Thread: Engine Braking
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Old 10-05-2012, 12:52 AM   #17 (permalink)
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Originally Posted by wobombat View Post
I don't think the point of highest torque always lines up with the point of highest efficiency as seen on BSFC charts. For example, my brother's civic has a peak torque at 7000 RPMs. My car has a peak torque at 4750RPMs. Yet the common knowledge around here seems to be that operating your car over about 3000 rpms tends to be very fuel inefficient. What could account for the difference, or is there actually no difference and these car models really do produce power the most efficiently at these numbers.
Peak efficiency is pretty much never at peak torque since peak torque is when your volumetric efficiency is highest. More air, but also proportionally more fuel injected. Highest volumetric efficiency can be achieved at any point in the rpm range with cams and manifold/header tuning.

Peak thermal efficiency rpm is where lower cooling losses from running the engine faster balances out the increased friction and parasitic loss from running the engine faster. For "Otto cycle" engines (non-"Atkinson cycle") the gain in power from increasing volumetric efficiency/load doesn't balance out the increased amount of wasted pressure not captured by the expansion stroke that is simply blown out the exhaust (aka, more heat energy ending up in the exhaust).

Last edited by serialk11r; 10-05-2012 at 02:28 AM..
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