. The RFGs also contain oxygenates. Metropolitan regions with severe ozone air quality problems must use reformulated gasolines in 1995 that;- contain at least 2.0 wt% oxygen, reduce 1990 volatile organic carbon compounds by 15%, and reduce specified toxic emissions by 15% (1995) and 25% (2000). Metropolitan regions that exceeded carbon monoxide limits were required to use gasolines with 2.7 wt% oxygen during winter months, starting in 1992. The 1990 Clean Air Act (CAA) amendments and CARB Phase 2 (1996) specifications for reformulated gasoline establish the following limits, compared with typical 1990 gasoline. Because of a lack of data, the EPA were unable to define the CAA required parameters, so they instituted a two-stage system. The first stage, the "Simple Model" is an interim stage that run from 1/Jan/1995 to 31/Dec/1997. The second stage, the "Complex Model" has two phases, Phase I (1995-1999) and Phase II (2000+), and there are different limits for EPA Control Region 1 (south) and Control Region 2 (north). Each refiner must have their RFG recertified to the Complex model prior to the 1/Jan/1998 implementation date. The following are some of the criteria for RFG when complying on a per gallon basis, more details are available elsewhere, including the details of the baseline fuel compositions to be used for testing [16,41,42,43,43a]. 1990 Clean Air Act CARB Simple Complex Phase 2 I II Limit Average benzene (max.vol.%) 2 1.00 1.00 1.00 1.00 0.8 oxygen (min.mass %) 0.2 2.0 2.0 2.0 1.8 - (max.mass %) - 2.7 - - 2.2 - sulfur (max.mass ppm) 150 no increase - - 40 30 aromatics (max.vol.%) 32 toxics reduction - - 25 22 olefins (max.vol.%) 9.9 no increase - - 6.0 4.0 reid vapour pressure (kPa) 60 55.8 (north) - - 48.3 - (during VOC Control Period) 49.6 (south) 50% evaporated (max.C) - - - - 98.9 93 90% evaporated (max.C) 170 - - - 148.9 143 VOC Reductions - Region I (min.%) 35.1 27.5 - - (VOC Control Period only) - Region II (min.%) 15.6 25.9 - - NOx Reductions - VOC Control Period (min.%) 0 5.5 - - - Non-VOC Control Period (min.%) 0 0 - - Toxics Reductions (min.%) 15.0 20.0 - - These regulations also specify emissions criteria. eg CAA specifies no increase in nitric oxides (NOx) emissions, reductions in VOC by 15% during the ozone season, and specified toxins by 15% all year. These criteria indirectly establish vapour pressure and composition limits that refiners have to meet. Note that the EPA also can issue CAA Section 211 waivers that allow refiners to choose which oxygenates they use. In 1981, the EPA also decided that fuels with up to 2% weight of oxygen ( from alcohols and ethers (except methanol)) were "substantially similar" to 1974 unleaded gasoline, and thus were not "new" gasoline additives. That level was increased to 2.7 wt% in 1991. Some other oxygenates have also been granted waivers, eg ethanol to 10% volume ( approximately 3.5 wt% ) in 1979 and 1982, and tert-butyl alcohol to 3.5 wt% in 1981. In 1987 and 1988 further waivers were issued for mixture of alcohols representing 3.7% wt of oxygen. 4.11 What are the effects of the specified fuel properties? Volatility This affects evaporative emissions and driveability, it is the property that must change with location and season. Fuel for mid-summer Arizona would be difficult to use in mid-winter Alaska. The US is divided into zones, according to altitude and seasonal temperatures, and the fuel volatility is adjusted accordingly. Incorrect fuel may result in difficult starting in cold weather, carburetter icing, vapour lock in hot weather, and crankcase oil dilution. Volatility is controlled by distillation and vapour pressure specifications. The higher boiling fractions of the gasoline have significant effects on the emission levels of undesirable hydrocarbons and aldehydes, and a reduction of 40C in the final boiling point will reduce the levels of benzene, butadiene, formaldehyde and acetaldehyde by 25%, and will reduce HC emissions by 20% [44]. Combustion Characteristics As gasolines contain mainly hydrocarbons, the only significant variable between different grades is the octane rating of the fuel, as most other properties are similar. Octane is discussed in detail in Section 6. There are only slight differences in combustion temperatures ( most are around 2000C in isobaric adiabatic combustion [45]). Note that the actual temperature in the combustion chamber is also determined by other factors, such as load and engine design. The addition of oxygenates changes the pre-flame reaction pathways, and also reduces the energy content of the fuel. The levels of oxygen in the fuel is regulated according to regional air quality standards. Stability Motor gasolines may be stored up to six months, consequently they must not form gums which may precipitate. Reactions of the unsaturated HCs may produce gums ( these reactions can be catalysed by metals such as copper ), so antioxidants and metal deactivators are added. Existent Gum is used to measure the gum in the fuel at the time tested, whereas the Oxidation Stability measures the time it takes for the gasoline to break down at 100C with 100psi of oxygen. A 240 minute test period has been found to be sufficient for most storage and distribution systems. Corrosiveness Sulfur in the fuel creates corrosion, and when combusted will form corrosive gases that attack the engine, exhaust and environment. Sulfur also adversely affects the alkyl lead octane response, and will adversely affect exhaust catalysts, but monolithic catalysts will recover when the sulfur content of the fuel is reduced, so sulfur is considered an inhibitor, rather than a catalyst poison. The copper strip corrosion test and the sulfur content specification are used to ensure fuel quality. The copper strip test measures active sulfur, whereas the sulfur content reports the total sulfur present. Manufacturers many also add additional tests, such as filterability, to ensure product distribution problems are minimised. Article: 151777 of rec.autos.tech From: B.Hamilton@irl.cri.nz (Bruce Hamilton) Subject: Gasoline FAQ - Part 2 of 4 Date: Wed, 18 Sep 1996 09:34:54 GMT Expires: 18 October 1996 00.00.01 GMT Organization: Industrial Research Limited Lines: 998 Archive-name: autos/gasoline-faq/part2 Posting-Frequency: monthly Last-modified: 17 September 1996 Version: 1.11 4.12 Are brands different? Yes. The above specifications are intended to ensure minimal quality standards are maintained, however as well as the fuel hydrocarbons, the manufacturers add their own special ingredients to provide additional benefits. A quality gasoline additive package would include:- * octane-enhancing additives ( improve octane ratings ) * anti-oxidants ( inhibit gum formation, improve stability ) * metal deactivators ( inhibit gum formation, improve stability ) * deposit modifiers ( reduce deposits, spark-plug fouling and preignition ) * surfactants ( prevent icing, improve vaporisation, inhibit deposits, reduce NOx emissions ) * freezing point depressants ( prevent icing ) * corrosion inhibitors ( prevent gasoline corroding storage tanks ) * dyes ( product colour for safety or regulatory purposes ). During the 1980s significant problems with deposits accumulating on intake valve surfaces occurred as new fuel injection systems were introduced. These intake valve deposits (IVD) were different than the injector deposits, in part because the valve can reach 300C. Engine design changes that prevent deposits usually consist of ensuring the valve is flushed with liquid gasoline, and provision of adequate valve rotation. Gasoline factors that cause deposits are the presence of alcohols or olefins [46]. Gasoline manufacturers now routinely use additives that prevent IVD and also maintain the cleanliness of injectors. These usually include a surfactant and light oil to maintain the wetting of important surfaces. Intake valve deposits have also been shown to have significant adverse effects on emissions [47], and deposit control additives will be required to both reduce emissions and provide clean engine operation [48]. A slighty more detailed description of additives is provided in Section 9.1. Texaco demonstrated that a well-formulated package could improve fuel economy, reduce NOx emissions, and restore engine performance because, as well as the traditional liquid-phase deposit removal, some additives can work in the vapour phase to remove existing engine deposits without adversely affecting performance ( as happens when water is poured into a running engine to remove carbon deposits :-) )[49]. Chevron have also published data on the effectiveness of their additives [50], and successfully litigated to get Texaco to modify some of their claims [51]. Most suppliers of quality gasolines will formulate similar additives into their products, and cheaper product lines are less likely to have such additives added. As different brands of gasoline use different additives and oxygenates, it is probable that important fuel parameters, such as octane distribution, are slightly different, even though the pump octane ratings are the same. So, if you know your car is well-tuned, and in good condition, but the driveability is pathetic on the correct octane, try another brand. Remember that the composition will change with the season, so if you lose driveability, try yet another brand. As various Clean Air Act changes are introduced over the next few years, gasoline will continue to change. 