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How much Air Conditioner affects MPG

Old 10-19-2004, 11:24 AM
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Found this on a website today (houston chronicle archives) indicating A/C usage will decrease MPG by 34 percent on the Honda Hybrids - they did not indicate Civic or Insight but the MPG numbers they quote are closer to the Civic:

"Air-conditioning issue : "I've heard that if you run the air-conditioning in a hybrid, it stays in the gasoline mode. In Houston, that would be 10 months of the year; may as well get a straight gasoline engine."

That was true of the previous generation of Prius cars — the 2000-2003 models — but "we fixed that little glitch," said Cindy Knight, environmental communications administrator for Toyota Motor North America in Torrance, Calif.

The air conditioning in the 2004 and 2005 Prius runs on the electric motor, which is powered by a battery.

However, if it's 90 degrees or hotter outside the car and the air conditioning is set at 65 degrees inside the car, the engine will go into gasoline mode to keep the cool air blowing, Knight said. And that cuts down on fuel efficiency.

The air conditioning can also force the car to run on gasoline if the battery is low and the AC setting is high, Knight said.

Honda's hybrids don't switch between gasoline and electricity like Toyota. They use both forms of power at all times and never turn off the gas engine, said Yuzuru Matsuno, a spokesman in Detroit for Honda North America.

But running the air conditioning does lower the Honda hybrid's fuel efficiency, he said. For example:

If a car with a regular engine got 20 miles per gallon and the hybrid engine got 50 miles per gallon, the regular car would need 5 gallons to go 100 miles without running the air conditioning and the hybrid would need 2 gallons.

If you turned on the AC, each vehicle would require one more gallon of gas to go that same 100 miles — the regular engine would use 6 gallons and the hybrid would use 3.

That additional gallon changes the regular car's mileage rating to about 17 mpg, a 16 percent decrease in fuel efficiency. The hybrid drops to about 33 miles per gallon, a 34 percent decrease.

That's still better than the regular engine but not the ideal netted in the Environmental Protection Agency's testing."
I wonder if that is true, and where or how they came up with the "extra 1 gallon used in 100 miles" calculation?
Any Ideas, gang?
Old 10-19-2004, 12:54 PM
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To me, this sounds somewhat ludicrous. What they're REALLY saying is that regardless of the car, it takes 1 gallon of gas to run the A/C for 100 miles. Period. That's a different percentage of mpgs for each car, and obviously the higher mpgs the car gets, the bigger decline it will be. If they were comparing a Honda Insight, 1 gallon would be closer to a 50% drop!! We all know that this isn't true. So each car does NOT use 1 gallon of gas to run the A/C for 100 miles.

Furthermore, even if it did, a more fuel efficient car would still be more efficient.

Statistics. I hate them, because most people misuse them.
Old 10-19-2004, 08:56 PM
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Hi Buzz70:

___Unfortunately, a 30 - 40 mpg drop when using A/C is a reality for Insight 5-speed hypermilers. For Insight owning non-hypermilers, they don’t receive EPA estimates anyway so a 15 - 25 mpg drop is about right … When the Insight first arrived on our shores, the NREL had one for testing and found an ~ 35% overall drop in fuel economy when using A/C under a variety of driving conditions.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___[email protected]
Old 11-25-2004, 03:59 AM
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I really disagree with all this. I achieve better than epa ratings with temps in the 90s and 100s. It is important to keep the battery compartment shaded. I got my glass tinted with huper optik. I only get 2 or 3 mpg better performance with more moderate temps. For all the loss of the AC compressor, the performance of the hybrid is also helped in some ways by the higher temps. Park in the shade for a smaller mileage hit due to overheating of the battery and the need to cool it with the auxilliary fan, which pulls AC air across the battery and dumps it outside.
Old 11-25-2004, 09:04 AM
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AC is an interesting killer for the Prius. If it's an extremely hot day, expect to get 30MPG until the car cools off. On a moderate day, it'll make a few MPG difference.

The biggest killer for the Prius, in my opinion, is traffic. AC and power demand trade off depending on the circumstances.
Old 11-25-2004, 07:12 PM
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Originally posted by Jason@Nov 25th 2004 @ 12:04 PM
The biggest killer for the Prius, in my opinion, is traffic. AC and power demand trade off depending on the circumstances.
Traffic is a killer, but let me tell you... cold weather is the worst-imaginable offender.

My current tank, I'm at 700 km, 4.4 showing on the MFD, and about due for a fill-up. Not bad. In fact, my second-best tank ever. However, most of the tank was done last week, with temperatures between 15 and 10 C. Prior to last Saturday, I was at 3.9, 500 or so KM, and hell-bent for a breakaway best tank ever. Even had the champagne ready to pop to celebrate.

And then the temperature dropped. And then the rain came. So after a couple of days of minimal driving -- but very similar mix of suburban traffic and low-speed (60/70 km/h) highways -- with the lower temperatures (3 to -3 C), I've tacked on .5 L/100 km to my tank total in around 200 km. Just takes too much to keep the system (and the driver) at a comfortable temperature.

And no, I'm not one of those types who are willing to just turn off the climate control and shiver for my drive to get the best-possible mileage out of the car.
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