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AC/HEAT and Fuel Economy

Discussion in 'Gen 2 Prius Technical Discussion' started by jmatuska, Jun 11, 2007.

  1. jmatuska

    jmatuska New Member

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    I have a couple questions in regard to the AC/Heat system in my Prius. First, I have heard the AC unit is totally powered off the hybrid battery only and not off a belt off of the engine as would be typical in a non hybrid.

    1. Does use of the AC during hot weather significantly effect milleage? I have not noticed any real affect so far, but have heard it can to an extent. I know on a non hybrid it can be a significant impact.

    2. How exactly does the heat work on a prius? Does it just put the AC in reverse like a heat pump? If so, does this affect milleage as well?

    3. Does running AC/heat put any additional stress on the engine while going up a steep hill. In the past driving non hybrids I always turn off AC when going up a steep hill in the summer due to the extra stress this typically would put on the engine and potentially cause overheating. Is this no longer an issue since the AC unit is not directly coupled to the engine?
     
    #1 jmatuska, Jun 11, 2007
  2. donee

    donee New Member

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    Hi Jmatsuka,

    No, the heat works like a traditional car, but with feedback control. There are also PTC (positive temperature coeffient) heaters that assist with cold morning quick heat. They run from the battery. That heat pump idea is a good thought!

    The AC serves to keep the batteries cool as well as the passengers. I run AC whenever its above 85 F here, after an open windows two-mile cool down. Use a sun shade and crack the windows when parked. If I try to run my 23 mile afternoon commute without the AC in this weather, the heat from the sun will bring the passenger compartment up so warm, the battery pack will tell the transmission its not going to give or take any electricty in certain conditions, towards the ends of the trip. In that situation, I loose the capability to electric cruise at speed, and the oportunities to glide (coast without electric or engine) and regenerate are reduced. This ends up using more fuel, I think. With the AC on, the car maintains a steady operating mode all the way home, and the fuel economy driving technigues all work. I pulled into home today at 60.3 mpg.

    The mileage is effected, but as the AC is close-loop (feedback) controlled, and you can set the temp to whatever you like, you can choose to minimize the impact. I run the AC at 80 to 83 F. The dry air coming out of the vents is enough to keep me cool enough. If I do not get stuck in traffic, the AC has so little effect on mileage, that traffic effects (on a cool day) are dramatically larger. If your driving with multiple people in the car, the impact will be worse, as the AC will need to remove the heat from them as well.
     
    #2 donee, Jun 11, 2007
  3. richard schumacher

    richard schumacher shortbus driver

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    In brief, the battery system is cooled from the cabin air through the vent to the right of the rear seats. Do not be afraid to use the A/C. If the driver is uncomfortably hot then the battery system is also uncomfortably hot, and fuel economy will be reduced.
     
    #3 richard schumacher, Jun 12, 2007
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