4.13 What is a typical composition? There seems to be a perception that all gasolines of one octane grade are chemically similar, and thus general rules can be promulgated about "energy content ", "flame speed", "combustion temperature" etc. etc.. Nothing is further from the truth. The behaviour of manufactured gasolines in octane rating engines can be predicted, using previous octane ratings of special blends intended to determine how a particular refinery stream responds to an octane-enhancing additive. Refiners can design and reconfigure refineries to efficiently produce a wide range of gasolines feedstocks, depending on market and regulatory requirements. There is a worldwide trend to move to unleaded gasolines, followed by the introduction of exhaust catalysts and sophisticated engine management systems. It is important to note that "oxygenated gasolines" have a hydrocarbon fraction that is not too different to traditional gasolines, but that the hydrocarbon fraction of "reformulated gasolines" ( which also contain oxygenates ) are significantly different to traditional gasolines. The last 10 years of various compositional changes to gasolines for environmental and health reasons have resulted in fuels that do not follow historical rules, and the regulations mapped out for the next decade also ensure the composition will remain in a state of flux. The reformulated gasoline specifications, especially the 1/Jan/1998 Complex model, will probably introduce major reductions in the distillation range, as well as changing the various limits on composition and emissions. I'm not going to list all 500+ HCs in gasolines, but the following are representative of the various classes typically present in a gasoline. The numbers after each chemical are:- Research Blending Octane : Motor Blending Octane : Boiling Point (C): Density (g/ml @ 15C) : Minimum Autoignition Temperature (C). It is important to realise that the Blending Octanes are derived from a 20% mix of the HC with a 60:40 iC8:nC7 ( 60 Octane Number ) base fuel, and the extrapolation of this 20% to 100%. These numbers result >from API Project 45, and are readily available. As modern refinery streams have higher base octanes, these Blending Octanes are higher than those typically used in modern refineries. For example, modern Blending Octane ratings can be much lower ( toluene = 111RON and 94MON, 2-methyl-2-butene = 113RON and 81MON ), but detailed compilations are difficult to obtain. The technique for obtaining Blending Octanes is different from rating the pure fuel, which often requires adjustment of the test engine conditions outside the acceptable limits of the rating methods. Generally, the actual octanes of the pure fuel are similar for the alkanes, but are up to 30 octane numbers lower than the API Project 45 Blending Octanes for the aromatics and olefins [52]. A traditional composition I have dreamed up would be like the following, whereas newer oxygenated fuels reduce the aromatics and olefins, narrow the boiling range, and add oxygenates up to about 12-15% to provide the octane. The amount of aromatics in super unleaded fuels will vary greatly from country to country, depending on the configuration of the oil refineries and the use of oxygenates as octane enhancers. The US is reducing the levels of aromatics to 25% or lower for environmental and human health reasons. Some countries are increasing the level of aromatics to 50% or higher in super unleaded grades, usually to avoid refinery reconfiguration costs or the introduction of oxygenates as they phase out the toxic lead octane enhancers. An upper limit is usually placed on the amount of benzene permitted, as it is known human carcinogen. 15% n-paraffins RON MON BP d AIT n-butane 113 : 114 : -0.5: gas : 370 n-pentane 62 : 66 : 35 : 0.626 : 260 n-hexane 19 : 22 : 69 : 0.659 : 225 n-heptane (0:0 by definition) 0 : 0 : 98 : 0.684 : 225 n-octane -18 : -16 : 126 : 0.703 : 220 ( you would not want to have the following alkanes in gasoline, so you would never blend kerosine with gasoline ) n-decane -41 : -38 : 174 : 0.730 : 210 n-dodecane -88 : -90 : 216 : 0.750 : 204 n-tetradecane -90 : -99 : 253 : 0.763 : 200 30% iso-paraffins 2-methylpropane 122 : 120 : -12 : gas : 460 2-methylbutane 100 : 104 : 28 : 0.620 : 420 2-methylpentane 82 : 78 : 62 : 0.653 : 306 3-methylpentane 86 : 80 : 64 : 0.664 : - 2-methylhexane 40 : 42 : 90 : 0.679 : 3-methylhexane 56 : 57 : 91 : 0.687 : 2,2-dimethylpentane 89 : 93 : 79 : 0.674 : 2,2,3-trimethylbutane 112 : 112 : 81 : 0.690 : 420 2,2,4-trimethylpentane 100 : 100 : 98 : 0.692 : 415 ( 100:100 by definition ) 12% cycloparaffins cyclopentane 141 : 141 : 50 : 0.751 : 380 methylcyclopentane 107 : 99 : 72 : 0.749 : cyclohexane 110 : 97 : 81 : 0.779 : 245 methylcyclohexane 104 : 84 : 101 : 0.770 : 250 35% aromatics benzene 98 : 91 : 80 : 0.874 : 560 toluene 124 : 112 : 111 : 0.867 : 480 ethyl benzene 124 : 107 : 136 : 0.867 : 430 meta-xylene 162 : 124 : 138 : 0.868 : 463 para-xylene 155 : 126 : 138 : 0.866 : 530 ortho-xylene 126 : 102 : 144 : 0.870 : 530 3-ethyltoluene 162 : 138 : 158 : 0.865 : 1,3,5-trimethylbenzene 170 : 136 : 163 : 0.864 : 1,2,4-trimethylbenzene 148 : 124 : 168 : 0.889 : 8% olefins 2-pentene 154 : 138 : 37 : 0.649 : 2-methylbutene-2 176 : 140 : 36 : 0.662 : 2-methylpentene-2 159 : 148 : 67 : 0.690 : cyclopentene 171 : 126 : 44 : 0.774 : ( the following olefins are not present in significant amounts in gasoline, but have some of the highest blending octanes ) 1-methylcyclopentene 184 : 146 : 75 : 0.780 : 1,3 cyclopentadiene 218 : 149 : 42 : 0.805 : dicyclopentadiene 229 : 167 : 170 : 1.071 : Oxygenates Published octane values vary a lot because the rating conditions are significantly different to standard conditions, for example the API Project 45 numbers used above for the hydrocarbons, reported in 1957, gave MTBE blending RON as 148 and MON as 146, however that was partly based on the lead response, whereas today we use MTBE in place of lead. methanol 133 : 105 : 65 : 0.796 : 385 ethanol 129 : 102 : 78 : 0.794 : 365 iso propyl alcohol 118 : 98 : 82 : 0.790 : 399 methyl tertiary butyl ether 116 : 103 : 55 : 0.745 : ethyl tertiary butyl ether 118 : 102 : 72 : 0.745 : tertiary amyl methyl ether 111 : 98 : 86 : 0.776 : There are some other properties of oxygenates that have to be considered when they are going to be used as fuels, particularly their ability to form very volatile azeotropes that cause the fuel's vapour pressure to increase, the chemical nature of the emissions, and their tendency to separate into a separate water-oxygenate phase when water is present. The reformulated gasolines address these problems more successfully than the original oxygenated gasolines. Before you rush out to make a highly aromatic or olefinic gasoline to produce a high octane fuel, remember they have other adverse properties, eg the aromatics attack elastomers, may generate smoke, and result in increased emissions of toxic benzene. The olefins are unstable ( besides smelling foul ) and form gums. The art of correctly formulating a gasoline that does not cause engines to knock apart, does not cause vapour lock in summer - but is easy to start in winter, does not form gums and deposits, burns cleanly without soot or residues, and does not dissolve or poison the car catalyst or owner, is based on knowledge of the gasoline composition. 