AC commits pre-meditated murder on MPG

How are you defining 'efficiency' of an A/C unit which is not same as 'effectively dropping the temperature'?

 

Exactly. My condo unit has a heat pump which provides A/C in the summer and heat in the winter. You can get the exact same unit in A/C-only configuration, which was my original intention (I was replacing some through-the-wall A/C units which were horribly noisy, inefficient, and A/C'd only part of the place!). When I found out that the heat pump version cost only a tad more, I got that instead and am extremely pleased with it.

 

Back onto topic, I used the A/C in my Prius again yesterday for a couple of 100-140mile trips. Ambient temps were 88-90F, and I set the A/C at 78-79F, mode upper vents only, fan AUTO, recirculate. Once again, there was no discernible MPG hit within the margin of error for wind direction and terrain.

Of course the A/C uses some energy, which is partly offset by the aero advantage of having the windows completely closed. But apparently this moderate usage of the A/C does not have a major impact on MPG, for me anyway.

 

We did the same when we replaced our system. We have a gas furnace and a heat pump and the furnace now only runs under 32 degrees or when a 4 degree change is requested.

It has saved us a ton of gas cost in the winter with not a large increase in electrical.

 

The heat pump I got is a Sanyo Mini ECO-i, and it's a bit unusual in that it supposedly works down to -4F. Lowest temps when I've been here have been down into the single digits and it ran perfectly. Haven't tested -4F yet. This condo complex doesn't have gas service.

 

I've never seen anyone who mainly drives in austin average 50 I am getting 43 in yearly average (17" wheels probably drop me 2 mpg versus you). Current tank is getting around 38mpg. Its way to hot to roll the windows down. The biggest hits are short hot trips. I keep my temp around 78. Too many days near or over 100, at least its cooler than '09.

I think my ac hit is larger in this car than my last, but that is likely because of the much hotter conditions than the rest of the country. Those that don't see a hit are either driving much farther, or in cooler temperatures. I do turn the fan down initially, and have a back window opened a little to blow out the hot air. Then I go closed windows and auto, eco mode once its cooled down.

 

Q1, I wouldn't know, it's too hot here most of the time, but even with the A/C on, I get about 2-3mpg better than winter.

Q2, no difference there, but ECO mode does help significantly with the A/C on.

 

ac kills

 

i call the ac the BEAR !!!! it eats allot of gas !!!! i think more than usual in other cars its amazing !!!!!

 

its like running a second engine !!!! i make 7+ litters per kilometer some times under certain circumstances !!!! its scary !!!!

 

Keep in mind that the actualy fuel used is still less than other vehicles. You are just seeing large numbers go down while the actual percentage loss is similar to small numbers going down slower.

So if the Prius at 50mpg drops 5% that equals 47.5mpg (2.5mpg loss).

A lesser car at 20mpg drops 5% that equals 19mpg (1mpg loss).

 

81F this morning. Car sits outside all day. 93F this evening. I'm expecting this tank's FE to, well, tank. This morning I pull into the lot at 60.9mpg (up from 60.7mpg the previous night) without running the AC at all. Tonight I run the AC on recirclate, lowest fan speed & arrive at home with 61.1mpg on the MID. Yeah, these temps are killing me. The car is happier in the 70's.

 

50 MPG = 2.00 gallons per 100 miles
45 MPG = 2.22 gallons per 100 miles

Is it really worth sweating over an additional 0.3 ounces of gasoline per mile?

And of course comparing MPG *differences* can be highly misleading:
25 MPG = 4 gallons per 100 miles
20 MPG = 5 gallons per 100 miles

Also a drop of 5 MPG, but 1.28 ounces per mile, more than four times worse than the A/C hit taken by a Prius.

 

Unless you're a Tibetan monk, or Wim Hof [ame="http://en.wikipedia.org/wiki/Wim_Hof"]Wim Hof - Wikipedia, the free encyclopedia[/ame].

Now to be on topic-- in my Subaru, AC drops my mileage in the summer by about 1-1.5 MPG, which is approximately a 5-7% drop. That's roughly the same drop that winter weather and gas blends drop me by, and about half as much as snowy, slushy roads drop it by.

Of course that makes sense since in snowy, slushy roads I'm running winter blended gas as well as AC for the defrost. Would that I were able to not use AC while keeping my windshield defogged. That "feature" is the bane of pizza guys everywhere!

 

There's one time (of many I've found) where ECO Mode helps. It tones down the A/C and doesn't blast you into the back seat.

 

There are lots of factors here. A big factor is outside temperature. Our compressor is variable speed. The hotter the air across the condenser the hotter the freon has to be in order to condense. Hotter freon means higher pressure, and that pressure is generated by the compressor. How much energy is required varies alot with how hard it is to compress.

Turning on the AC on an 85 degree day is cheap. The compressor discharge pressure is low, so energy consumption is low too. It runs at low speed (rather than cycling) for greater efficiency.

The other big factor is driving style, stop-and-go or highway. The Prius is great at stop and go because it can basically shut down when stopped and use no fuel...unless the AC is on. Then the stopping thing starts to have a penalty, because the AC is using energy (regardless of whether or not the engine runs), and it'll have to be replaced by burning gasoline eventually. That's why when I drive in the city on a 100 degree day, it costs me 10 MPG, but on the highway on that same 100 degree day, it's only 1 or 2 MPG.

Regarding the system efficiency, I can assure everyone here that Toyota did everything they could to make the system thermally efficient. The compressor has a variable frequency drive to enable variable speed operation. Only a few ultra high-end home AC units have this and they're in the 15 grand+ range. The compressor itself is a high-efficiency scroll design. Scrolls are still new for automotive applications.

Even the evaporator has an innovative pressure recovery design (Denso technology) to reduce compressor power demand. I've never seen that on any commercial refrigeration system.

Climate Control / Technology | DENSO CORPORATION

Technologically, there isn't much else they could have included to raise efficiency of the AC.

 

So if the Prius at 50mpg drops 5% that equals 47.5mpg (2.5mpg loss).

A lesser car at 20mpg drops 5% that equals 19mpg (1mpg loss).[/QUOTE]


i see what your saying ... what i am saying is that from 5,0lt/100km without ac i end up going up to 7.+5range that's allot for ac my friend i am a cab driver all i do is crunch gas numbers and this is a great difference in my opinion ! mind you when the heat is moderate it isnt as bad but when it gets really hot the BEAR GETS THIRSTY AND THERE IS NO QUENCHING IT's THIRST HAHAHAAAA
this weekend the temp is going to hit 40 degrees in athens greece i bet i am going to waist more than 8 liters to the hundred km

 

i see what your saying ... what i am saying is that from 5,0lt/100km without ac i end up going up to 7.+5range that's allot for ac my friend i am a cab driver all i do is crunch gas numbers and this is a great difference in my opinion ! mind you when the heat is moderate it isnt as bad but when it gets really hot the BEAR GETS THIRSTY AND THERE IS NO QUENCHING IT's THIRST HAHAHAAAA
this weekend the temp is going to hit 40 degrees in athens greece i bet i am going to waist more than 8 liters to the hundred km[/QUOTE]

I don't disagree that you lose MPG with the A/C on. What I am saying is that the loss is less than with almost any other vehicle when you do the math. It's just that those high numbers (40-55mpg) make the loss seem higher. Most people see a 7mpg loss and they think it's the same as losing 7mpg no matter what your average MPG started off at. A 7mpg loss is huge when you are only getting 20mpg but it it is not such a big deal when you are getting 50mpg. That is why percentages work better.