hybrid bicycle with electric CVT?

Discussion in 'General' started by NiHaoMike, Jan 6, 2009.

  1. NiHaoMike

    NiHaoMike Well-Known Member

    Has anyone thought about building a hybrid bicycle with an electric CVT? (Basically, it would work like a scaled down version of the CVT in the Prius, with the pedals instead of the engine.) One motor is directly coupled to a wheel, and the other motor has the shaft coupled to the rear wheel, the casing to the pedals, and slip rings used for the electrical connections. Two bidirectional DC/DC converters connect the motors to the batteries, with a microcontroller managing power flow.
     
  2. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi NiHaoMike:

    ___I am actually looking forward to what Honda comes up with in terms of their upcoming hybrid motorcycle in about 2.5 years. IMA on a bike should easily be able to propel from the pack and if the pack is large enough... :D :D :D

    ___Good Luck

    ___Wayne
     
  3. 99HXCivic

    99HXCivic Well-Known Member

    I think the worst thing with electric bicycles is the some state laws limit their speed to a slow 20 mph, - which is a road biker's unassisted cruising speed!
     
  4. NiHaoMike

    NiHaoMike Well-Known Member

    I actually started to think how the controls would work. To ensure it rides like a normal bicycle, the "gear shifting knob" will control the proportion of pedal speed to wheel speed. In other words, the microcontroller will measure the wheel speed and calculate a pedal speed based on the "gear ratio" selected by the knob.
    * If the pedals are rotating faster than that, one DC/DC converter will pull power from the split motor to load down the pedals to slow them down to that speed while the other DC/DC converter will supply power to the wheel motor to (by default) net zero battery current.
    * If the pedals are slower than the calculated speed, the motors will be inactive and the bike will "glide".
    * If the pedals are going backwards, the bike will regen at a rate proportional to the speed of the pedals going backwards. (In other words, pedal backwards to regen!)
    * Optionally, if the pedals are moving while the brake is engaged, use that to recharge the batteries.

    To make use of electric boost, a twist handle will be used to control battery power used for boost just like a regular electric bike. (Optionally, the handle can twist backwards to engage regen, as an alternative to pedaling backwards. It can also allow for regen while pedaling forwards.)

    The display panel will be based on a small video LCD, so the display layout and data will be defined by software. (A cheaper alternative can be a plain text LCD, although small video LCDs are cheap nowadays, often cheaper than some text LCDs! However, text LCDs are orders of magnitude easier to interface than video LCDs.) It can be programmed to look like a panel of meters (either digital or "analog"), a pictorial diagram, a combination, or whatever else you like! What I think would be interesting would be an animated diagram with icons of the wheel, batteries, and pedals arranged in a triangle. Arrows of varying sizes would show the flow of power when in operation (pedaling, boost, regen, and pedal recharge). For additional information, a few meter displays would be used. The diagram can also include the motors to be more accurate, but that would probably clutter it up too much.

    The microcontroller would have to be advanced to accommodate the sophisticated system, so it would likely be something like the Parallax Propeller as it would be well suited for the many threads needed.

    I don't think I'll actually build such a bike as bikes are of very limited use in my current neighborhood, but I will be happy to help with the development if someone else wants to build one.
     
  5. abcdpeterson

    abcdpeterson Well-Known Member

    Hope this helps.

    When I looked around for an electric assist bike I found one that had the ability to do regen. I don’t know where and I can’t find it again.
    But…
    Everything I found on it basically said (just like I found with the Prius) You don’t really want to spend energy into charging the battery.
    If you have the energy to pedal – pedal. That way you get 100% of your energy going to move the bike, without the losses in the charging system.
    When costing down hill your better off keeping the speed and costing farther. You will cost farther than the power you generate would take you.
    One web site said the regen would eat up a lot of your energy and at best you might get 5% charge back form it.

    I might not recommend the regen side but I would spend time looking at the assets side.

    There are a number of electric assist bikes that will allow you pedal or not while the electric motor is in use. You can have the motor help you or let the motor do all the work. If you help you go farther.

    What would be nice is an electric assist that went through the same gearing as the peddles, or had it own changeable gearing. Most electric assist bikes do not have gearing that will change on the electric assist side. With out that gearing you end up with a choice, more high end speed or low end torque.
    Most people go with torque, as hills are where the help is needed most.

    That is where I think you could make the most improvement. On the assets side, giving torque when needed and higher top end to get longer distance when the torque is not needed.

    IF you make a system that will allow for speed you need to be mind full of just what 99HXCivic said.
    If the electric is what gets you past 20mph you need to classify it as a scooter or motor cycle, in most all states. Then you no longer ride on sidewalks or trails only roads.
    You can go over 20mph if it is You doing it and not motor.

    In the end the biggest problem is the battery.
    This is the choice for batteries: Heavy or expensive.


