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Efficiency of engine generated electricity

Discussion in 'Gen 1 Prius Plug-in 2012-2015' started by Redpoint5, Aug 7, 2015.

  1. Redpoint5

    Redpoint5 Senior Member

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    Generally I run in EV mode during the first 1 mile of any trip as its mostly downhill with lengthy stops at lights before getting onto the freeway.

    Just before accellerating onto the freeway, I switch to HV mode and the engine kicks on, however it doesn't supply any of the initial motive power. Instead, 1.5 miles or so will come from the battery to get me up to speed until the engine reaches sufficient operating temperature. Once this occurs, the engine then begins to recharge the amount that was lost during warmup.

    My questions are:

    How efficient is it to regenerate this energy from the engine?
    Is it more efficient to toggle EV/HV mode to cancel the recharge process?
    Is the purpose of waiting for the engine to warm up before supplying motive force just to reduce pollution, or does it affect drive train reliability? In other words, will any harm be done to the Prius by fooling the computer into thinking it is already at operating temperature?
    If it is efficient to regenerate electricity during peak engine efficiency operation, then why doesn't the Prius do this all the time when the battery drops below a certain threshold?
     
  2. Jeff N

    Jeff N The answer is 0042

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    Based on what I've read it's probably about 10-15% less efficient to charge the battery with the gas engine (and then use then use that battery power again later) than it is to just drive the wheels without charging the battery. That's not too bad. If that recharged power helps prevent another cold engine start later on then it was well worth it.

    The strategy of leaning on the battery and electric motors during cold engine startup is mostly about emissions control but also likely helps reduce engine wear.

    Actually, the Prius routinely pulls some battery power and then recharges it while driving down the road just like it does in the non-plugin Prius so this is nothing new.

    As another example, a non-plugin Prius does the same thing while it is bringing the engine up to operating normal temperature when you first start the car and begin driving.

    Toyota has thought this through already. Don't worry about it.
     
  3. Redpoint5

    Redpoint5 Senior Member

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    As I think more on this, it would also depend on how the generated electricity is then used compared to how the gas is used. If the gas motor generates electricity, and then the electric energy is used to accelerate or go up a steep hill, then it's very inefficient. However, if the generated electricity is spent putting around in parking lots or other low speed/low power situations, it increases the overall efficiency.

    Gas engines are most efficient at near full throttle, and very inefficient at low throttle. It makes sense to generate electricity at high throttle openings, and burn the electrons during low throttle operation.

    Again, I wonder what the gain or loss is in overall useful energy when running the gas motor at peak efficiency to generate electricity that is later used in low demand driving.
     
    #3 Redpoint5, Aug 7, 2015
    Last edited: Aug 7, 2015
  4. gallde

    gallde Active Member

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    Remember that the Prius engine is an Atkinson-cycle engine that has widely-variable, computer-optimized valve timing, so the efficiency curve is atypical of a conventional ICE.
     
    Redpoint5 likes this.
  5. roflwaffle

    roflwaffle Member

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    Like gallde said, the combination of optimized valve timing and PSD allows the car to keep the engine in an efficiency sweet spot across a wide range of loads.

    The motor and inverter are crazy efficient at modest loads (for them), so I wouldn't worry too much about them.
    http://ecee.colorado.edu/~ecen5017/notes/OakRidge_2010Prius.pdf
    The PSD in the first gen has losses that are pretty much a function of (I think) MG1 speed.
    http://web.ornl.gov/~webworks/cppr/y2001/rpt/121813.pdf

    I just try to save EV for the extremes (steep uphills and extremely low load situations) and keep the engine in the eco portion the rest of the time.