4.14 Is gasoline toxic or carcinogenic? There are several known toxins in gasoline, some of which are confirmed human carcinogens. The most famous of these toxins are lead and benzene, and both are regulated. The other aromatics and some toxic olefins are also controlled. Lead alkyls also require ethylene dibromide and/or ethylene dichloride scavengers to be added to the gasoline, both of which are suspected human carcinogens. In 1993 an International Symposium on the Health Effects of Gasoline was held [53]. Major review papers on the carcinogenic, neurotoxic, reproductive and developmental toxicity of gasoline, additives, and oxygenates were presented, and interested readers should obtain the proceedings. The oxygenates are also being evaluated for carcinogenicity, and even ethanol and ETBE may be carcinogens. The introduction of oxygenated gasoline to Alaska and some other areas of the USA resulted in a range of complaints. Recent research has been unable to identify additional toxicity, but has detected increased levels of offensive smell [54]. It should be noted that the oxygenated gasolines were not initially intended to reduce the toxicity of emissions. The reformulated gasolines will produce different emissions, and specific toxins must initially be reduced by 15% all year. The removal of alkyl lead compounds certainly reduces the toxicity of exhaust gas emissions when used on engines with modern engine management systems and 3-way exhaust catalysts. If unleaded gasolines are not accompanied by the introduction of catalysts, some other toxic emissions may increase. Engines without catalysts will produce increased levels of toxic carbonyls such as formaldehyde and acrolein when using oxygenated fuels, and increased levels of toxic benzene when using highly aromatic fuels. There is little doubt that gasoline is full of toxic chemicals, and should therefore be treated with respect. However the biggest danger remains the flammability, and the relative hazards should always be kept in perspective. The major toxic risk from gasolines comes from breathing the tailpipe, evaporative, and refuelling emissions, rather than occasional skin contact >from spills. Breathing vapours and skin contact should always be minimised. 4.15 Is unleaded gasoline more toxic than leaded? The short answer is no. However that answer is not global, as some countries have replaced the lead compound octane-improvers with aromatic or olefin octane-improvers without introducing exhaust catalysts. The aromatics contents may increase to around 40%, with high octane unleaded fuels reaching 50% in countries where oxygenates are not being used, and the producers have not reconfigured refineries to produce high octane paraffins. In general, aromatics are significantly more toxic than paraffins. Exhaust catalysts have a limited operational life, and will be immediately poisoned if misfuelled with leaded fuel. Catalyst failure can result in higher levels of toxic emissions if catalysts or engine management systems are not replaced or repaired when defective. Maximum benefit of the switch to unleaded are obtained when the introduction of unleaded is accompanied by the introduction of exhaust catalysts and sophisticated engine management systems. Unfortunately, the manufacturers of alkyl lead compounds have embarked on a worldwide misinformation campaign in countries considering emulating the lead-free US. The use of lead precludes the use of exhaust catalysts, thus the emissions of aromatics are only slightly diminished, as leaded fuels typically contain around 30-40% aromatics. Other toxins and pollutants that are usually reduced by exhaust catalysts will be emitted at significantly higher levels if leaded fuels are used [55]. The use of unleaded on modern vehicles with engine management systems and catalysts can reduce aromatic emissions to 10% of the level of vehicles without catalysts [55]. Alkyl lead additives can only substitute for some of the aromatics in gasoline, consequently they do not eliminate aromatics, which will produce benzene emissions [56]. Alkyl lead additives also require toxic organohalogen scavengers, which also react in the engine to form and emit other organohalogens, including highly toxic dioxin [57]. Leaded fuels emit lead, organohalogens, and much higher levels of regulated toxins because they preclude the use of exhaust catalysts. In the USA the gasoline composition is being changed to reduce fuel toxins ( olefins, aromatics ) as well as emissions of specific toxins. 4.16 Is reformulated gasoline more toxic than unleaded? The evidence so far indicates that the components of reformulated gasolines ( RFGs ) are more benign than unleaded, and that the tailpipe emissions of hydrocarbons are significantly reduced for cars without catalysts, and slightly reduced for cars with catalysts and engine management systems. The emissions of toxic carbonyls such as formaldehyde, acetaldehyde and acrolein are increased slightly on all vehicles, and the emission of MTBE is increased about 10x on cars without catalysts and 4x on cars with catalysts [55]. When all the emissions ( evaporative and tailpipe ) are considered, RFGs significantly reduce emissions of hydrocarbons, however the emissions of carbonyls and MTBE may increase [55]. There has been an extensive series of reports on the emissions from RFGs, produced by the Auto/Oil Air Quality Improvement Research Program, who measured and calculated the likely effects of RFG [18,19,20,58,59,60,61]. More research is required before a definitive answer on toxicity is available. The major question about RFGs is not the toxicity of the emissions, but whether they actually meet their objective of reducing urban pollution. This is a more complex issue, and most experts agree the benefits will only be modest [18,19,20,61,62]. 4.17 Are all oxygenated gasolines also reformulated gasolines? No. Oxygenates were initially introduced as alternative octane-enhancers in the 1930s, and are still used in some countries for that purpose. In the US the original "oxygenated gasolines" usually had a slightly- modified gasoline as the hydrocarbon fraction. The US EPA also mandated their use to reduce pollution, mainly via the "enleanment" effect on engines without sophisticated management systems, but also because of the "aromatics substitution" effect. As vehicles with fuel injection and sophisticated engine management systems became pervasive, reformulated gasolines could be introduced to further reduce pollution. The hydrocarbon component of RFGs is significantly different to the hydrocarbon fraction in earlier oxygenated gasolines, having lower aromatics contents, reduced vapour pressure, and a narrower boiling range. RFGs do contain oxygenates as the octane-enhancer, but have different hydrocarbon composition profiles [34,41,42,43,44]. ------------------------------ Subject: 5. Why is Gasoline Composition Changing? 5.1 Why pick on cars and gasoline? Cars emit several pollutants as combustion products out the tailpipe, (tailpipe emissions), and as losses due to evaporation (evaporative emissions, refuelling emissions). The volatile organic carbon (VOC) emissions from these sources, along with nitrogen oxides (NOx) emissions >from the tailpipe, will react in the presence of ultraviolet (UV) light (wavelengths of less than 430nm) to form ground-level (tropospheric) ozone, which is one of the major components of photochemical smog [63]. Smog has been a major pollution problem ever since coal-fired power stations were developed in urban areas, but their emissions are being cleaned up. Now it's the turn of the automobile. Cars currently use gasoline that is derived from fossil fuels, thus when gasoline is burned to completion, it produces additional CO2 that is added to the atmospheric burden. The effect of the additional CO2 on the global environment is not known, but the quantity of man-made emissions of fossil fuels must cause the system to move to a new equilibrium. Even if current research doubles the efficiency of the IC engine-gasoline combination, and reduces HC, CO, NOx, SOx, VOCs, particulates, and carbonyls, the amount of carbon dioxide from the use of fossil fuels may still cause global warming. More and more scientific evidence is accumulating that warming is occurring [64,65]. The issue is whether it is natural, or induced by human activities and and a large panel of scientific experts continues to review scientific data and models. Interested reader should seek out the various publications of the Intergovernmental Panel on Climate Change (IPCC). There are international agreements to limit CO2 emissions to 1990 levels, a target that will require more efficient, lighter, or appropriately-sized vehicles, - if we are to maintain the current usage. One option is to use "renewable" fuels in place of fossil fuels. Consider the amount of energy-related CO2 emissions for selected countries in 1990 [66]. CO2 Emissions ( tonnes/year/person ) USA 20.0 Canada 16.4 Australia 15.9 Germany 10.4 United Kingdom 8.6 Japan 7.7 New Zealand 7.6 The number of new vehicles provides an indication of the magnitude of the problem. Although vehicle engines are becoming more efficient, the distance travelled is increasing, resulting in a gradual increase of gasoline consumption. The world production of vehicles (in thousands) over the last few years was [67];- Cars Region 1990 1991 1992 1993 1994 Africa 222 213 194 201 209 Asia-Pacific 12,064 12,112 11,869 11,463 11,020 Central & South America 800 888 1,158 1,523 1,727 Eastern Europe 2,466 984 1,726 1,837 1,547 Middle East 35 24 300 390 274 North America 7,762 7,230 7,470 8,172 8,661 Western Europe 