View Single Post
Old 07-15-2008, 05:52 PM   #38 (permalink)
Master EcoModder
Join Date: Jun 2008
Location: NorCal
Posts: 451
Thanks: 1
Thanked 40 Times in 26 Posts
Originally Posted by MechEngVT View Post
Using Mean Piston Speed is the effective average speed of the piston as it travels through the given stroke over the given time. It doesn't matter how wide the engine's operating range is the only factor affecting mean piston speed is the physical engine design (considered fixed) and the actual engine speed....

I don't think rod/stroke ratio affects maximum piston speed. It will have a *huge* affect on maximum piston acceleration and therefore wristpin loading (which will limit maximum RPM), but generally pistons accelerate from 0 at BDC to max speed halfway up the cylinder and decelerate to 0 at TDC. When piston speed is maximum the rod is perpendicular to the crankshaft throw and the piston's maximum linear speed at that engine rotational RPM is independent of connecting rod length. The higher the rod/stroke ratio the closer the piston's actual velocity approaches a sinusoidal variation, and lower ratios cause less dwell (i.e. sharper acceleration into and out of) the top/bottom centers..

So succinctly, the average piston speed for a given engine RPM is independent of rod length, just as the formula quoted earlier indicates.
I think we agreed/I understand, independant of average piston speed for a given rpm.

But, I should have said piston speed (average, max, and profile) is stroke dependant, right? Also, the other key is piston speed has a profile that is rod ratio dependant. For a given rpm, the bigger the engine's stoke, the greater the piston speed. For a longer stroked engine for instance, the piston has a longer distance to travel over the same time, so piston travel in feet per minute (speed) must be greater.

In your expanding pressure wave example you used, that wave has a pressure profile. With dwell, max piston speed, the piston must have a speed profile we could graph over 180 crankshaft degrees. Short rod race engines are known for yanking the pin out of the piston boss from extreme acceleration rates, for example. Short rod race engines are also known for 'outrunning' the expanding pressure wave at very high rpms. Extreme acceleration rates are the result of high bore-to-rod angles present in a high rod ratio engine. If I think about a graph, I think about the (piston speed) magnatude is stroke dependant, and the speed profile is rod ratio dependant. The shape of the curve from TDC to where the rod is perpendicular to crankshaft throw, is rod ratio dependant.

Why do you care?

Some choices must be better for for extracting the maximum motion energy from that expanding pressure wave.

If what I've said is true, and average piston speed changes with stroke, to answer the question "what rpm do I want to acheive XX average piston speed" one needs more information... what's the stroke?

There may very well be a magic average piston speed, but not a magic rpm without considering the engine's stroke.

I would argue there must be an optimal piston speed profile (which include speed, decel and accel rates) to take the most advantage of a given pressure wave profile, but I also think there is a lot more to the puzzle. I also belive for every piston speed profile, the pressure wave profile changes.

You should want to find the best engine specific RPM for best FE, with so many variables I think you need an engine dyno to tell you that.

Last edited by metromizer; 07-15-2008 at 07:00 PM..
  Reply With Quote