37 replies to this topic

[GBORSARI]

I'm hvaing an arugument with the neighbor boy. He's just out of college (no tech or science classes) and we don't agree on car air condit.


I blast it all the time ! he says that it takes too much power away from the engine and ruins the gas milage.

he has a 2003 Toyota camry 4 cyl.

So . . . does any one have some good scientific evidence about HP loss with the air/con on ? I would figure that it can't be more than 1-2% power loss and milage loss


Thanks

Mark

[Kjetil]

I was told the aircondition on my car used around 5-6 hp. I guess they counted the compressor connected to the engine, and the electrical cooling fan behind the radiator and maybe the ventilation fan. The electrical cooling fan runs only when the car is driving slowly, so the numbers is probably lower on the road.

[JwS]

I read somewhere, a while ago, that if you are driving on the highway the drag from the AC would be less than the drag from open windows. Now I have always assumed that they meant all the windows open, so the whole thing is open to interpretation. Personally I dislike AC and will virtually never use it if I am alone, I'd rather sweat if needed and get fresh air in. If someone is in the car I will run it. Of course if you are "blasting" it, you may well be using more power than if you just used it to "condition" the cabin.
JwS

[alexbiker]

Being thorough bordering on obsessive/compulsive, I compared using A/C and not for my mileage in the recent UK heatwave.

No difference noticeable.

Alex

[ciaoduc1]

Don't know if you've seen the show "Myth Busters". They did a comparason to get the scoop on a similar question. Their "myth" was that it's more fuel efficient to drive with the windows up but a/c on than to drive with the windows down and a/c off. Theory being that the improved aerodynamics of raised windows would out weigh the drag of the a/c compressor.

Anyway, they took two identical cars (might have been SUV's) and drove them around a NASCAR track. In the end, the car with the windows down had better fuel economy but only marginally. After a whole tank of fuel, the winning car did an extra lap or two. Maybe 4-6 miles per tank.

Cost of adding an extra 4-6 miles per tank of fuel: 1-3 dollars
Cost of knowing you're doing a teeny-tiny bit to help protect the environment: negotiable
Cost of traveling in comfort, being able to carry on a conversation or easily listen to your music and not being wind blown when you arrive? Priceless

[Deepak]

In my country and it gets really hot (avg 38-40C in April - May). I really cant drive without the A/C on but it really puts a lot of strain on the engine. I can actually feel the power come down when cruising in top gear. I guess it all depends on also how good the compressor on the A/C is.

Rest of the time I prefer to keep my windows up and turn the blower on.

[Engineguy]

The methods used for the Mythbusters test were hotly debated. The test was done with Ford Explorers: a brick whose aero was probably not affected by windows down as much as an aerodynamic car would be. The did the test on a very small tight oval where severe tire scrub was quite evident even at their 45 MPH test speed. Their fuel consumption measurement method was horrendus and full of pitfalls. I like the show, but in this case they proved absolutely nothing.

They mentioned on the show that at 55 MPH the Explorers' trip computer readouts contradicted their test, and showed less consumption with A/C on than with windows down.

I recall (because I bought one) that when Dodge introduced the 1993 Intrepid, they bragged that the A/C had enough capacity for a 1700 square foot (i.e 3-4 bedroom ranch) house. That implies a home unit with a 2 to 3 HP compressor motor and a 1/4 HP fan motor. Some cars declutch the compressor at high RPM and do likewise intermittantly as needed to maintain idle speed. All cars cycle the compressor on only as long as needed to keep the evaporator temp at a certain level.

In any case, IMHO... put another 1 psi in your tires and you'll make up what you lose from running the A/C. And you'll be cool

[BRG]

Originally posted by Engineguy
In any case, IMHO... put another 1 psi in your tires and you'll make up what you lose from running the A/C. And you'll be cool

That's assuming that you don't fall off the road or lock up and slide into someone as a result. And that the resultant uneven wear on the tyres doesn't mean you have to replace them earlier than otherwise.

With my 1400cc Corsa with fairly limited power reserves, I can feel the difference with the a/c on - it is certainly more sluggish. It doesn't matter when you are cruising, but in traffic or when overtaking it makes a difference. But I've never noticed mcuh difference between windows open and closed. But then all the extra noise from open windows makes you FEEL that you are going faster anyway!

[Pils1989]

The a/c slows down my 1.5 Scion xB, less on my former 2.0 Renault Clio 172. Bigger the engine, less "power loss" feelings I've noticed with a Jag XKR or a Corvette ZR1.

[Wingman]

Originally posted by BRG
That's assuming that you don't fall off the road or lock up and slide into someone as a result. And that the resultant uneven wear on the tyres doesn't mean you have to replace them earlier than otherwise.

With my 1400cc Corsa with fairly limited power reserves, I can feel the difference with the a/c on - it is certainly more sluggish. It doesn't matter when you are cruising, but in traffic or when overtaking it makes a difference. But I've never noticed mcuh difference between windows open and closed. But then all the extra noise from open windows makes you FEEL that you are going faster anyway!

