A/C On then Off

[DRW]

Originally Posted by sandalscout

You know, I hadn't really thought about it, but my Galant VR-4 also had 2 stage A/C, as did the Laser parts car I owned. and of course, both of my Mirages did too. I'm just dissappointed that Hyundai didn't carry that over to their own cars after building off of Mitsus for so long.

Ah yes, a Galant VR-4 is a rare car. Very nice.
I've often thought of doing a motor+tranny swap between my Laser and an early 90's Colt/Mirage. Big motor in a little car, it's hard to resist the temptation to have good power in a small light chassis.

[Telco]

They actually make an ice chest air conditioner, you put a bag of ice in and turn on the fan, and it's supposed to last several hours before it's no longer cool. All it is is a new lid for an ice chest with a fan that blows into the chest and a vent that blows out. I was looking into one of these for the wife who delivers mail in an un-air conditioned vehicle, but she doesn't want one because she has little extra room in the truck. Anyone here wanting to do away with their air conditioner altogether could get one of these, would just need a steady ice supply for it. So far as weight goes, removing the AC components and drag on the engine would more than make up for it. You could even go down to your car a couple hours before quitting time and turn the fan on, and have a nice, cool car when you leave for the day if you wanted. The fans are 12V, so a small solar cell in the windshield should be enough to run it.

On cycling the AC, seems to me like you would get more out if it running the blower full blast during the cooling cycle to get as much cold air in as possible, then run the fan on low when the compressor is off to make the residual cold last as long as possible. Doing this would be like high speed P&G, where you lower the temperature more then glide longer.

[trebuchet03]

This would actually be a decent experiment. Keep in mind that your compressor is most efficient in steady state conditions - that is, when the high pressure side and low pressure side have normalized. Initial start up will consume more energy per unit time and consumption will ramp down until you've reached steady state.

What I'm getting at is -- there may be a critical point where if you wait until after, you'll see a gain whereas if you don't wait long, you'll see a loss.

[rh77]

I've significantly reduced the number of trips I use the A/C this summer, but there are times where I have to use it for social/work purposes.

I do have some actual SG and fill data from manual cycling of the A/C: (not completely Scientific: 1-2 trips of each arm thus far...)

1) First: Used the A/C to maintain a decent temp ("recirc"-on)
2) Employed usual methods of hypermiling, including Engine-off coast (yes, while still stressing the batt with headlights and HVAC fan on 1/2-3/4)
3) A/C cycle method: after cool, cycled the compressor to 'off' up grades, and to 'on' downhill.
4) On level ground, the system is cycled based on frigidity or sauna-ness

Baseline HOT summer FE with no A/C: ~36 mpg
Full A/C with hypermiling: ~31 mpg
Manual Cycling w/hypermiling: ~32.5 mpg

*Gain = ~4.6% with A/C Manual Cycling vs. Not
*A/C vs No A/C Loss (with manual Cycling) = ~ -9.7% (without man. cycling) = ~ -19.4%

Yeah, I lose a LOT of power and FE with the A/C on.

Of Course, YMMV...

RH77

[bfg9000d]

thanks for the info.

Sounds like cycling does give you slightly better FE. It could give you even more depending on how hot it is outside. If its not as hot you might be able to leave the compressor off a little longer.

[Telco]

Originally Posted by theclencher

Wouldn't the a/c system itself know the best (optimal) times to cycle?

Only based on its own design. The AC will cycle on and off according to the pressure on the system, not according to fuel economy. The AC isn't that smart, and the computer doesn't care. If AC is called for, then it will produce air as cold as possible. Even the variable compressors do this, the compressor will produce air as cold as possible for each setting. As far as the computer is concerned on the computer controlled systems if you want the AC on you must want to cool the air, not save fuel.

[bfg9000d]

If I can gain and extra 2 mpg from it. To me its well worth it. Maybe one of these days they will make a compressor. So that it monitors cabin temp and cycle on an off according to what temp you want it to get up to in the cabin.

Anytime you accelerate you should turn the compressor off. The compressor drags the engine down quite a bit. Try this start accelerating up to 60 mph. With compressor on then when you get up to about 30 mph or so. Turn the compressor off notice how your engine will surge ahead quickly. Then naturally you start applying less gas to achieve the same amount of acceleration that you had with the compressor on. The only time the compressor cycles off when accelerating is when you floor it or get close to flooring it.

[trebuchet03]

Originally Posted by bfg9000d

If I can gain and extra 2 mpg from it. To me its well worth it. Maybe one of these days they will make a compressor. So that it monitors cabin temp and cycle on an off according to what temp you want it to get up to in the cabin.

Anytime you accelerate you should turn the compressor off...

It's called climate control A lot of cars have them as options Sometimes they are hidden under fancy names such as Climatronic (VW). It also cycles blower speed among other things

My old Mazda 626 (2L) had a throttle switch (in the ECU) that would kill the compressor when you accelerate over 85% (or so) throttle.

[popimp]

When my family rides with me we use the AC. I normally shut it off going up hills, which helps the FE.

[bfg9000d]

Since I have done the cycling I havent really been keeping tabs on my mpg like I should. But I have notice I'm getting a heck of allot more mileage on one tank. My last tank I got 450 miles on one tank. Versus the 322 to 340 I normally get. Plus my car is still pretty cool considering the 100 plus degrees outside it is. Even when cycling I never sweat in the car. So it deffintley worth doing.