Deciding on when to shut off your engine is no idle matter


April 03, 1991|By Susan McGrath | Susan McGrath,Los Angeles Times Syndicate

YOU JUMP in your car. Turn it on, fasten the seat belt. Oops, forgot to turn on the answering machine. Leap out of the car, into the house, back again, engine still running, off you go. Uh-oh. Looks like there is road construction up ahead. Wait . . . wait . . . wait. On your way again. Pull into a fast-food joint. Drive through, of course. Wait in line, listen to the radio.

The American Automobile Association estimates that, depending on the make and condition of your car, you burn between one-half and one gallon of gasoline an hour while your engine is idling. It adds up.

More important, while you are going nowhere, your car is spewing its usual gaseous soup of chemical pollutants into the air. In fact, it is probably spewing a larger-than-usual serving of that soup. According to data collected by California's South Coast Air Quality Board, a car moving 5 miles an hour pollutes about three times as much as a car moving 55 miles an hour does. A car that is idling pollutes even more.

Clearly, if you are going to be idling for any length of time, it would be better to turn off your engine and restart it when you are ready to move. But what is that length of time? Syndicated auto columnist Bob Sikorsky says that if you're going to be idling for more than 30 seconds, you will save on gas by switching off.

But what about emissions? The Air Quality Board came up with these figures: While idling, a theoretical average car emits 0.66 grams of reactive hydrocarbons (HC) and 7.04 grams of carbon monoxide (CO) a minute. Restarting a warm engine puffs 3.08 grams of HC and 15.57 grams of CO into the air. So you'd have to be stopped longer than 4.67 minutes to gain on HCs, but 2.21 minutes of waiting justifies turning off your engine for CO emissions.

The Air Quality Board reckons you should split it more or less down the middle, and switch off if you expect to be idling for three minutes or longer. A New York City law already requires that.

What does this mean for your car? Preston Sheriff, technical service specialist for the American Automobile Association of Washington state, suggests that some people aren't sure their cars will restart if repeatedly turned off and restarted again while still hot. Furthermore, he points out, if you do this every few minutes, say at a succession of long traffic lights, restarting can drain the battery without giving it time to recharge.

Idling isn't great for your engine, either. The cooling system doesn't function as well, and the richer mixture your car runs on when idling can allow gas to get past the rings to contaminate your crankcase oil.

For the well-being of your car, Sheriff recommends turning off the engine if you expect to be idling longer than four minutes.

So there you have it. Next time you are waiting in line and you think it'll be more than three or four minutes, turn off your engine.

Once you know that it is best not to idle for more than a few minutes, the sight of idling engines all around you may begin to drive you to distraction. I can't recommend telling strangers to turn off their cars. However, if you have access to a company newsletter or bulletin board, use it to let other people know they can save gas, improve air quality and spare their cars by switching off their engines.

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