Same with my Lupo SDI. Some weeks ago, when I was driving uphill with the endlessly powerful 60HP car, I played with the AC. And when I switched it off, it almost felt like I pressed a boost button

[Greg Locock]

Typical rated power for the compresssor is 5 kW.

That'd be a direct hit on crankshaft hp.

BUT, most of the time your compressor is disengaged to allow the cold end to defrost, or destroked. So, the average load on the engine will be much less than 5 kW. When we're calibrating engines in cars we do have a telltale to tell us when the compressor is actually engaged.

Both 'modern' systems I've worked on had variable displacement compressors, which match the power usage to the operating conditions, somehow.

I think the Harrison V5 uses some mechanical feedback mechanism so that it works at a constant output pressure, for example. If you ever see one in a scrapyard pick it up, it is a nice little 5 cylinder variable stroke engine.

I saw the Mythbusters show in question, it was rather disappointing, the lack of experimental rigour was obvious, and there are plenty of cool ways they could have done the test rather than what they actually did. However, I think they showed that the results were pretty close.

I've done more or less the same experiment, and over 10 km at 150 kph couldn't see much difference, between windows up, a/c on, and windows down, no a/c, but it needs to be done properly. The results would change with speed as well.

[TDC Racing]

Ive found this is only applicable to small engined cars. I have a cousin with a toyota echo and she turns the ac off when going up hills. But with an engine that small, maybe she should also turn the radio off too

Im pleased i dont have that problem with my 5.7L Commodore, but i do pay for it in fuel

[Deepak]

Originally posted by TDC Racing
Ive found this is only applicable to small engined cars. I have a cousin with a toyota echo and she turns the ac off when going up hills. But with an engine that small, maybe she should also turn the radio off too

Im pleased i dont have that problem with my 5.7L Commodore, but i do pay for it in fuel

Hey...the radio wont effect the power of the engine. It only drains the battery a bit more ;)

[hydra]

Yes, but the alternator charging the battery drains power from the engine ;)

[McGuire]

Always run at 100% WOT for maximum volumetric efficiency.

[Pils1989]

The Echo has the same 1.5 engine than my car and the a/c is the only problems in the Appalachians.
I've noticed I was going faster than big SUV or other cars just because I wasn't braking for the curves!

[J. Edlund]

Opened windows add somewhere around 0.067 to the drag coefficient. For a race car such as IMSA, NASCAR or similar one can expect a slightly lower value of 0.025. There will also be a small increase in lift.

For a typical large car with a frontal area of 2 m^2 the power required to overcome this amount of extra drag should look something like this, power in kW and velocity in km/h.



Average consumption from an A/C compressor is probably not much more than one or two kW if not the temperature outside is very high or the temperature in the car is high and must be taken down by the A/C. Based on that the A/C should give the lowest fuel consumption when going faster than 90-100 km/h.

In any case, at the speeds we talk about any difference in fuel consumption between windows down and A/C on would most likely come from other factors such as driver style, tire pressure, difference between cars and so on. And if one really want to see the difference one should trust the readings from the engine management system which with good accuracy know the amount of injected fuel.

[TDC Racing]

We got some big hills in our area......the comment about the radio was tongue in cheek

[clSD139]

In real hot areas also cooling down the intake air can be effective, I think. If a closed circuit with a cooling medium makes charge air denser power can be gained in my view. Also accumulated ambient air (like wind) doesn't gets colder anymore around 40 celcius.

[Deepak]

Originally posted by hydra
Yes, but the alternator charging the battery drains power from the engine ;)

Thanks

[ciaoduc1]

Has anyone ever used an a/c evaporator as an intercooler?

[hydra]

To tell you the truth I was planning on using either that or a motorcycle radiator for a custom water/air intercooler setup...

[Engineguy]

Originally posted by ciaoduc1
Has anyone ever used an a/c evaporator as an intercooler?

I seem to recall there was a special edition Ford Lightning pickup truck that had a charge cooler under the Eaton supercharger that was chilled by the A/C refrigerant rather than water. Seems like it was in addition to the standard water/air charge cooler, and was activated only under 100% throttle. I don't recall how many additional HP it was credited with.

[NTSOS]

Actually for street use, a good way to extend the capacity of an auto air conditioning unit time wise when used as a charge air cooler would be to place the evaporator in a holding tank and simply pump the chilled liquid to an air to liquid intercooler. 98% of the time the system would be chilling the liquid until you put your foot into it. It's not primarily about increasing horsepower....though it certainly will, it's about keeping the motor from detonating. A twin turbo 406 SB with 800 hp is fairly easy to achieve on pump gas these days.....in this case, who would really care if the AC system ate up even 20 hp to power it?

John

[bobqzzi]

Originally posted by NTSOS
Actually for street use, a good way to extend the capacity of an auto air conditioning unit time wise when used as a charge air cooler would be to place the evaporator in a holding tank and simply pump the chilled liquid to an air to liquid intercooler. 98% of the time the system would be chilling the liquid until you put your foot into it. It's not primarily about increasing horsepower....though it certainly will, it's about keeping the motor from detonating. A twin turbo 406 SB with 800 hp is fairly easy to achieve on pump gas these days.....in this case, who would really care if the AC system ate up even 20 hp to power it?

