AC impact on MPG

<div class='quotetop'>QUOTE(Grasshopper 01 @ Aug 2 2007, 03:01 PM) [snapback]489513[/snapback]</div>

It depends on a lot of factors.

Hot hot is it outside?
How cool do you want it in the car?
How cloudy is it?
What time of day is it?
Are you driving in a flat area with low buildings?
What is your MPG without A/C?
How many people are in the car?
How much are you using the battery?

Those are just a few of the variables that come to mind right away.

I've found that at noon on a 90 degree F day with the car parked in full sun, no people in the car, and the A/C set to 76 degrees, my Prius uses a bit less than 0.1 gallons of gasoline every 30 minutes. Now some of that gas is being used to keep the electronics of the car running even if the A/C is off. If you assume about 0.075 gal is being used to compensate for the A/C, and you assume the rest of the variables aren't affected significantly then:

Driving down the expressway at 60 MPH in a Prius that gets 55 MPG:
without A/C = 1.09 gallons
with A/C = 1.24 gallons = 48.4 MPG
a loss of about 6.6 mpg

Driving down the expressway at 60 MPH in a "Prius" that gets 25 MPG:
without A/C = 2.4 gallons
with A/C = 2.55 gallons = 23.5 MPG
a loss of about 1.5 mpg

Notice that in both cases both cars used an extra 0.15 gallons of gasoline per hour, but this is a much larger percentage of total fuel used in a car that is chugging the stuff vs. a car that is sipping it.

Since 90 degrees in full sun at noon is about the worst I can expect in my area 99.5% of the year, I figure that on average I'll use less than 0.1 gal per hour to run the A/C. Even on the worst days and even if gas gets up to $4.00 /gallon, planet be darned, I'm more than willing to spend the extra $0.40 per hour to be comfortable while driving in my car.

 

It's worth mentioning that for highway travel, it's better to drive with A/C on than with the windows down in terms of gas milage. Also it's important to keep the batteries cool. I use the A/C whenever it's over 75 degrees outside and/or I'm driving more than 40mph. The A/C in the Prius is electronic, not mechanical, in other words it has an electric pump rather than one attached to the engine with a belt, so the impact it will have on gas milage is less than a normal car.

 

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Without A/C my commute is good for 61-62mpg consistently. When I turn on the A/C that drops to 57-58mpg. (This is steady-state driving on 55 and 60mph roads.) That's about 8% by my reckoning.

 

Boise's had the hottest July on record (days & days of 100 degrees plus) and I've been running the a/c full-on most days. The mpg has plunged from near 50** to around 42. The black plastic dash is a huge factor in this - once you take the sunshade off, it stays hot forever. Think I'll perform a search for dashmats.

**and Darrelldd, please don't jump on my head because our avg is so low -- been there, done that! <_<

 

<div class='quotetop'>QUOTE(screaming red @ Aug 2 2007, 09:03 PM) [snapback]489687[/snapback]</div>

I live in Florida, where I need to run the A/C quite often. Being a "native" I can cope w/o the A/C on until the mid to upper 80's. That means I can drive w/o the A/C on here at night and before 9 AM or so in the summmer time. I will open the windows on both front side down about 4 inches or so for cross-ventalation.

The past two days we have had a lot of rain here, and I did not need to run the air. I can tell you from my experiecne that I lose about 5 MPG or so with the A/C on. I think this comes into play with P&G. W/O the A/C on I am more likely to get "green bars" on my battery, giving me more P&G power and range. I rarely if ever get into "green" with the A/C on. When I have the A/C on, it's set anywhere between 65 and 70, and either on low or one setting above low. The only exception is if it's the middle of the day and the car has been parked in the sun. Then I will put the A/C on medium until it cools some. I never run it above medium. I don't like to set the A/C above 70 degrees, as it's too hot here at 90 degrees for that higher setting to be very effective. Most all of my driving is city/suburban roads, with an average speed limit of 40-45 MPH. If I do any interstate driving, I am in the far right lane doing 50 to 55 MPH to save gas and get the highest possible MPG average for a tank.

I hope this long winded explaination was more helpful than confusing.....

Brad
Tampa Bay Florida

 

<div class='quotetop'>QUOTE(Grasshopper 01 @ Aug 2 2007, 04:01 PM) [snapback]489513[/snapback]</div>

I have an 07 Prius. The temps here are in the high 70's to high 80's. I keep the AC set to 75. My lifetime average MPG is 54.9 (only over 1800 miles --new car). My best tank so far was 58.6mpg. That is amaising FE, why bother turning off the AC?

 

<div class='quotetop'>QUOTE(scm2000 @ Aug 7 2007, 03:23 PM) [snapback]492006[/snapback]</div>

Well, if we assume that you could have a lifetime average of 57.7 MPG if you didn't use the A/C. . . .

After 150,000 miles you could have saved an extra 143 gallons of gasoline used to run your A/C

At an average price of $3.15 per gallon, you would have saved $450

You would have dumped 1.4 tons less CO2 into the atmosphere.

On the other hand, planet be darned, I personally find it worth it to spend the extra $0.45 per hour or so to keep comfortable.

 

Wait a few months till you have to have the heat on then you'll be wishing you had the AC on LOL. Cold weather and running the heat have a much bigger hit on mpg's then running the A/C.

 

<div class='quotetop'>QUOTE(paprius4030 @ Aug 7 2007, 03:35 PM) [snapback]492020[/snapback]</div>

In the winter I can pile on clothes and a coat to keep warm. There's only so much I can legally take off in the summer to keep cool, but I can always throw more on in the winter to keep warm. I'm hoping that the EBH I just ordered will help reduce the MPG hit I take in the winter.

 

It seems to be about a 5 - 10 % decrease for me.

 

Huge drain, I'm constantly searching for the optimal temp. Got tired of adjusting & just leave it on either 76 or 85 now, depending on how long been in sun. Lowered mpg to 48 or worse. Some days I couldn't care less just give that excellent cold air.

 

I recently discovered that even with a 20% tint on the hatch window, the hatchbox gets really (REALLY) hot when the car sits under the sun for a while (even with sunshades on and both front windows wide open). When I opened up the trunk to work on my stereo, the amp was so hot that I first thought it'd been running by itself!

I'll start using the tonneau cover from now on while driving to see if that helps keep the trunk cooler, as well as reduce the volume of air that needs A/C. I've also fashioned a sunshade out of cardboard that fits snugly inside the hatch window. One of these days I'll make a proper heatshield for it using leftover insulation from my garage door project. Wonder if there's one ready made on the market already...

 

I'd also like to know what the fuel cost of the AC is, but it terms of gallons per hour rather than mpg. Unless I'm mistaken the energy and consequent fuel consumption of the AC should depend roughly linearly with time (after initial cooldown) but should not depend much on vehicle speed or miles traveled.

As a starting point, does anyone know the wattage of the AC compressor (operating at full capacity) and a rough conversion factor for gallons of gasoline to watt hours of DC output? (Clearly the compressor won't operate at full capacity much of the time and the efficiency of ICE and mechanical to electrical (to battery to electrical) conversions will vary with operating conditions.

 

link for theorist:

http://www.epa.gov/cppd/Presentations/Mats...%20inverter.pdf

Gallon of gasoline has about 36 kwh, and Prius is thirty something % efficient at extracting that energy.

DAS