# 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!

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.

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.

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.

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.

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.

Not only that, but the main air quality hazards in Texas come from oil refineries and chemical plants.

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.

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.

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)).
© 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©…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©/BTU(g)=gallon(s) of gas required to run A/C for 1 hour.

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.

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.

I remember asking my Internal Engines instructor about this after a lecture of his related to this. Like your comment, I was convinced that AC would reduce fuel consumption at idle because it lowered engine rpm. He explained to me why my logic was flawed (but I can’t remember his reply - it was 36 years ago).

How do you know his logic wasn’t flawed? Too many of us, especially when we are impressionable young adults, think that being in a position of authority means you can walk on water.
One of the most dangerous things for someone in a position of authority to say is “I don’t know” so they quickly learn to BS their way through questions that they really don’t know the answer to instead of admitting they don’t know.

How do you know his logic wasn’t flawed?
It very well could have been. He was a nerdy guy who was very knowledgeable with the physics of the internal workings of an engine. And I was just a lowly 21 year old student.

People sitting in roasting cars during a Texas summer are going to do a lot more laundry. That won’t help the planet.

Let’s explore this one. I like the question.

Assumption: carb, no “kick up” solenoid.

That being the case, the engine will either maintain the same RPM or stall (if the idle is insufficient to support the extra load).

The amount of gas being drawn into the cylinders (for given atmospheric conditions) is based solely upon the volume of air being drawn through the venturi. That volume of air will remain the same, since it’s based solely on the idle speed and the carburator settings. Added load does not automatically drop the idle speed.

I believe we’re looking at the problem backwards. The engine doesn’t need more gas when the compressor is engaged, it simply stops wasting so much. In essence, it’s operating inefficiently at idle with the compressor not engaged.

If there were insufficient gas being drawn in at idle to operate the added load, the engine would simply stall.

My thinking is this:

Assumption: Carb, no “kick up” solenoid. When the AC compressor is turned on, the idle speed will drop, but not stall.

I agree the amount of gas being drawn into the cylinders is based on the volume of air being drawn through the venturi. I believe the added load of the AC does automatically drop the engine idle speed. At the lower idle, less air will be drawn through the venturi - and to me that means less full will be drawn as well.

When you put a small load on an idling engine, the engine does slow down a little which in return results in a reduction in manifold vacuum.

less manifold vacuum is the same thing as more manifold pressure

Since the manifold pressure rises, the cylinders fill more completely with air/fuel and the engine makes more torque. The engine slows down until the manifold pressure is high enough for the engine’s torque to match the torque needed to turn the load on it.

Since there is less vacuum in the manifold, less air is drawn through the venturi. The cylinders reload more completely but less often.

This makes a gasoline engine’s idle speed very stable and they don’t need idle governors like diesel engines normally have. The power consumed fighting the intake manifold vacuum is also why gasoline engines have a very high idle fuel consumption compared to diesel engines.

No, there’s no need to conduct an experiment. This is junior high school stuff. A little thing called thermodynamics. Idling engine with or without ac on. Only source of energy is gasoline. The engine is just sitting there, and suddenly it has to power another machine, the air conditioner. More ac, more gas consumed.

Since a gallon of gasoline weighs about 8 pounds, the idea of it producing 20 pounds of carbon dioxide, leaving aside the water product of combustion, is, well, let’s just say “extraordinary” but a good way to hook up with college chicks at anti-globalization protests.

I like that. Let the engineers and politicians save the planet with our support in other ways. Let’s figure away to run the air more efficiently and not decide whether we want to use it or not. I bet in Texas there’s more than enough wind power and solar energy to power every AC, directly or indirectly in the entire state. At least there was when I was going through basic training there.