Prius Heating/Cooling & Fuel Economy

[moltenmetal]

The last heatiing/cooling discussions I found in searches were in 2004 and didn't answer my questions completely, hence the new thread.

I'd like to know if anything has changed between then and the 2008 model year, particularly in terms of the heating system.

The IC engine has a conventional heater core, correct? That's where I'll get most of my watts in winter- or so I hope. That's basically free heating, as it's waste heat from the IC engine that would normally go to atmosphere anyway.

Are there supplementary electrical resistance air heaters which are heated via the traction battery? Do they just operate for a time after start-up, or are they continually sapping fuel efficiency from me while I drive during the (long) heating season here in Canada? I seem to notice some heat almost immediately on start-up, even when the car is started first thing in the morning. It's tough to determine this by experiment, and the Owner's Manual is useless in explaining this system.

I can turn the AC on and off manually while the heating/cooling system is in AUTO. Does the AC function at all as a heat pump in the heating direction? Or is it purely a cooling device? I've never seen the AUTO cabin temperature control de-select the AC automatically, even when the cabin temperature setpoint is well above the outdoor air temperature. Unfortunately, actual cabin air temperature is not displayed as far as I can see- only interior setpoint and exterior temperature.

I notice that the AC tends to over-cool when the outdoor temperature nears the setpoint. Does the AC compressor cycle on and off, or is it variable speed (modulating) based on the cooling duty required?

Your help in understanding a car which I otherwise LOVE completely would be greatly appreciated!

[David Beale]

Yes, thanks for reminding me. There are two 400W heating elements in the air vents under the dash. I noticed them for the first time on Saturday. As I'm sure you know, in a Canadian winter 800W isn't going to heat much! But it was just "cool" on Saturday morning and I had left the A/C on so I got some heat immediately when I started Pearl.

In modern cars with "climate control", such as the Prius, "A/C" means both heating and cooling.

Lets start with the "cooling". The compressor is variable speed and runs off the traction battery (HV). If it runs long enough without the engine running it will cause the engine to start (stopped waiting for someone, for example - don't use "N", use "P" - the battery won't charge when you are in "N"). I haven't noticed it start the engine while waiting for a light.
It is purely a cooling device.
If you are very observant you will notice a slight drop in mileage in the summer from energy lost to cooling the interior, but you want the interior comfortable, as the traction battery also uses that air to cool itself.

On to heating. Prius uses a "normal" in-car engine coolant heater. In winter it will cause the engine to run much more than it would in summer. It's a good idea to block the grills to help keep the engine warm. I recommend getting a Scangauge II so you can accurately monitor the coolant temp. if you do block the grills. If you do not block the grills the Prius becomes a "cold winter car". It just will not warm up. But even with grills blocked, you also will not be able to warm up the interior by letting the car idle. You HAVE to drive a few blocks. Pearl blows nice hot air after about 3 blocks at city speeds, even when it's -30C ambient. The engine is so efficient you will see the coolant temp drop at lights if the ambient is below -20C. So I suppose it could be argued it's not "waste heat". I'm not sure if the two 400W heaters stay on continuously, but they do go off if the cabin temp reaches or gets close to the "set point" (the temp you set the auto climate control to).

[moltenmetal]

Thanks David: your post has been very helpful. Thanks particularly for the mention that the AC symbol doesn't really mean AC...My last car was a '99 Civic hatchback base model with power NOTHING and no AC, so indeed I had no idea what AC means on a "modern" car! I just want to keep the actual refrig compressor from running when I don't really need it for comfort, as it definitely saps fuel economy in a measurable way. The cabin temperature controls appear not to be a straightforward feedback control on cabin temperature even in purely cooling mode, but since there's no cabin air temperature display I can't really tell.

Any idea at what speed it becomes more efficient to run the AC versus opening windows? I know it's true at highway speeds, but how about at 50 km/hr?

Toronto is a darn sight warmer than Edmonton, so I don't know if blocking the grilles will become necessary. What you're implying is that in winter there might be enough airflow across the engine itself to cool the engine too much via just the cabin heater core plus losses directly from the engine surfaces to the atmosphere (ie. air cooling), since the thermostat valve will not be sending coolant to the radiator under those conditions.

If the engine's at temperature, the heater core should be sufficiently large to permit adequate heating of the cabin without the need for the 400+400W of electric air heaters- or at least I hope so! 800 W is a fair bit of wasted energy, considering the little 50 watt-hour regeneration symbols on the fuel economy display- trusting these as accurate, those heaters could easily eat most of the regenerative braking energy recovery from my typical hour's commute if they stay on for the whole trip!

[archae86]

Originally Posted by moltenmetal

Any idea at what speed it becomes more efficient to run the AC versus opening windows? I know it's true at highway speeds, but how about at 50 km/hr?

I'm completely sure that at 50 kilometers per hour open windows is better than air conditioning. I suspect that the crossover speed is quite a lot higher than that, probably higher than "highway" speeds.

I've seen astonishingly little numeric evidence of the windows versus air conditioning comparison, and have never seen any acknowledgment at all of the option of opening the windows less than all the way.

Here in New Mexico, so long as the car has not sat out in the sun first, our famous "dry heat" means that just opening two windows on one side about 2 inches provides a good cabin air circulation at higher speeds. I seriously doubt the drag from that is worse than air conditioning up to the absolute maximum speed of the car.

