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How much gasoline is used if I idle my car with the air-conditioning on vs. air-conditioning off?

I'm trying to prove my case that idling in the car pool line is BAD NEWS. I've found a lot of data supporting that every gallon of fuel that is burned produces about 20 pounds of CO2, and even some that says we use about 0.026 gallons of gasoline for every 10 minutes of idling, but no data that suggests how much gas is burned when the A/C is on - which is our problem here in Texas. Please help save our planet and our little children's lungs and help me answer this question!


  • edited September 2010
    In order to have an answer to your question, someone is going to have to conduct an experiment. Since you have the interest, it might as well be you. Please help save our planet and our little children by conducting a simple experiment and let us know how it turns out.
  • edited September 2010
    I've wondered about that myself. However I think you're worrying about it too much. People having been idling cars whilst stationary for years and people aren't keeling over and dying of CO2 poisoning. You know we all also exhale CO2. Maybe we should just all hold our breath for the good of the planet.
  • edited September 2010
    I would not sacrifice comfort in the brutal Texas heat for the sake of a little C02. The savings in fuel would be miniscule at best.
  • edited September 2010
    I really doubt (but don't know for a fact) that it makes more than a few pennies of difference between having the AC on or off if the engine is idling anyway. When you are idling, most of the gas is wasted just keeping the engine going. Adding a small load to that probably doesn't change much.
    In any event, the treatment for the resulting heatstroke will cost far more than you would save. Just think about how much gas the ambulance would burn.
  • edited September 2010
    You want people to turn off the A/C? In TEXAS?!? Are you really serious? I'd be more worried about the resulting increase in "Road Rage" and related incidents. People get very short-tempered when they're in the sweltering heat.
  • edited September 2010
    Not only that, but the main air quality hazards in Texas come from oil refineries and chemical plants.
  • edited September 2010
    No disrespect meant, but do you really think that turning off your AC for 10 extra minutes while you sit in line will compensate for thet 60,000 pound tractor-trailer from Mexico that just blew by you leaving huge clouds of black smoke behind? That tractor trailer that's pushing out black smoke 16 hours a day, 7 days a week?

    You'd probably have to get 10 million people to turn their AC units for 10 minutes to compensate for that one truck. Good luck in your quest.
  • edited September 2010
    Depends on the vehicle, engine, etc. A 1969 Road Runner 440 6pack will burn considerably more fuel idling with the AC on than off. A 2009 Honda Civic 1.8L 4cyl won't burn much at all either way.
  • edited September 2010
    Two ways to figure this out:
    1. (Free, but lotsa math):
    (a) Determine avg. BTU/hr to cool a car. (Call it BTU(a)).
    (b) Guesstimate efficiency of a/c at turning crank HP into cool air (Call it eff(a)).
    (c) BTU(a)/eff(a)=BTU(b)...BTU/hr loss at the crank.
    (d) Guesstimate efficiency of engine at idle, turning gasoline into crank HP (call it eff(b)).
    (e) BTU(b)/eff(b)=BTU(c)...BTU/hr of fuel burn attributable to A/C.
    (f) Determine BTU content of 1 gal gasoline. [gimme: it's 114,000 BTU]. (Call it BTU(g)).
    (g) BTU(c)/BTU(g)=gallon(s) of gas required to run A/C for 1 hour.
    [ an OBDII reader w/ real-time data access.]
  • edited September 2010
    On old carburettor cars, the AC might actually reduce idle fuel consumption because the AC load reduces the engine's idle rpm, unless there is an idle kick up solenoid to raise the engine's idle speed a little when the AC is on. Ditto with leaving it in drive.
    When you put a load on an idling engine, its rpm lowers and that reduces the intake manifold vacuum, the lower vacuum sucks less air through closed throttle and that means the carburettor mixes less fuel with the airflow.

    Modern EFI cars have an adaptive idle airflow, the throttle opens slightly to offset the extra load of AC on or being in drive and so there is a measurable difference in idle fuel consumption in both cases. On my car, the idle speed actually jumps up a few rpm when the AC is on.
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