    Sounds like your having fun with the idea. Keep working on it. maybe you come up with something marketable.
     
  6. NiHaoMike

    NiHaoMike Well-Known Member

    It can be designed so electric boost alone cannot accelerate it to more than 20MPH under normal conditions. That'll also reduce the battery requirements.
    The bike will, by default, "glide" just like a normal bike when not pedaled. The twist handle or pedaling backwards can override that on demand. In other words, if you need to stop, you can manually engage regen. The implementation in the DC/DC converter would be simple as synchronous rectified (buck/boost) converters are bidirectional. And in a situation when the load is light, you can engage regen while pedaling to get a little more exercise while recharging the batteries. In the case of a bicycle, high efficiency is sometimes not a good thing. (Especially if you're trying to be as skinny as Shannon Liu or Jennie Chen!)
    What sets my idea for a hybrid bike apart is the use of an electric CVT - effectively a scaled-down version of the Prius transmission. Electric motors perform well both in terms of torque and in terms of speed. In fact, at low speeds, the torque might have to be electronically limited to avoid skidding the wheels. The fact that you can pedal at normal speeds even while the bike is stationary means that it should be able to accelerate quite well even without boost.
     
  7. Elixer

    Elixer Well-Known Member

    When it comes to a bike you want to have direct drive. I have to say that a lot bikes these days have far more gears than I ever really use. It's very very rare that when on a well designed bike I feel like I don't have the gear that I need. 30 speeds is more than enough. A well oiled bike transfers the mechanical motion from the petals to the wheels with >95% efficiency.

    However looking at going electric:

    You're only looking at real efficiency of maximum 90% for the generator and the motor, and if you're really lucky 90% for the AC to DC/DC to DC conversion (depending on what type of generator you use).

    .9*.9*.9 = .73 = 73%

    Your battery is going to sink some current too so I think you'll get no more than 70% of your energy out that you put in, and only when your output power is the same as your input power. When they're not equal you have to take in the charge/discharge losses of the battery as well: ~80% effecient. Keep in mind the numbers I've used are close to what you'll see ideally, you're more likely to see even worse numbers in many cases.

    Then you have to manage the whole system electrically: The "battery charger" has to connect and disconnect the generator appropriately.

    Overall there's definite reason to think about building a regen system as that's a large amount of energy simply lost to the brakes that you can save. However there's absolutely no reason to go through the electrical system to drive the wheels from the petals. You're looking at huge losses, and a complicated system to do something that can be done cheaply and efficiently with a cheap bike chain.

    Motors are generators (more or less) so it shouldn't be complicated to build a system which would allow regen-braking from the existing electrical motor. This is how current hybrids practice regenerative braking.
     
  8. NiHaoMike

    NiHaoMike Well-Known Member

    DC/DC converters easily exceed 90% efficiency. And it isn't a pure series hybrid. The torque applied to the split motor goes on to the wheel (just at a lower speed). Think of a smaller version of the Prius transmission. And if you're looking for efficiency, you try to minimize battery usage. Use it to get boost under high loads and use it to buffer excess energy under low loads.

    I wonder how the electric CVT compares to implementing a mechanical CVT on a bike. Toyota chose an electric CVT for their Prius while Honda chose a mechanical CVT for their Civic, and they're both similarly efficient. I don't see why it would change when scaled down. The electric CVT would have the benefit of being more or less software-defined.

    And I can think of some extras that can be thrown in for fun. One of which would be to use the ambient temperature sensor (used to compensate LCD drive voltages and possibly to improve accuracy of battery meter) to change the clothes on the figure of the person on the animated power flow diagram. (Possibly something like from a bikini in hot (above ~85F) weather to a winter coat in cold (below ~40F) weather.) Also add in some appropriate warnings (heat and ice) as well as change the behaviour of the drive system, such as more pedal recharge in cooler weather and less in hot weather.
     
  9. Xringer

    Xringer Older Member


    We have a 25 MPH operating speed limit on Mopeds here in Mass.
    http://www.mass.gov/rmv/forms/20017.pdf
    [​IMG]
    With a maximum speed capability of 30 MPH..


    So, 20 MPH max for an E-bicycle sounds about right to me..
    [​IMG]

    Maybe 20 MPH is a road biker's unassisted cruising speed but the remaining 85%
    of us bicyclists cruise at half that speed.. And 12 MPH when we are in a hurry.

    And yes, I get very uncomfortable coasting down hills at anything over 22 MPH..
    I know what my stopping distance is.
    And I know that a tiny shard of glass in a tire at that speed can put me into a world of hurt.
     
  10. abcdpeterson

    abcdpeterson Well-Known Member

    well you did get me looking around at bikes again. :)
    Need new battery for mine. :(

    found this.... it may give you idea's
    The Bionx (pronounced by-on-ex) electric bike motor conversion kits
    [​IMG]

    Too spendy for me.
     
