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Engine Revving Shortly After Startup

Discussion in 'Gen 3 Prius Technical Discussion' started by Yaesu, Dec 8, 2020.

  1. Yaesu

    Yaesu Member

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    2010 Prius, 30,000 miles.

    The vehicle was parked in a 55 degree garage for a few days. It started fine, and the engine ran at a normal RPM for a minute or so while I backed out of the garage. I headed down the driveway and onto the road and all was normal. About a half mile further I let off the throttle and applied the brakes to make a turn onto another road, and as soon as my foot came off the throttle, the engine revved and kept revving until I applied the throttle. Then the engine went back to a normal speed. I tried releasing the throttle several more times and each time the engine revved. The drive battery was one bar below full.

    I pulled into a side road to turn around and head back home, but the moment I shifted into reverse the engine stopped revving. I drove the vehicle for about twenty-five more miles, stopping once. No problems, all seemed normal.

    Does anyone have an idea about why the engine was revving so high?
     
    #1 Yaesu, Dec 8, 2020
  2. ChapmanF

    ChapmanF Senior Member

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    That usually means that the state of battery charge was as high as the management system happened to want it at that moment, so the recovered energy as you decelerated was disposed of in the engine instead of sent to the battery.

    Most of the management decisions my car makes, I just go with.
     
    #2 ChapmanF, Dec 8, 2020
  3. Yaesu

    Yaesu Member

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    Thanks for your quick reply. As the high revving was happening, the bar graph for MPG was at the very top (100 MPG). How does the recovered energy of deceleration get disposed of in the high-revving engine?
     
    #3 Yaesu, Dec 9, 2020
  4. bisco

    bisco cookie crumbler

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    30k? really?

    if it keeps happening, i would look into low mile problems that commonly occur
     
    #4 bisco, Dec 9, 2020
  5. ChapmanF

    ChapmanF Senior Member

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    By revving it. :) In that mode, fuel is not being used to spin the engine. The car's kinetic energy is used, through the transmission, to spin the engine (just as happens in a conventional car if you downshift to slow down). The engine is being used then as a large vacuum pump, sucking against the mostly-closed throttle; that converts the energy of the car's motion into noise and pumped air. As no fuel is needed to do that, the car does not inject any. Your pegged-at-the-top MPG reading is entirely consistent. The car is covering distance without using any fuel, so if the MPG graph could go to ∞, that's what it would do right then.
     
    #5 ChapmanF, Dec 9, 2020
  6. Yaesu

    Yaesu Member

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    ChapmanF,

    Interesting. I wish I had let the vehicle stop for a few seconds to see if the revving also stopped. But when I turned around, I stopped and quickly shifted into reverse and that's when I noticed that the engine wasn't revving as I backed up.

    I attempted to duplicate the situation this morning, by repeating what had been done yesterday. The engine didn't rev at all, but yesterday the battery level was one bar higher, one from the top of the symbol, as I went down the driveway.

    Here's what I think might have happened to cause the battery to have such a high state of charge. Before turning into the driveway the night before, the vehicle coasts down a long gradual hill for about one mile and then the seven hundred foot driveway is steep enough so the engine powers the vehicle up to the garage, adding charge to the battery. The next morning, (when the problem occured) the battery was only one bar from the top when I headed down the driveway, which added charge by braking and also the engine was running to warm the engine. Does any of this make sense?
     
    #6 Yaesu, Dec 9, 2020
  7. ChapmanF

    ChapmanF Senior Member

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    Yes, that makes sense. The car can also have other reasons for limiting the use of the battery at times, such as when it is extra cold or extra warm. The management strategy will make gentler use of the battery until it has reached a preferred temperature (which a bit of gentle use can bring it to).

    It sounds like the temperature was mild in your case, so that probably wasn't a big part of the picture that time. Being at a high state of charge was probably most of it.

    As I said, the car makes a lot of management decisions like that on its own. It makes them competently, and I generally don't bother trying to micromanage it.
     
    #7 ChapmanF, Dec 9, 2020
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