fuel cost of idling

We've been down this road before. If you take the idle rpm, the displacement, and the typical air/fuel ratio, you should be able to calculate more or less what the fuel consumption would be.

What it comes down too? Idle for X amount of time vs. Start up enrichment and use of electric causes higher idle. It's all theory but the one I like is, if 5 mins of idle is enough to prevent another start cycle, then that is worth it. More than 5 mins of idle wastes gas, money.

disston

Hardly. Cars with OBD-II capability can report their actual fuel rates in gallons per minute.

Not hardly, you're not operating at 100% VE at idle. In fact, the throttle is nearly completely closed, so very little of the "displacement" is actually displaced when the engine is idling.

nate

Not hardly, you're not operating at 100% VE at idle. In fact, the throttle is nearly completely closed, so very little of the "displacement" is actually displaced when the engine is idling.

nate

It is an estimate. The cylinders will be displaced by the piston no matter what, so that is not totally correct. If you can figure out how much gasoline has to enter each cylinder to keep the thing running, then you can estimate an idling fuel consumption.

Better still would be to use a calibrated fuel cylinder and measure it.

Which mode would this be shown in?

Matt skrev:

The figures I've seen are around .5 liters/hour for 2 - 2.5 liter engines. My cars probably consume ~1 l/h, or 1/4 gal/hr.

Assuming a car gets 10 L/100km (23.5 mpg) at 100 km/h (62 mph), it consumes 10 liters per hour, or ~2.6 gal/hr.

So, basically, the more you idle, the more gas you save per hour...

Ulf

One can plug an appropriate computer with LCD into the connector, for instance this one called ScanGauge II:

compared to driving in circles :-)

Thanks.

So you're saying a car burns fuel ten or twenty times as fast cruising as it does at idle?

I guess we are assuming a warm engine.

Actually my question is motivated by my rather poor gas mileage. A few times I warmed the car while idling to diagnose the cooling system. I was wondering if it hurt my measured gas mileage much. There might have been an hour of idling in those few warmups over a week when I burned maybe six gallons and drove 90 miles, all on the highway.

If you idled it for an hour, it definitely made a significant impact on your mileage. You don't say what your normal highway mileage is on this car, but its a good bet that much idling halved it or worse.

It should make about 25 mpg on the highway.

My usage of 6 gallons was say half a dozen 10-20 mile trips (total 90 miles, all at about 60 mph) plus about three warmups at idle, the warmups probably totaling not more than an hour.