Prius Heating/Cooling & Fuel Economy

[moltenmetal]

David: got a photo of your grilles plugged up?

Hmmm, that Scangauge is getting a pretty healthy sales pitch here. Might have to check it out, once my wallet stops smarting from the Canadian purchase price premium for the vehicle...

Thanks for the explanation about plugging the grilles- I understand now why this is such a good idea! Forgot about that engine shutting down all the time- the thing should cool to the atmosphere right quick with all that air rushing by, and won't be doing me any good in the cabin with the water pump not turning!

miscrms has me thinking about a block heater. Man, I'd look like an idiot with one of those here in Toronto, but not nearly as weird as someone in Phoenix must look! With a good timer I'd imagine it would provide some net environmental benefit rather than burning more gas to get the engine hot, and it's sure to help with the winter comfort.

My guess was that 50-60 km/hr was a safe bet for the windows vs AC thing, and it sounds like I might be able to push that upward a bit but not up to my 115 km/hr cruise speed. I agree about the simulator- it's cool, but just 'cause it's in a computer don't make it accurate!

I'm curious as to why diagonally opposed partially open windows would give you less drag than, say, both front windows open an inch? More effective cross flow, perhaps, but I'm not seeing the obvious reason for the reduced drag.

[JimboK]

Originally Posted by archae86

That's interesting. Do you have any idea what the simulator used for input information to provide the drag estimates that went into this?

I feel quite sure it's not running a computational fluid dynamics simulation of the car in real time, so rather likely it has a numeric model somebody put together. Unless the model is sound the results are just a picture of a guess.

Yes, the simulator uses mathematical modeling, but supported with extensive real world test data. From the site I linked previously, the words of the developer:

"I had to fit two different Prius vehicles with anemometers, wind direction equipment & data loggers for more than 30,000 miles (people thought I was a storm chaser) to come up with the formulas & factoring values used in this program."

I suggest giving the site a good read. That should give you a pretty good sense of his methodology and thoroughness.

[David Beale]

I suspect your car -has- a block heater. When I purchased Pearl last year, as I was about to leave, I remembered and asked if she had it installed. The salesman replied "Of course it has a block heater!" It's a 400W device that plugs into a cavity in the aluminum block.
My grill blocking is considered here as uglier than some others. It looks better if you find the 3/4" soft stuff used for air conditioning and cut it in half lengthways, then fold those in half and insert between each grill gap. I just used the ethafoam stuff, 3/4" for the top grill uncut, and I think it was 2 1/2" for the lower grill uncut, both gray from Rona. Two zip ties for each. Pic attached.
radblock2007 small.jpg

[skruse]

I try not to run the AC and generally open the driver's window 3 centimeters and the right rear passenger window 3 centimeters. I try to imagine air flow through the vehicle and into the rear vent. All windows closed makes the car too stuffy, even with various configurations on the MFD. Only on extreme cold or hot days are all windows up with the AC or heater with air recirculating. Even in vary hot and sun intense conditions, conditions are tolerable with the cross-window configuration and not running the AC. I have a block heater for winter use (early morning pre-warmup). I ordered the BH from Canada and had to argue to convince the local dealer to install it (who saw no need for the BH whatsoever).

[archae86]

Today I conducted some mpg testing on my 2002 Audi A4 1.8T. I'll describe the methods and results in some detail because I have in mind doing similar testing on the Prius.

I compared mpg in fifth gear at speeds from 30 through 80 mph, at a standardized summer air conditioning condition:


I compared most interesting windows/Aircon configurations at 70 mph, and compared two at 30 and 40.


Regarding the configuration questions so often debated:

1. For this car under these test conditions, 70 mph was well under the crossover point for moderate airconditioning to give better mpg than even full open windows.
2. My preferred two windows opposite side partial opening had so little additional drag at 70 mph that it was below the measurement resolution of my test (I observed an 0.1 mpg improvement--presumably measurement reproducibility error).
3. The air conditioning penalty, by contrast was easily detected, amounting to about 10% even at 70mph, and much higher percentage at lower speeds.

My conclusions and advice:

1. At city speeds, if you find fully open windows make you comfortable, don't think you are wasting fuel by preferring them to aircon--they are much better.
2. At highway speeds, even fully open windows may be competitive (on my car they win), and if you can find comfort with a pair of moderately open windows, you likely win over aircon at all conditions at which you are likely to cruise.

Method Details:

The test site is on U.S. route 60, about 10 miles east of the I-25 intersection at 34d25mN, 106d39W, altitude 5200 feet msl. It is typical New Mexico U.S. route--two medium width lanes with very small paved shoulders, decently paved and graded.

