Quote:
Originally Posted by AdamP
Sorry for dragging up an old thread but I was looking for mpg comparisons and stumbled across it.
Coasting uses less fuel under some circumstances and more for others. Engine frictional losses increase with the (almost) square of engine speed, therefore you lose more kinetic energy to engine 'braking' at 1600rpm than you input in fuel to idle the engine at 800rpm, even at 40% thermal efficiency. Therefore if you are maintaining a speed it is more energy efficient to coast. However if you are trying to slow down it is obviously not efficient to continue to add energy (fuel) so engine braking is more efficient. My experience is that the Volvo system is pretty good at deciding when to coast and when not to, as for whether is makes a significant difference to mpg I couldn't say!
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Except that zero fuel is injected at the over-run. Zero. You assume that there is. And in top gear there is very little actual engine braking.
Coasting will always use fuel to maintain an idle speed. Certainly in a current Volvo. There are some American cars that shut off the engine completely during coasting, using the electric system to continue to power the steering and brakes while the engine is switched off while travelling. This is specifically to avoid idle speed fuel consumption, which is the same reason stop-start is fitted.
Do these things save any significant fuel? No. None of them are likely to be cost effective, but that is not their point.