Air Conditioning Fuel Consumption!

Denny B schrieb:

On my 2001 PT I can hear a slight decrease in rpm when I turn on while the car is in idle ...

It's simple ... everything you turn on in your car puts more load on the engine and that means that it lowers the gas mileage since the engine has to use more energy for different stuff than moving the car. I wouldn't say you get poor mileage, when you turn on the a/c; I once read in the german ADAC magazine that using the a/c eats up around 2 liters of gas per filling (and that's not much).

It would be different though, if you turn on every gimmick to max in your car ... :-)

In the 2001 PT, turning on the AC does not give you poor gas mileage, it gives you POORER gas mileage!

It may cause a decrease in gas mileage, it really depends on the vehicle. Only way to find out is to try it with and without the AC operating.

Yes. That would be the AC clutch engaging. You might also hear the AC compressor turning and the electric cooling fan engage.

The AC is driven by the engine via a rubber drive belt. It takes a small amount of horespower to drive the compressor,which places a load on the engine. The AC clutch is an electromagnetic clutch. The alternater must produce enough elctricity to power the AC clutch as well as power the blower motor and condensor fan. The alternater is driven by the engine also and places an additional load on the engine as well. These additional loads may result in a decrease in mileage. You may notice a 1-2 mpg drop in fuel mileage.

First off, a VERY basic rule.

It takes power to run A/C

Now some other obvious rules that apply to this.

An aerodynamic car gets better mileage

Going faster decreases mileage

Running your A/C will lower the mileage over not running. How much? With some cars it's very minor. With others it can be a major hit. Depends on your vehicle.

If the tradeoff is A/C vs opening the windows, then it's a tradeoff of how much the aerodynamic change of opening the windows vs the A/C. And the windows can be affected even more by the speed.

There are claims that at certain speeds, under certain conditions, SOME modern cars get better mileage with the A/C as opposed to opening the windows, but the bottom line is the best mileage will be with the A/C off and the windows closed.

On the other hand, if you're cruising at 90+, the A/C probably won't be a significant hit at all.

It all depends on the comparison. If you compare "A/C on" versus "A/C off with windows rolled up" then absolutely you will lose mpg with the A/C.

But, imagine it is 90 degrees or more outside and it is 100 degrees inside the car. Not too comfortable. The A/C or windows are there to make it comfortable in the car. If you don't use the A/C you would most likely need to open all the windows to cool it off enough to stand it inside.

At low speeds (45 and below) the windows open don't effect the mpg much, but if you get on the highway it will (I know all you guys drive 70 or more.) The loss of mpg due to drag that is caused at these highway speeds partially outweighs the loss due to A/C. Best bet, as you start you drive open the windows for initial cooling and then close the windows and use the A/C at highway speeds. I bet being comfortable is worth the 1% loss in mpg.

As for the PT Cruiser in one reply, yea, your stuck. They suck for mpg, though I get pretty good mpg (for a PT) on flat roads with the cruise control on (A/C or not.)

Denny B wrote:

Not necessarily "poor", but not as good as if the a/c weren't running.

Yes. And there's no real better way to describe it than that.

It takes energy to run the a/c compressor. That comes from the gasoline.

Last I heard, the claim was that while a/c hurts gas mileage, it's not as bad as driving with your windows down.

Denny,

Others have already provided enough info to confirm what you heard and the qualifications such as windows up or down, etc... but I just wanted to add that low tire pressure and lack of maintenance/tune up could actually affect your MPG more than the A/C.

Bob

And note that if that's Fahrenheit rather than MPH, you won't *care* what it does to your gas mileage!

Hmm, I didn't intend it that way, but yes, that fits perfectly!

Having a 92 Dodge Shadow, I can also say from experience that having the AC switched from Freon to R-134 will make a BIG difference, not only in the performance of the AC, but also in mileage. My advice...DONT SWITCH! Bite the bullet and stick with Freon. You will be cooler, and the AC will run less.

D

While the conversions are less than optimum, there's no reason to stay with R-12. R-12 is expensive, and only going to get more so as supplies dry up. Conversions from R-12 to R-134 aren't going to be that efficient. But the first thing I would wonder is if you convert, have it done right, not just try to fill the system with the wrong stuff after changing connectors.

Repairing an R-12 system will be expensive and only done by certified 'freon handlers'. And R-12 systems are old, and going to continue to break and need repaired. And refilled.

With R-134 you can repair it yourself. I put a new condenser on my Pontiac when it ate a bird and eventually leaked. Took it to the shop, and the guy hooked up, evacuated for 30 minutes, then filled it for $50. Try that with R-12. If I had an adapter to pull a vacuum, I could have done it myself, then filled it with cans of R-134 from Wal-Mart.

If the system is done right, there's no reason R-134 can't give performance similar to an older system. My 93 Intrepid came with R-134, and that was the coldest A/C I've ever had in a car.

