I found a web sites says 1 hour = 1 gallon. Not sure if this is realistic for our trucks. Not that I'm planning on idling all day. It's just good info to have if your heading into snow country.

http://www.consumerenergycenter.org/myths/idling.html

prob realistic figure.

>>>*It depends on the vehicle of course, more modern engines are getting pretty conservative.

I remember some GPM readings we took way back during some testing, a 1983 carbed 22R, it was running just under 3 GPH at 2625 (50% of torque/H/P cross) but only .68 GPH at idle.

Well, according to my math, anyway....*EB

read somewhere once that you might be better off to let one run continuous instead of run/stop/run/stop, if the engine has time to cool off enough to trigger cold start enrichment again. That can double the fuel burn/min, easy...

Um... my daily driver is a 2007 Volvo 630 with a 13 liter Cummins.

At idle, it burns a little less than .75 gph. I'm pretty sure your gas engine will be closer to .4 gph unless it has some kind of problem.

>>>*There is also the cost of fixing the dent in the rear end a person can get when they delay taking off after the light changes...*LOL**...*EB

I figured there was some sort of rough formula like displacement x RPM for most modern engines.

I'll dig around a bit more.

My Miata has 550 cc/min injectors, running sequential, and datalogs show that commanded injector pulse widths at idle are about 1.7 ms, so with an injector dead time of 1.0 ms that means 0.7 ms of fuel per squirt. Hopefully I won't screw up the math too badly.

injector flow rate of 550 cc/min = 9.167 cc/sec
x .0007 sec = .0064 cc of fuel per cylinder per revolution
x 4 cylinders x 850 rpm = 21.8 cc of fuel per minute
x 60 min in an hour = 1309 cc/hr
1309 cc is 0.35 gallons burned per hour at idle.

If I go by the datalogged injector duty cycle % instead of injector pulse widths I come up with 1584 cc/hr = 0.42 gallons/hr. Not sure which is closer to reality, but 0.35 to 0.42 is probably a good ballpark range.

This is a 1.8L engine idling at 850 rpm and 15:1 AFR. It's also using mid-late 80s RX-7 injectors which have crappy spray patterns compared to modern tech, though the Megasquirt does idle them pretty well.

fueleconomy.gov says .25 to .5 gallons per hour on passenger type cars and trucks. I think your math was pretty good. <img src="/forums/images/graemlins/cheers.gif" alt="" /> Most of the info I found was for large trucks with formulas based on displacement and HP drain of the accessories.

I like Scott's math, too. You can back into it on an obd2 system or any system that gives you units off the maf. My astrovan reads out maf in lb/min. IIRC, it runs about .60lb/min at idle. At 15:1, that's about .04lb of fuel/mim, or roughly .24gal/min.