The thought that using the AC is more expensive in an economy (or hybrid) car versus a larger car (such as the Expeditions used on Mythbusters) is misleading via the way we compute fuel consumption -- miles per gallon, rather than gallons per mile. The AC will consume power (and thus gasoline) based on difference in temperature, number of passengers and sun intensity, and between different cars is effected by interior volume, window size and position, color of car, and window tinting, but it's effect is best measured in gallons per hour. Speed and distance have a minor effect.

Lets say that the hybrid AC consumes .25 gallons/hour, that it gets 40 mpg at 60 mph. Thats 1.5 gallons/hour. Adding the AC gives 1.75 gallons/hour and converting back we get 34.3 mpg. That's a loss of 5.7 mpg, or 16%.

Lets say the big Expedition, which has twice the interior volume, has an AC that consumes .5 gallons/hour. It gets 12 mpg at 60 mph. Thats 5 gallons/hour. Adding the AC gives 5.5 gallons/hour and converting back we get 10.9 mpg. That's a loss of 1.1 mpg or 9% which sure sounds much better.

But if we look at gallons per hour (or gallons per mile) we find that, of course, the AC is more costly in the Expedition, in fact twice as costly.

You can also see that the faster you go, the less the AC costs per mile. If you open windows, the drag will cost more at higher speeds. So there is probably a certain speed that represents the crossover point. I'd guess that in town the windows should be open and on an open highway they should be closed.

 

This is mostly accurate given the recent data I've been seeing from my Graham miniscanner. In the case of the older 2001-03 Prius, at speeds below 42 mph, the engine has to run to operate the compressor. So typical idle, engine air flow runs about 2 g/sec but with the compressor, 2.45 g/sec. The problem is we don't see as much 0 g/sec with the engine off. But that changes once the car is going over 42 mph.

Over 42 mph, the engine runs regardless of load. This means the 0.45 extra g/sec is pretty much just part of the ordinary operation. However, it also means that as the engine load increases with higher speeds, the AC can push the engine into inefficient modes. What this means is cruising at 65 mph w/o AC is closer to cruising at 60 mph with AC.

It is important to use air recirculate to prevent cooling hotter external air than internal. Also, set the internal temperature control to as warm as is comfortable . . . avoid seeing condensation on the exterior glass.

As for air drag effects from open windows, if they are partially open so as to get a breeze versus a full blast, the effects appear to be minimal although I don't have numbers, yet.

GOOD LUCK!
Bob Wilson

 

Might be interesting for someone to repeat this little exercise with one of the side window fairings like those from Lund (Link = http://www.autoventshade.com/home_avs.aspx. They claim an aerodynamic improvement, and come in chrome or tinted poly. I have thought about it, but it just doesn't get hot enough, long enough in the Puget Sound area.

 

Even with an electric compressor, and the ICE off, the power has to come from somewhere, so there will be additional consumption later when the ICE is on again and recharging the traction battery. In fact, there will be a loss in efficiency (powering via the battery) so the average gallons/hour of AC use will increase where there is low speed EV operation mixed in. As you have observed with a 2001-2003 Prius, low speed AC operation is particulary onerous as you have about 5x the airflow (=gas consumption) you would need to run the AC alone! Even more reason to open the windows around town.

 

Recently we've had some +100F days and it was unsafe to drive around without AC. So instead of taking the access road at 38 mph, I jumped on the divided, limited access expressway and tooled along at 50 mph. I only saw low 50s MPG but I was fresh, alert and it worked. Had I stayed on the access road at 38 mph, I'd have only gotten middle 50s with AC and heat stroke with without AC.

Bob Wilson

 

Ahh.... I love the extreme heat. I was in Vegas two years ago when it was 119! I loved it! At midnight I went out to walk around and get a drink.... it was 112! Man I loved it.

It was 100+ here last week and I rolled the windows down and cruised on home. I wish it were like that more often.

 

Pardon my ignorance Bob; but I've got two questions:

1) What are the units for g/sec? Grams/second?
2) How do you correlate air flow to fuel consumption? I think I know, to maintain a stochiometric ratio a higher air flow means also a higher fuel flow. Am I correct?

 

Hi,
Sorry, I'm often so buried in the technology I often forget to 'speak human.'

Air flow is in grams per second based upon a sensor located just in front of the throttle plate. It is a critical part of injector timing to maintain a stochimoetric, 14.7 to 1, air to fuel ratio.

Take the air flow in grams per second and divide by 14.7 and you'll get the fuel flow rate in grams/second. However, this typically becomes a pretty small number:

2.00 g/sec -> 0.136 grams of fuel / second
2.45 g/sec -> 0.167 grams of fuel / second

When I posted the air flow I realized that it is technically more correct to post the fuel rate but it is often harder to visualize the difference between 0.136 and 0.167. So I used the source data, the air flow rate, which by inspection shows a 20% increase: 0.20 -> 10% of 2.00, thus 0.45 ~> 20% increase over 2.00. But it was somewhat sloppy engineering not to show the exact units and explain the relationship between air and gas flow.

Bob Wilson

 

We can even enjoy some old fashioned engineering fun and re-calculate these units into something else.

Since gasoline's specific weight is about 0.7 grams/milliliter and there are 3785 milliliters per gallon, this translates into 2649 grams per gallon of gasoline.
Therefore 0.136 grams per second translates into 0.00005134 gallons per second, and 0.167 translates into 0.00006304 gallons per second.
Or put another way 19,477 seconds to consume one gallon of gas in the former case, and 15,862 seconds in the latter.

You may want to check my calculations, they were done in a hurry.

 

Thanks for doing the hard work. In more useful units that would be .184 gallons/hour and .227 gallons/hour. The AC uses 0.042 gallons/hour, so if you are paying $3/gallon, that means less than 13 cents/hour for air conditioning. I would guess we could all afford the luxury!