Date: Wed, 29 Nov 1995 20:59:56 -0800
From: drussell@cisco.com (David Russell)
Subject: Re: Long stroke vs Torque.
I liked the "leverage of the crank" idea. I had never thought of this
before and thought it explained the situation well. On the other hand I
think your explanation is the "balderdash".
If you have a given engine displacement, with a given amount of energy
entering the combustion chamber and exploding, it should release the same
amount of energy irrelavent of the stroke/piston diameter ratio. As you
said valves and timing can change all this but say they are constant in
comparing our "long stroke plonker" with a "short stroke revver". So let's
start by agreeing we have the same input of energy to our experiment. We
have the same energy to play with in the short stroke versus the long
stroke engine.
In the short stroke engine the energy sends the piston rattling down the
cylinder, and it doesn't have far to go to the bottom so it gets there
quickly, thus spinning the crank 180 degrees in no time and we have high
rpm. In the long stroke engine the piston has a long way to go so it takes
longer to get to the bottom, spinning the crank in a longer time, producing
lower rpm.
So we have used the same amount of energy but produced in the short stroke
motor a high rpm, and in the long stroke motor low rpm. But hold on, we
need conservation of energy here. From my physics class I remember that
Power = torque * rpm, so if power is the same, due to intake energy being
the same, then what the long stroke engine has given up in rpm it must have
gained in torque.
This all sounds like gearing to me. Same amount of energy but either high
speed/low torque or low speed/high torque. Reminds me of pedalling my
bicycle, or changing down for a hill, or using a slow turn on a long handle
to turn a winch. I think the leverage of the crank, or crank radius, does
have effect on torque vs. rpm.
I guess racing cars like high rpm motors because in general they have
engine displacement constraint rules, and less energy consumption
constraint rules. A high rpm engine can consume fuel much faster (internal
combustion engines are really pumps), thus producing more power albeit at
high revs and relatively lower torques. Good examples are kart racing and
motorcycle racing engines, they might be restricted to 125cc capacity, but
can burn gas at any rate, therefore you want to design that motor to have
high rpm's, i.e. design it with a short stroke, and use gearing to make up
for the lower torque.
David.
>A question showed up awhile ago, asking why long-stroke motors produce more
>torque. The answer that was posted talked about the lever-arm advantage a
>long-stoke has.
>
>Sorry, but that is all balderdash.
>
>First, long stroke motors DON'T necessarily product more torque. It is much
>more a function of cam timing, intake tract tuning, ignition, exhaust, etc.
> The "common knowledge" that long stroke motors product greater torque came
>about by comparing high-revving racing engines against plonkers.
> Historically, racing engines always worked in a limited RPM band and were
>designed for max HP. Higher RPM almost always produces greater HP, so race
>engines were tuned (via the cam shaft and ignition timing) to produce
>high-rpm HP at the expense of low-rpm torque. High rpm could only be
>achieved (without over stressing rods, rings, etc.) via short-stroke motors.
> Following this logic "backwards" produced the common but incorrect concept
>that long-stroke = torque. (This is less true today, where race drivers have
>finally realized the advantages of a wide torque-band compared to maximum but
>narrow HP.)
>
>Now, as to the "physics" of a long leverage arm producing more torque.
> First, torque is the force exerted by the piston acting through a vector to
>the lever-arm (the crank throw). The force generated by the pistion is the
>pressure in the cylinder multiplied by the piston area.
>
>Now, FOR A GIVEN ENGINE DISPLACEMENT (and that is important!) if you INCREASE
>the stroke you must DECREASE the piston area by the same amount! In other
>words, anything you gain by increasing the stroke you lose by decreasing the
>piston area. And since torque is proportional to the crank-throw times the
>piston area, the two effects cancel.
>
>The major advantage slow-reving long-stroke motors have over short-stroke
>motors is their superior low-rpm velocity of the incoming charge, which DOES
>product better combustion chamber filling which DOES produce better
>combustion which DOES produce higher cylinder pressures which DOES product
>better torque. (Even this isn't so true anymore with variable valve timing,
>variable intake runners and four valve heads.)
>
>Sorry, but it has nothing to do with the leverage of the crank.
>
>- Doc Dreher
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