  11. Fuzzo

    Fuzzo New Member

    Regen does have some value

    The idea of regen braking is perfectly sound for e-bikes, it's just more complicated and therefore more costly. Not sure with CVT - it seems like it could work, but doesn't seem necessary. Chain drives are well established and extremely efficient.

    On the flat, regen would be mostly valueless, unless you are starting and stopping a lot, in which case, the inertia factor would be high, and the reclaimed energy would be of value. But for hill climbing I can really see the point. Yesterday I went for a ride on my own electric bike (200 watts - the max legal down here in NZ before it falls into the category of bikes requiring registration and periodic safety checks). I headed straight uphill for about an hour and 20 mins, by which time my batteries (and my legs) were pretty much exhausted. I stopped at a cafe on the mountain I had climbed. Then I headed home, running pretty much on brakes for the 10 mins or so it took me to descend.

    I don't think it's true that only a small amount of energy would be captured by a regen system in such a scenario, compared to just using inertia. It would also be a lot safer.

    If you just use inertia, then you basically have to go fast downhill. This means you rapidly hit your friction balance point, since most of the friction increases as you go faster. When you hit that speed, any further inertial energy you have is lost. It doesn't matter if you are descending 100 meters or 10 kilometers - at the bottom you will coast for the same distance.

    But if you have regenerative braking, then it is better if you descend slowly, with the regen on it's most aggressive setting. Then it really would make a difference how far down you were going. The further down, the more you would charge the battery. I could have got to the bottom of the hill with a partially recharged battery and a whole lot more rubber on my brake pads.

    Of course, this is exactly the opposite of what most cyclists are used to doing, because they want the inertia, and the speed itself is also exciting (if dangerous). That is probably a large part of the relative unpopularity of regenerative braking on electrics - it goes against the grain to be slowing yourself down on a hill - isn't coasting fast downhill one of the greatest joys?
     
  12. bomber991

    bomber991 Well-Known Member

    Here's the CvT biking stuff:
    http://www.fallbrooktech.com/Nuvinci.asp

    I think it's a hub gear. Problem is that most EV bikes also use hub motors.

    Anyway ways, regen is doable for EV bikes, but it's much better to just glide.

    Also, top speed is "limited" to 20mph, but there's this one crazy guy on youtube that took his bike up to 50 something mph. Now I don't have the link to his videos but they're on there somewhere.

    And here's the perfect forum for bicycles with motors added onto them:
    http://www.motoredbikes.com/
     
  13. nervousmini

    nervousmini Well-Known Member

    The car dealership I work at sold something called he Ebike about 10 years ago or so - it had some kind of tie in to Chrysler and or Lee Iacocca. They sold probably 20 of these things, and they were a disaster. If I remember right that were about $1000.00. I rode one around the parking lot, it was horrible - the bike geometry was all out of whack to make room for the giant(heavy) battery pack and the drive motor, and so heavy if the battery was dead (it only had a range of like 15 miles) you were better off pushing it than pedaling. I also remember some sort of recall on them because the battery packs were catching fire (how un-chrysler like....)

    I'll keep my old Trek 8000, thanks.
     
  14. NiHaoMike

    NiHaoMike Well-Known Member

    After thinking more about this, I realized that there are some other advantages:
    * Since power flow is managed electronically, it can very accurately measure the effort actually used to pedal the bike. Most bikes estimate energy usage based on speed, but accuracy is lost going up and down hills and with winds. (In the extreme case, a regular bike would think it's using lots of energy when coasting fast downhill!)
    * The "virtual gear ratio" can be changed at different points throughout every pedalling cycle! Since the amount of power that can be supplied to the pedals depends on the angle of the pedals, it can freewheel through (or even motor through) inefficient angles and extract power from more efficient angles a greater percentage of time.
    * When pedalling backwards to regen, the split motor would spin faster than it would if the pedals were still. Which means it would be able to regen at speeds too low for the direct motor to extract any useful power.
    * The bike can be used as a pedal generator simply by using something to hold it stable and adding a connection to allow regened power to power external loads. In contrast, a normal (IMA-like) hybrid would need something to hold it stable and something to allow the rear wheel to spin free. (And of course, a normal bike would need both of those and a generator.)

    One main problem is that near the "fixed ratio point" (the gear ratio if the split motor were locked in place), performance would suffer as the split motor would be turning too slowly. That can be solved by either designing that point to be unlikely reached in operation or by adding a clutch to lock the split motor. Of course, when locked, the accurate measurement and dynamic ratio capabilities would be lost.
     
  15. Xringer

    Xringer Older Member

  16. NiHaoMike

    NiHaoMike Well-Known Member

    I'm sure that one, as advanced as it is, is based on an IMA-like system. Not HSD or eCVT.
     

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