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Old 11-17-2011, 10:02 PM   #64 (permalink)
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Originally Posted by Frank Lee View Post
VE is nice and all, but I was after the bsfc peak... the reason for an optimal piston speed has been mentioned earlier in the thread (thanks guys ) but I resurrected it just cuz I found this:

Sfc measures how efficiently an engine is converting fuel into work. The graph of an engine's sfc plotted against engine RPM will be "U" shaped. At high speeds the sfc increases due to increased friction, while at lower speeds the sfc also increases due to higher heat losses from the combusted gas to the cylinder and piston wall because of the increased time the hot gases remain in the cylinder. This is also the reason why air cooled engines are usually less fuel efficient than water cooled engines.
Wow its been a long time since I posted in this thread. Frank I think your explanation of the U shape is oversimplified at least with respect to the lower RPM. The curve is U shaped in part because the torque curves in general are inverse U shaped. Its been over a decade since I learned the term "mechanical efficiency" wrt to engines but... The maximum torque is like the engines maximum potential, when the engine is operating below that it still carries the frictional losses associated with that RPM but its out put is less so it is mechanically less efficient.
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