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Automotive Conversion of CVT Scooter to Hybrid

  1. May 23, 2017 #1
    Hello friends,

    My team is working on a hybrid bolt on setup for a competition. We will be given a 110cc, CVT driven scooter (producing 7.80 bhp @ 7,500 rpm and 8 Nm @ 5,500 rpm). This is the scooter- https://www.bikewale.com/tvs-bikes/..._kKbSX6goR4u50g2XY4mjq9SqVcNjmKu-9hoCzD7w_wcB
    also, https://www.tvsjupiter.com/techspec
    (Competition is organised by SAE India in association with TVS)

    We have come across quite a few problems, and since we are all new to this, we aren't able to solve it, so I thought I would ask here.
    1-- Does mating an electric motor to a CVT reduce its efficiency? What transmission would you suggest?
    2-- Why do motors often use direct drive? (single gear ratio) Or do they?
    3-- Is it possible to couple the 2 power sources (engine+ motor) using any transmission? How would planetary transmission work? Is there a way?
    4-- How are the power and torque characteristics of an electric motor? Is it like constant torque throughout the RPM and max power at max RPM, or otherwise? (I could be wrong)
    5-- The rules state that power during electric motor mode should NOT go to the engine crankshaft. So, we have decided to add an electromagnetic clutch at some point in the CVT where centrifugal clutch is located (so that when in EV mode, if we use direct drive to rear wheel, we don't have to worry abour CVT running alongside and reducing efficiency). Thing is, we aren't sure of how it would work out, and if it would even work in the first place, because CVT is a tight fit, and messing with it is risky, given our limited budget. Any idea whether it would work?
    And does anybody know any other ways of disengaging power?

    Thanks in advance.
  2. jcsd
  3. May 25, 2017 #2


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    Most of your questions 1-4 can be answered with some internet searches, what information have you found so far? I think you will get more useful answers if you ask specific questions one at a time after having done some basic research yourself.

    Not sure if there's a question in #5 or not, but I would emphasize trying to keep the application as simple as possible. Electromagnetic clutches and the like seem complicated and difficult to implement if you need to try modifying the existing transmission. Have you considered keeping your hybrid system totally separate and just depending on the centrifugal clutch to disengage when the engine is idling? When you say "bolt on" what is the scope of that requirement? I would think of it as "simple to install with minimal work" in which case modified transmissions and clutches don't sound "bolt-on" to me...
  4. May 25, 2017 #3
    Thanks for replying.

    1-4 were not answered satisfactorily. We have been at it for about 2 months, no answer specific, or even close enough to solve our problems. They just confused us further. We know that a direct drive is used in 2 wheelers, but we aren't sure how to make the gear ratios- motors have constant torque and power modes. Ideally, a CVT would be good for a motor, but the efficiency reduces, apparently(again, not sure. We have received contradictory replies), so we don't know.
    4 was partially sorted, but we wanted the characteristics for calculation of power and torque requirements before we move on to motor and battery selection.
    3 is the major problem, in conjunction with 5. All need a clutch.

    You see, that's the thing. Centri can disengage when idling ONLY. Rules demand that in EV mode, that's say when you are going at 35+, and you come to 25, which is when we plan on engaging EV mode, then motor takes over and engine must be disconnected completely. And centri clutch won't disengage unless you hit idling. So we need something to disengage at will.
    Ok, maybe using the term "bolt on" was wrong. My bad.
  5. May 25, 2017 #4

    jack action

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    Why would your engine be going faster than idling speed when in EV mode? I would even consider completely turning it off.
  6. May 25, 2017 #5


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    I think the point is the centrifugal clutch will be engaged when accelerating up to 25 mph, at which point it won't be able to disengage if you coast?
  7. May 26, 2017 #6
    Yes. Exactly.
    Centri's are designed to only engage and disengage at idling.
  8. May 26, 2017 #7
    I can kill the engine, but, because it is a centri clutch, which engages and disengages only at engine idling speed, and here it is rotating at speed equivalent of 25kmph, it will stay connected. Fuel supply could be disconnected, but you will still be running the engine crankshaft, and hence moving the pistons up and down, which is parasitic on the motor.
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