Just be aware that any repairs that require recharging will be way more expensive with R-12 than with R134A (both because of the artifically high price put on R-12 and the shops will charge more in general because it is not the standard, so they have to give it special handling, and they want to encourage you to abandon it or convert). The downsides of switching an R-12 system over are that the operating pressures are higher, so the likelihood of a hose (which by definition already has quite a bit of age on it - unless it's recently been replaced) rupturing is greater. As you point out, the R-12 system was not optimized for R134A, so it will be less effective and less efficient.

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

The compressor clutch.

The A/C runs off engine power, both the compressor and the electrical fans.

On my '95 3.3L Concorde I hardly notice the mileage difference with A/C on. I suggest you only use the A/C when necessary and not set the temperature too low. I just direct the cool air on me if I'm alone in the car.

That isn't true of all cars.

R134 systems require a larger condensor. Some cars have pretty small AC condensors, just barely big enough for a R12 system. And some cars have adequately sized condensors but have poor airflow through them.

If the body is the same, you can get a larger condensor from a R134 system in a newer model in a wrecking yard.

To properly switchover an R12 system you basically have to do this:

1) replace compressor (your never going to get all the old oil out) 2) Replace dryer 3) Replace all A/C hoses with barrier hose 4) Replace all o-rings with newer o-rings that are compatible with PAG oil 5) completely clean out the condensor/evaporator and all metal A/C lines by running at least a quart of mineral spirits through them. There is a fairly inexpensive flush gun on the market that does this. You need an air compressor for this.

I have done one of these conversions on a 1984 Chevy. It worked OK. The AC only didn't keep up when the car was idling in traffic. Unfortunately the particular model chevy I did it on used the famous GM DA-6 compressor and these compressors have a max lifetime of 6 years for a brand new one, 3 years for a rebuild. Mine lasted 3 years after the conversion then fragged.

Generally it's not worth doing the conversion unless your going to have to replace the compressor because of it failing. Since a proper compressor replacement requires flushing the system, it's not that much more to do the conversion.

Ted

Agreed. I've converted my '66 and '69 both to R-134a, did it RIGHT (including a full flushing of the system) and they both cool BETTER than they did with R-12.

Yeah, but there's another reason for that. The 93 LH cars (Intrepid, Vision, Concorde) were among the first cars sold that were designed for R-134a from the ground up, combined with the fact that they had more glass area than any car produced for several decades at the time. I think Chrysler engineers decided to err on the side of caution and head off any possible complaints about R-134a performance, and they MASSIVELY over-sized the AC systems on those cars. I remember one commercial at the time claimed that the A/C system was powerful enough to cool a

4-bedroom house. That would be about a 5-ton unit, which coincidentally was about the same as the very first Chrysler Airtemp air conditioners back in the early 50s on New Yorkers and Imperials- and were also known for how cold they blew. Personally, I love it. My wife's '93 has 250,000 miles, and will still freeze you out faster than any new car. IMO one of the huge shortfalls of the current LX cars (Magnum, 300, Charger) is that they don't have ENOUGH air-conditioning capacity.

The 93 LH cars (Intrepid, Vision, Concorde) were among the first cars sold that were designed for R-134a from the ground up, combined with the fact that they had more glass area than any car produced for several decades at the time

You want glass??? I had a 1976 AMC Pacer. I think that it had close to a ton of glass on it. I'll tell you what though, I got more car for the money than any other car. I was going to get a Malibu, six cylinder, three on the tree, rubber floor mats, heater and an AM radio. About $2700. But I found out I could get a Pacer. Six cylinder, four on the floor, carpeted, blue vinyl seats, AC for a couple hundred bucks less. Both were new cars. Great car. With the AC on and carting around all that weight, the 258 six got much better mileage than my four cylinder Cruiser does.

Your wrong.

It's illegal to manufacture R12 in the US

It is perfectly legal to continue to maintain OLD systems that use R12 by using R12 manufactured before 1994 (the cut off date) The big problem is importation - while R12 is illegal to manufacture in any country that is a signatory of the Montreal convention, there are some countries that still make it. (think Mid-East, you get the picture) So in many parts of the world R12 is still plentiful. Getting it smuggled into the US is the hard part.

NO a/c manufacturers are designing NEW systems that use R12 or R22 now. So every year, more and more old A/C systems go out of commission and are replaced by brand new units, so the demand will drop.

R22 is the next one on the list - it's used in virtually all home A/C systems. If you have central air and a central AC system you have an R12 system. You might consider now, buying a bottle of R22. See why here:

Ted

When the LHs first came out and the glass area was quoted as being so large, I immediately thought of the Pacer. I *think* that it turned out that the LH cars actually have more total glass area than even the Pacer had. They just don't look badly proportioned and swollen the way the Pacer did. :-)

All kidding and Pacer-bashing aside, AMC was way ahead of its time in many respects with that car.