Quote:
Originally Posted by hackish
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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I know this is an old thread, but I ran across it doing research, and I noticed that noone ever answered this question.
I had the same question myself, and it seems counter-intuitive, but there actually is a very good answer to it!
Here is why it works:
An ICE engine is, inherently, inefficient. Even in a prefect world there is a theoretical upper limit to any heat engine's efficiency.
In a typical gas engine, it is something like 25%.
In other words fully 75% of the energy in the fuel is used just to overcome internal resistance, to turn the engine itself.
On top of that, belt driven engine accessories and drivetrain losses absorb another 5-10% of the energy in the fuel.
That means all together, when the engine is running, more than 3/4 of the energy never even reaches the wheels.
And that is in optimal conditions, at the peak of the BSFC curve!
Any time the engine is off, you are not wasting that 75%.
However, the internal losses don't change (a significant amount) in acceleration vs stead state.
So, if you can P&G with a 1:3 ratio (10 sec pulse, 30 sec glide) the engine is off 3/4 of the time. Over that 40 sec span, the 75% of potentially wasted fuel energy is conserved.
In order for that not to save fuel compared to stead state, it would have to require 4x more fuel to accelerate - and of course that is before even taking into account pumping losses or the fact that acceleration puts the engine in a more efficient part of the BSFC curve.