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Old 07-15-2008, 08:59 AM   #37 (permalink)
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Originally Posted by metromizer View Post
Piston speed isn't constant, right? The piston stops at top and bottom dead centers, therefore accelerates and decelerate in between, or every 180 degrees of crankshat rotation. So using 'average piston speed' is kinda like using average speed while calculating fuel economy: Great if you only vary a few mph, but not if your commute invovles stop-n-go traffic.

The variable of rod ratio governs max piston speed for a given crankshaft rotational position, so wouldn't average speed be the same for every rod ratio if the same rpm is used?
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. It's similar to using mean effective pressure to talk about combustion's not REALLY how much pressure is in the cylinder since theoretically at TDC pressure is tremendously large and drops off very quickly as the piston descends on its power stroke, but there is an average effective pressure that would complete the same amount of work as the actual combustion does and that (the MEP) is a useful design tool.

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.
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