Does Anyone Know How Exhaust Heat Recovery and Recirculation Works?

Discussion in 'Gen 3 Prius Technical Discussion' started by GregP507, Jul 3, 2014.

  1. GregP507

    GregP507 Senior Member

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    I've tried to research this myself, but so far all I can find are sketchy explanations of how this works.

    I'm not sure how it's possible for the cooling system to extract heat from the exhaust system without sometimes overheating the coolant. Over an extended period of driving with the gas engine running, there would be a significant amount of heat built up in the exhaust system, so what protects the cooling system?
     
  2. RRxing

    RRxing Senior Member

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    See attached...
     

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  3. GregP507

    GregP507 Senior Member

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    That explains it perfectly.
    Thanks
     
  4. retired4999

    retired4999 Prius driver since 2005

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    I think if you check out Bob Wilson's post's. He has a neat picture of the system of the actual engine, with the system exposed so you can see how it hook's up and returns to engine and exhaust.
     
  5. El Dobro

    El Dobro A Member

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  6. Mendel Leisk

    Mendel Leisk Senior Member

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    There's some valve that shuts off coolant flow once temp has climbed sufficiently. At least it's supposed to. Someone here posted in the last few days about that valve failing, and an ensuing overheat issue?
     
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  7. El Dobro

    El Dobro A Member

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    Is the valve electric or mechanical, like a thermostat?
     
  8. Mendel Leisk

    Mendel Leisk Senior Member

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    Excerpt from Repair Manual. Seem there's a mechanical, temperature sensitive actuator, and electronic monitoring. Also an override to prevent to much back pressure during heavy acceleration with cold engine.

    Exhaust Heat Recirculation System Circuit

    DESCRIPTION

    In the exhaust heat recirculation system, coolant is warmed up using conventionally wasted exhaust gas heat to accelerate engine warm-up time, enhancing fuel efficiency and heater performance.

    The heat recirculator is positioned in the front exhaust pipe assembly after the catalyst. Coolant from the engine flows around the heat recirculator and then returns to the engine. If the engine is started while the engine is cold, the exhaust pipe gas control actuator rod is contracted and the exhaust flow control valve is closed, routing the exhaust gas around the heat recirculator to warm up the coolant.

    After the engine coolant temperature rises and the engine has warmed up, the heat of the coolant expands the thermostat and the exhaust pipe gas control actuator rod extends. This opens the exhaust flow control valve to switch to the normal exhaust gas path.

    The exhaust flow control valve can also be opened by exhaust gas pressure to prevent insufficient acceleration performance when the engine is cold. In addition, to monitor the engine coolant temperature, a engine coolant temperature sensor (for exhaust heat recirculation system) is provided between the engine and the heat recirculator. The engine coolant temperature sensor (for exhaust heat recirculation system) has a built-in thermistor with a resistance that varies according to the temperature of the engine coolant. When the engine coolant temperature becomes low, the resistance of the thermistor increases. When the temperature becomes high, the resistance drops. These variations in resistance are transmitted to the No. 3 meter circuit plate as voltage changes. If the engine coolant temperature is excessively high (overheating), the water temperature indicator light in the No. 3 meter circuit plate illuminate to inform the driver of the malfunction.
     

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  9. Feri

    Feri Active Member

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    One of the few problems I have had with the Prius was a stick puncture of the rubber inlet pipe to the exhaust cooler.
     
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