A/C On then Off
 

On my car I have the ability to turn the compressor off so that it no longer compresses and blows cold air. The air stays cool for about a few more minutes due things being residually cooled down. Of course the minute I turn the compressor off I feel a slight power boost. So my main question is. How much FE would you gain if you kept cycling your A/C Compressor on and off when needed. Of course the best thing is not to run A/C at all but when it gets up to 103 to 113 thats just not and option.

Well, if the compressor decreases your FE by 10% (which is the usual estimate) and you run it for 50% of your drive, then you'd gain 5% over running it the whole time. Just tweak the numbers for how often you run it.

what's the "d" for BFG9000d? I always preferred the SSG or RL. (actually might play some Doom tonight) Anyway, I had the same question. Like anything when you turn it on, does it use extra energy to turn it on? So would it not be more efficient to leave it on rather than turning it on and then off and then on and off etc? I do this sometimes: I'll put the A/C on low, but as cold as possible--then I'll turn it off and put the blower on high. When it becomes too warm again, I put the A/C on low again on frigid and then after about 5 minutes turn it off and put the blower on high again. The idea is that I'm using the residual coolness and cycling it through a lot of air, but it doesn't last that long; only a minute or two. Is that more efficient than leaving it on low? I only have the blower on for about a minute or two before I put it back to low and turn the A/C on (for about 5 minutes). Maybe we could dub it the compress and blow method... then again, maybe not. I don't always do this, but I wonder if it actually saves any gas or maybe uses more?

Be careful not to ruin the clutch in the ac compressor. Those compressors are expensive. I just bought one a couple of weeks ago.

My old Mitsubishi Mirage had the coolest thing that I think I've ever seen that was A/C related, and I've wanted it on every car since. It actually had 2 levels of A/C, there was a push button. First click it went 1/2 way in, and a yellow LED lit up. In this mode it would actually cycle the A/C automatically, and I almost always used this mode. One more push to get a green LED and it was full tilt, and then one more push to turn A/C off.

anybody with a scanguage figure this out.

I know I always get better gas mileage with it turn off. But I cant tolerate being in a car windows rolled up in 105 degree weather. I either run the A/C all the time or cycle it. The question being does it help to cycle it. As far as the clutch goes if anything you are saving the clutch more. Because normally the a/c compressor clutch turns on and off during system operation. By leaving it off for a minute or two and then turning compressor back on for a short time. You are reducing the amount of time the clutch turns on and off.

Hey my car has the same thing! My car is a Plymouth Laser, which is the same as a Mitsu Eclipse. My other car also has the same two step A/C. It's an Eagle Talon, which is the same as a Mitsu Eclipse. :) I can feel the AC cycle on and off when I'm holding steady throttle. I got in the habit of turning the AC on full blast when I was coasting to a stop. That way the energy from coasting in gear would drive the AC, not engine power.

The compressor probably cycles on and off using that clutch already, so doing it manually would probably result in fewer cycles...

Well my best guess is that you are saving more gas. The compressor cycles on and off anyways so start energy for it to turn on is minimal or not enough to effect FE. Plus keeping the compressor off for a short period of time and then cycle the compressor back on. That alone should save a fair amount of gas. Atleast some part of your trip you where riding with the compressor off. Which would result in a slight FE increase. I guess unless you got some really exotic compressor. Its the only way I see cooling the car down via the A/C as far as getting the most out of FE. The only other way would be to take a ice cooler fill it with water chill it so its one big block of ice. And plug a little fan in your car and have it blow on you. The big ice block should last a while. Although you would have to account for the extra wait in the car from the block of ice. So I dont know.:confused:

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.

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.

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.

Hill Climb / Cycling Data
 

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 :o

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% :eek:

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

Of Course, YMMV...

RH77

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.

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.

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.

It's called climate control ;) A lot of cars have them as options :thumbup: 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.

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

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.

For in-traffic use, try installing a second brake-light switch so it comes on when you barely touch the brake pedal. It trips a relay and switches the A/C on. This enables you to regulate your speed in traffic. Keep the a/c fan on and the air stays mostly cool. But instead of turning inertia into powdered brake-linings, you'll start turning it into cool air.

The compressor takes the lease load when first turned on because there is equal pressure on both sides of the pump. Turning it on just after it stopped with full pressure uses the most energy and in 110AC powered cooling systems the motor may actually fail to start and kick in the thermal current overload switch and that really wastes power. In a car with a clutch it presents the worse shock load to the clutch and belt when you kick it on and off a lot.

Best to keep the fan on low the temp to the coldest setting and shut it off when you need to accelerate or rev the engine more. Turn it on when slowing down or going down hill and then kick up the fan as well to create more braking action and make the compressor work harder when you want to slow down. Most important is to recirculate the air in the vehicle so that it dries out the air and can cool you more as well as to keep cooling the already cool dry air in the vehicle instead of constantly taking in the HOT outside air. Keep on eye on engine temps as well because the AC can cause more engine heating when stuck in traffic. Also keep some distance from the vehicle in front of you to allow more cool air to reach the radiator and condensor or else you get the heat of the one in front of you and reduce the cooling efficiency. I barely notice the AC in my Scion when used in this way and the impact is maybe a mile per gallon which is a small price to pay for a lot of comfort and not sweating up the seats.

Remember to let a little fresh air in once in a while if using recirculate or the vehicle will get a little stuffy - low on O2 high on CO2 !