HV/EV Strategy for pulling out of a room temp garage into freezing cold temps?

I say you are over thinking this. Did you worry about pulling out of the warm garage in a traditional car?

There may be a temperature low enough that the engine will come on, but unless you are trying to tough it out with no cabin heat, the engine will come on to provide heat. The heat pump needs enough heat energy outside in order to provide cabin heat. At 14F and lower, there isn't enough, so the engine will fire up. The heat pump also can't heat and dehumidify at the same time. When the front defroster is turned on, the heat pump will dehumidify, and the engine comes on for heat.

 

i'd just put it in hv mode and let it warm, then switch to use ev as necessary.

 

Just drive it.

There's nothing to be concerned about, as I documented with video to address questions later... just like yours:

 

Are you sure the A/C system can't heat and dehumidify at the same time, without the defroster on? The Prime has an incredibly complex heat pump system that I have to admit I don't fully understand...

How does the heat pump work in a Toyota Prius Prime plug-in hybrid?

I dread the day my heat pump fails - finding someone who knows how to work on that system will be a challenge!

 

That's exactly what I did almost entire cold winter months from late Dec to early Mar. Even with heat OFF and defroster OFF, when the ambient temp was less than 10F (-12C), my car's ICE started immediately after I pushed the START button. (I park my car outside, so the initial temp of the car is same as ambient temp.) Yeah, if you keep your car in warm heated garage, you maybe able to fool the car to think it is in warmer climate and be able to start out in EV mode, but as you said, it will last only a few minute once the car is out in the cold. My daily commute is ~18 miles one way, and I don't have way to charge at work. During cold month EV range on full charge plummeted to less than 20 miles on my PRIME, so I can only drive on EV one way. Since ICE comes on anyway in the cold morning, I just drove my car 100% HV on my way to work in morning, and saved all of EV range for afternoon commute back home when day temp is usually higher than 14F (-10C).

 

The heat pump can only work as AC or a heater, but not both at the same time. To dehumidify most cars run the heat and AC at the same time in defrost mode. The Prime will do that too by using the engine for heat, and presumably using the heat pump in cooling mode to dehumidify. But that's probably unnecessary in very cold weather, and the AC won't be able to run much anyway if the inside coil drops below freezing. It's most helpful for warmer humid weather, like a rainy day.

The heater alone will tend to dehumidify, since cold air doesn't hold much moisture, and since warm surfaces won't condense as much moisture. Toyotas seem to blow some air out the defroster vents when the vents are set on feet mode (I think the Prime does too), so that can defrost the windshield a little if you don't need full defrost. If you want to stay in EV mode, and if conditions permit (above 14 F), set the vents to feet and blast the heat to clear the windshield. It won't work as well as defrost mode because there's less airflow to the windshield, and the air is a little more humid.

 

Use a block heater for a couple of hours, start out in Hybrid mode, save the Electric mode for later?

 

Actually, it works just fine using the ordinary blower mode. No defrost required, as this cold & moist video clearly shows...

 

Yep, the manual states the engine comes on for defrosting
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Heat pumps are basically air conditioners and dehumidifiers. They just have a set of valves that allow them to work in reverse when it comes to moving heat around. They all remove humidity from the air by having it condense out on the cold coil. When a heat pump is heating, the cold coil is the one outside.

When defrosting, the Prime heats the air by the engine, and then dehumidifies it by having the heat pump cool its inside coil. I guess the engine won't come on if the temperature is set below ambient, like when defogging the windshield during the summer, but that won't help with defrosting it.

 

"Defrost" is rarely actually needed.

Again, the basic blower for the window will keep it clear in all but uncommon situations. I know this well living in Minnesota driving a Prime.

 

That's how most cars work, but I think the Prime is more sophisticated than that. See the article I linked to. I understand the engine comes on when you explicitly select Defroster mode, for heavy duty defrosting, but in the US southeast I never use that mode, and yet do seem to get heating AND dehumidification from the system in our typical low temps of 30's and 40's (F).

Posted via the PriusChat mobile app.

 

In my experience, even with A/C set to off, if you have the fan on, the heat pump will apply heat if the outside temp is below your temp setting. As you can't set the temp below 60F, this effectively means if the blower is on, the heat pump is working when it's below 60F outside. The only way to be sure it's off is to turn the system (including fan) completely off, in which case you get no airflow at all. Thus there is really no plain-old-vent mode...

Posted via the PriusChat mobile app.

 

That article isn't the clearest on how it works. Going just by the diagram it appears that the system would be heating the cabin air all the time. There was a thread on just how the heat pump worked here when the Prime first arrived.

Keep in mind that humidity is dependent on air temperature; the warmer air is, the more water it can hold. When cold, humid air is heated up, the humidity level drops. The warmed up air feels drier, but no water has been removed from it.

 

Looks like the red herrings are getting out of hand. The windshield is cleared without need of the gas engine under most circumstances. What else is there you need to know?

 

I agree the gas engine/defrost mode is not needed in most situations. But I'm still peeved there's no plain-old-vent mode, in case you couldn't tell

Posted via the PriusChat mobile app.

 

That would annoy me too.

 

ENERGY MONITOR clearly shows when the pump is on. You can see the flow of electricity from the battery-pack.

Just turn off the A/C and reduce the temperature to "LO" to get outside air... your plain-old-vent.

 

It is not obvious from the linked drawings, but in the Prime, there is a refrigerant coil in the cabin that always has hot, compressed refrigerant, regardless of whether it is operating in heating or cooling mode. This is the first place the refrigerant goes from the compressor. Dampers control the cabin air flow over this coil, so the air does not go through this coil unless cabin heating is desired. It is entirely possible to run the system in dehumidify mode by running the system in cooling mode, so the evaporator in the cabin is cold and the condenser in the engine compartment is hot, and opening the damper on the internal condenser to reheat the air coming off the cold coil. No engine heat is required for this, unless you really need the extra heat.

Unlike in conventional heat pumps, refrigerant always flows in the same direction in the Prime; heating vs cooling is determined by which of several expansion valves are opened. In cooling mode, the valve in front of the "condenser" coil in the engine compartment is open and the one in front of the "evaporator" in the cabin is (mostly) closed, and thus high pressure, hot refrigerant is in the external coil, and the cold, low pressure refrigerant is in the interior evaporator, cooling the cabin. The damper on the interior condenser is closed, so no heat exchange occurs there in this mode.

In heating mode, the valve in front of the external coil is (mostly) closed and the one in front of the inside "evaporator" is open, and thus the external coil is cold (it is actually acting as an evaporator). Also, the damper on the internal condenser is opened to heat the cabin. I think the interior "evaporator" is bypassed in heating mode and the refrigerant goes directly back to the compressor. There is also a coil with engine coolant also controlled by dampers for traditional engine heating.

 

Does putting the HVAC in defrost cause the engine to always come on then, or can it stay off in some conditions?