Air conditioning

The more the batteries are used for air conditioning, the more the engine needs to run to recharge them. It's more efficient than most systems, but of course it still requires power to operate.

 

All power (electrical or other) used to run anything has to come from somewhere (i.e. matter/energy cannot be created or destroyed). Since you never plug in your Prius, the only source of power comes from the gasoline engine. So, if you power something in your car, you must ultimately draw that power from the gasoline engine.

Practically, what does this mean? If you use some energy from the battery to run the A/C, you'll eventually have to charge that battery again by running the gasoline engine.

 

Yes, the AC is powered electrically from the HV battery. But, the battery is not a source of energy, it is device to temporarily store energy from the actual energy source (the gas engine, and from regenerative braking). So, ultimately the AC is powered from the gas engine, just via a different method. Thus, it absolutely will affect your mileage.

Nothing is free (well, OK, you could consider Solar and Wind to be free). If you want AC that doesn't affect your mileage, move to the Arctic (and install an engine block heater).

If you're interested, you can search other threads on PriusChat for more complete descriptions of the AC, such as what makes it slightly more efficient than conventional automotive AC.

 

Brand new Newbie here... I have been checking out this website off and on for quite some time, and finally decided to register today. Hello all!

I understand how and why running the a/c can and does decrease my mpg, and I still have questions. It sounds like maybe if I set my a/c temperature higher (between 70 and 80), it will decrease my mpg less than if it is set below 70. Is this true? Or does running the a/c simply reduce mileage by about 2mpg, no matter what the temp setting?

And, along those same lines, does it make any difference whether the fan is on high, medium, or low?

Temps in the summer where I live run in the high 90's and low to mid 100's, causing the inside temp of a parked vehicle to reach above 140 in no time. The town I live in is very small, and trips from home to work, or the other end of town, are generally between 5-10 minutes at the max, and I realize this alone reduces my gas mileage. But if I don't set the temp to about 68 and blast the fan on high, not only does my chapstick melt, but I get real grumpy.

Please share your experiences and advice. Thanks!

Hey, these smiley faces are fun!

Tread Lightly on the earth; it's the only one we've had for millions of years...

 

Setting the A/C below 80'F or so will reduce MPGs more (but see next para). For most people it's not worth the loss of comfort to worry about 1 or 2 MPGs.

But! Note that the battery system gets its cooling air from the passenger cabin (through the vent to the right of the rear seat). Somewhere above 80'F there is an increasing chance that the battery system will get too hot to operate normally. It will start to throttle itself, and that will also reduce MPGs. Rule of thumb: if you are too hot then the battery system is probably too hot also.

 

I run my AC at about 76, and get right at 50 mpgs for 99% highway driving at 70-75 mph. It all depends on your driving habits, driving route and outside temp. Its about 90-100° here in summer and I drive over a couple of mountain passes on my 45 mile commute each way. I don't like sweating and I don't like driving slow. I'm very happy with my Prius AC over the last 66,000 miles. Your mileage may vary.

 

I've read here that the way to tell if your battery is getting warm is hearing the battery fan come on. I've never heard this even though it's been warm in the car frequently. I presume the battery fan draws air through the vent on the right rear seat?

 

from my observations sitting in my car on my lunch break with car in full ready for 30 minutes:

the choice that the prius makes whether or not to run off the engine or battery seems to rely on the climate contol system.

