|
Actually, the max BSFC typically seems to fall below the peak torque RPM. It seems to be somewhere around 2500 RPM in most cases for gasoline engines, while peak torque RPM varies a lot from engine to engine--in many cases, though, it is over 3000 RPM.
The question of what speed to drive is not really the same question as what the most efficient RPM is, though. When you move a car, you have to use power to overcome friction, air resistance, rolling resistance, to warm up the engine parts, and so on. At anything over in-town speeds (guessing, 40 MPH?) the largest factor is air resistance, which goes up by the square or cube of the speed, depending on what kind of drag you're talking about.
So the slower you can go, the less power needed to push your car through the air. But if you don't need to make much power, you have to throttle the air going into the engine way way down. And that isn't so efficient. So there's a trade-off. But the trade-off seems to generally be weighted so that lower speed is more important than the throttling losses, so for steady-state cruising the most efficient speed is typically "in top gear as slow as you can stand". For me, that is anywhere from 50 to 65 MPH, depending on a lot of factors.
-soD
|