air conditioning and fuel consumption

I was told that it did so when I'm down to fumes I turn the A/C off. Someone will come along and straighten this out

YES, the air conditioner compressor eats much engine power

so technically speaking why does this happen?

the ac compressor runs off a fan belt directly from the crankshaft of the engine which reduces the horsepower output of the engine therefore requiring the engine to "work harder" to maintain the same road speed. this would result in the typical car engine achieving about 3 to 5 less m.p.g..of course the more horsepower the engine has to begin with, the less it will be effected by the ac compressor. or least that's what I believe.

Ahhh, the Rennaissance man answers.

And if you are stuck in traffic and your car is overheating to the extent of boiling over,turn the a/c off.Taking the load of the motor may be enough to stop it boiling over

Gordy, even better. I used to turn the heat full on to keep the radiator from boiling. Made driving in Florida torture.

More like .3-.5MPG. An normal car A/C compressor uses 10-15HP to turn. If the fuel consumption increase is noticeable, it will be in the 1MPG range.

Incidentally, the A/C causes less drag than open windows on the highway.

hey thanks again you guys know your stuff

Jarlaxle's got it right. A modern, aerodynamic vehicle will expend less energy, and consume less fuel, at any speed over about 20 Mph with its windows closed and airconditioning engaged than would be the case if the a/c were shut off and the windows opened. The disruption of airflow around, over, and past the vehicle's body caused by open windows greatly increases the vehicle's drag coefficient, effectively multiplying the apparent cross-section presented to the air through which the vehicle is moving.

Incidentally, the windshield defroster system actually engages the airconditioner of vehicles so equipped; the cooling of the air reduces its moisture content, enhancing the clearing effect as the airstream is directed onto the inside windshield. Even if the heater is engaged, the air directed to the windshield first passes over the airconditioner's cooling coils.

Speaking of the heater, vehicles with liquid-cooled engines actually have 2 radiators; the one which cools the engine coolant fluid, and another, called the heater core, which is the source of cabin heat. Engine coolant circulates through the heater core independently of the main radiator, with rate-of-flow regulated by a valve controlled through the temperature-setting knob on the dashboard. The heater core itself is simply a multi-row finned radiator over which the heater fan blows air. Engaging the heater adds cooling surface to the engine cooling system, and will reduce the engine's operating temperature.

so its more effecient to have a car with air con on ,on a hot day than to have a car with no air con but the windows down

For most of today's cars, that's it. The body designs are calculated to present the least amount of drag. In actual practice, however, the fuel economy differences will be all but neglible in normal driving. A well tuned engine with clean filters, tires properly inflated, a steady throttle foot without frequent speed changes (think cruise control ... but by foot), brisk but not jackrabbit starts, reasonable speeds ... a vehicle travelling 75 may use anywhere from 30 to 50% more fuel than at 55, in many cases, and well-planned decellerating and braking to stops will give much better results than just about anything else you can do. To drive cheap, drive smooth.

I wonder if the drag I'm getting from my Flinstone-style, open-floor, added-acceleration system is offsetting any benefits...

its funny that because my new van i drive at work is as aerodynamic as a brick,on the motorway it can be as slow as 60mph going uphill whereas the old van i drove could easily do 70 mph uphill.sorry for the people in usa i dont know what that is in kmph.its a real pain in the ass,good job i dont drive with the windows down as well as it would take me forever to get anywhere.

A work or delivery van prolly isn't gonna be all that aerodynamic to begin with, I s'pose; ain't much you can do to streamline a rolling storage container

true but what about painting some go faster stripes on the side???

Whatcha need is a rear-wing spoiler. And one of them noisy coffee can mufflers. Maybe some fancy rims. And a sub-woofer. Yeah, then your van will be fast.

The weight of the air conditioner alone is enough to decrease the fuel economy slightly. The guys at the shop warned me when I bought my car, and they were right.

Oh? I wonder if the conventional wisdom mostly applies to the Belchfire 8 of yore? Surly, hanging an A/C on my dinky 4 cylinder would knock down a bigger percentage than on the land cruisers used by the OK highway patrol to prove the efficiency of air conditioned cars.

Hmmm. Maybe the tests were performed by officers that wanted airconditioned cars. Windows down, no ac, and cruise at 70mph, then compare mpg to a 60 mph cruise in high gear at 60.