The site was chosen from Google Earth, where I searched for a level location with good sight lines, opportunities to turn around, and low traffic.

It was an hour drive from Albuquerque. I preconditioned the interior (and thoroughly warmed the engine) during the drive down, so first ran the tests at a comfortable automatic aircon setting. The controls were set to 69F, though the actual cabin air temperature was considerably warmer than that. The Audi cycles the aircon compressor unless it is maxed out. I believe I had a stable condition in the range many would find comfortable, though far too warm for a Texan.

I standardized each data point, averaging mpg over 60 seconds using the Audi's excellent trip computer function once in each direction. The West-bound runs all started at an obvious visual marker--the only speed limit sign in miles. At the end of the first West-bound run at a given speed, I created a GPS marker, which I used to match the road used for the East-bound run. I stabilized speed before the initial point and engaged the Audi's magnificent cruise control. Nearly all runs at a given speed (in particular all the 70 mph runs) used the remembered cruise control set point from the first run at that speed. I relied on the displayed value (mentally averaged for the few tenths of mph variation) on a Garmin GPS V running with WAAS engaged for speed. Most runs appeared to be within 0.2 mph of intended speed.

The 2-way method helped compensate out error due to my failure to find perfectly level road (this is not western Ohio, nor even Kansas) and any wind. In the event, wind appeared to be negligible during the test. The tall grass was not waving, which puts it well below 5 mph. The ambient temperature reported by the Audi varied from 90 to 92, but was almost always at 91.

I did a few primitive tests of reproducibility, which suggested that within this couple of hour period my results might be reproducible to about 1 or 2%.

On the trip down, I compared comfort from the left forward, right back window partially down to my own preferred counterpart, which leaves both driver side windows full up, and lowers the two passenger side windows a little. At equal opening I found I prefer my method, which to my taste gives a broader cabin circulation, with less annoying blast right at my left ear. I doubt the air drag differs appreciably. I carried a bit of scrap wood to calibrate the window lowering to 2 inches--perhaps 30% more than my usual actual practice.

I think the Audi (and my former BMW 325i) aircon is on the weak side. Higher output units run at full on other cars might well impose larger mpg penalties. This Audi is pretty slippery (though less so than the Prius or the Insight). Most other cars would probably have a worse degradation of mpg with speed.

I'm going to this painful detail partly because I'd like testing results on the Prius to be creditable and credited, so seek feedback on things which trouble you, or just reasonable improvements.

My own intended changes and improvements:

I won't again do the multi-speed testing with automated aircon--it is clearly the least reproducible of the configurations. Even though my method averaged 120 seconds of operation, the fraction of compressor on time probably varied appreciably. I'll use the 2 inch down for passenger side configuration instead.

I'll drink water during the test next time.

[9G-man]

All to often, turning off the a/c has as adverse impact on MPG, because as the traction battery heats up, the less electric only operation (stealth and warp stealth) potential you're going to have. (Which of course, is where the staggering fuel economy comes from).
That far outweighs the negligable, indirect decrease in economy by running the A/C. Especially in warmer climates.
Does the Prius Sim take battery temp potential into account?

[Ozzyprv]

Originally Posted by David Beale

I just used the ethafoam stuff, 3/4" for the top grill uncut, and I think it was 2 1/2" for the lower grill uncut, both gray from Rona. Two zip ties for each. Pic attached.
radblock2007 small.jpg

Hi David, where did you get the ethafoam from? I went to Home Depot and all they have is a self-adhesive black stuff.

Thank you.

[David Beale]

I got it from Rona, but it's the same as the black self-adhesive stuff from Home Depot. Except Home Depot didn't have any in stock when I looked last fall so I went to Rona. They had it in gray, and it matched my car. They also had a better selection of sizes. I was shocked.

[moltenmetal]

David: just phoned my dealer, and NO, Toyota doesn't install a block heater in the Prius' sold in Ontario as a matter of course. $150 for the part. That, plus the MSRP drop between June (when I bought it) and July, has me steaming... The dealer's service rep of course had no idea why anyone in Ontario would need a block heater in a vehicle!

[David Beale]

I guess it was just my dealer (Kingsway Toyota). NOT installing an EBH in a new car is just being a cheapskate. And you can quote me! They have a generous profit margin when they sell a Prius here! There is no good reason to cheap out and not install an EBH. It is -needed- in Canada, though I will admit I've never actually used mine. I'm retired, and can't predict exactly when I'll be driving Pearl. She sleeps in the unheated but well insulated garage. The coldest I've ever measured it in there is -10C. That was after a few weeks of -30 to -40C weather. Yuk! Last winter it never went much below -30C. Tropical!