[MR2] General Car Question
wbrandt1 at sbcglobal.net
Thu Aug 5 00:57:21 EDT 2010
Joel - I think we have swerved to an interesting subject - I did catch
myself after the first post with the realization of a firing order (duh,
something can't fire simultaneously if there is a sequential order ;-) -
although with distributors on the way out and computers controlling
everything that would be possible.
Then we have a few engines that can turn off sets of cylinders at will for
fuel economy - Cadillac tried that in the 1980s with disastrous results but
Chrysler's Hemi V8 is doing that now - although I think with all the
complexity of such a system the fuel savings isn't that great.
And do the valves still open and close even though the cylinder isn't
----- Original Message -----
From: "Joel" <priamyd at comcast.net>
To: "William Brandt" <wbrandt1 at sbcglobal.net>
Cc: "MR2 List" <mr2 at mr2.com>
Sent: Wednesday, August 04, 2010 9:48 PM
Subject: Re: [MR2] General Car Question
>I will counter only 1 small portion of Bill's reply (in my usual long
> Based upon the firing order in the 4AGE (1 - 3 - 4 - 2), it is highly
> unlikely that cylinders 1 and 3 fire at the same time, and the same would
> go for cylinders 2 and 4.
> I believe that the cylinders fire at 180 degrees from each other.
> Granted, cylinders 1 and 3 to reach TDC at the same time (as do 2 and 4),
> but if both 1 and 3 were to fire at the same time (and 2 and 4 alike), it
> would cause massive power surging on just those portion of the rotating
> stroke, and leave the other 2 cycles of the 4 stroke cycle to basically
> wait for inertia to rotate the assembly towards its optimal firing
> Inversely, if you have 1 cylinder firing at a time, - think of a clock,
> you'd have a firing at 12, 3, 6 and 9 - and not just 12 and 6. This would
> create a constant and fluidly moving rotating assembly that will balance
> itself. With ignition happening at 12 and 6, half of your rotation would
> be, uh, useless and the engine rock considerably and lend itself to
> violent and unstable behavior.
> If you were riding a bike and ONLY applied your leg muscle (ignition) when
> the pedals reached 12 and only stopped all torque forces at the '3'
> position, you would be waiting for the other pedal for 1/4 turn - a 1/4
> turn of wasted motion and possible forward momentum. An engine operates
> on the same principle, but with 4 'pedals' instead of 2. :)
> Again, I do not say this from a seat of absolute knowledge, so please do
> not take it as such - this is just my hunch and guess. :)
> I have noticed the same thing as Bill - an I4 cylinder engine at low RPM
> is very rough compared to an I6 engine. My old 1987 Supra was butter
> smooth at 800rpm, whereas my Mr2 was considerably rougher.
> - Joel -
> William Brandt wrote:
>> Wayne - I was thinking of trying to answer this but 4 stroke engines are
>> always a bit of a mystery to me. I know, they shouldn't be.
>> In a 4 stroke engine isn't there only one power stroke?
>> RPMs and power strokes - well here's a guess - 6 cylinders - 3000 rpm -
>> 4 stroke - each piston is producing 125 power strokes per minute.
>> (3,000 / 6 / 4)
>> As to your other point I believe in most designs 2 pistons are always
>> producing a power stroke at the same time for balance and smoothness.
>> When Mercedes came out with the 5 cylinder diesel in the mid 70s it was
>> a revolution and something many engineers thought "couldn't be done" - in
>> that engine the middle "odd" cylinder has its own power stroke while the
>> other 4 are going 2 at a time (I think!). Usually the 2 power stroke
>> pistons are at opposite points in the engine for balance (again, "I
>> Here's a good explanation in Wikipedia -
>> I also don't understand why inherently next to a V12 an I6 is by design
>> the most balanced engine. I know when my 2 is idling at 700 rpm it is a
>> bit rough. I know my old MB I6 - when the motor mounts are decent - is
>> so smooth at idle as to be almost undetectable.
>> I remember from old school engine class it was every other stroke was a
>> power stroke (so 1500 times) but so much has changed. For instance I
>> know how they make a 5 cylinder engine run...fairy dust I think.
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