Cold Weather Interior Heating and ICE Operation

The new Prius is a bigger vehicle and uses more heat. The ICE runs to supply that heat. You may have A/C selected, but the A/C doesn't supply heat, only cooling and dehumidification.

There are times that the ICE will run to ONLY supply heat and you won't see it doing anything on the MFD even though you know the ICE is running.

Hey, We take a hit on MPG in the winter. You discovered one of the reasons why. Not too bad at highway speeds, a bummer in town. Pray for spring.

One way to minimize the ICE running for heat is to lower the interior temp and train the air ducts onto you. The same thing can be done in the hot summer with the A/C, except the temp gets set higher under that condition. Or just live with the MPG hit.

 

I don't have my Prius yet.. but was wondering..

Does the heater/defroster work off electric... meaning... basically the minute I start the car and drive away, I got heat on my window and feet?

Or... is it like any other car where I'm 10 minutes down the road when I start to feel heat?

 

This is a misconception! AUTO AIR simply indicates that the climate control system is on.

Denso who makes the system says:
Air-conditioning systems should not rob torque from hybrid powertrains on hot summer days. And they need to be able to heat interiors on cold days with little help from engine heat. For the Toyota Prius hybrid car, we developed an ultraefficient air-conditioning system that consumes minimal power.

Our new subcool condenser and scroll compressor, equipped with oil separators, allow the air-conditioning system to run on 30% less power than conventional systems. And a two-layer flow heating, ventilating, air conditioning (HVAC) unit saves energy by recirculating heat already in the cabin.

Another efficiency-raising component is a new, straight-flow core that we developed for the heater. In conventional systems, engine cooling water flows in a U-shaped route through an aluminum core. We have raised heating efficiency by routing the warmed water in a one-way flow through the aluminum tubing. Our straight-flow aluminum heater core is 22% smaller and 25% lighter than conventional cores.

The Prius's main water pump loses its source of mechanical energy when the vehicle is running on battery power alone and the engine is off. So, we developed an auxiliary electric water pump for the car. Our electric water pump provides a constant flow of heat to the cabin even when the mechanical pump is stopped. The mechanical pump operates when the engine is running, which reduces the load on the battery. And a bypass function reroutes water flow to avoid resistance from whichever pump isn't running.

For a complete description of the system see:

http://www.denso.co.jp/AR-e/1998/TW/climate.html

 

Well, you're both right of course, but both talking about different things. Brian K is simply saying the only electric component (other than a couple of wimpy heater elements) is the refrigeration unit, which sorry to say, people like to call the air conditioner, even though "air conditioning" is part of a larger concept. That component is even labeled "A/C" on the climate control. It only confuses people to refer to it as anything else.

In any case, that unit only cools and dehumidifies, as he said. The car engine provides the heat, with a little help from a couple of heater elements under special circumstances. So, like everyone is saying, the engine has to run to provide heat.

Anyway, really would like to see people refer to the refrigeration unit as the air conditioner, because that's what it is.

I like both your explanations, however.

To answer daronspicher, the Prius heats a little quicker than a normal car because it stores some of its coolant in a thermos, and under some circumstances supplements the heat with two small electric heating elements. So, 'kinda' like a normal car, but a little quicker. B)

 

How about the auxiliary electric water pump? It provides a constant flow of heat to the cabin even when the mechanical pump is stopped.

 

I think the heater is the dominant factor in lower mileage in the winter. Tonight as an experiment (and since I was dressed appropriately for the outside temperature), I tried my commute home with no heat. I set the climate control to Defrost (via steering wheel button), Max Cold, and minimum fan speed. This kept the windows from fogging up with no detectable heating (i.e. like using the vent procedure, but pressing front defrost on the steering wheel first to direct the air to the defrosting ducts).

It was -5 C to -7 C outside for the 1/2 hour commute, but I got mileage that seemed as good as what I would get in the summer, though judging from the bar graph it took longer for the ICE to warm up. I got signficantly better mileage than my reverse trip in the morning with the heater on. With the heater off, the ICE stayed off for long periods, whereas it would rarely turn off with the heater set to the lowest possible temperature (18 C).

Note that cold weather should have some positive factors for mileage, once the ICE is warmed up. A given fuel will be denser, and therefore have more energy per volume (this is offset somewhat by winter gas having more volatile components, making it less dense at a given temperature). Colder intake air will be denser and therefore expand more when its temperature is raised by combustion. Operating in a lower temperature is like operating at a lower altitude, because fuel and air are more dense. I think it's generally observed that one gets better mileage at low altitude than high (assuming the same temperature).

 

I agree -- I've actually had cars in that past that would overheat and boil over in a matter of minutes, and I was able to delay or stop that behavior by simply putting the heat on 'max', even if it was in the middle of summer.

The heater is a heck of a heat sink!

 

That is quite correct. The Prius system using a fuzzy logic algorithms the system conditions the air, heating and cooling and controlling humidity. You can force the system by using Max Heat (I guess heat is still heat) and Max AC (cooling, why does this have to be confusing). All of this effect the efficiency of the car to some degree or another.

 

Afternoonnap, that was a neat little experiment. I never thought of doing that. It's great to have so many minds thinking about similar things.

Kram, ayuh, near as I can figure the ICE cabin heating system is mostly the same "waste heat" system that's been in use for decades. Obviously in the Prius it isn't waste heat, but heat produced specifically by the ICE to heat the cabin. A fairly inefficient system. Bets that it's something that's corrected in gen 4 Prius? It'd be nice to not have to take the hit in mpg just for cabin heat.

 

Thanks! The scary thing is I've taken to driving without heat fairly often now that I know it makes a significant difference to mileage (but not with passengers on board). However for longer highway trips, where the ICE is running anyway, I find I get just about as good mileage leaving the heater on auto with the temperature control set to its second lowest setting (i.e. the one above MAX COLD).

BTW, if it's snowing or snow is expected soon, I find it useful to use cold defrost a few minutes before stopping. That helps prevent ice formation on the windshield, since when cold the windshield doesn't melt the snow when the car is parked. I did that with my previous vechicle too.