Argonne National Laboratory

Ask a scientist: When is it more efficient to turn off my car instead of idling?

April 3, 2017

When I pick up my kids after school, should I let my car idle or should I shut down and restart a few minutes later?

Linda Gaines, Argonne transportation systems analyst: You should shut off your engine. Unless you drive a vintage, carburetor-equipped vehicle, you’ll save fuel and reduce carbon dioxide emissions by turning it off. Some drivers think that idling uses less fuel than restarting, but our research has found that drivers save fuel and reduce emissions by shutting down for stops as brief as 10 seconds. That being said, we don’t recommend turning your car on and off in stop-and-go traffic; driving safely means being able to respond quickly to traffic conditions.

Won’t I wear out my car’s starter with the extra restarts?

We actually published a study on that topic last year. For typical drivers (i.e., 10 or fewer starts per day), the starter motor is unlikely to need to be replaced during the vehicle’s life. Today’s starters are more robust than those in older cars.

What about winter? Don’t car engines need to be warmed up?

Assuming that your windows are free of ice—safety first, of course—run the vehicle for about 30 seconds and drive gently (i.e., no hard acceleration). The engine will warm up more quickly by being “at work” than by idling, which will also help the car’s interior warm up more quickly. Also, the catalytic converter, which reduces harmful emissions, reaches operating temperature much more quickly if the car is driven rather than idled. Most auto manufacturers recommend against idling even on the coldest days.

I see a lot of trucks idling— what about them?

Some vehicle types do need power while stationary, such as emergency vehicles and long-haul trucks stopped for drivers’ overnight rest periods, and idling the main engine is one way to provide that power. Increasingly, though, there are alternatives to idling for this power. For example, you can use smaller, auxiliary power units, including those powered by batteries, or you can tap into grid power.

To learn more, check out “IdleBox”—a resource developed by me and my team for the Department of Energy’s Clean Cities program and now available to the public.

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