Why my car switches to EV mode at 40-45 mph? Seems inefficient.

Discussion in 'Gen 3 Prius Main Forum' started by Troy Heagy, May 23, 2014.

  1. Troy Heagy

    Troy Heagy Member

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    I notice my Prius sometimes turns-off the engine & runs pure EV when cruising at 40-45 miles/hour. Sometimes for a few miles! That seems very inefficient vs. taking power directly from the engine:

    - Gasoline--->burn---> transmission--->wheels
    - 1 state transition (chemical-to-mechanical)

    - Gasoline--->burn--->transmission--->generator--->store in battery--->extract from battery--->motor--->wheels
    - 5 state transitions

    My Hondas don't have a full-speed EV mode, so the electric motor remains idle at 40-45 miles/hour (except for several seconds of assist or regen braking). This Prius operation is new to me.
     
  2. drysider

    drysider Active Member

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    Much of the energy stored on the battery comes from the regeneration systems and is pretty much free. The Synergy firmware is really good at figuring out the best combinations to maximize mileage. The engine almost never runs just to charge the battery. The firmware will use excess engine capacity to charge the battery if necessary.
     
  3. hybridbear

    hybridbear Member

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    You are partially correct. Remember though that the ICE is only 35-40% efficient at turning the energy stored in gasoline into mechanical energy. An electric motor is 90+% efficient. That's why electric vehicles can go 80 miles on the equivalent kWh of 0.75 gallons of gas. The electric operation is most efficient at low speeds.

    If you want some detailed analysis of this, check out this thread at the Fusion Energi Forum. One member has done extensive analysis of the efficiency of the system and how it operates. It's really fascinating. The Prius system doesn't have as many variables, but it's still helpful to understand what's going on.
     
  4. alanderego

    alanderego Junior Member

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    I can force the Prius to go into EV mode at 60 mph by slacking the throttle and then slowly pressing the throttle to bring my mph up.
    It is fun to show people that it can be done, but it is hard to hold it at that speed on batteries.

    If your Prius goes into EV mode and you don't slack gas, you better have it checked out.
    I don't think that is normal.

    Alan
     
  5. Troy Heagy

    Troy Heagy Member

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    I'm using cruise control so I don't touch the gas pedal. The most anyone has extracted from an ICE car is 45 on the EPA highway test (50+ for a three-cylinder diesel). The EPA figure for the Focus electric is 99 highway MPGe, so the electric car is about twice as efficient.
     
  6. Stratman

    Stratman Member

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    Not sure I quite understand any of this either. There have been many times on my commute home I have had no arrows in either direction at over 50mph and when I get home it shows 99mpg and $.07 total usage in fuel. My EV indicator is on with no ICE shown at this speed even when I am gently accelerating going up a slight uphill grade. I coast to charge on downgrades to charge. It does seem to take more of a "touch" on the pedal but I don't have too much trouble achieving this. I have gotten where I quit second guessing the thing and just accept it.
     
  7. Stratman

    Stratman Member

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    The efficiency may be better but it's range specific. I'm not sure I would be comfortable driving more than 60 miles without having to stop and plug in. Having a hybrid that mimics an all gas vehicle can't be beat when I'm driving 390 miles to check on my 85 year old father or going on vacation. I prefer it's "sustainability". For the want of a better word.
     
  8. SageBrush

    SageBrush Senior Member

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    Wow. We are going to lose Bob Wilson because of that Larry fellow ;)

    Truly good work
     
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  9. fuzzy1

    fuzzy1 Senior Member

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    [I replied in the earlier copy of this thread, but it appears that this thread copy will be the main one. So I'm pasting over here.]

    If this is news to you, it helps explain why you achieved only 40 mpg in LA city traffic.

    When this operation is sustained for a few miles, it means the needed propulsion power is quite low (likely including some minor downhill slope, not necessary perceptible to the driver), so the car can almost glide.

    Yes, some energy is lost in the extra energy conversions of EV. But the ICE consumes some minimum overhead power just to run at all, so it cannot operate near to its optimum efficiency when traction power is under about 10 HP. When the needed traction power is much less than that, the ICE overhead loss exceeds the extra overhead loss of EV. If the traction battery has enough charge to support EV, then it becomes worthwhile to shut off the ICE.

    At these lower speeds, best efficiency comes from alternating between ICE and glide/EV, not sticking to ICE-only or EV-only. The car does very well when making its own choice on its own schedule. Drivers using Pulse & Glide or similar strategies can do significantly better by incorporating knowledge about conditions ahead (traffic, required stops, slope, etc.) that the car alone cannot know.

    At higher speeds, where the air drag and traction power are high enough to keep the ICE operating in or near its peak efficiency zone, then ICE-only operation becomes efficient. Geeks can derive this minimum power from the engine's BSFC chart (frequently linked in other postings), and monitor actual power output with many OBDII-type scanners (ScanGauge, Torque, etc.)
     
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  10. hybridbear

    hybridbear Member

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    I've gotten to meet him in person since he lives close to us and he is truly a genius when it comes to engineering. He is incredibly smart.
     
  11. Troy Heagy

    Troy Heagy Member

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    Yes I suspected that was the case..... that it was more efficient to run EV then to run the ICE at a bare power minimum. Makes logical sense. In the Honda hybrids this problem was solved with "lean burn". The engine never shutsoff, but instead adjusts the fuel/air mixture from 1:15 downto 1:25. The Instant MPG displays shows 120 in that state.

    I'm not sure why my "lack of understanding" would have given my 40mpg in city driving. Even if the driver is clueless, the computer in the car should automatically choose the best operating state (EV or engine) to conserve fuel at the low speed. I still suspect the EPA City test is not an accurate representation of real world city block driving.
     
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