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Originally Posted by hackish
Finally I ran the pulse and glide idea by the engineer and he flatly said it is entirely false. Obviously people on here have managed to record gains with their pulse and glide thing but his description of the physics involved make sense. I'll try to describe it the best I can.
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What to believe? ABA data or some "engineer"? I'm so confused.
Quote:
Originally Posted by hackish
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Now, pulse and glide you accel to some speed, shut the engine off and coast down to some other speed then resume. The problem is that nearly all engines are designed to have the best BSFC at cruise, not accelration. In fact, EGR normally turns off during acceleration.
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Nearly all engines are most efficient at their torque peak, under a heavy load. Lighter loads have more throttling losses and are also at a higher BSFC.
Quote:
Originally Posted by hackish
Since v=d/t the pulse/glide person is going to be travelling at some average speed. For some time they consume 0 fuel and for some time they consume an above average amount of fuel accelerating. The amount of work is the same whether you go fast or slow but BSFC, the engine's energy output and the losses (pumping, air resistance etc) are different.
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The efficiency is great enough under acceleration to offset the difference in aerodynamic losses, and if you use hills properly, that can mostly be negated. And aerodynamic mods make P&G that much more effective.
Quote:
Originally Posted by hackish
At this point I got a bit lost in the calculus but since air resistance is exponential by the speed it turns out the friction from air resistance is greater when you are pulsing than the amount you save when your glide falls below the average speed.
Now, you've got more friction losses than someone cruising constantly at the average speed. The engine has to output a little more total energy than the average speed case. At higher outputs the engine is less efficient. At the end of the day more power out under less efficient conditions - P&G cannot possibly work.
I wonder why those who do it are recording gains or if anyone has ever done an objective test by figuring out their average speed with P&G and compared it to driving the same average speed. Or is it just some vehicles that did better while most do not? Maybe something older that doesn't have features like DBW and EGR?
The complicated engineering answer seems to say P&G cannot work. I've always questioned it myself. Can anyone describe how it possibly could work? Maybe it's a component of an overall driving style that as a whole provides gains?
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The fact remains that it works, and very well. Plug some real live numbers into it, not that easy, and you will be able to verify it does. Better yet, try it doing yourself and then experience will teach you.