Air conditioner usage

[bokone]

Question on A/C usage and mileage. With the "traditional" belt driven car, the common thought was below 40mph it was better to have the windows down because of the power draw on the engine and above 40mph (or so) the aerodynamic drag was far greater with the windows down. So A/C was a better choice. This is just an approximate because drag from the windows is going to be greater, on say a Corvette vs. a Hummer.
So here is my question: with the belt-less electric A/C compressor on the gen 3 Prius, wouldn't it be ALWAYS more efficient to drive with the A/C on??

[Paradox]

It wouldn't always be more efficient because the a/c runs off the battery at low speeds when the engine is not running and that energy has to be replaced by running the engine to recharge the battery pack.

[V8Cobrakid]

but sustaining a constant inside temp Should be more efficient than turning an A/C unit on and off all the time. Humidity control comes to mind too (all the same thing really)...

[timo27]

OK, what I'm about to say admittedly is *completely* anecdotal--but my experience, in 2+ months of ownership, is that, in all but the most completely stagnant city driving with lots of (and long) stops and very low speeds (20-25 mph max), I seem to do best with the windows rolled up, the car in ECO mode, AC on and the temp set on Auto at as high as I can find it comfortable (78 feels delightful on a humid, 95-degree day). I think the car is just so aerodynamic that rolling down all the windows really messes with it. Wind noise also suggests this. Plus, it's just a lot more comfy

Should I descend (further?) into utter geekdom, I'll do some controlled runs and post the results.

As a (slight) aside, I think the whole idea that the car does it's best in city driving is bunk. My best mileage seems to occur during rolling, slow-but-not-stop rush-hour oozing gridlock.

Cheers,
Tim

[a64pilot]

I don't how important cooling air is for the traction battery, but I have to think it could cool itself better with 78 degree air than 95 to use your example. I run the air in ECO and recirc
Oh and I would think any driving that causes you to run about 20 or 25 mph without stopping would return the best mileage. Sounds like city driving to me, but I guess it depends on what the definition of city driving is.

[qbee42]

It's an issue of speed. The efficiency of the Prius A/C and the good aerodynamics of the Prius will move the break-even point, but it still comes down to speed. Look at the limiting case: standing still. Obviously when standing still it is more efficient to roll down the windows, as there is no aerodynamic drag. As speed increases, so does aerodynamic drag, but not in a linear fashion. At low speeds the extra drag from open windows makes no practical difference. At high speeds it makes a world of difference. Somewhere in between it breaks even.

Tom

[bokone]

Originally Posted by qbee42

It's an issue of speed. The efficiency of the Prius A/C and the good aerodynamics of the Prius will move the break-even point, but it still comes down to speed. Look at the limiting case: standing still. Obviously when standing still it is more efficient to roll down the windows, as there is no aerodynamic drag. As speed increases, so does aerodynamic drag, but not in a linear fashion. At low speeds the extra drag from open windows makes no practical difference. At high speeds it makes a world of difference. Somewhere in between it breaks even.

Tom

The "break even thing" is kind of my point/question. I know in ANY car that eventually the aero drag is greater than the A/C load, but with an electric compressor wouldn't that speed be really really low?

[qbee42]

Originally Posted by bokone

The "break even thing" is kind of my point/question. I know in ANY car that eventually the aero drag is greater than the A/C load, but with an electric compressor wouldn't that speed be really really low?

No. The fact that the compressor is electrically driven is irrelevant. The fact that the Prius A/C is fairly efficient lowers the break even speed, but not by a lot. Even if the A/C compressor were 100% efficient, it takes a lot of energy to move heat. Don't lose sight of the fact that work is being done to cool the car.

Also recall that aerodynamic drag is not linear in respect to speed. Aerodynamic drag is exponentially related to speed, meaning a small increase in speed makes a big difference, but only after you get up to some nominal speed. Below 30 mph aerodynamics have little effect. At highway speeds they make a big difference. Your answer is somewhere in between.

Tom

[timo27]

Originally Posted by a64pilot

I don't how important cooling air is for the traction battery, but I have to think it could cool itself better with 78 degree air than 95 to use your example. I run the air in ECO and recirc
Oh and I would think any driving that causes you to run about 20 or 25 mph without stopping would return the best mileage. Sounds like city driving to me, but I guess it depends on what the definition of city driving is.

I agree about the speed issue. Unfortunately, here in Baltimore, the traffic patterns, for whatever reason, virtually ensure that in the city you spend at least 1/2 the time at a complete and utter dead stop. Rush hour, midnight, it doesn't seem to make that much of a difference. I know the situation is better in some other cities (and worse in some). Now, on our local Interstates on the other hand, that 20-25 is somewhat easier to achieve

[anne1965]

Originally Posted by qbee42

No. The fact that the compressor is electrically driven is irrelevant.

Yes, it is relevant. The fact that the compressor is electrically driven means that it can be driven at exactly the right load.

A belt driven compressor is much more crude. When not much cooling is necessary, some hot air is mixed in with the cold air from the A/C to keep the interior from cooling too much. The Prius A/C will throttle back and reduce consumption.

Lower average consumption from A/C use means the break even point will also be lower. But of course the break even speed will never be 0.