To Idle, or Not to Idle?

A cold snap seems to divide Alaskans into two classes; all it takes is a drive to the grocery store. Once a parking spot is found, the segregation begins: some people shut off their car engines; some people prefer to let their cars idle.

I've heard non-idlers fantasize about driving unoccupied, running, vehicles to another parking spot, just to let the idler know the excess exhaust isn't appreciated. Idlers counter with the statement that in keeping the engine warm, they're polluting less than the shopper who comes out to a cold car. Who's right?

First, a bit about cold car chemistry. When a car is cranked to life after the engine has cooled to below about 20 degrees Fahrenheit, the tailpipe spews out a high amount of carbon monoxide. Abbreviated CO, carbon monoxide is a colorless, odorless gas that reduces the blood's ability to carry oxygen. CO occasionally collects in Fairbanks and Anchorage in concentrations greater than nine molecules of CO per million molecules of air, which violates federal air quality standards.

Gasoline isn't fully burned during cold starts, and that contributes to more CO entering the air from a vehicle's exhaust pipe, according to Robert Andres, a University of Alaska Fairbanks research assistant professor in engineering.

If a perfect engine existed, it would force out the tailpipe the same gases we exhale--water vapor and carbon dioxide. But engines aren't perfect. Besides water vapor and carbon dioxide, car exhaust contains a few forms of nitrogen oxides, carbon monoxide, and "a whole soup' of hydrocarbons (partially burned gasoline)," Andres said.

A car that's been running for about 10 minutes emits considerably less CO because the heat of the car's engine has warmed the gasoline, Andres said. Because of this warming, gasoline easily changes from a liquid to a vapor. Gasoline vapor ignites better than tiny droplets of cold gasoline, so a warm engine forces less CO out the tail pipe.

Both Andres and Dave Veazey, a Geophysical Institute graduate student who did his masters thesis on carbon monoxide emissions in Fairbanks, agree it takes most vehicles about five minutes for the engine to warm up enough to get carbon monoxide emissions to a minimum. Veazey said it takes that much time for emission control devices to begin operating properly.

The question remains: To idle, or not to idle?

Veazey said the results of his study show that clean-running vehicles--which comprise perhaps 90 percent of all vehicles in the Fairbanks North Star Borough, according to his study--could be left idling for hours and still not spit out the volume of CO it would in the first five minutes after a cold start.

The "dirty" 10 percent, which Veazey pointed out are not always old cars, but ones with faulty or maladjusted fuel-system components, always emit high CO levels whether the vehicle is warmed up or not. Clean cars emit a lot of CO for the first five minutes after a cold start, but the amount drops dramatically after they warm up.

Veazey said a clean car would have to idle for about 10 hours to put out the same amount of CO as it would in the first five minutes of a cold start.

So, the idlers--if their engines are efficient--may indeed be pumping less carbon monoxide into the air than those who turn their engines off. But the non-idlers will return to a happier gas gauge, and a ticket-free windshield---it's against state law to leave an idling car unattended.