Thread: Engine Braking
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Old 10-06-2012, 12:16 AM   #22 (permalink)
serialk11r
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Quote:
Originally Posted by kennybobby View Post

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 right 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...
BSFC charts are few and far between. The point of highest volumetric efficiency pretty much dictates the torque peak as I said, and volumetric efficiency and thermal efficiency are very very different. As you noticed, minimum BSFC is usually below the WOT line. This is because running at WOT has the lowest throttling loss, and also the highest mechanical efficiency, but the last bit of power comes at a greater cost because the extra heat can't be extracted efficiently by the piston.

The reason you're seeing lowest specific fuel consumption close to the torque peak is because most engines are made "street friendly" with a small rev range and low-medium torque peak usually in the 3000-5000rpm range. Coincidentally, the size of almost all street engines (per cylinder displacement range on most engines is like in the 0.35L to 0.7L range and bore/stroke has a similar range) makes it so that their peak efficiency occurs in the 2000-3500rpm range. However a torque peak in the 4000s is not uncommon at all, especially for smaller cars, so you really can't go by torque peak.

Again there's very few BSFC charts out there but the ideal shift point is probably usually in the low-mid 2000s to low 3000s, depending on engine. If the efficiency drops a lot below 2000 then shifting from 1st to 2nd gear should probably happen at a higher engine speed to avoid dropping too low.
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