13,688 13,286 13,097 11,141 12,851 Total World 37,039 34,739 35,815 34,721 36,289 Trucks ( including heavy trucks and buses ) Region 1990 1991 1992 1993 1994 Africa 133 123 108 101 116 Asia-Pacific 5,101 5,074 5,117 5,057 5,407 Central & South America 312 327 351 431 457 Eastern Europe 980 776 710 600 244 Middle East 36 28 100 128 76 North America 4,851 4,554 5,371 6,037 7,040 Western Europe 1,924 1,818 1,869 1,718 2,116 Total World 13,336 12,701 13,627 14,073 15,457 To fuel all operating vehicles, considerable quantities of gasoline and diesel have to be consumed. Major consumption in 1994 of gasoline and middle distillates ( which may include some heating fuels, but not fuel oils ) in million tonnes. Gasoline Middle Distillates USA 338.6 246.3 Canada 26.8 26.1 Western Europe 163.2 266.8 Japan 60.2 92.2 Total World 820.4 1029.0 The USA consumption of gasoline increased from 294.4 (1982) to 335.6 (1989) then dipped to 324.2 (1991), and has continued to rise since then to reach 338.6 million tonnes in 1994. In 1994 the total world production of crude oil was 3209.1 million tonnes, of which the USA consumed 807.9 million tonnes [68]. Transport is a very significant user of crude oil products, thus improving the efficiency of utilisation, and minimising pollution from vehicles, can produce immediate reductions in emissions of CO2, HCs, VOCs, CO, NOx, carbonyls, and other chemicals. 5.2 Why are there seasonal changes? Only gaseous hydrocarbons burn, consequently if the air is cold, then the fuel has to be very volatile. But when summer comes, a volatile fuel can boil and cause vapour lock, as well as producing high levels of evaporative emissions. The solution was to adjust the volatility of the fuel according to altitude and ambient temperature. This volatility change has been automatically performed for decades by the oil companies without informing the public of the changes. It is one reason why storage of gasoline through seasons is not a good idea. Gasoline volatility is being reduced as modern engines, with their fuel injection and management systems, can automatically compensate for some of the changes in ambient conditions - such as altitude and air temperature, resulting in acceptable driveability using less volatile fuel. 5.3 Why were alkyl lead compounds removed? " With the exception of one premium gasoline marketed on the east coast and southern areas of the US, all automotive gasolines from the mid-1920s until 1970 contained lead antiknock compounds to increase antiknock quality. Because lead antiknock compounds were found to be detrimental to the performance of catalytic emission control system then under development, U.S. passenger car manufacturers in 1971 began to build engines designed to operate satisfactorily on gasolines of nominal 91 Research Octane Number. Some of these engines were designed to operate on unleaded fuel while others required leaded fuel or the occasional use of leaded fuel. The 91 RON was chosen in the belief that unleaded gasoline at this level could be made available in quantities required using then current refinery processing equipment. Accordingly, unleaded and low-lead gasolines were introduced during 1970 to supplement the conventional gasolines already available. Beginning with the 1975 model year, most new car models were equipped with catalytic exhaust treatment devices as one means of compliance with the 1975 legal restrictions in the U.S. on automobile emissions. The need for gasolines that would not adversely affect such catalytic devices has led to the large scale availability and growing use of unleaded gasolines, with all late-model cars requiring unleaded gasoline."[69]. There was a further reason why alkyl lead compounds were subsequently reduced, and that was the growing recognition of the highly toxic nature of the emissions from a leaded-gasoline fuelled engine. Not only were toxic lead emissions produced, but the added toxic lead scavengers ( ethylene dibromide and ethylene dichloride ) could react with hydrocarbons to produce highly toxic organohalogen emissions such as dioxin. Even if catalysts were removed, or lead-tolerant catalysts discovered, alkyl lead compounds would remain banned because of their toxicity and toxic emissions [70,71]. 5.4 Why are evaporative emissions a problem? As tailpipe emissions are reduced due to improved exhaust emission control systems, the hydrocarbons produced by evaporation of the gasoline during distribution, vehicle refuelling, and from the vehicle, become more and more significant. A recent European study found that 40% of man-made volatile organic compounds came from vehicles [72]. Many of the problem hydrocarbons are the aromatics and olefins that have relatively high octane values. Any sensible strategy to reduce smog and toxic emissions will also attack evaporative and tailpipe emissions. The health risks to service station workers, who are continuously exposed to refuelling emissions remain a concern [73]. Vehicles will soon be required to trap the refuelling emissions in larger carbon canisters, as well as the normal evaporative emissions that they already capture. This recent decision went in favour of the oil companies, who were opposed by the auto companies. The automobile manufacturers felt the service station should trap the emissions. The activated carbon canisters adsorb organic vapours, and these are subsequently desorbed from the canister and burnt in the engine during normal operation, once certain vehicle speeds and coolant temperatures are reached. A few activated carbons used in older vehicles do not function efficiently with oxygenates, but carbon cannister systems can reduce evaporative emissions by 95% from uncontrolled levels. 5.5 Why control tailpipe emissions? Tailpipe emissions were responsible for the majority of pollutants in the late 1960s after the crankcase emissions had been controlled. Ozone levels in the Los Angeles basin reached 450-500ppb in the early 1970s, well above the typical background of 30-50ppb [74]. Tuning a carburetted engine can only have a marginal effect on pollutant levels, and there still had to be some frequent, but long-term, assessment of the state of tuning. Exhaust catalysts offered a post-engine solution that could ensure pollutants were converted to more benign compounds. As engine management systems and fuel injection systems have developed, the volatility properties of the gasoline have been tuned to minimise evaporative emissions, and yet maintain low exhaust emissions. The design of the engine can have very significant effects on the type and quantity of pollutants, eg unburned hydrocarbons in the exhaust originate mainly from combustion chamber crevices, such as the gap between the piston and cylinder wall, where the combustion flame can not completely use the HCs. The type and amount of unburnt hydrocarbon emissions are related to the fuel composition (volatility, olefins, aromatics, final boiling point), as well as state of tune, engine condition, and condition of the engine lubricating oil [75]. Particulate emissions, especially the size fraction smaller than ten micrometres, are a serious health concern. The current major source is from compression ignition ( diesel ) engines, and the modern SI engine system has no problem meeting regulatory requirements. The ability of reformulated gasolines to actually reduce smog has not yet been confirmed. The composition changes will reduce some compounds, and increase others, making predictions of environmental consequences extremely difficult. Planned future changes, such as the CAA 1/1/1998 Complex model specifications, that are based on several major ongoing government/industry gasoline and emission research programmes, are more likely to provide unambiguous environmental improvements. One of the major problems is the nature of the ozone-forming reactions, which require several components ( VOC, NOx, UV ) to be present. Vehicles can produce the first two, but the their ratio is important, and can be affected by production from other natural ( VOC = terpenes from conifers ) or manmade ( NOx from power stations ) sources [62,63]. The regulations for tailpipe emissions will continue to become more stringent as countries try to minimise local problems ( smog, toxins etc.) and global problems ( CO2 ). Reformulation does not always lower all emissions, as evidenced by the following aldehydes >from an engine with an adaptive learning management system [55]. FTP-weighted emission rates (mg/mi) Gasoline Reformulated Formaldehyde 4.87 8.43 Acetaldehyde 3.07 4.71 The type of exhaust catalyst and management system can have significant effects on the emissions [55]. FTP-weighted emission rates. (mg/mi) Total Aromatics Total Carbonyls Gasoline Reformulated Gasoline Reformulated Noncatalyst 1292.45 1141.82 174.50 198.73 Oxidation Catalyst 168.60 150.79 67.08 76.94 3-way Catalyst 132.70 93.37 23.93 23.07 Adaptive Learning 111.69 105.96 17.31 22.35 If we take some compounds listed as toxics under the Clean Air Act, then the beneficial effects of catalysts are obvious. Note that hexane and iso-octane are the only alkanes listed as toxics, but benzene, toluene, ethyl benzene, o-xylene, m-xylene, and p-xylene are aromatics that are listed. The latter four are combined as C8 Aromatics below [55]. Aromatics FTP-weighted emission rates. (mg/mi) Benzene Toluene C8 Aromatics Gas Reform Gas Reform Gas Reform Noncatalyst 156.18 138.48 338.36 314.14 425.84 380.44 Oxidation Cat. 27.57 25.01 51.00 44.13 52.27 47.07 3-way Catalyst 19.39 15.69 36.62 26.14 42.38 29.03 Adaptive Learn. 