Quote:
Originally Posted by AdamP
No fuel is injected on overun, the engine is turned by the kinetic energy of the vehicle. The engine doesn't suddenly turn friction free because its using no fuel! It doesn't matter what is turning the engine, it uses more energy to turn it at 1600rpm than it does to turn it at 800rpm. I've also ignored the fact that when there is torque through the gearbox during overun 'engine braking' there is increased frictional losses too.
Essentially it uses more fuel to engine brake and then accelerate to regain the lost kinetic energy than it does the coast on idle. Physics  |
If it is on the over-run, or coasting, it is running down hill or slowing anyhow and probably will be braked to further slow the vehicle as often or more than just accelerating just from coasting. The mass of the vehicle keeps the car rolling along in top for ages on even a slight downhill in top whether coasting or on the over-run. It uses NO fuel on the over-run but uses the normal fuel to idle [which is mitigated usually by the stop-start system when not rolling, so must be somewhat significant]. Friction and pumping losses on the over-run are insignificant. You ignore the potential energy and acceleration of gravity and you also ignore the fuel and power required when coasting to charge batteries, run the aircon and power the steering, none of which are assisted by gravity when coasting with the engine at idle.
The higher the gear, the less engine braking there is, and since coasting is only programmed to happed at over 40mph, when switched off the over-run at those speeds is invariably in 7th or 8th gear where engine braking is almost ineffective and revs will indeed be very low relative to speed. Even with large commercial engines of 6 litres and above, there is a massive fuel saving on the over-run where fuel consumption on the monitors falls to zero.
Just done a search for some references for my absolutely correct information and come up with this video. I hope this has been educational to you to help you understand why you are quite wrong.
[YOUTUBE]
https://youtu.be/GOc-0l3lfrY[/YOUTUBE]