(COOL)-------------------- (72)-------------------------------- (HOTTEST)
more charging-------------- more battery less charging---------- more engine

the sweet spot for me, actually sitting in my car 5 days a week seems to be AUTO 72, when the car is kept at that setting, a fully charged prius battery can keep the entire car going in full ready mode 95% of a 30 minute time frame (in park and just sitting there) without burning a drop of gasoline.

the warmer the climate control system is, the more it will run its gasoline engine purely to heat the cabin area, as there is no other way to warm up the cabin area

the colder the climate control system is, the more likely the faster the traction battery will be depleted, requiring the engine to turn over and burn gasoline.

yeah there is a mpg hit, i am not hear to argue about how the power has to come from somewhere, but 5 days a week, sitting in my car, the engine only seems to come on for a minute or two during the entire 30 minute period, sitting in park, in full ready mode, with occasional charging of laptops, etc.

the reason i do this and not just sit in accessory mode and listen to the radio is that i do not really trust the battery that is used to start the prius. i have found that after having the car in accessory mode for 45-60 minutes is really pushing it, and likely to leave me stranded.

i have already had to replace the 12 v battery (free under warranty) once

 

That's it. I wouldn't rely on being able to hear the fan. In any case it won't damage itself, so all you have to do is decide whether or not you want to sweat while reducing the MPGs :_>

 

As an owner of a Prius, you are already part of the 45 mpg club. Use you AC as you wish and enjoy the ride.

 

YMMV. In PA 90 degree heat & humidity I had the AC on at around 78 or 80 and the engine came on about every 5 minutes. I also brought my mileage (first part of the first tank) down from 44 to 40 in one or two parking lot sessions.

 

Welcome fellow Arizonan

Here in AZ I find you can think of the AC load in summer in a similar way that most Prius owners think of the warm up cycle in winter. The HVAC system in auto mode throttles not only the fan, but the electric compressor to match the conditions. You will see a pretty significant hit while the AC is running full blast trying to bring the cabin temperature down to wherever you have set it. Once it reaches that point things get much better. The lower you set the temperature, the longer it will take to throttle back. In the case of very low temperature or short trips, it will never get to the point of throttling back resulting in a pretty big MPG hit.

I haven't gotten to drive the Prius much this summer (can't get the wife to give it back ) but here are some observations I've employed. I had been getting to where I could hit mid-50s on most tanks in a mix of around town and freeway driving. When temps first went up into the 110 territory I quickly found my self down in the low to mid 40s. Using these techniques I am at least back up to 48-52mpg average.

1. Use AC auto mode. In manual I tend to set it in the medium high range on max cold and leave it there. Takes a big chunk out of mpgs.

2. Set AC auto as warm as you can stand. In the dry southwest I find 82 or even 84 pretty comfortable. Don't overdo it, you do need to keep the battery temps from getting too high and it does use cabin air.

3. Park in shade with windows cracked whenever possible. Makes a huge difference in cool down time.

4. Early in cool down cycle you may actually find highway driving more efficient than city. AC running at high on battery at stops is a pretty big mpg penalty.

5. Shut off AC compressor if you have to stop. As long as internal temp isn't really hot, make sure you're set to recirculating. Air will remain cold for 10-20 seconds, fan blowing keeps it comfortable for a minute or two. Usually long enough to get through a traffic light.

6. P&G probably won't work. I have used pulse and coast (green recharge arrows in stead of no arrows) to some success, as the regen on coasting covers some of the AC usage. Same goes for gliding to stop lights, seems better to coast when AC is running. In general your best bet is probably steady state cruising in the low to mid 40s mph at about 50-55mpg. The ICE always stays on, but you are going fast enough to amortize out the overhead and get pretty good numbers.

7. Avoid heavy acceleration for the first few minutes after starting up. If the battery is hot and limiting current output, then the ac will be getting power directly from MG1 by loading the ICE. Due to this extra load and no real power boost from MG2, the ICE will be much more likely to rev up into the higher RPMs where it is much less efficient.

8. Avoid sitting with the AC running whenever possible. This is by far the biggest killer of MPGs. My wife often ends up sitting in the car with the AC to nurse our 3 month old, and she ended up her last tank at 40mpg.

9. Once cabin temp reaches target and throttles back to min fan speed, you should be able to drive pretty much like you did w/o AC and get almost as good results (I still kill the AC comp at stops or on glides though).

Not all of these things are possible all the time of course, but doing whatever you can under current circumstance should make a difference.

Rob