A question about efficient use of A/C in the Prime

Thanks... appreciate everyone's reply (really!) but... this is the mystery then, as the manual (and Lee Jay) seem to suggest otherwise. But with NO other car I've had is this the case. In any of the 5 other cars our family has had (3 Hondas, 2 Corollas), you could feel a definite shift in the engine/engine power when you put the AC on -- but NO such change when putting on JUST the fan (with the AC not on) -- ie, recirculating hot air. And NO change could ever be felt when (with the AC on), moving the FAN switch between speeds ( med, low). This made no difference.

So one would logically say that the fan speed, in and of itself, doesn't affect MPG at all.

The only reason I could see this would be different, is, with the Prime -- unlike other cars -- if the fan speed somehow tells some software program that you want the AC unit to work harder. Otherwise, just the difference of the rotations per minute of the fan isn't going to affect the MPG, correct?

Sorry, I'd really like to know this, as in the past if I had to turn the AC on, I'd always blast it to high (since in all other cars the fan speed made no difference -- may as well get the most out of it while it's on), then turn it back off to save gas.

 

A heat pump is a refrigeration unit that can switch its condenser and evaporator functions. The condenser throws off heat (when the hot gas refrigerant condenses to a cool liquid). The evaporator takes in heat (cools) (as the cool liquid refrigerant evaporates to a cold gas). In an auto air conditioner the evaporator is inside the cabin and cools the air that passes across it. The condenser is under the hood and rejects the heat collected by the evaporator. OK, reverse those functions in the winter. The unit under the hood is now the evaporator that is drawing heat out of the air (down to a practical limit). The condenser is inside the cabin giving heat to the air passing across it. A home heat pump works the same way.

The faster the fan runs the more warm air is pushed across the evaporator. This may cause the compressor to run more often (or faster) to keep its temperatures and pressures at the set points.

 

You're confusing yourself right here: "JUST the fan (with the AC not on)"

The fan draws a small amount of electricity but probably not enough to feel the difference in load on the alternator in a conventional car.

But, if the AC is on, changing the fan speed changes how hard the compressor has to work to keep the air cool. In a conventional car that can only turn the compressor on and off with a clutch, that just means it runs more often when the fan is set higher. In a Prius, with its variable-speed compressor, that means more electricity is drawn from the traction battery to spin the compressor faster when the fan is set higher.

So, fan speed doesn't affect mileage very much when the AC is off, but it does when the AC is on by forcing the compressor to work less or more depending on fan speed.

 

I'd say an emphatic yes to that. Getting into a hot, been-sitting-in-the-sun car, I would suggest:

Set the system to "Auto" with AC on (if AC is optional on yours), and set temp high enough to prevent the fan from racing. This might be as high as 85F. Then, as the interior temp starts to drop/stablilze, and the fan slows, bring the set temp down. I would actually aim for around 75F, as a final setting. Try it.

Also, do you need to be hermetically sealed, air-conditioned, all the time? I appreciate that question gets increasingly debatable in the southern latitudes, but try it: roll the windows down, just like the good 'ol days, once in a while. Preferably when not stuck in traffic behind a diesel bus.

 

What app is this?
thanks!

 

Carista

 

Carista – OBD2 vehicle diagnostics, customizations, coding & tools

 

The air conditioning/heat pump in the Gen-4 Prius (including the Prime) is extremely efficient, using sophisticated thermodynamic techniques, and since the compressor is electrically powered, it can vary its load. In conventional cars, the A/C is powered by a belt to the engine, and a clutch has it either on or off.

 

Wouldn't you also want to set the ventilation to recirculate mode during the cooldown?

 

I usually don't. If anything, the inside air is hotter, better to be exhausted/replaced by outside air. Also, I get the sense the air flow is increased when set to outside, not sure though.

 

I can see that, although based on what Lee Jay pointed out maybe start out with windows down to blow out the hot air, assuming you are not at highway speed. Then windows up and turn on AC with the vent set to recirculate. Then when the temperature stabilizes turn off recirculation. This method would use a little bit of electricity lowering and raising the windows although I'm guessing not much. To reduce that maybe just lower the driver window and right-rear passenger window (might as well put yourself in the breeze).

I guess great minds think alike, even one minute apart!

 

The energy it takes to raise and lower the windows is indistinguishable from zero in the overall scheme of things.

 

One nice thing about Gen 4 and Prime is that all of the windows are one-touch open and close. Not sure about Gen 3 but I know that on my Gen 2 only the driver's window is one-touch (but we are mighty glad to have it!)

Another nice thing is it looks like the key fob can be programmed by the dealer so that if you press and hold the door unlock button it will lower the windows. So as you are approaching and getting into into your car you will already have a head start on getting rid of the heat.

 

yup, and you have to hold the unlock button until the windows are completely down otherwise they stop going down midway through the process

 

Inner secrets of the Toyota-Denso heat pump revealed - SAE International

Downloaded JP/EN "feature guide" May 10 but can't find the url so I'll attach the file.
Also explains the heat pump system.

Also saw this Japan JDM production quantity news release.
新型プリウスPHV 受注状況について | トヨタグローバルニュースルーム
2,500 per month but 12,500 already ordered as of 3-17

 

James, thank you for the link! Very informative. Just to clarify, 2,500 is the sales target of the first month of sale in Japan, or so it looks to me.

Posted via the PriusChat mobile app.

 

Interesting that the PDF for the Prius PHV shows "target cruising range" of 68.2 km (42.3 miles) EV which it says is "Based on the JC08 Japanese test cycle for EV mode (on a fully charged battery) and verified by the Ministry of Land, Infrastructure, Transport and Tourism of Japan". And it shows hybrid mode fuel efficiency as 37.2 km/L (87.2 mpg) "JC08 mode fuel efficiency. Ministry of Land, Infrastructure, Transport and Tourism of Japan evaluation value"

Compared to the Toyota North America's Prime brochure which lists the EPA numbers of 55 city/53 hwy/54 comb mpg and EV range of 25 miles.

https://www.toyota.com/content/ebrochure/2017/priusprime_ebrochure.pdf

JC08 test cycle is apparently limited to 51 mph so maybe that is part of the reason for the difference.

Fuel economy in automobiles - Wikipedia

 

The Japanese test cycle is known to provide very generous numbers.