If you’re a frequent city driver who often encounters heavy traffic and stop lights, you may wonder whether it is more fuel-efficient to let your car idle or to shut it off. In fact, a Journal of Energy Policy study cited by Engineering Explained found that almost every single American they questioned would drastically overestimate how long they should idle before shutting off their car. By understanding exactly how much fuel is used by idling as opposed to restarting the engine, you can become a more informed, fuel-efficient driver.
Letting a Car Idle
While many people imagine that it uses more fuel to restart an engine than it does to simply let it idle, this is actually untrue. The Journal of Energy Policy’s study found that, on average, people thought that it was more fuel-efficient to let the car idle for three minutes before shutting it off.
However, by dividing the amount of fuel it takes to start the engine by the amount of fuel the engine uses per second while idling, you can determine how many seconds your engine can idle before it becomes more fuel-efficient to simply shut the engine off. The truth? Idling is only fuel-efficient for approximately seven seconds—after that, it’s better to shut off your engine.
To answer this question, researchers in a 2004 study published in the journal of SAE took two identical cars and equipped them with fuel flow meters, allowing the researchers to measure the amount of fuel being used by the engines. They first analyzed both cars as they idled for 90 minutes — and, to see if it made a difference, the researchers put one car in Park and the other in Drive. They found that an idling car consumes, on average, 0.63 liters of fuel per hour. Contrary to popular belief, putting the car in Park did not significantly decrease the amount of fuel wasted.
To determine whether it really takes more fuel to start an engine than it does to let the engine idle, the researchers stopped and started the same engine repeatedly until they had used the same amount of fuel that was burned during the 90 minutes the cars spent idling. They found that it takes about 1.2 milliliters of fuel to start a 1.5-liter engine—significantly less fuel than would be used if the car was left to idle for several minutes.
In the final part of this study, the researchers wanted to determine whether start-stop engines actually saved fuel. They equipped one of the cars with a start-stop engine and left the other one to run with no alterations. They then took both of these cars and drove them along the same routes, making sure to put the vehicles through varying levels of traffic. After completing this experiment, the researchers found that the start-stop car was significantly more efficient, saving an average of 8.7% of fuel in heavy traffic and four to six percent of fuel in lighter traffic when compared to the car that simply idled.
Don’t Let Your Car Idle
Although not all engines are the same, larger engines will require more fuel both to restart and to idle, meaning it is logical to apply the study’s findings to many different engines. While it may not be feasible to constantly turn off your car in traffic, there’s no doubt that having an accurate understanding of how much fuel you’re using is beneficial in the long run—because, as it turns out, idling simply wastes a lot more fuel than most people realize.