Virtual Mythbusters by American Society of Mechanical Engineers Florida Section

Contact ASME Florida Section at email address: floridasection@asme.org

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.

We asked our Florida Section members to perform a simple experiment with their vehicles

equiped 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/gal gas are as follows:

Idling with no air conditioning on             0.5 gal used in 90 minutes ($1.50 in 90 minutes)

Idling with A/C on an 88 degree F day      0.5 gal used in 60 minutes ($1.50 in 60 minutes)

Restarting uses about the same amount of gasolene as idling for 6 seconds with

the A/C on.  See graph of fuel used per minute of idle below.

One researcher reported experiencing about 10 minutes idling at stop lights when

driving 25 miles round trip to work in stop and go traffic.

These stop lights 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 get moving, you would only save $0.25 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 a V6 restart takes about the same fuel as 5 seconds of idling.

We expect a V8 to save even 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 $0.25 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 lets not forget the fact that you’re A/C won't be cooling you during that 1 minute

stoplight.  Normally, you’re 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 you’re A/C fan keep running, the blowing air gets warm quickly.

Within about 23 seconds, you’re 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 2000 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 definetely 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.