Low MPG, Why?!?

Discussion in 'Prius c Fuel Economy' started by Fastfret7, Aug 27, 2019.

  1. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Okay now I see what you’re getting at. Yes, the gas engine and electric motors are mechanically connected to the wheels. This occurs at the time of vehicle assembly. That status does not change until somebody bothers to disassemble the vehicle.
     
    #21 Leadfoot J. McCoalroller, Sep 25, 2019
    Last edited: Sep 25, 2019
  2. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Sam, in D, the engine is resisted by one of the two electric motors; this provides resistance to push against, and it can assist in propelling the car.

    In N, no gearing changes*, the computers do not power either electric motor. With nothing to push against, the engine cannot help propel the car.

    This is not a change in how the engine is geared, unlike most cars.

    *the parking pawl in P is the only physical gearing change ever, the rest is software.
     
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  3. sam spade 2

    sam spade 2 Senior Member

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    I guess I still really don't understand.
    It seems to me that would imply that any time the wheels are turning, so are the motors and the engine.
    Which would further imply that there is no real NEUTRAL.

    Turning the motors shouldn't be a problem since their resistance would be pretty close to zero when not energized.
    But trying to turn the engine when it is OFF seems like a problem to me.
     
  4. PriusCamper

    PriusCamper Senior Member

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    Show me the actual law that says that... Until you do so it's just another SamSpade preposterous unsubstantiated claim.
     
  5. sam spade 2

    sam spade 2 Senior Member

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    It is not "preposterous" and is not "unsubstantiated".
    I will not do the research necessary to educate YOU however.
    Do it yourself.

    The reason is: With no engine breaking to help slow the vehicle down, the brakes can overheat and become mostly worthless in less than a minute. At that point, most drivers would be afraid to put it back into gear......and you have a run-away vehicle and likely a really NASTY accident.

    Try a Google search on "coasting downhill in neutral".
    You might actually learn something.
     
  6. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Correct.

    Motor/Generator 2 is going to turn if the wheels turn, just as always. M/G1 and the engine are more random; if the engine was off when you went into N, nothing can start it. M/G1 acts as the starter and it gets no power in N. If the engine was on, then nothing can stop it. (it hurts my brain that being in N may mean the engine idles unnecessarily) It mostly does not matter what M/G1 is doing as it is freewheeling, without resistance.

    Avoid N, it is a silly place.

     
  7. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Yep, you’re starting to get it.

    Jimbo’s explanation is quite good- here’s another take on it, just in case.

    Your statements are absolutely correct. If the wheels are turning, then MG1 and/or the engine is also turning. There is no real neutral.

    I’ll start with the idea that you already understand how a differential works in the classic sense, like an old 1940s truck. A driveshaft on the front connected to the transmission, and 2 wheels on shafts on either side touching the road. 3 shafts. If you do something to one of those shafts, the torque will flow through to some combination of the other two, right? OK.

    The Prius transaxle is very much like this- except the engine is connected to one shaft, MG1 to another. MG2 and the wheels are on the third.

    So… you are in your prius at the top of a hill and you start down. The wheels turn. MG2 is carried along because it is directly coupled. This is turning one of the shafts into the differential. This means that all of that rotational energy must go out to the other shafts- maybe all to one, all to the other, or some random split of the two.

    Now in the truck axle example- say both wheels are on the ground, and we will assume that each wheel gets traction. That means each one is ready to push back on any torque imparted on it. So if you spin the driveshaft, the truck moves. The road is the medium providing that balancing force pushing back on both wheels. If you lift one or both wheels off the ground, there is no balance, nothing to push back against, and the lifted wheel just spins in the air when you try to drive, right?

    Ok, back to the Prius rolling down the hill. We have rotational force coming in through one of the shafts- the one connected to the wheels and MG2. That force is going to [engine + MG1.] When you are in Drive, there is an electrical circuit between MG1 and MG2. Regulating the current flowing through that circuit allows you to steer whether the incoming torque goes more easily to MG1 or the gas engine. That electrical circuit is fulfilling the same role as the road in the truck axle example.

    If you put the Prius in Neutral, it simply shuts down the power to both MGs. Energy-wise it’s just like lifting both tires off the road in the truck example. So now the Prius is in neutral, rolling down the hill. The rotational energy is being randomly passed through to both MG1 and the gas engine, with no control. Whichever one spins easiest winds up going faster- nothing on the car can or will attempt to regulate the speeds of either device while you are in Neutral, so a dangerous over-RPM is theoretically possible.

