MPG with & without AC... tested!

Using my average trip speed of 38 mph, and the trip fuel economy of 59.3 mpg and 67.9 mpg for ac and no ac respectively, I get a fuel burn of 0.64 gph with ac and 0.55 gph no ac.

That's a 0.09 gph difference for ac. Your calculation was right on for my experience!

 

I planned on doing that exact scenario on my road trip to TEXAS in about 6 weeks. When I stop to take a nap I was going to record the values. Won't be at low speeds, but the drive will be far enough to give good data.

The problem is it will only give an accurate number for the fuel it takes to run the AC at idle (which is a number I DO want to know for when the car idles and runs the AC). However, it will not convert into a number that can be applied to driving. The reason is that the engine is ALREADY running when you're driving, so the losses to run the engine (drive train, pumping, coolant heating, and friction losses) are payed for by the fuel already being burned to move the car. The fuel to run the AC is 'extra' energy production, so it's MUCH more efficient.

Running the AC at idle, fuel must be burned to overcome all of the losses mentioned above JUST to power the AC. Of course, it should be much more efficient than a belt driven system because the engine can play 'catch-up' with the battery and then turn off.

While I plan on doing the experiment, the numbers are not apples to apples.

 

I agree with almost everything you've said. No one is claiming this is THE test and THE fuel burn number, LOL... this is a starting point! While I agree that the energy needed to run the AC will vary considerably depending on MANY factors, I do believe we can test the fuel used to power the AC for a GIVEN situation reasonably well. The general repeatability of my daily commute mpg and that of many others is evidence of the general repeatability of tests. How do you know there are too many variables to get reliabe results? How many controlled runs have you done to test this theory? My guess is none... though I could be wrong!!!

I put a lot more confidence in one reasonably well tested run, numerous daily commutes at the same time over the same route, and the comments from others that this reflects their observations, than anecdotal testimony.

I actually hope it turns out that the fuel used to run the AC in my test run was only 2-3 mpg difference (I'll feel less wasteful using the AC), but I doubt that will be the case. I'll explaine why in my next post.

 

Here is my HIGHLY unscientific reason why I strongly doubt we only burn 2-3 mpg more under the speeds and conditions I tested.

I have kept track of my fuel economy for almost every tank of gas I have put in my cars since 1996 (I could count on one hand the number of tanks on which I didn't check the MPG). That's over 400,000 miles of records and observations. My data includes two different makes of gasoline cars, 1 diesel, and two different makes of hybrids.

In the gasoline cars I averaged around 23 mpg in both cars (an '89 Ford Taurus SHO 5-speed, and a 1998 Buick Regal GS). When I would run the AC I would average around 20 mpg. About a 3 mpg difference. Again, this varied with the type of driving... but with the type of driving I did overall, these are the results. In the buick on the realtime mpg indicator, I would see about a 4 mpg drop when the compressor was on... but the compressor did not run all the time, hence the 3 mpg overall drop.

3 miles is 0.15 gallons of fuel at 20 mpg... for the moment lets assume the Prius AC has the same efficiency as the two cars here. That same energy that cost the two cars above 3 mpg would cost a prius that's getting 68 mpg around 9 mpg!

Obviously the prius has a more efficient AC system (maybe 30% more efficient overall from what I've read). Assuming a full 30% greater efficiency, that would mean a loss of 6.1mpg. Far more than 2-3 mpg. There is no way the prius AC is 75% more efficient than the typical car AC... just no way.

My driving style in the other cars involved time on the interstate and rural highways, plus city and stop and go driving... not the same as the scenario I tested with the prius. From a steady state cruising point of view, I picked what I believe would be a worse case scenario for the prius. Higher speeds like the way the other cars were driven would yield a somewhat smaller number of MPG lost for the prius.

As I mentioned earlier, my 2003 HCH would lose as much as 13 mpg with the AC in the city!!! Though 8 mpg loss was typical with higher speed driving, and as low as 6 mpg at 78 mph interstate cruising.

In conclusion, I think what my test lacked was the resolution and proper type of driving to reveal the maybe 30% greater efficiency of the prius AC vs. normal cars. However, I think the loss I recorded on my test drive and daily commute is in the right ball park... for the conditions tested.