Does the A/C reduce mileage?

[Jakabite]

Someone told me that the A/C compressor is ran off an electric motor (not directly off the engine)?? Is this true?

Then running the A/C will only draw from the batteries...?

Does this really decrease your gas mileage??

I would think it would a little, because it draws off the battery and that could provide more power to the electric motor... But how much of a decrease are we really talking about here?

Any thoughts?

Thanks.

[efusco]

<div class='quotetop'>QUOTE(Mountain_Prius @ Oct 4 2007, 07:24 PM) [snapback]521439[/snapback]</div>

Someone told me that the A/C compressor is ran off an electric motor (not directly off the engine)?? Is this true?[/b]

Yes

Then running the A/C will only draw from the batteries...?
[/b]

Yes

Does this really decrease your gas mileage?? [/b]

Yes

I would think it would a little, because it draws off the battery and that could provide more power to the electric motor... But how much of a decrease are we really talking about here?[/b]

It varies depending upon how cold or hot you have it set, how hot or cold it is outside. Usually you're talking about around 2-5mpg loss. If you go to Wayne Brown's Prius Simulator Site you can download his simulator and enter various data to see how the AC temp setting will impact FE for various outside temps at various speeds.

[dallas2727]

<div class='quotetop'>QUOTE(efusco @ Oct 4 2007, 09:03 PM) [snapback]521465[/snapback]</div>

Yes

Yes
Yes

It varies depending upon how cold or hot you have it set, how hot or cold it is outside. Usually you're talking about around 2-5mpg loss. If you go to Wayne Brown's Prius Simulator Site you can download his simulator and enter various data to see how the AC temp setting will impact FE for various outside temps at various speeds.
[/b]

I've driven entire tanks with AC on (set to auto) during temps in the 90s, and I've done tanks with it off. I didn't see a lick off difference in MPG, and I pay close attention.

[Tideland Prius]

It's yes to all of your questions.

As for how much of a decrease, it depends on the ambient temperature and how cold you want it inside and traffic. Sit in traffic with the A/C at 65 and you'll see the battery down to two bars with 5 mins. Bump it up to 72-75 and it'll last longer.


dallas, it won't make a difference if you're on the highway (since you're charging the battery anyway) so you might as well use A/C.

[Tech_Guy]

In really basic terms...

The ulitmate source of all power in the Prius (and other automobiles) is the gasoline in the tank. Whether the air conditioner is run directly from the engine (as in a regular automobile from a belt connected to the crank shaft) or by electric motor (as in the Prius) attached to the electrical system, the ultimate source of power is still the gasoline engine.

The air conditioner requires a only a couple of horsepower which is a relatively small fraction of power required to make the car go. Consequently in most cases you will not see very much, if any, reduction in gas consumption. (I suspect that if you ran two identical Prius automobiles side by side (one with air conditioning on, and the other with air conditioning off and windows closed) over a long distance, you might see some evidence in increased fuel consumption by the vehicle using air conditioning .

Keith

[PriusOwner004]

No difference on the fuel economy if you're on the highway. Little difference if you're in the city, as long as you set it to 78?F/Low. That's sufficient to keep you cool even in the desert heat. If you set it to Max and full blast and drive in the city, I assume it would have a more noticeable impact on the MPG.
No A/C in a HOT day doesn't mean just torture for you, it also tortures the car. The battery needs to be cool to perform at its best.
So if you need the A/C turn it on at a comfortable setting (around 78?F and low fan) and enjoy your drive!

[hyo silver]

Hadn't we come to the conclusion in a previous thread that windows up/cool air on used less gas at highway speeds?

[Bob64]

All I have to say is that the energy has to come from somewhere. Any amps going into the AC is less amps going into the MG's.

[efusco]

<div class='quotetop'>QUOTE(Bob64 @ Oct 4 2007, 10:25 PM) [snapback]521544[/snapback]</div>

All I have to say is that the energy has to come from somewhere. Any amps going into the AC is less amps going into the MG's. [/b]

That's true and for the most part that's all you need to know. But, like so many things, it gets more complicated than that.

Let's assume outside air temp is 100F and the AC is set to 72F when used...

Highway
Option 1) AC windows up-more aerodynamic, mild FE drop due to AC parasitic drain, battery stays cool and works in near optimal temperture/performance range.
Option 2) No AC windows up-aerodynamic, no drop due to AC, but battery temp rises, battery cooling fan runs at max and decreases performance which induces drop in FE. But it's too hot for human comfort.
Option 3) No AC windows down-Poor aerodynamics, No drop from AC, battery warm but still in reasonable operating range due to inflow of outside air...may induce slight drop in performance.

Conclusion--most feel that Option 1 makes most sense at highway speed. While most of us observe a small drop in FE compared to the other 2 options it's more than worth it.

City/Rural (low speed)
Option 1) AC windows up-more aerodynamic but not a big factor due to lower speeds, moderate FE drop due to AC parasitic drain which continues even when in stealth or at a stop, battery stays cool and works in near optimal temperture/performance range.
Option 2) No AC windows up-aerodynamic but not a big factor, no drop due to AC, but battery temp rises, battery cooling fan runs at max and decreases performance which induces drop in FE. But it's too hot for human comfort.
Option 3) No AC windows down-Aerodynamics not significantly impacted due to low speed, No drop from AC, battery warm but still in reasonable operating range due to inflow of outside air...may induce slight drop in performance.

Conclusion--This is more of a personal preference/comfort issue. At 100F outside temp I choose to use AC and eat the MPG hit. At temps closer to 80F and sometimes up to 90F I'll suffer the heat a little and drive with the windows down to minimize the hit...in this situation it clearly makes a significant impact on FE to use AC, but personal comfort has value too.

[seasalsa]

Great answer Doc! You really put it in perspective.