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Engine Braking, is it possible to over rev the engine?

Discussion in 'Prius v Main Forum' started by wstt, Jul 11, 2013.

  1. wstt

    wstt Member

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    I was driving down a long steep hill on the highway going about 65. I put it into engine braking, B, because I didn't want to just ride the brake the whole way down. The engine was revving pretty loud and with no RPM guide, I had no clue if it was OK, so I ended up going back into D and just rode the brakes all the way downhill.
    Is it possible to over rev the engine when going down hill, or will the car just choose a higher gear?

    Also when costing from Neutral, do I have to worry about over revving when I switch into Drive from Neutral?
     
  2. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    1) No, the computers will not overrev your engine.
    2) Avoid N at any time, neither M/G1 nor M/G2 has any power in N. No charging happens in N, the gas engine cannot start in N.

    There are odd conditions where N can hurt your MG/1 motor, in Gen1 and Gen2, coasting in N from bellow 42 MPH to above 42 MPH would over rev MG/1 as the gas engine cannot be started in N. This limit SHOULD be 62 MPH in Gen 3 but I have never seen a test of that.
     
  3. wstt

    wstt Member

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    Wow, I thought neutral is great for down hill where it is not so steep you don't need to brake and where it has too little slope where just taking your foot off the accelerator in drive causes the car to slow down.
    I saw some video from Wayne Gerdes where he keeps having everyone put it in neutral while driving. I assume MG/1 is the hybrid motor and MG/2 is the normal engine (I'm new to this and just got my prius V 2 days ago).
    Is shifting in Neutral ok if the motor is on? I like coasting down hills but often the hill isn't steep enough so I have to give it some gas if I am in D.
    But I'm glad to hear the engine breaking can't over rev the engine.
     
  4. Lee Bridges

    Lee Bridges Junior Member

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    I'm still enjoying tripling my MPGs in going from a pickup to the Prius, so I can't vouch personally, but I understand the correct technique for max MPG is to feather the as pedal so that no bars appear in the regeneration part on the left of the upper dash display.
     
  5. fuzzy1

    fuzzy1 Senior Member

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    Real Neutral is a wonderful tool in traditional manual transmissions, but unnecessary on a Prius. It is OK on mild slopes where you don't speed up. Uncle Wayne has students use it so that they can quickly get the feel of a true glide. But they are supposed to practice achieving the same thing by just putting light pressure on the gas pedal and not shifting into Neutral, saving time and hand motion and avoiding risk. His lessons just don't have enough time to gain proficiency at it, so on a Prius, Neutral is a stopgap to help pack in more lesson in the brief time available.

    The Hybrid system is more complex than that. MG1 is typically a generator, passing some of the propulsion power on the electrical path to the motor MG2. The rest of the power flows through the mechanical path. But there are several other operating modes, and each MG gets its turn both as a motor and as a generator.

    Toyota warns to not shift to Neutral while moving, but doesn't go into detail about what the risks really are. (Whatever they write, someone will misunderstand and botch it, damaging the transaxle.) Some of us do it on occasion anyway, believing we know how to avoid the risky spots, but mostly nudge the gas pedal to achieve essentially the same result. Neutral is also an important tool to know in case of a real runaway engine, something that has been alleged on numerous car models over the decades, but in the modern era is mostly caused by pilot error instead of true mechanical problem.
     
  6. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Motor/Generator 2 is linearly attached to the wheels, if they are stopped, it is stopped, it is always going the same speed at 60MPH (or any speed) regardless of load. Viewed from the perspective of M/G2 your Prius has a one speed transmission with no clutch.

    Motor/Generator 1 has a function as your car's starter, and, frequently, as your car's alternator, but is used by the computers to help adjust engine speed. By spinning plus or minus zero to 10,000 RPM, it can force the engine RPM to be what the computers believe is ideal for current conditions.

    The ratio between M/G 1, the internal combustion engine (ICE), and M/G 2 is fixed (no gears ever shift) and there is no clutch. (the gears always turn when moving) Yet Federal law requires a N gear, and there is not one. Toyota emulates Neutral by cutting power to both M/Gs (with nothing to push against, the engine can never move the car)

    Since neither M/G has power, no electricity is being generated in N.

