Windows Down vs A/C

It is 98F and humid out today. There is NO QUESTION.

 

I just got home from an 86mile trip (43 each way). On the way there, outside temp was a few degrees either way of 90F. Most of the way there I had the AC on set at 82F, but I shut it off and opened the windows at both ends of the route, when I was on <40mph roads. Brisk 90 degree cross wind judging from several flags. MPG was 71.2 on the HSI when I arrived at my destination. I did not zero it.

On the way back, temperatures gradually dropped from about 88F at the start, down to 77F when I arrived home. But I ran the AC most of the way, set at 82F, until most of the way home when the outside temp showed 82F, when I shut it off. Winds dropped to nil as far as I could tell. When I got home, the HSI showed 74.3 MPG for the entire trip.

So I'd say (a) the Prius loves warm weather and (b) not setting the interior temp at 72 F probably helps.

P.S. I probably could have managed without AC at all but I had my dog with me, and she has a permanent thick fur coat and can't cool herself by sweating.

 

It would be nice if there was a way to directly dial in the "cooling effort" and then run the fan at max.

 

Easy, your hot and sweaty with the windows down, put them up and use the A/C. Comfort is first!. Saving a few MPG's is great, But not at the expense of your body and well being.
A lot of good INFO in the previous replies . Like crack your windows just a little when parking the sun, drive first with windows down until all the hot air is out and started to cool some of the solid surface materials in the car. Then slowly start to use your A/C this will help take some of the load off the energy used to cool the car. Its that first couple of miles that use the most gas, after that the MPG loss is less.

A little window tint goes a long ways to help the A/C

tom of Lech Auto Air conditioning.

 

And also cooling requirements. With a variable speed A/C like that on the Prius, the load varies depending on need. On a cool day the A/C load will be zero. On a blisteringly hot day it will maximum.

Older A/C systems were either on or off. If you used them at all, you paid for the full load.

Tom

 

Not true. They cycle on and off to regulate temperature. It doesn't regulate temperature as well and there's wear and tear on the starting relay (if not electronic) and compressor bearings, but it will use less power if less cooling is demanded.

 

2009Prius just posted new data he collected recently which supports the idea that the mpg hit due to heavy AC use can be quite large.

How to Reduce MPG loss Because of A/C Use | Page 3 | PriusChat

 

Of course it affects MPGs, not as badly as using heat in winter, but still very obvious! Spring and Fall are the seasons to enjoy max mileage, when you don't have to use the HVAC at all!

 

So for those of you that are using windows down even on hot days and are monitoring your battery temps...what temps do you need to keep the batteries below?

 

Running the A/C has a bigger impact on mpg. That said, I live in the south and I would rather be comfy than sitting there sweating, trying to eek out a few more mpgs for the sake of fuel economy. Would you guys sit in a house that is over 80 degrees in the summer and not turn on the A/C? My elderly parents might because they're always cold, but the average person would not.

 

Funny you should mention that: I'm sitting in my condo right now and the inside temp is 80F, AC is off, very comfy, hadn't even thought about turning on the AC. When the temps get into the upper 80's or the humidity is high, I turn on the ceiling fans. Usually no AC until it's into the 90's, whether at home or in the car.

If you insist on 72F then yes you'll burn a lot of energy at home or in the car, and in the car it will be more than "a few" mpg. It's really a question of humidity as much as heat. The trick is to turn on the AC with the temp set a few degrees below ambient. That way you get the increased comfort of dehumidification (which is why it's called "air conditioning" rather than "refrigeration") without using the extra energy needed to cool the air a lot.

Also be sure the system is set to recirculate air. Takes a lot less energy to handle partly dehumidified and cooled air than to process hot/humid outside air. The Gen III Prius switches to recirc when AC is turned on but I think it can be overridden if you hit the recirculate button by mistake.

 

Isn't the rule that if the driver is comfortable the traction battery is comfortable? I thought I read that somewhere. Anyway, if the humidity is high, the A/C is on for me. Otherwise, the windows are open. I also like to keep the fan on the lowest setting to keep the evaporator dry no matter what.

Regarding Mythbusters, they did a show one time about tying a cable onto a rear axle so it would pull out when driving away. If you've seen American Graffiti or Leave It To Beaver, you'll know what I'm talking about. Well, they tried it and the axle wouldn't pull off the car, therefore it was fiction. The only problem was that the car they used was a newer car with a four link rear suspension. The cars back then had the axle bolted to leaf springs. I e-mailed them, but never got a reply.

 

I don't have A/C in my house. It's the exception here, rather than the rule. When it's in the 80's I open all the windows and I'm quite comfortable.

I'll admit that when it gets up into the upper 90's, I've been known jerry rig a swamp cooler, provided the humidity allows. But that's rare here and doesn't last more than a few days.

 

There are forums set up where all they do is discuss each episode. Yes, obviously it is set up for entertainment. But they do a pretty good job of testing stuff in the real world. And they are making science cool for a ton of kids and people, which is extremely awesome.

 

+1

 

When I run the A/C in my Prius I set the temp to somewhere around 74 to 77, depending. If it's in the 90's I don't need the car to be ice box cold, I just need enough of a difference to ensure my comfort. I don't think I've ever run the A/C down to 72 degrees beyond a few minutes, because it feels very cold at that level. It's variable of course. Getting the humidity out of the air helps a lot. Since I'm in a humid environment that becomes an important consideration for me.

 

If it's not oppresively hot and it's lower speed driving, we'll typically just roll the windows down, a bit or fully, depending. Same story with short trips in hot weather: by the time the car interior cools down you're at your destination anyway.

With longer runs, higher speeds, muggy/hot weather, we use the AC, keeping the temp just low enough to be comfortable.

AC is an effective defogger in rain too, just if the ventilation gets overwhelmed.

One caveat: over the years with a succesion of Honda vehicles, the Owners Manuals always cautioned to run the AC periodically (at least monthly), to circulate oil within the system, keeping seals from drying out and leaking.

 

If it's like most other cars, the compressor usually starts up if you use the front window defroster. It probably won't start up if the outside temp is below 40 degrees F.

 

I played around with A/C consumption gauges today (while in PowerMode) and was pretty astonished to see that my A/C was drawing 1800watts for the first few minutes. It then dropped to 1600watts for another couple minutes and eventually settled at about 800-1000 watts for most of the drive. By the time I got home, 38miles later, the consumption dropped to 650 watts. 98% of this trip was freeway at 70mph with outside temps of 97F-99F. It must be stated that the car was per-cooled from a prior trip to the mall in that town so I wasn't even starting with a hot interior. The Torque app claimed my interior was 87F upon starting the trip.

 

That sounds quite a bit higher than what it would be if you were in ECO. Thanks for biting the bullet and checking this out for us.