Red Light: Idle or Off?

Idling prohibited sign image

Image Source: Found_Drama

Dear Pablo: If I am stopped at a red light should I idle my car or turn it off?There are several occasions when people idle. In cold and hot climates people may let their cars idle to acheive a comfortable temperature while they wait in their home. Some people also believe that it is better for their car to let the engine warm up, a myth that I have written about previously. Other times people may let their cars idle while they run into a store for "just a minute." In fact Mayor Michael Bloomberg has told his drivers to cut out the idling after The Associated Press reported it observed his SUVs idling throughout the city. In another form of idling, the one the reader asked about, your car continues to run while you are waiting for the light to change, a train to pass, or a traffic jam to clear. While the first two types of idling are completely dependent on convenience and personal comfort, and therefore easily avoidable, most of us find ourselves idling against our will at least a few times a day.

In order to boost fuel economy further hybrid vehicles automatically turn off when you come to a stop and seamlessly restart once your foot returns to the gas pedal. New technologies are also being developed to allow non-hybrid vehicles to do this as well, such as Mazda's Smart Idling Stop System, an internal combustion engine that is outfitted with idle-stop technology can save fuel by shutting down the engine when the vehicle stops, and nearly instantly restarting it when the driver takes his foot off the brake pedal. But for the rest of us does it actually make sense to turn the car off at red lights?

Using a device called Scangauge II, which plugs directly into a car's computer to read performance characteristics, speed, fuel consumption and numerous of other information, I can observe that my car, a 1.8 liter 4-cylinder uses 0.21-0.27 gallons per hour while idling between 667 and 778 RPM. I can also calculate that the car consumes 0.00053 gallons every time I start it up. So the fuel consumption of leaving it running appears to be far more than the fuel used in starting the vehicle.

A critically-thinking reader may point out that most of the energy used in starting a car comes from the battery and not from the gasoline, which is true. As a result of drawing electricity from the battery for the starter, the alternator needs to capture energy from the engine to recharge the battery. The elevated fuel use for this contributes 0.00225 gallons to the ignition fuel use, for a total of 0.00278 gallons per start-up. If we convert the gallons per hour used in idling (0.21) to gallons per second (0.0000583) and divide the start-up fuel use (0.00278) by it we find that idling longer than 38.6 seconds (in my car) is less efficient than turning the car off an restarting it. The results will vary from car to car but I would expect them to fall into a reasonably close range. In Europe many red lights and railroad crossings where the wait is longer than 30 seconds are equipped with lighted signs telling you when to turn your engine off an on again.

Of course there are many more factors involved in the exact answer, such as "evaporative soak emissions," and the impact on the performance of the catalytic converter as it cools while the engine is off, but in general it is not good TreeHugger practice to idle your car longer than about 30 seconds. I don't need to remind you that the "greenest" car trip of all is the one that is avoided, replaced by alternative means, combined with other trips, or done as a carpool.

Additional Resources for TreeHuggers on the Go:
Hybrid-Electric Cars: How They Work, Battery Technology, and More
Should I Cash In On My Clunker
Is My Electric Bicycle Lame?