Air On Or Off??

Hi NYPrius1,

On a sunny day, warmer than 82 F, I will crack the windows to keep the car from heat soaking in the afternoon. I also use a front window shade, and the car is parked pointed towards the sun. The in-car temp indicator (reading the front grill temp) is 90 F or hotter just before leaving work. Then, I will run will all four windows wide open for the two miles of stop and go to the highway (speed up to 30 mph, then stop , repeat few times). Then at the light before the highway I roll up all the windows, and turn the AC to 80 F setting in automatic mode. If the AC pops the recirc open, I reclose it. As I am only in the car for 45 minutes to an hour, the humidity does not get that bad.

This routine only effects the MPG a 2 or 3 mpg or so, unless I get stuck in a stop and go situation in traffic. In which case, the battery will run down to the pinkish purple battery state of charge (SOC) bars. And it takes a long steady speed run to get the battery over 5 bars. 6 bars or better are really needed to do the hypermilling techniques, other than slow braking.

There were multiple accidents on Friday in the Chicagoland western suburbs, and people were driving crazy, thinking they would get home quicker that way. There was no rythm to the accells and decells, making it very difficult to pulse and glide. Allot of people ended up on routes they do no normally take, and were not familiar with (me included) as well. So, its was just messy. All in 93 F weather, with no clouds and mostly out on an unshaded high road my trip took 90 minutes, or about twice as long. My 60.1 mpg average I had going in the morning was down to 58.2 at the end of Friday, with 117 miles on the tank, after the 26 or so miles on that drive home. So, for the drive home on Friday the mpg was only 52.4 mpg. In normal traffic conditions, the FE average might drop down down to 59.5 MPG, which works out to a 23 mile average of 57.2 mpg for the evening trip home.


Hi NYPrius1,

Yes, I found on sunny days above 82 F, driving home on a totally unshaded interstate, up above the average terrain, if I do not use the AC, the car will get into a mode, where it will not enter stealth or warp stealth towards the end of the commute. That is, it wont use the battery to maintain speed. It runs the engine. The battery fan sometimes comes on in this condition. Gliding for prolonged periods is also different. It wants to run the engine.

Using the AC, this does not happen.

 

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<div class='quotetop'>QUOTE(NYPrius1 @ Aug 5 2007, 10:52 AM) [snapback]490910[/snapback]</div>

My thoughts On the AC.......
I see some that say try not to use it cause it hurts MPG. Possibly 1-3 MPG loss at most. If you don’t use AC and open the windows the additional drag will nearly equal any MPG gain.

Then I see some who say you need to use it to keep the battery cool. Or that will hurt MPG. True, there have been several posts about not being able to pulse and glide when battery temp is high.

Bottom line is that if you are comfortable your battery will be comfortable.

 

Hi NYPrius1,

On this last friday, I forgot to point out. That 1/3 (or 30 minutes) of the trip was spent stopped in traffic, above the tree line, in the sun, with no shade. Where a normal commute the total stop time is more like 10 minutes. The other extra 1/2 hour was spent backtracking, and crawling along in slow traffic, or suck at the extra stop lights that the only available routes had in them. The A/C only really hurts when you are stopped in traffic with it on. Otherwise the impact is marginal.

 

Both parties are correct... for the most part. Here's my opinion/observation.

Using AC always hurts my mpg. The only exception may be if you don't use AC on a very hot day and your battery temp gets too hot, then your mpg will suffer. What is too hot? I do not know. I'm sure others with a better technical understanding of the Prius can comment here. However, I have driven my Prius home from work on many afternoons when the air temp has been in the low-to-mid 90's (and after the car has been parked in the sun all day) and I have always gotten better fuel economy with the AC off (and all windows down).

Also, in my experience, it never make sense (from a fuel economy standpoint) to drive at highway speeds with the AC on versus with it off and all windows down. I have never found the additional air resistance from having some or all windows down more detrimental to FE than using AC (even at the lowest fan speed).

One other important thing to consider is if you choose not to use your AC at all, then your AC system will eventually loose it's effectiveness.

 

I have driven exactly the same distance (160 miles) with the air conditioner off and on (set at 75 degrees). Without air- 56 mpg, with air- 51 mpg.

Using the air conditioner to keep the main battery cool makes little sense because its temperature is not influenced so much by cabin temperature as by how much use the car demands of the battery. I have driven up the mountains in 70 degree weather and the battery fan has come on, and I've driven in the city with 93 degree weather and the air conditioner off, and the cooling fan has stayed off.

 

<div class='quotetop'>QUOTE(catgic @ Aug 5 2007, 01:39 PM) [snapback]490929[/snapback]</div>

You are a wise man. I assume you are also wise enough to keep quiet when she rolls down the windows in December? (although where you live that probably isn't a big deal, but here in Chicagoland, it is!)

 

Who cares about mpg when its 90 degrees outside. In Texas you need A/C in the summer. No way around it. Its would be like driving at night without your lights on to save energy.

Who cares about mpg when its 90 degrees outside. In Texas you need A/C in the summer. No way around it. Its would be like driving at night without your lights on to save energy.

