Quote:
Originally Posted by wobombat
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.
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My point about using the RPM speed of the torque peak as the shift point was meant as guidance for running the engine up into a high(er) efficiency region for acceleration in order to minimize fuel consumption.
To do this you will spend less time accelerating overall and do it at a high enough power level to be operating the engine in it's most fuel-efficient region.
The Bosch Automotive Handbook has a section of empirical graphs and formula for internal combustion engines both gasoline and diesel. The torque-power-speed curve for gasoline engines shows the peak torque occurring at the minimum specific fuel consumption point at WOT. In addition the formula for peak torque is engine displacement times the peak cylinder pressure (BMEP).
The speed of maximum volumetric efficiency occurs when the airflow has the least restrictions and the cylinder filling is greatest and the pressure thereby peaks. Hence the torque peak occurs at the point of maximum volumetric efficiency.
Most of the BSFC charts that i have seen seem to show a tiny island of the absolute minimum bsfc slightly below the WOT line and slightly to the left of the torque peak-- but the point to be made here is to get up into those lower bsfc bands at the higher torque regions during those times when you accelerate (the "pulse" phase). You can never operate your car when cruising down the road (the "glide" phase) in that tiny zone of lowest bsfc--the power level is too high for gliding...