English is currently available.
Automotive Engineering DISCUSSION
There is a long descent on my commute. One colleague insists that shifting to neutral saves fuel because the engine drops to idle; another says that modern engines use no fuel at all when you stay in gear with your foot off the accelerator.
Which is correct for a fuel-injected petrol car, what is the mechanism, and does the answer change on a gentle slope where the car slows down in gear?
In gear with the throttle closed, a fuel-injected engine goes into deceleration fuel cut-off. The engine control unit sees a closed throttle and an engine speed above a threshold, usually somewhat above idle, and stops firing the injectors. The wheels turn the engine through the gearbox, so it keeps rotating with zero fuel. When the revs fall towards idle or you touch the accelerator, injection resumes.
In neutral the engine is disconnected from the wheels and has to burn fuel to keep itself idling. So on a descent steep enough to need braking anyway, staying in gear uses less fuel: none, against idle consumption.
To put a size on it: a warm petrol car engine idles on something of the order of 0.5 to 1 litre per hour, depending on displacement and accessories. At 60 km/h, 1 L/h corresponds to 1.7 L/100 km. That is what neutral costs on the descent, and what fuel cut-off saves. Many trip computers show it directly: instantaneous consumption reads zero on the overrun in gear and a small non-zero value in neutral.
The gentle slope is the case where the answer can reverse. In gear, the engine's pumping and friction losses act as a brake. If the slope is not steep enough to hold speed against that drag, you will have to press the accelerator again sooner, and then you are burning fuel to make up for energy the engine braking threw away. Coasting in neutral keeps the car's kinetic energy for longer at the price of idle fuel.
So: if you would otherwise need the brakes, stay in gear. If you would otherwise need the accelerator, coasting can come out ahead, which is why some automatic transmissions have a sailing function that decouples the engine on its own.
The fuel difference is small either way, and the control argument favours staying in gear. Engine braking shares the work with the friction brakes on a long descent, which keeps the brakes cooler and less likely to fade, and drive is available at once if you need to accelerate. Coasting downhill in neutral is also prohibited by traffic rules in some places.
One caveat on the mechanism: most carburetted engines have no fuel cut-off of this kind and keep drawing fuel on the overrun, so the zero-fuel statement applies to injected engines.
Back to Automotive Engineering community
