"Idle Vs. Restart," Input/Output, ME Departments, June 2007

input/output

Idle Vs. Restart

The myth we are trying to investigate is the one that says it is better to let your engine idle because shutting it down and restarting it wastes more gas.

The ASME Florida Section members were asked to perform a simple experiment with their vehicles equipped with a miles-per-gallon meter. By using this meter and a special technique, we could determine the gallons used per minute of idle with only about 5 minutes of actual idle. The preliminary results with a V6-equipped vehicle and $3-per-gallon gas are as follows:

Idling with no air conditioning on: 0.5 gallon used in 90 minutes ($1.50 in 90 minutes). Idling with A/C on an 88¼F day: 0.5 gallon used in 60 minutes ($1.50 in 60 minutes).
Restarting uses approximately the same amount of gasoline as idling for 6 seconds with the A/C on.

One researcher reported experiencing approximately 10 minutes idling at stoplights when driving 25 miles round trip to work in stop-and-go traffic. These stoplights required anywhere from 15 seconds to as much as 1 minute of idling.

If you could conveniently turn off your engine at a stoplight (similar to hybrid vehicles) and restart it just prior to when the line of cars gets moving, you would save only 25 cents of gas per day, assuming you average 10 minutes sitting at stoplights.

If you believed the myth that restarting takes more fuel than idling, the myth is busted. Our research showed that a V6 restart takes about the same fuel as 5 seconds of idling. We expect a V8 to save more and a 4-cylinder less.

But just because you might save gas by shutting off your engine instead of idling, should you shut off your engine at stoplights? Based on the minimal cost of gas saved, probably not. And there are other reasons.

Restarting your car with the automatic transmission in drive is prevented by a lockout switch. To restart, you must move the transmission to neutral, restart the car, and then place the car in gear to get moving. Plan on at least 4 to 5 seconds to get ready to move again. After a few traffic lights, you will get tired of this or likely forget. Hybrid cars have intelligent computers that sense your speed and your foot on the brake pedal to do this. Your vehicle was probably not designed for such frequent restarts, about 10 times or more than what was anticipated by the manufacturer. If you save 25 cents per day in gas but have to replace the car battery or starter, you could likely wipe out a major portion of your savings with the cost of a major repair.

And let's not forget the fact that your A/C won't be cooling you during that 1-minute stoplight. Normally, your A/C air is at 50°F to keep you cool and dehumidify the air. Tests showed that even if you turn your ignition back on without restarting the engine and let your A/C fan keep running, the blowing air gets warm quickly. Within about 23 seconds, your A/C air will be at 70°F, which won't feel very cool in a hot car. And it will take about a minute, even at 2,000 rpm after the restart, for the A/C to get back to 50°F. And running the A/C fan while your engine is stopped will definitely load your battery even more, so plan on replacing it sooner.

Note that the above discussion is true for normal vehicles. Hybrid cars have been designed from the beginning to use this strategy of shutting down the engine at stoplights. The electric motor/battery stops the vehicle by recovering the energy of the moving vehicle. The batteries used are designed for frequent charge/discharge and have the capacity to run the A/C when the car engine is stopped.

Just like the Mythbusters from the Discovery Channel, the investigators are trained mechanical engineers. Do not try this at home.

—ASME Florida Section

This article was originally published on the ASME Florida Section's Web site, http://sections.asme.org/florida. You can reach the section by e-mail at floridasection@asme.org.