    Leave it in D and feather the pedal going down that same hill- now you get exactly the same mechanical result: The wheels+MG2 will cause both MG1 and the gas engine to rotate. The difference is that you and the car’s ECU get to decide how to steer the torque- more to MG1 or more to the engine? This way MG1 can be kept from overspeeding, and the engine can be employed for traditional engine braking, just by controlling the electrical current between MG1 and MG2.

    It’s really weird and it took me a while to wrap my head around it… but I love it! No clutches, drums or synchros to wear out like in traditional transmissions. Just a single gearset that gets meshed at the factory and split apart by the crusher, hopefully with a long service life in between.

    Hope this helps!

    Edit: adding- the only reason the Prius even has an “N” gear is because there is a federal law that says automatic transmissions in USA cars must have one available on the shifter. They basically had to hack this power cut in as a kluge for that law.

    Japanese-market Priuseses don’t bother with it, because it really is useless.

    Edit 2: (sorry gotta snark somewhere)
    -said every Lucas brand starter motor
     
    #27 Leadfoot J. McCoalroller, Sep 25, 2019
    Last edited: Sep 25, 2019
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  8. sam spade 2

    sam spade 2 Senior Member

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    Thank you.
    I think all of that means that the coupling is through a set of planetary gears.
    But until your post, I hadn't really considered the overall effect.

    P.S. In the models without an N position on the shifter, would it then be true that just releasing the parking pawl with the engine OFF would effectively be a neutral ?
    Then if you needed to push a dead one, how would you accomplish that ??
     
  9. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Flatbed Tow Truck.
     
  10. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Yup. MG1 is the sun at the center. The gas engine is coupled to the planet ring, and MG2+wheels are connected to the outer ring.

    It would be more accurate to say that in all models, the only mechanically defined states are:

    Parking pawl on = Park
    Parking pawl off = Reverse, Neutral, Drive & Brake

    …because the differences between Reverse, Neutral, Drive & Brake are all electrically defined.

    How dead is the dead one you are pushing? Somehow you must release the pawl, either with the electric system normally used, or maybe with a mechanical override (I’ve never looked for one but it may exist)

    If the gas engine is seized, then MG1 will freewheel while you push. If MG1 is seized, the engine will freewheel (yes it will resist considerably because of compression) and if both are seized then you’ll have to lift or drag the car.
     
  11. Matt H

    Matt H Member

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  12. PriusCamper

    PriusCamper Senior Member

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    Lol, what a busy body you are... I'm in Washington state not Oregon... Did you look at the law and notice that it was written in 1965 when brake technology was incredibly primitive and lacked redundancy and failure was more common? And how are they going to know I'm in neutral if I was? Are they going to drive next to me and try to see my dash board?

    Congratulations on doing so much hard work on something that's totally irrelevant and devoid of context of the advancement of 55 years of braking technology.

    And maybe if there was a major accident and at the time of it my Prius was in neutral, but a 2007 doesn't record real-time data and a major accident would probably ground fault the hybrid system, which would find the car in neutral when the accident investigation was done; regardless of what position the shifter was in at the time of the accident.

    And perhaps the stupidest part of this whole discussion, I don't have a need to go down hill in neutral more than once every couple years at most. But hey, if it feeds the online trolls and gives them a job because they don't have a real one, maybe the point is worth making, right?
     
  13. dubit

    dubit Active Member

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    You are aware that your the one that asked for "proof". Doesn't matter if it was written in 1965. If it's still the law, it's the law. For what it's worth, I remember learning this in Drivers Education 40 years ago. Oh, and go ahead and coast down that hill in Neutral. Find a REALLY big one with a sharp turn at the bottom just to prove to yourself the law is stupid and irrelevant because of advancement in brake technology. I've seen toasted brake pads/rotors. But of course your the most knowledgeable here on the subject. So when you get done with that test, let us know how it turns out.

    Funny you talk about online trolls. PC, I've seen you do it here more than a few times. You may not realize your doing it, but many times it comes off that way. But that's cool. I enjoy trolling from time/time myself. Especially the snowflakes on twitter. :p
     
  14. PriusCamper

    PriusCamper Senior Member

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    If you're implying the car handles a bit more light on a corner when it's not in gear, that's true and sometime that's ideal... When it comes to non-prius cars and the obsession of hyper-miling / getting every last mile possible out of a tank of gas, a 5 speed transmission is ideal. In fact the entire art of hyper-miling depends on a great deal of time coasting with the car out of gear. And if you think that's unsafe, you don't understand how slow people who hyper-mile drive most of the time.

    Of course when it comes to Prius, high MPG is how the vehicle runs in the first place so there's not much you can do other than a little pulse and glide to improve that. There's not real need to run a Prius in neutral.