The impact of A/C on MPG

Forgive me for being nieve - but, the engine doesn't run the A/C - so there really isn't a "load" per se on the system. granted it uses power - and the ICE has to regenerate that power with mg2, or regenerative breaking...
however - with the aerodynamics of the car - running with no A/C and windows down would result in worse mpg loss due to drag than running an electric a/c compessor - would it not?

on a standard ICE - with a belt drive clutch actuated a/c on a hot day - can pull 10-15 hp from the ICE ... to turn the compressor... but on a prius - it's electric like your home ac - so there's no loss of power on the ICE - only charging load.

i don't see where it can make that much diference. someone chime in and call me crazy???

 

Lets make it simple, If you pull into In and Out, wait in line for your burgers, run the AC so you are nice and comfey, you will pretty much suck the HV battery so low it will start the ICE and begin wasting gas to recharge the HV battery and keep the AC running so the is car cool.

Don't kid yourself, using AC robs you blind of good MPG.....But at least you are cool about it!

Edit, you are in Indiana, substitute Rallys or some other drive thru hamburger joint!
It's too bad, In and Out rocks!!!

 

Anything that uses power is really a load, regardless of how it is connected (crankshaft or generator output or battery), and will reduce mpg.

The mpg loss under discussion is not AC vs. windows down. It is AC vs. no-AC with the windows still closed.

While the Prius AC is more efficient than those of standard nonhybrid cars, it is still a fairly heavy load. The Prius' low basic fuel consumption, combined with the very nonlinear nature of the American MPG scale, magnifies the apparent size of this load. If we instead used Gallons Per 100 Miles, analogous to the liters/100km scale used by the much of the world, the Prius AC impact would look much smaller.

 

I haven't checked any of this on the Prius, probably because I don't need it nearly as often as you. Up here, ordinary vent mode with no added heat is enough most of the time. Or at least it was until that silver-roofed 2010 was replaced with a black-roofed 2012.

On my Subaru, ScanGauge shows a normal warm idle drawing 0.30 gph. When the AC compressor cycles on, it jumps to 0.50 gph, falling right back to 0.30 when the compressor cycles off. I haven't watched this at highway speed, where the more efficient operating point may pull a smaller gph increment. Nor have I tried to measure compressor duty cycle, so this figure doesn't lead directly to an average fuel consumption increase / mpg decrease.

 

In my experience, if you are driving the car at a steady highway speed of 50 mph or so, the impact of using AC is minimal, in the order of 1-3 mpg. Now if you are in stop and go traffic, and going from the hamburger drive in window, to the bank drive-up teller, and sitting eating your lunch in the car with the AC on...then the sky is the limit.

 

"...the impact of using AC is minimal, in the order of 1-3 mpg."

Some people on this site would almost throw their kids overboard to get that much of a mileage increase.

I was on a long-ish trip this weekend and fooled around with the a/c to see how much of a hit it took. 3-4 mpgs at highway speed. Prius C. Using a scangauge.

That's quite a bit in most people's book(s).

 

D**n, Mr. incredible, I can see the bodies adrift in the wakes of Priis! <grin>

 

Arrrr! (squinty pirate look)

 

Using a ScanGuage II, I have testing the A/C draw during the different times of the day here in Phoenix (Xguage ACw). In the mornings at 79f, the A/C is drawing @200watts. Fan speed does not matter. In the afternoons with the outside temp @100f, the A/C draw is @800 -1000watts. When I first start the car, the A/C runs at @1800watts for a while. Typically the MPG in the afternoons in city driving is about 2 - 3mpg lower for me. I'm wondering if its the increased A/C draw, though there is much heavier rush hour traffic to deal with. Using the freeway, I didn't see much differance mornings vs. afternoon. For a 60 minute drive in my GenIII in 100f at 65mpg with the A/C on, I got @60MPH. Not to bad for an auto cranking out cold air in sunny Phoenix Arizona.

Can anyone comment on what kind of wattage is consumed by the car in ICE and EV modes. I wonder what percentage of the total amount of energy comsumed by the car is drawn from the A/C. From my vantage, not too much. In a typcial gasoline engine, when you turn on the a/c and you can feel the instant load on the rpm/momentum of the auto. This is not the case with the hybid Prius directly or indirectly.

 

A/C consumes power (as you found out) from 400 - 1800 W. How much of this impacts your MPG depends on your average speed, if you are driving on the highway with 60 mph impact will be smaller than in city driving average of 20 mph.

You can also convert electricity (Wh) to fuel consumption because electricity comes from fuel. Lets say efficiency of conversion fuel to electricity is 30% that means 1 l will give you 2,7 kWh. One liter of fuel will cool your car for 6.75 to 1.5 hours or average 3 hours at 900 W. If your average speed is 20 mph (32 kph) A/C impact is 1 l/100 km but if your average speed is 60 mph (96 kph) A/C impact will be only 0.35 l/100 km. Also when you calculate MPG the difference will be different at 40 MPG or at 60 MPG, so you can not just add a number to your MPG (Google: l/100 km to mpg).

I usually see A/C draining around 500 W, because I don't live in hot climate, I didn't even use A/C in the morning so my impact in the summer was only around 0.2 l/100 km, that represents 2.8 MPG at my 56 MPG summer best.

 

Great data, thanks everyone. I had no ideal that ac had such an effect on mpg. I have a 2000 mile round trip coming up in about a week and a half and I will be really experimenting with my ac settings on the highway. I usually run mine on low and high blow all the time here in Houston but I will cut back and see what happens. Another reason it is so much fun driving a Prius.

 

I agree, on a long trip, fuggetaboutit. Make yourself comfortable. Use ECO mode and the effect is lessened even more.

 

thanks rdgrimes and F8L. I will do that then. Maybe once I hit New Mexico and head toward Colorado it will cool down outside and then I can be comfortable running on vent mode.

 

I've definitely noticed the hit around town on very hot days. But on long trips, virtually nothing material.
Now in ECO mode vs Power mode? I've played around with it several times. No question ECO mode will make it easier to get a higher mpg number. One of the biggest reasons is pedal pressure. But on longer trips, I can't stand it unless on cruise control, and even then I shut the cruise control off to avoid the near heart attack the car gets uphill

I've also noticed less of a hit if I wait for the car to warm up a little bit.

I'm averaging 42 easily with no sweating, so I'm fine with that. I'm still amazed at my two longest trips getting 46 and over 45. Those were not trips on flat highways either.

 

^^^
ECO mode also lowers the drain on the system from the A/C.