 

<div class='quotetop'>QUOTE(Texas911 @ Aug 5 2007, 11:23 PM) [snapback]491078[/snapback]</div>

Hummmm,
I wonder how people who lived in texas in the late 1800's were able to survive without A/C? The people that were our great grandparents were tougher than we are today. Much tougher! Just making a point. Not trying to ruffle your feathers. I enjoy my A/C set at a comfy 76 F with the fan speed between low and medium. I am sure the traction battery appreciates it also

 

This applies to A/C systems with belt driven compressors. The A/C must be run to circulate the coolant/lubricant mixture so that the various seals do not dry out and leak coolant. The Prius A/C is entirely sealed, so this is not necessary.

Another benefit in using A/C exclusively, and recirc only except as needed during window defrosting, is that it minimizes dust and keeps everything cleaner. Heat exchangers inside the A/C itself and in the battery system are less effective when dirty.

 

<div class='quotetop'>QUOTE(D Rock @ Aug 6 2007, 09:30 AM) [snapback]491205[/snapback]</div>

Oh gee, I don't know, they also did without TV, internet, medicine, dental hygiene, running water, plumbing, cars. Go ahead and head to work all sweaty and stinky to save 2-3 mpg. I set my auto a/c to 75 during the summer and it seems to work. I have set it to 72-3 when it was night after a rain and it was super humid. I'm impressed with the auto a/c function.

 

<div class='quotetop'>QUOTE(richard schumacher @ Aug 6 2007, 11:34 AM) [snapback]491246[/snapback]</div>

Well, if that's the case, then I think I'll be running the AC even less!
Thanks for the clarification, Richard.

Let's all keep in mind that while 90 F may be considered hot to some, others may find it a very comfortable. So... as long as you are comfortable with the ambient temp, why bother turning on the AC? It's all relative.

 

<div class='quotetop'>QUOTE(Texas911 @ Aug 6 2007, 10:44 AM) [snapback]491251[/snapback]</div>

I'll second that. I moved from hot and steamy Florida to hot and steamy Texas. My Mercedes E320 AC worked really well and I was bracing myself for a let down in the Prius but MUCH to my surprise ... NO! It works REALLY well! In fact, I have to turn the temp up higher than I remember in the Benz, usually run around on 76. I am very much impressed and continue to grow even more enamored with this car every time I push that button! I'm afraid I am just a guy who willingly trades a couple of mpg for cooler temps.

 

<div class='quotetop'>QUOTE(MarcInDentonTx @ Aug 6 2007, 03:51 PM) [snapback]491550[/snapback]</div>

I used to work for a heavy duty truck manufacturer that happens to have a manufacturing plant in your town. I've spent some time there in late July and August.

Here's a memory I will never forget:

You land at DFW airport. As soon as you step on to the jetway, you feel a blast of hot, humid air. You go get your rental car and as soon as you turn the key, you get hit with the hot humid air from the AC in the rental car. The previous renter left the AC on MAX AC with the Fan running at the highest setting. You never touch the AC controls, leaving the car on MAX AC all the time even at night. When you return the car to the airport, you leave the AC on the MAX setting with the Fan at the highest speed for the next lucky recipient.

If I lived there, I would also trade a few mpg for cooler temps.

 

I thought the A/C in the Prius is running off the electric motor? So wouldn't that have a minimal impact on mileage versus a belt-driven A/C?

 

<div class='quotetop'>QUOTE(Soylent @ Aug 7 2007, 02:27 PM) [snapback]491969[/snapback]</div>

And where do you suppose the electricity comes from?

It either comes from running the gasoline powered internal combustion engine (ICE) to generate the electricity or slowing the vehicle with regenerative braking after accelerating it with the ICE.

Either way gasoline is used to create the electricity, and any electricity used by the A/C is no longer available for the propulsion system to use for acceleration. Therefore, additional gasoline will need to be used to accelerate the vehicle where the electricity otherwise could have been used.

As to the original poster's question, it really depends on the situation. If the battery gets hot enough (somewhere above 100 deg. F but below 130 deg. F) the Prius doesn't seem to get into glide mode very well anymore, and this can hurt MPG. A few hypermilers around here have mentioned that if they then turn on the A/C the battery cools down enough to allow glide mode again.

Since the best MPG comes from using the battery as little as possible, the fact that the battery may operate less efficiently at warmer temperatures shouldn't significantly affect MPG. The biggest affect on MPG seems to come from when the Prius attempts to protect the battery from "overheating".

 

Well, it's a little subtler than that. All of the energy comes from the fuel, of course, but even with generator and battery conversion losses it's more efficient to run the electric compressor in a Prius than to run the engine merely to drive the belt-driven compressor in a conventional car. Using an electric compressor the Prius is (usually) free to run the engine and charge the battery only when it can do so most efficiently. Stand somewhere for a while in Park with the A/C running, and you'll see the engine running only intermittently: engines are more efficient when running at a moderate-to-high load, and not at a near-idle.