Is it More Fuel-Efficient to Turn Off the Engine at a Red Light or Keep It Idling?

[DiDaniella]

Here is the relevant information about turning off the engine versus keeping it idling at a red light: 1. It depends on the duration: The fuel consumption for one engine start is approximately 5 milliliters. Therefore, theoretically, if the waiting time at a red light exceeds 8 seconds, turning off the engine is more fuel-efficient than idling. However, this is under ideal conditions. Engine starting is not instantaneous but a process, and the actual fuel consumption is much higher than the theoretical value, usually around 10 milliliters. Additionally, considering the wear and tear on the engine and the impact on the starting system and battery, it is generally not recommended to turn off the engine for such short periods. Some vehicle manuals recommend turning off the engine after idling for more than 40 seconds, while others suggest doing so after idling for more than a minute. 2. Engine idling fuel consumption: The fuel consumption during engine idling mainly depends on factors such as the engine's design level, manufacturing process, combustion method, displacement, and oil viscosity. For example, improving engine thermal efficiency, using lean-burn technology, gasoline direct injection technology, or low-viscosity oil can all reduce fuel consumption during engine idling.

[DeTheo]

I've been driving for ten years and have specifically tested this scenario. Turning off the engine at short red lights is really unnecessary because starting up consumes more fuel and damages the battery. It only makes sense to turn off the engine if the red light lasts more than 30 seconds, and typically, traffic lights on regular roads last just tens of seconds. Frequent engine shutdowns and starts cause significant wear to the engine and starter motor, and the repair costs far outweigh the small amount of fuel saved. Plus, how uncomfortable is it to turn off the engine with the AC running! I usually just check the countdown on the red light and shift to neutral while braking for short waits. Keeping the engine running at red lights is actually more convenient and less strenuous, overall better for the car and safer too.