19.77 20.39 29.98 29.67 35.01 32.40 Aldehydes FTP-weighted emission rates. (mg/mi) Formaldehyde Acrolein Acetaldehyde Gas Reform Gas Reform Gas Reform Noncatalyst 73.25 85.24 11.62 13.20 19.74 21.72 Oxidation Cat. 28.50 35.83 3.74 3.75 11.15 11.76 3-way Catalyst 7.27 7.61 1.11 0.74 4.43 3.64 Adaptive Learn. 4.87 8.43 0.81 1.16 3.07 4.71 Others 1,3 Butadiene MTBE Gas Reform Gas Reform Noncatalyst 2.96 1.81 10.50 130.30 Oxidation Cat. 0.02 0.33 2.43 11.83 3-way Catalyst 0.07 0.05 1.42 4.59 Adaptive Learn. 0.00 0.14 0.84 3.16 The author reports analytical problems with the 1,3 Butadiene, and only Noncatalyst values are considered reliable. Other studies from the Auto/Oil research program indicate that lowering aromatics and olefins reduce benzene but increase formaldehyde and acetaldehyde [20] Emission Standards There are several bodies responsible for establishing standards, and they promulgate test cycles, analysis procedures, and the % of new vehicles that must comply each year. The test cycles and procedures do change ( usually indicated by an anomalous increase in the numbers in the table ), and I have not listed the percentages of the vehicle fleet that are required to comply. This table is only intended to convey where we have been, and where we are going. It does not cover any regulation in detail - readers are advised to refer to the relevant regulations. Additional limits for other pollutants, such as formaldehyde (0.015g/mi.) and particulates (0.08g/mi), are omitted. The 1994 tests signal the federal transition from 50,000 to 100,000 mile compliance testing, and I have not listed the subsequent 50,000 mile limits [28,76,77]. Year Federal California HCs CO NOx Evap HCs CO NOx Evap g/mi g/mi g/mi g/test g/mi g/mi g/mi g/test Before regs 10.6 84.0 4.1 47 10.6 84.0 4.1 47 add crankcase +4.1 +4.1 1966 6.3 51.0 6.0 1968 6.3 51.0 6.0 1970 4.1 34.0 4.1 34.0 6 1971 4.1 34.0 6(CC) 4.1 34.0 4.0 6 1972 3.0 28.0 2 2.9 34.0 3.0 2 1973 3.0 28.0 3.0 2.9 34.0 3.0 2 1974 3.0 28.0 3.0 2.9 34.0 2.0 2 1975 1.5 15.0 3.1 2 0.90 9.0 2.0 2 1977 1.5 15.0 2.0 2 0.41 9.0 1.5 2 1980 0.41 7.0 2.0 6(SHED) 0.41 9.0 1.0 2 1981 0.41 3.4 1.0 2 0.39 7.0 0.7 2 1993 0.41 3.4 1.0 2 0.25 3.4 0.4 2 1994 50,000 0.26 3.4 0.3 2 TLEV 0.13 3.4 0.4 2 1994 100,000 0.31 4.2 0.6 2 1997 LEV 0.08 3.4 0.2 1997 ULEV 0.04 1.7 0.2 1998 ZEV 0.0 0.0 0.0 0 2004 0.125 1.8 0.16 2 It's also worth noting that exhaust catalysts also emit platinum, and the soluble platinum salts are some of the most potent sensitizers known. Early research [78] reported the presence of 10% water-soluble platinum in the emissions, however later work on monolithic catalysts has determined the quantities of water soluble platinum emissions are negligible [79]. The particle size of the emissions has also been determined, and the emissions have been correlated with increasing vehicle speed. Increasing speed also increases the exhaust gas temperature and velocity, indicating the emissions are probably a consequence of physical attrition. Estimated Fuel Median Aerodynamic Speed Consumption Emissions Particle Diameter km/h l/100km ng/m-3 um 60 7 3.3 5.1 100 8 11.9 4.2 140 10 39.0 5.6 US Cycle-75 6.4 8.5 Using the estimated fuel consumption, and about 10m3 of exhaust gas per litre of gasoline, the emissions are 2-40 ng/km. These are 2-3 orders of magnitude lower than earlier reported work on pelletised catalysts. These emissions may be controlled directly in the future. They are currently indirectly controlled by the cost of platinum, and the new requirement for the catalyst to have an operational life of at least 100,000 miles. 5.6 Why do exhaust catalysts influence fuel composition? Modern adaptive learning engine management systems control the combustion stoichiometry by monitoring various ambient and engine parameters, including exhaust gas recirculation rates, the air flow sensor, and exhaust oxygen sensor outputs. This closed loop system using the oxygen sensor can compensate for changes in fuel content and air density. The oxygen sensor is also known as the lambda sensor because the actual air-fuel mass ratio divided by the stoichiometric air-fuel mass ratio is known as lambda or the air-fuel equivalence ratio. The preferred technique for describing mixture strength is the fuel-air equivalence ratio ( phi ), which is the actual fuel-air mass ratio divided by the stoichiometric fuel-air mass ratio, however most enthusiasts use air-fuel ratio and lambda. Lambda is the inverse of the fuel-air equivalence ratio. The oxygen sensor effectively measures lambda around the stoichiometric mixture point. Typical stoichiometric air-fuel ratios are [80]:- 6.4 methanol 9.0 ethanol 11.7 MTBE 12.1 ETBE, TAME 14.6 gasoline without oxygenates The engine management system rapidly switches the stoichiometry between slightly rich and slightly lean, except under wide open throttle conditions - when the system runs open loop. The response of the oxygen sensor to composition changes is about 3 ms, and closed loop switching is typically 1-3 times a second, going between 50mV ( lambda = 1.05 (Lean)) to 900mV (lambda = 0.99 ( Rich)). The catalyst oxidises about 80% of the H2, CO, and HCs, and reduces the NOx [76]. Typical reactions that occur in a modern 3-way catalyst are:- 2H2 + O2 -> 2H2O 2CO + O2 -> 2CO2 CxHy + (x + (y/4))O2 -> xCO2 + (y/2)H2O 2CO + 2NO -> N2 + 2CO2 CxHy + 2(x + (y/4))NO -> (x + (y/4))N2 + (y/2)H2O + xCO2 2H2 + 2NO -> N2 + 2H2O CO + H20 -> CO2 + H2 CxHy + xH2O -> xCO + (x + (y/2))H2 The use of exhaust catalysts have resulted in reaction pathways that can accidentally be responsible for increased pollution. An example is the CARB-mandated reduction of fuel sulfur. A change from 450ppm to 50ppm, which will reduce HC & CO emissions by 20%, was shown to increase formaldehyde by 45%, but testing in later model cars did not exhibit the same effect [32,58, 59]. This demonstrates that continuing changes to engine management systems can also change the response to fuel composition changes. The requirement that the exhaust catalysts must now endure for 10 years or 100,000 miles will also encourage automakers to push for lower levels of elements that affect exhaust catalyst performance, such as sulfur and phosphorus, in both the gasoline and lubricant. Modern catalysts are unable to reduce the relatively high levels of NOx that are produced during lean operation down to approved levels, thus preventing the application of lean-burn engine technology. Recently Mazda has announced they have developed a "lean burn" catalyst, which may enable automakers to move the fuel combustion towards the lean side, and different gasoline properties may be required to optimise the combustion and reduce pollution [81]. Mazda claim that fuel efficiency is improved by 5-8%, while meeting all emission regulations, and some Japanese manufacturers have evaluated lean-burn catalysts in limited numbers of 1995 production models. Catalysts also inhibit the selection of gasoline octane-improving and cleanliness additives ( such as MMT and phosphorus-containing additives ) that may result in refractory compounds known to physically coat the catalyst, reducing available catalyst and thus increasing pollution. 5.7 Why are "cold start" emissions so important? The catalyst requires heat to reach the temperature ( >300-350C ) where it functions most efficiently, and the delay until it reaches operating temperature can produce more hydrocarbons than would be produced during the remainder of many typical urban short trips. It has been estimated that 70-80% of the non-methane HCs that escape conversion by the catalysts are emitted during the first two minutes after a cold start. As exhaust emissions have been reduced, the significance of the evaporative emissions increases. Several engineering techniques are being developed, including the Ford Exhaust Gas Igniter ( uses a flame to heat the catalyst - lots of potential problems ), zeolite hydrocarbon traps, and relocation of the catalyst closer to the engine [76]. Reduced gasoline volatility and composition changes, along with cleanliness additives and engine management systems, can help minimise cold start emissions, but currently the most effective technique appears to be rapid, deliberate heating of the catalyst, and the new generation of low thermal inertia "fast light-up" catalysts reduce the problem, but further research is necessary [76,82]. As the evaporative emissions are also starting to be reduced, the emphasis has shifted to the refuelling emissions. These will be mainly controlled on the vehicle, and larger canisters may be used to trap the vapours emitted during refuelling. 