The Diesel engine has had it right from the word go, the petrol engine received it largely with the introduction of the petrol injection. The fuel shut-off
not only saves fuel, it also improves the efficiency of the engine brake.
Previously, in the Diesel engine, the regulation occurred mechanically. Nowadays, the engine-control device is responsible for all these processes. Depending on the vehicle, it cuts off the fuel injection above a
certain RPM between 1000- and 2000 revs if the gas pedal is not operated. From this point on, the engine takes in only pure air, compresses and then expels it. This process is used as an engine brake
when down-shifting. It is important however, that the injection takes place again somewhat above the idling RPM. The shut-off- and re-activation RPM must not necessarily be identical.
Let's now have a look at the fuel- and brake-pad saving behaviour when driving downhill. A steep slope is best managed in the lowest possible gear. As long as the engine does not over-rev, one should accept the
louder engine noise. The engine suffers a lot less, than when it is revved to the red line when accelerating in the lower gears. There are of course, very steep hills where despite the fuel supply shut-off the engine will
still tend to over-rev. In this case, using the foot-brake is absolutely necessary.
One should, under no circumstances, switch the engine off (ignition off) while driving down a gentle slope to avoid the idling consumption. This can only be done with very few specially constructed vehicles (e.g.,
Ecomatic). It could cause the brakes, the steering and other supporting functions to fail. There is also the danger of the steering locking when the ignition switch is turned and that, of course, could have fatal
results. If the speed limit allows it, one should let the vehicle freewheel out towards the end of the slope, this way, momentum is gathered and energy is saved on the following flat stretch.
No combustion engine injects above approx. 1500 RPM if the gas pedal is not operated. Something new is the partial fuel shut-off when changing gears, this is triggered by the clutch-switch.