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Old 07-13-2009, 01:08 AM   #1 (permalink)
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Default Is it normal to lose 15+ MPG running the AC nonstop?

I drive the same route every day with a few minor changes and average 220-240 miles per day. It is about 50/50 on city/freeway driving.

When I drive without AC, I generally get 68-71 MPG overall.

When I drive with the AC, I average 53-56 and sometimes even lower (like today when it was 115). On my afternoon route (when the compressor literally never shuts off), the mileage is sub-50.

I realize that the compressor running continuously will cause a major hit, but I'm wondering if this is abnormallly high. The engine really seems to struggle at times.

The difficulty in getting into and staying in lean burn is also an issue as well. Generally, I can only do it on the freeway.

Mainly I'm worried about the AC compressor, though, and whether or not this huge MPG hit is simply a fact of life or if I should have it checked out.
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Old 07-13-2009, 01:41 AM   #2 (permalink)
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The difficulty in getting into and staying in lean burn is also an issue as well. Generally, I can only do it on the freeway.
The compressor isn't putting a 15mpg load on the car, it is putting a small load that keeps it from getting into lean-burn. That's what is costing you 15mpg.
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Old 07-13-2009, 02:02 AM   #3 (permalink)
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I would agree with the above statement.

CVTs don't have lean burn. However, according to our experience over the last 8 years, perhaps our lean burn-deprived CVTs don't seem to suffer the mileage hickey using A/C that MTs do. I say this based on the fact our CVT Insight has over 53K miles now, and except for a few (fewer than ten) times that My Beloved or I have driven with the windows open, due to a stunningly beautiful day or somesuch, we have had the A/C turned on with a setting of Econ/72 and the fan on two bars (out of four) continuously since January of 2001, when we acquired it new. She drives it like a Civic, although I at least try to achieve maximum mileage and drive it 90-95% of the time. Yet our LMPG is right at 55.6, which is within a stone's throw of the original estimated EPA highway mileage. So apparently the use of A/C is not such an onerous mileage penalty.

MTs always get such fantastic mileage compared with us slushboxers. Seems to me the difference in mileage wouldn't be 10-15 mpg... unless the use of A/C impacts your ability to enter into lean burn and its fabled mileage-enhancing properties. Otherwise, you're "just" driving a 55 mpg economy car...

Surely others will chime in with some intelligent observations...
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Old 07-13-2009, 09:59 AM   #4 (permalink)
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Default AC mpg effect

I agree with the last post. Most loss is probably due to the loss of lean burn. I would guess you are losing 10-12mpg due to this and 3-5 from the actual AC usage. It rarely gets much above 80 here, but when it does and I try out the AC, I have a heck of a time getting into lean burn. The smallest incline or even a rougher road surface knocks it out. I guess its like driving at 10 or 15 miles per hour faster, the extra wind resistance is similar to the AC burden.???
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Old 07-13-2009, 10:34 AM   #5 (permalink)
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i'll jump on the boat, and agree with all of the above

it rarely, if ever, gets up to 115 here, but whenever its hot out and im on the freeway. i keep the windows up, the climate control in econ mode with all 4 bars of fan, and the setting to either face, or face and feet. this gives me as much air blowing on me as fast as possible to help keep me cool. i also turn on recirculate to keep the cool air in the car. and i cycle the a/c manually. turning it from econ ac off to econ ac on and back. i will turn the a/c on when going down a hill, or if necessary, when coasted on a level surface. whenever possible, i will make sure the a/c is off when going up a hill. recirculating the air helps keep cool air blowing at you for at least a short time in between a/c bursts.

whenever im off the highway, i keep the same settings, but with econ ac off. and rolling the windows down.

max mpg is achieved(mythbusters proved it) by a/c on the highway, or above a steady 35-40mph, and a/c off the highway

of course, this might not work at 115 degrees, lol
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Old 07-13-2009, 11:02 AM   #6 (permalink)
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In order for me to lean burn at highway speeds with the AC on, I have to draft off of another vehicle.

Where I realy hate using the AC it is around town. It's impossible to drive the car reasonalby without useing a fair amount of IMA under these conditions. Enough consecutive days of in town driving with the AC and I'm in forced regen.
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Old 07-13-2009, 03:20 PM   #7 (permalink)
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Well, now that the Texas temps are hitting triple digits on multiple consecutive days, and birds are spontaneously combusting in mid-air, you have to have your A/C on. You just have to. I guess I'm getting too comfortable in my old age for anything else.

For decades, I've wondered why manufacturers never seriously considered an easily-replaced, lightweight electrical motor to power the A/C compressor, instead of a difficult-to-replace, belt-driven pump sapping energy from the ICE. I just know there are wonderful, common-sense, financial and technical reasons for it, but I can't think of many. It would eliminate complexity, extra belts, tensioners, weight... In our little cars, I'm surprised Honda didn't do it from the get-go...
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Old 07-13-2009, 11:59 PM   #8 (permalink)
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Thanks for the replies, they made a lot of sense. I try to avoid using the AC but once it hits 105, I can't take it anymore.

This is also the one place on the planet that I can grumble about getting only 55MPG and be taken seriously.
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Old 07-14-2009, 12:00 PM   #9 (permalink)
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This is also the one place on the planet that I can grumble about getting only 55MPG and be taken seriously.
thats why i love our cars
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Old 07-14-2009, 01:21 PM   #10 (permalink)
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Quote:
Originally Posted by boogetyboogety View Post
For decades, I've wondered why manufacturers never seriously considered an easily-replaced, lightweight electrical motor to power the A/C compressor, instead of a difficult-to-replace, belt-driven pump sapping energy from the ICE. I just know there are wonderful, common-sense, financial and technical reasons for it, but I can't think of many. It would eliminate complexity, extra belts, tensioners, weight... In our little cars, I'm surprised Honda didn't do it from the get-go...
Actually Honda did it in the Civic Hybrid. And went a step further: it's dual-drive, electrical and mechanical, and I believe variable-rate. I suppose the reasoning is that it's more efficient to run AC directly off the ICE (no losses converting to electricity and back) as long as the ICE is running, but Autostop can still be used because the AC switches to electrical when the ICE is off.
Not sure how the Prius II and III manage AC.
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