Is it More Fuel-Efficient to Turn Off the Engine at a Red Light or Keep It Idling?
4 Answers
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.
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.
When researching fuel consumption issues, I've read a large amount of data. Whether turning off the engine at red lights saves fuel depends on the duration. A cold engine start consumes fuel equivalent to 3 seconds of idling, but if the red light lasts more than 7 seconds, turning off the engine saves more fuel than idling. However, real-world traffic lights vary in duration, and frequent engine shutdowns can affect driving smoothness. Many modern cars now come with auto start-stop functionality, which is designed based on this principle, yet most drivers prefer to manually disable this feature, indicating that actual user experience matters significantly. Personally, I believe fuel efficiency should come with peace of mind.
As a daily commuter, I face dozens of red lights every day. From practical experience, I never turn off the engine at stops. For short red lights, shutting down isn't worth it - the fuel consumption spikes during restart. My car has an i-Stop system, but I manually disable it because the frequent vibrations are unbearable. Especially in summer with AC running - shutting off means sweating buckets from the heat. Rather than fussing with that, easing off sudden brakes saves more fuel. The tiny fuel savings aren't even enough for a coffee - driving comfort is what really matters.