5.8 When will the emissions be "clean enough"? The California ZEV regulations effectively preclude IC vehicles, because they stipulate zero emissions. However, the concept of regulatory forcing of alternative vehicle propulsion technology may have to be modified to include hybrid or fuel-cell vehicles, as the major failing of EVs remains the lack of a cheap, light, safe, and easily-rechargeable electrical storage device [83,84]. There are several major projects intending to further reduce emissions from automobiles, mainly focusing on vehicle mass and engine fuel efficiency, but gasoline specifications and alternative fuels are also being investigated. It may be that changes to IC engines and gasolines will enable the IC engine to continue well into the 21st century as the prime motive force for personal transportation [77,85]. There have also been calls to use market forces to reduce pollution from automobiles [86], however most such suggestions ( increased gasoline taxes, congestion tolls, and emission-based registration fees ) are currently considered politically unacceptable. The issue of how to target the specific "gross polluters" is being considered, and is described in Section 5.14. 5.9 Why are only some gasoline compounds restricted? The less volatile hydrocarbons in gasoline are not released in significant quantities during normal use, and the more volatile alkanes are considerably less toxic than many other chemicals encountered daily. The newer gasoline additives also have potentially undesirable properties before they are even combusted. Most hydrocarbons are very insoluble in water, with the lower aromatics being the most soluble, however the addition of oxygen to hydrocarbons significantly increases the mutual solubility with water. Compound in Water Water in Compound % mass/mass @ C % mass/mass @ C normal decane 0.0000052 25 0.0072 25 iso-octane 0.00024 25 0.0055 20 normal hexane 0.00125 25 0.0111 20 cyclohexane 0.0055 25 0.010 20 1-hexene 0.00697 25 0.0477 30 toluene 0.0515 25 0.0334 25 benzene 0.1791 25 0.0635 25 methanol complete 25 complete 25 ethanol complete 25 complete 25 MTBE 4.8 20 1.4 20 TAME - 0.6 20 The concentrations and ratios of benzene, toluene, ethyl benzene, and xylenes ( BTEX ) in water are often used to monitor groundwater contamination from gasoline storage tanks or pipelines. The oxygenates and other new additives may increase the extent of water and soil pollution by acting as co-solvents for HCs. Various government bodies ( EPA, OSHA, NIOSH ) are charged with ensuring people are not exposed to unacceptable chemical hazards, and maintain ongoing research into the toxicity of liquid gasoline contact, water and soil pollution, evaporative emissions, and tailpipe emissions [87]. As toxicity is found, the quantities in gasoline of the specific chemical ( benzene ), or family of chemicals ( alkyl leads, aromatics, olefins ) are regulated. The recent dramatic changes caused by the need to reduce alkyl leads, halogens, olefins, and aromatics has resulted in whole new families of compounds ( ethers, alcohols ) being introduced into fuels without prior detailed toxicity studies being completed. If adverse results appear, these compounds are also likely to be regulated to protect people and the environment. Also, as the chemistry of emissions is unravelled, the chemical precursors to toxic tailpipe emissions ( such as higher aromatics that produce benzene emissions ) are also controlled, even if they are not themselves toxic. 5.10 What does "renewable" fuel or oxygenate mean? The general definition of "renewable" is that the carbon originates from recent biomass, and thus does not contribute to the increased CO2 emissions. A truly "long-term" view could claim that fossil fuels are "renewable" on a 100 million year timescale :-). There was a major battle between the ethanol/ETBE lobby ( agricultural, corn growing ), and the methanol/MTBE lobby ( oil company, petrochemical ) over an EPA mandate demanding that a specific percentage of the oxygenates in gasoline are produced from "renewable" sources [88]. On 28 April 1995 a Federal appeals court permanently voided the EPA ruling requiring "renewable" oxygenates, thus fossil-fuel derived oxygenates such as MTBE are acceptable oxygenates [89]. Unfortunately, "renewable" ethanol is not cost competitive when crude oil is $18/bbl, so a federal subsidy ( $0.54/US Gallon ) and additional state subsidies ( 11 states - from $0.08(Michigan) to $0.66(Tenn.)/US Gal.) are provided. Ethanol, and ETBE derived from ethanol, are still likely to be used in states where subsidies make them competitive with other oxygenates. 5.11 Will oxygenated gasoline damage my vehicle? The following comments assume that your vehicle was designed to operate on unleaded, if not, then damage such as exhaust valve seat recession may occur. Damage should not occur if the gasoline is correctly formulated, and you select the appropriate octane, but oxygenated gasoline will hurt your pocket. In the first year of mandated oxygenates, it appears some refiners did not carefully formulate their oxygenated gasoline, and driveability and emissions problems occurred. Most reputable brands are now carefully formulated. Some older activated carbon canisters may not function efficiently with oxygenated gasolines, but this is a function of the type of carbon used. How your vehicle responds to oxygenated gasoline depends on the engine management system and state of tune. A modern system will automatically compensate for all of the currently-permitted oxygenate levels, thus your fuel consumption will increase. Older, poorly-maintained, engines may require a tune up to maintain acceptable driveability. Be prepared to try several different brands of oxygenated or reformulated gasolines to identify the most suitable brand for your vehicle, and be prepared to change again with the seasons. This is because the refiners can choose the oxygenate they use to meet the regulations, and may choose to set some fuel properties, such as volatility, differently to their competitors. Most stories of corrosion etc, are derived from anhydrous methanol corrosion of light metals (aluminum, magnesium), however the addition of either 0.5% water to pure methanol, or corrosion inhibitors to methanol-gasoline blends will prevent this. If you observe corrosion, talk to your gasoline supplier. Oxygenated fuels may either swell or shrink some elastomers on older cars, depending on the aromatic and olefin content of the fuels. Cars later than 1990 should not experience compatibility problems, and cars later than 1994 should not experience driveability problems, but they will experience increased fuel consumption, depending on the state of tune and engine management system. 5.12 What does "reactivity" of emissions mean? The traditional method of exhaust regulations was to specify the actual HC, CO, NOx, and particulate contents. With the introduction of oxygenates and reformulated gasolines, the volatile organic carbon (VOC) species in the exhaust also changed. The "reactivity" refers to the ozone-forming potential of the VOC emissions when they react with NOx, and is being introduced as a regulatory means of ensuring that automobile emissions do actually reduce smog formation. The ozone-forming potential of chemicals is defined as the number of molecules of ozone formed per VOC carbon atom, and this is called the Incremental Reactivity. Typical values ( big is bad :-) ) are [74]: Maximum Incremental Reactivities as mg Ozone / mg VOC carbon monoxide 0.054 alkanes methane 0.0148 ethane 0.25 propane 0.48 n-butane 1.02 olefins ethylene 7.29 propylene 9.40 1,3 butadiene 10.89 aromatics benzene 0.42 toluene 2.73 meta-xylene 8.15 1,3,5-trimethyl benzene 10.12 oxygenates methanol 0.56 ethanol 1.34 MTBE 0.62 ETBE 1.98 5.13 What are "carbonyl" compounds? Carbonyls are produced in large amounts under lean operating conditions, especially when oxygenated fuels are used. Most carbonyls are toxic, and the carboxylic acids can corrode metals. The emission of carbonyls can be controlled by combustion stoichiometry and exhaust catalysts, refer to section 5.5 for typical reductions for aldehydes. Typical carbonyls are:- * aldehydes ( containing -CHO ), - formaldehyde (HCHO) - which is formed in large amounts during lean combustion of methanol [90]. - acetaldehyde (CH2CHO) - which is formed during ethanol combustion. - acrolein (CH2=CHCHO) - a very potent irritant and toxin. * ketones ( containing C=0 ), - acetone (CH3COCH3) * carboxylic acids ( containing -COOH ), - formic acid (HCOOH) - formed during lean methanol combustion. - acetic acid (CH3COOH). 