Windows versus air conditioning is a pet peeve of mine and I have intentions of doing real tests on my Audi A4 before summer is over. My personal opinion is that the great majority of people who were driving in a condition where some degree of open windows makes them actually comfortable will find that option gives them better mileage than using the air-conditioning.

For the Prius, and not other cars, a complicating factor would be the balance between gasoline saving for higher cabin temperature vs. loss of ultimate traction battery life. But I suspect that in highway cruise in non-hilly country the traction battery is not in much need of cooling assistance.

[nyty-nyt]

I find that the best arrangement for me, alone in the car most of the time I admit, is to keep the driver side window open and the right rear window cracked a couple of inches. I find the airflow cools the cabin as much as I would expect, and I imagine the airflow providing cool air to the battery cooling intake.
I've kept myself reasonably comfortable, and have used the A/C only twice this summer.
I average 3.8 liters per hundred k in the summer.

[David Beale]

I've done some simple tests to see how much drag open windows causes. It's a lot. I haven't done drag vs speed tests, but I suspect above about 70 km/hr the drag is a greater loss than the power fed to the compressor motor.

My tests show that on a particular very long downhill (mountain decent) the car will coast well in excess of 130 km/hr windows up and compressor on or off. The Prius takes off on downhills so be careful. Use "B" on long ones to control your speed.

Windows all the way down limited the descent speed to about 110 km/hr (still well in excess of the 80 km/hr posted limit - so what's that, a $500 fine?).

Using "B", of course, limited the speed to around 90 km/hr. But I was testing the aerodynamics, honest, officer!

Oh, and if you think this windows up for efficiency thing is Prius specific, it is not. The anti-Prius (2001 Nissan Pathfinder) worked the same only better. I could hold its' speed on that downhill to 95 km/hr with just the windows open! And it weighted a -lot- more than Pearl!

One last thing. You WILL want to block the front grills in the winter. My experience with the Scangauge II as monitor last winter showed it's safe up to about 10C ambient. At that temp and above you should have the Scangauge to monitor the temps. We use pipe insulation to block the grills. Simple, inexpensive, and effective. I use zip ties to hold mine in, but lots here just use the friction of the foam insulation in the grill. Easy to insert and easy to remove. Bonus, it protects the rad and A/C coil from salt corrosion, something we don't have to worry too much about here in Alberta, but you do in the east.
You will also want good winter tires. I use Nokian WR all year round. Your stock tires may be ok, as the Canadian cars are now all "touring models", so you got reasonable tires, not the junk Goodyear Integrites my car came with last year.

[JimboK]

Originally Posted by moltenmetal

Any idea at what speed it becomes more efficient to run the AC versus opening windows? I know it's true at highway speeds, but how about at 50 km/hr?

The Prius MPG Simulator performs modeling using a variety of user-specified criteria, including speed, outside air temperature (OAT), automatic climate control (ACC), and the following combinations of window positions: all up, all down, two halfway down, and two diagonally opposed 1" down.

The chart below shows results with 90F (32C) OAT, with and without ACC set to 75F (24C), and all window position models. The models assume the following:

• Steady speeds
• Level terrain
• No wind
• 190' elevation
• 30.00 barometric pressure
• Smooth dry pavement
• Michelin Hydroedges inflated to 50/48 PSI
• E10 fuel
• 300# load, driver + cargo

(Forgive me for not doing all the metric conversions. I speak US measures. )

The conclusions:

• All windows up and no ACC gives the best results across all speed ranges -- for the brave soul who can tolerate it.
• Two diagonally opposed windows 1" down is next best, and might be tolerated by some at high speeds.
• Two windows halfway down is superior to ACC at speeds below the low 50s in MPH (low 80s in km/hr).
• All windows down is superior to ACC below 40 MPH (64 km/hr).

Specific values of course will change somewhat with different combinations of typical summertime OAT and ACC thermostat settings, but I doubt the overall relationships change much.

[David Beale]

Nice Jim. Just one correction, as 50 MPH = 80 Km/hr the cross over point is in the 70s km/hr area. About where I guessed.

Oh, and you still want the interior air to be "comfortable" so the battery can cool itself. So "toughing it out" with no A/C is probably not a good idea. It might overheat the traction battery and that would definitely shorten its' life.

[miscrms]

Originally Posted by David Beale

You WILL want to block the front grills in the winter.

Even here in Phoenix I had it half blocked last winter and noticed a good improvement. I know people that use block heaters here in Phoenix as well and notice a difference. The Prius is a unique beast since the engine keep shutting off while your driving. Once that happens it cools off pretty fast, and then you have to run the ICE just to get it warmed back up. That eats into your mpgs pretty quick.

Rob

[archae86]

Originally Posted by JimboK

The Prius MPG Simulator performs modeling using a variety of user-specified criteria, including speed, outside air temperature (OAT), automatic climate control (ACC), and the following combinations of window positions: all up, all down, two halfway down, and two diagonally opposed 1" down.

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.

The post from David Beale is about as close to real data as I've seen, while even for that I would caution that the comparison needs to be fairly closely at the same wind condition and that one can't measure the A/C effect that way. As the test seems to have been on a standard route for him, I have hope that he was sensitive to that concern.