John

This is how the Lightning system worked. A/C systems/evaporators don't have enough capacity to cool an intake charge on a real time basis.

[michaelab]

In low powered small cars (such as are very common in Europe, but much less so in the US) the effect of AC on the engine output is very noticeable as previous posters have already pointed out. Switching the AC on results in a very obvious drop in power. On more powerful cars (like my BMW M3 ) the effect is undetectable (at least by me). Also, based on just causally observing the on-board fuel consumption display (on my M3) I've never detected any increase in consumption due to having the AC on.

Michael.

[J. Edlund]

Originally posted by michaelab
In low powered small cars (such as are very common in Europe, but much less so in the US) the effect of AC on the engine output is very noticeable as previous posters have already pointed out. Switching the AC on results in a very obvious drop in power. On more powerful cars (like my BMW M3 ) the effect is undetectable (at least by me). Also, based on just causally observing the on-board fuel consumption display (on my M3) I've never detected any increase in consumption due to having the AC on.

Michael.

Most newer cars using "torque demand" engine management adds the torque required by the AC (calculation based on AC pressure) when turned on, therefore one should not be able to notice the difference.

Older cars usually turns the AC off automaticly when full throttle are given and some also offer some extra torque at low load using the idle air valve.

[DSP]

Modern A/C compressors are very, very eficient these days. Manufacturers usually state that the power loss (or consumption increase, if you prefer) averages to about 1%.

This is a figure that makes it negletible consumption wise.

Regarding opened windows beeing an alternative to cool the car, may i remind the more distracted, that at exterior temperatures above 35º Celsius, there is not much cooling coming from opening the windows.

(And of course opened windows only provide a confortable journey to the ones that already have become deaf by frequently using this method..... )

[wegmann]

The top speed of my Pontiac Sunfire (a weak 4-cylinder) differs by at least a couple MPH depending on whether the A/C is on.

I also overinflate the tires by a couple PSI. With normal street driving, the tread wear looks pretty even to me ... not sure how much it affects MPG, but of course it feels a little rougher.

[MikeTekRacing]

I drive a Daewoo Cielo (I think in other countries it's Nexia), with 1.5 SOHC 75hp engine.
you feel a lot of power loss when the AC is on.
if i floor the gas the AC is turned down for a while, you hear a click and just feel that the engine has more into it. After the 5-10 seconds, the AC kicks back in, another "click" and it feels like a little brake.
don't know exactly the power loss, but from what I feel I don't think it's smaller than 5hp.
before i overtake somebody i turn off manually the AC, because i need all the power i have.

on my father's car (2.0 HDi Xsara 90hp) the sensation is a lot smaller, it could be because of the big low-rpm toque of the engine that you just don't feel too much loss with AC.

[Scorg]

Been using the Air Con in a Fiat Stilo 1.6(103bhp) here. Cant feel any performance loss even under heavy acceleration and its econ has been maintaining over 39mpg (going by the cars computer), basicly staying the same as if it wasnt being used

[Collective]

Peugeot 206 (at least the chilean assembled version we get in Mexico) has an independent compressor... good for them!

[MikeTekRacing]

what do you mean by independent? it has to use energy one way or another it translates into lost engine power

[TDC Racing]

At a guess, i think it runs off the current to the motor of a compressor rather than from the belts

[McGuire]

On the small-displacement econoboxes, when you throw the A/C switch at highway speed you will feel a parachute effect...but it's sort of misleading as the high load is really only there for a moment and quickly levels out. Modern A/C systems are very efficient, with constant cycling and variable-displacement compressors.

[voice_of_reason]

2 anecdotes:

During my university years in the late eighties, a near neighbour on campus was the proud owner of an early sixties relic - an HA Vauxhall Viva. He was also known to be tight as *********. This was confirmed when he drove from Derbyshire (North of England) to Southampton (South of England) on a blistering summer's day with the heater on full bore (to compensate overheating engine with stuck thermostat) and windows shut (to avoid fuel consumption penalty of windows down to offset heater in summer). He arrived pink smelly and sweaty, drank any cost saving in the bar and had no joy with the girls that evening (probably unrelated to the incident).

On the Italian Autostrada from Imola , late for a flight departing Malpensa (yeah I know its the wrong side of Milan), the trusty Ford Ka hire car got to be Vmax tested. Result 147 km/h indicated, which became 151 km/h A/C off, mirrors folded. A/C on full cool was not able to win over Italian afternoon sun, so compessor presumably at full tilt. Base model Ka with prehistoric tiny pushrod engine => compressor significant relative to engine. Test repeated ABAB to be sure. Results corrected for gradient effects. I made the flight!

[Pioneer]

My Hyundai Excel decelerates noticeably when its AC compressor kicks on while driving on the interstate. I have to turn it off sometimes to make it up a large hill without holding up traffic.

[SimonW]

There are results of some fuel consumption tests at http://www.ademe.fr/...lim_auto_gb.pdf

However they were done a few years ago.