5.14 What are "gross polluters"? It has always been known that the EPA emissions tests do not reflect real world conditions. There have been several attempts to identify vehicles on the road that do not comply with emissions standards. Recent remote sensing surveys have demonstrated that the highest 10% of CO emitters produce over 50% of the pollution, and the same ratio applies for the HC emitters - which may not be the same vehicles [91-102]. 20% of the CO emitters are responsible for 80% of the CO emissions, consequently modifying gasoline composition is only one aspect of pollution reduction. The new additives can help maintain engine condition, but they can not compensate for out-of-tune, worn, or tampered-with engines. There has recently been some unpublished studies that demonstrate that the current generation of remote sensing systems can not provide sufficient discrimination of gross polluters without also producing false positives for some acception vehicles - more work is required, and in some states I&M emissions testing using dynamometers is being introduced to identify gross polluters. The most famous of the remote sensing systems is the FEAT ( Fuel Efficiency Automobile Test ) team from the University of Denver [99]. This team is probably the world leader in remote sensing of auto emissions to identify grossly polluting vehicles. The system measures CO/CO2 ratio, and the HC/CO2 ratio in the exhaust of vehicles passing through an infra-red light beam crossing the road 25cm above the surface. The system also includes a video system that records the licence plate, date, time, calculated exhaust CO, CO2, and HC. The system is effective for traffic lanes up to 18 metres wide, however rain, snow, and water spray can cause scattering of the beam. Reference signals monitor such effects and, if possible, compensate. The system has been comprehensively validated, including using vehicles with on-board emissions monitoring instruments. They can monitor up to 1000 vehicles an hour and, as an example,they were invited to Provo, Utah to monitor vehicles, and gross polluters would be offered free repairs [100]. They monitored over 10,000 vehicles and mailed 114 letters to owners of vehicles newer than 1965 that had demonstrated high CO levels. They received 52 responses and repairs started in Dec. 1991, and continued to Mar 1992. The monitored fleet at Provo (Utah) during Winter 1991:1992 - selected years Model year Grams CO/gallon Number of (Median value) (mean value) Vehicles 92 40 80 247 91 55 1222 90 75 1467 88 85 1651 86 100 300 1563 84 125 1206 82 170 639 80 220 500 551 78 420 584 76 770 317 75 760 950 163 Pre 75 920 1060 878 As observed elsewhere, over half the CO was emitted by about 10% of the vehicles. If the 47 worst polluting vehicles were removed, that achieves more than removing the 2,500 lowest emitting vehicles from the total tested fleet. Surveys of vehicle populations have demonstrated that emissions systems had been tampered with on over 40% of the gross polluters, and an additional 20% had defective emission control equipment [101]. No matter what changes are made to gasoline, if owners "tune" their engines for power, then the majority of such "tuned" vehicle will become gross polluters. Professional repairs to gross polluters usually improves fuel consumption, resulting in a low cost to owners ( $32/pa/Ton CO year ). The removal of CO in the Provo example above was costed at $200/Ton CO, compared to Inspection and Maintenance programs ($780/Ton CO ), and oxygenates ( $1034-$1264/Ton CO in Colorado 1991-2 ), and UNOCALs vehicle scrapping programme ( $1025/Ton of all pollutants ). Thus, identifying and repairing or removing gross polluters can be far more cost-effective than playing around with reformulated gasolines and oxygenates. A recent study has confirmed that gross polluters are not always older vehicles, and that vehicles have been scrapped that passed the 1993 new vehicle emission standards [102]. The study also confirmed that if estimated costs and benefits of various emission reduction strategies were applied to the tested fleet, the identification and repair techniques are the most cost-effective means of reducing HC and CO. It should be noted that some strategies ( such as the use of oxygenates to replace aromatics and alkyl lead compounds ) have other environmental benefits. Action Vehicles Estimated % reduction % reduction Affected Cost per $billion (millions) ($billion) HC CO HC CO Reformulated Fuels 20 1.5 17 11 11 7.3 Scrap pre-1980 vehicles 3.2 2.2 33 42 15 19 Scrap pre-1988 vehicles 14.6 17 44 67 2.6 3.9 Repair worst 20% of vehicles 4 0.88 50 61 57 69 Repair worst 40% of vehicles 8 1.76 68 83 39 47 ------------------------------ Article: 151779 of rec.autos.tech From: B.Hamilton@irl.cri.nz (Bruce Hamilton) Subject: Gasoline FAQ - Part 3 of 4 Date: Wed, 18 Sep 1996 09:35:20 GMT Expires: 18 October 1996 00:00:01 GMT Organization: Industrial Research Limited Lines: 996 Archive-name: autos/gasoline-faq/part3 Posting-Frequency: monthly Last-modified: 17 September 1996 Version: 1.11 Subject: 6. What do Fuel Octane ratings really indicate? 6.1 Who invented Octane Ratings? Since 1912 the spark ignition internal combustion engine's compression ratio had been constrained by the unwanted "knock" that could rapidly destroy engines. "Knocking" is a very good description of the sound heard from an engine using fuel of too low octane. The engineers had blamed the "knock" on the battery ignition system that was added to cars along with the electric self-starter. The engine developers knew that they could improve power and efficiency if knock could be overcome. Kettering assigned Thomas Midgley, Jr. to the task of finding the exact cause of knock [24]. They used a Dobbie-McInnes manograph to demonstrate that the knock did not arise from preignition, as was commonly supposed, but arose from a violent pressure rise *after* ignition. The manograph was not suitable for further research, so Midgley and Boyd developed a high-speed camera to see what was happening. They also developed a "bouncing pin" indicator that measured the amount of knock [9]. Ricardo had developed an alternative concept of HUCF ( Highest Useful Compression Ratio ) using a variable-compression engine. His numbers were not absolute, as there were many variables, such as ignition timing, cleanliness, spark plug position, engine temperature. etc. In 1927 Graham Edgar suggested using two hydrocarbons that could be produced in sufficient purity and quantity [11]. These were "normal heptane", that was already obtainable in sufficient purity from the distillation of Jeffrey pine oil, and " an octane, named 2,4,4-trimethyl pentane " that he first synthesized. Today we call it " iso-octane " or 2,2,4-trimethyl pentane. The octane had a high antiknock value, and he suggested using the ratio of the two as a reference fuel number. He demonstrated that all the commercially- available gasolines could be bracketed between 60:40 and 40:60 parts by volume heptane:iso-octane. The reason for using normal heptane and iso-octane was because they both have similar volatility properties, specifically boiling point, thus the varying ratios 0:100 to 100:0 should not exhibit large differences in volatility that could affect the rating test. Heat of Melting Point Boiling Point Density Vaporisation C C g/ml MJ/kg normal heptane -90.7 98.4 0.684 0.365 @ 25C iso octane -107.45 99.3 0.6919 0.308 @ 25C Having decided on standard reference fuels, a whole range of engines and test conditions appeared, but today the most common are the Research Octane Number ( RON ), and the Motor Octane Number ( MON ). 6.2 Why do we need Octane Ratings? To obtain the maximum energy from the gasoline, the compressed fuel-air mixture inside the combustion chamber needs to burn evenly, propagating out >from the spark plug until all the fuel is consumed. This would deliver an optimum power stroke. In real life, a series of pre-flame reactions will occur in the unburnt "end gases" in the combustion chamber before the flame front arrives. If these reactions form molecules or species that can autoignite before the flame front arrives, knock will occur [21,22]. Simply put, the octane rating of the fuel reflects the ability of the unburnt end gases to resist spontaneous autoignition under the engine test conditions used. If autoignition occurs, it results in an extremely rapid pressure rise, as both the desired spark-initiated flame front, and the undesired autoignited end gas flames are expanding. The combined pressure peak arrives slightly ahead of the normal operating pressure peak, leading to a loss of power and eventual overheating. The end gas pressure waves are superimposed on the main pressure wave, leading to a sawtooth pattern of pressure oscillations that create the "knocking" sound. The combination of intense pressure waves and overheating can induce piston failure in a few minutes. Knock and preignition are both favoured by high temperatures, so one may lead to the other. Under high-speed conditions knock can lead to preignition, which then accelerates engine destruction [27,28]. 