    Toyota Prius - Power Split Device
    Down the page, is an emulator that allow you to change M/G 2 RPM (and so Prius speed) and engine RPM. If the line connecting M/G 2, the ICE, and M/g 1 goes red, you have chosen an impossible combination. (Try leaving the engine off and traveling more than 42 MPH)
     
  7. thymara

    thymara Junior Member

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    I am finding it almost impossible to just nudge the gas pedal to achieve the equivalent of neutral for more than just a second or 2. Once all arrows have disappeared what are you supposed to do next?
     
  8. Jzerocsk

    Jzerocsk Member

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    Try to keep your foot in that position as steadily as you can (it's easier in ECO mode). For Gen3 (and Prius-v) the Energy display often doesn't show no arrows for very long, but the consensus from those with scangauges seems to be that even 1 bar above zero on the HSI generally works out to a glide.
     
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  9. fuzzy1

    fuzzy1 Senior Member

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    In a GenIII liftback (sorry, I'm not familiar with the 'v' displays), don't bother with the arrows. (It actually is impossible at some speeds.) Instead, use the HSI display, and aim to cut the HSI bar close to zero. Close is good enough, there is no need for 0.01% accuracy.

    The 'no arrows' display is useful in the GenII, where it is easily achievable. But for some unknown reason, Toyota cut the 'close enough' window in the GenIII display down to a very impractically narrow spread.
     
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  10. wstt

    wstt Member

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    For this glide (supposedly equivalent to neutral) if you are going 50+ mph is the ICE on? It sounds like I'd be giving it gas by my lightly pressing the pedal which defeats the purpose of neutral, right?
    Finally, I've tried to attach a pic of the prius v dash, where would this neutral equivalent glide be?
     

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  11. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    On my v, two green bars is about right. It is not worth my life to study if that is precise.
     
  12. Jzerocsk

    Jzerocsk Member

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    When you take your foot completely off the gas, there is a little bit of regen braking applied to help simulate engine drag as on a non-hybrid car. The HSI will show one pip in the charge area. You're applying just enough pedal pressure to override that regen. At highway speed in this situation, the ICE will be turning (because it has to prevent the MGs from spinning too fast) but no fuel is burning.
     
  13. wstt

    wstt Member

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    The prius and the glide are all new to me. I am coming from manual car where when I'm on a slight hill I would have to keep on the gas in order not to slow down, but by shifting into neutral I could coast with no gas pedal pressure needed. Am i understanding this correctly, that if you put your foot slightly on the gas in a prius, the regeneration wont happen and this also won't give extra gas to the engine even though the pedal is pressed? Are the engine gears not engaged and slowing down the car if you're barely pressing the gas? If you press the pedal a little more, do the gears engage so it will actually slow the car?
    Sorry, but I'm just trying to understand this.
     
  14. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    There is only one gear. It is engaged ALL the time. There is no clutch.
    P adds a pawl not to let the gear move.
    R is electric only, with the Motor/Generators turning 'backwards'.
    N does not send or accept electricity from the Motor/Generators
    B adds real engine braking, so gets lower MPG.
    D allows the Motor/Generators and the Engine to be blended to achieve the best MPG.

    If you imagine a manual 'stuck' in 3rd, with no clutch, you will see that the computers do an excellent job.
     
  15. crpriusv

    crpriusv Junior Member

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    When applying light throttle take your cue about gas usage from the instantaneous mpg reading. As long as it's displaying 99.9 mpg no gas is being used.
     
  16. kithmo

    kithmo Couch Potato

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    Don't you have the picture with the arrows on down on the Nav screen, press the CAR button.
     
  17. Mendel Leisk

    Mendel Leisk Senior Member

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    At lower speeds that's true. Somewhere above 70 km/hr though, the engine HAS to run. The car will persist in showing infinite mileage, but in fact it's idling, albeit using very little gas.

    It's too bad the Prius doesn't display rpm; you can see this with a ScanGuage.
     
  18. DAVEJAN

    DAVEJAN Junior Member

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    If you have the radar cruise control you can set the cruise and when you need to brake, say for a bend, you drop the speed setting down the 5 mph increment. The cruise will hold you at the set speed with regenerative braking, no foot brake needed. We live in Colorado and come down some very steep canyon roads, almost 100% without using the brakes.
     
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  19. thymara

    thymara Junior Member

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    The magic speed is 42 MPH or 67.6 KPH. If you go to this link you can see the relationships between ICE and MG2. You can use the MG2 slider to verify the 42 MPH

    You will discover later on in this thread that the link I have provided is for Gen II and that due to gearing changes the speeds are now different.
     
  20. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    It certainly was in the Gen 1. I suspect it varies in the Gen 2, Gen 3, c, v, and PHV.