6.3 What fuel property does the Octane Rating measure? The fuel property the octane ratings measure is the ability of the unburnt end gases to spontaneously ignite under the specified test conditions. Within the chemical structure of the fuel is the ability to withstand pre-flame conditions without decomposing into species that will autoignite before the flame-front arrives. Different reaction mechanisms, occurring at various stages of the pre-flame compression stroke, are responsible for the undesirable, easily-autoignitable, end gases. During the oxidation of a hydrocarbon fuel, the hydrogen atoms are removed one at a time from the molecule by reactions with small radical species (such as OH and HO2), and O and H atoms. The strength of carbon-hydrogen bonds depends on what the carbon is connected to. Straight chain HCs such as normal heptane have secondary C-H bonds that are significantly weaker than the primary C-H bonds present in branched chain HCs like iso-octane [21,22]. The octane rating of hydrocarbons is determined by the structure of the molecule, with long, straight hydrocarbon chains producing large amounts of easily-autoignitable pre-flame decomposition species, while branched and aromatic hydrocarbons are more resistant. This also explains why the octane ratings of paraffins consistently decrease with carbon number. In real life, the unburnt "end gases" ahead of the flame front encounter temperatures up to about 700C due to compression and radiant and conductive heating, and commence a series of pre-flame reactions. These reactions occur at different thermal stages, with the initial stage ( below 400C ) commencing with the addition of molecular oxygen to alkyl radicals, followed by the internal transfer of hydrogen atoms within the new radical to form an unsaturated, oxygen-containing species. These new species are susceptible to chain branching involving the HO2 radical during the intermediate temperature stage (400-600C), mainly through the production of OH radicals. Above 600C, the most important reaction that produces chain branching is the reaction of one hydrogen atom radical with molecular oxygen to form O and OH radicals. The addition of additives such as alkyl lead and oxygenates can significantly affect the pre-flame reaction pathways. Antiknock additives work by interfering at different points in the pre-flame reactions, with the oxygenates retarding undesirable low temperature reactions, and the alkyl lead compounds react in the intermediate temperature region to deactivate the major undesirable chain branching sequence [21,22]. The antiknock ability is related to the "autoignition temperature" of the hydrocarbons. Antiknock ability is _not_ substantially related to:- 1. The energy content of fuel, this should be obvious, as oxygenates have lower energy contents, but high octanes. 2. The flame speed of the conventionally ignited mixture, this should be evident from the similarities of the two reference hydrocarbons. Although flame speed does play a minor part, there are many other factors that are far more important. ( such as compression ratio, stoichiometry, combustion chamber shape, chemical structure of the fuel, presence of antiknock additives, number and position of spark plugs, turbulence etc.) Flame speed does not correlate with octane. 6.4 Why are two ratings used to obtain the pump rating? The correct name for the (RON+MON)/2 formula is the "antiknock index", and it remains the most important quality criteria for motorists [39]. The initial knock measurement methods developed in the 1920s resulted in a diverse range of engine test methods and conditions, many of which have been summarised by Campbell and Boyd [103]. In 1928 the Co-operative Fuel Research Committee formed a sub-committee to develop a uniform knock-testing apparatus and procedure. They settled on a single-cylinder, valve-in-head, water-cooled, variable compression engine of 3.5"bore and 4.5" stroke. The knock indicator was the bouncing-pin type. They selected operating conditions for evaluation that most closely match the current Research Method, however correlation trials with road octanes in the early 1930s exhibited such large discrepancies that conditions were changed ( higher engine speed, hot mixture temperature, and defined spark advance profiles ), and a new tentative ASTM Octane rating method was produced. This method is similar to the operating conditions of the current Motor Octane procedure [12,103]. Over several decades, a large number of alternative octane test methods appeared. These were variations to either the engine design, or the specified operating conditions [103]. During the 1950-1960s attempts were made to internationally standardise and reduce the number of Octane Rating test procedures. During the late 1940s - mid 1960s, the Research method became the important rating because it more closely represented the octane requirements of the motorist using the fuels/vehicles/roads then available. In the late 1960s German automakers discovered their engines were destroying themselves on long Autobahn runs, even though the Research Octane was within specification. They discovered that either the MON or the Sensitivity ( the numerical difference between the RON and MON numbers ) also had to be specified. Today it is accepted that no one octane rating covers all use. In fact, during 1994, there have been increasing concerns in Europe about the high Sensitivity of some commercially-available unleaded fuels. The design of the engine and vehicle significantly affect the fuel octane requirement for both RON and MON. In the 1930s, most vehicles would have been sensitive to the Research Octane of the fuel, almost regardless of the Motor Octane, whereas most 1990s engines have a 'severity" of one, which means the engine is unlikely to knock if a changes of one RON is matched by an equal and opposite change of MON [32]. I should note that the Research method was only formally approved in 1947, but used unofficially from 1942. 6.5 What does the Motor Octane rating measure? The conditions of the Motor method represent severe, sustained high speed, high load driving. For most hydrocarbon fuels, including those with either lead or oxygenates, the motor octane number (MON) will be lower than the research octane number (RON). Test Engine conditions Motor Octane Test Method ASTM D2700-92 [104] Engine Cooperative Fuels Research ( CFR ) Engine RPM 900 RPM Intake air temperature 38 C Intake air humidity 3.56 - 7.12 g H2O / kg dry air Intake mixture temperature 149 C Coolant temperature 100 C Oil Temperature 57 C Ignition Advance - variable Varies with compression ratio ( eg 14 - 26 degrees BTDC ) Carburettor Venturi 14.3 mm 6.6 What does the Research Octane rating measure? The Research method settings represent typical mild driving, without consistent heavy loads on the engine. Test Engine conditions Research Octane Test Method ASTM D2699-92 [105] Engine Cooperative Fuels Research ( CFR ) Engine RPM 600 RPM Intake air temperature Varies with barometric pressure ( eg 88kPa = 19.4C, 101.6kPa = 52.2C ) Intake air humidity 3.56 - 7.12 g H2O / kg dry air Intake mixture temperature Not specified Coolant temperature 100 C Oil Temperature 57 C Ignition Advance - fixed 13 degrees BTDC Carburettor Venturi Set according to engine altitude ( eg 0-500m=14.3mm, 500-1000m=15.1mm ) 6.7 Why is the difference called "sensitivity"? RON - MON = Sensitivity. Because the two test methods use different test conditions, especially the intake mixture temperatures and engine speeds, then a fuel that is sensitive to changes in operating conditions will have a larger difference between the two rating methods. Modern fuels typically have sensitivities around 10. The US 87 (RON+MON)/2 unleaded gasoline is recommended to have a 82+ MON, thus preventing very high sensitivity fuels [39]. Recent changes in European gasolines has caused concern, as high sensitivity unleaded fuels have been found that fail to meet the 85 MON requirement of the EN228 European gasoline specification [106]. 6.8 What sort of engine is used to rate fuels? Automotive octane ratin