I'm coming back around to this idea, having looked at it a while ago when I was thinking of building a hybrid velomobile, as Miles has mentioned.
The first thing we need to do is to try and estimate just how much power we actually need. For a true hybrid, with range only limited by the quantity of fuel carried, then I think the answer is probably around 200 to 300 watts for a bike, perhaps two or three times this for something like my electric motorcycle project.
200 to 300 watts is pretty easy to do, I think, if we can find the right components at a reasonable price. The GX25 is a good starting point, as it delivers about 750 watts when run flat out. Running it at about 1/3 throttle should make for a reasonably quiet and efficient unit. The down side of running at part throttle is the lower rpm, which makes finding a suitable motor to use as a direct drive generator tricky.
If we opt for using the motor/generator to start the engine, then the clutch and pull start can be discarded, saving 1/2 kg or so. It might well be worth stripping off all the casings as well and making a new housing to both quieten the thing down further and direct cooling air to the right areas.
I've looked around at potentially suitable model aircraft motors and few seem to have a kV low enough for direct drive. Assuming we need around 50V or so available to charge a 36V pack fully and that the engine runs at around 4000rpm, then we'd need a motor with a kV of about 80, which seems too low for anything I've found so far.
The way around this small problem might be to drive the motor/generator via small belt reduction, perhaps around 3:1. An alternative might be a model-type gearbox, I believe that these are fairly readily available with the right sort of ratios although I don't know how noisy they are, or how they would tolerate being run backwards. This gets us up to a required motor/generator kV of about 240 or so, which is far more promising. It would also allow for easier electric starts, as the motor wouldn't be required to deliver as much starting torque.
A brush motor, rather than brushless, would keep things simple but suitable motors seem hard to get hold of, based on a hour or so of happy Googling. Brushless motors of around 240 - 260kV, with the right sort of current ratings, are fairly easy to find. This
http://www.hobbycity.com/hobbycity/...uct=6773&Product_Name=Turnigy_e-drive_5335/10 motor looks OK, and costs $56. It has a kV of 230 and a current rating of far more than is needed for a small generator.
The snag with using a brushless motor is that the electric start option is harder to rig up, as a brushless controller needs to be switched in to the three phase wires, via a fairly hefty three pole relay. The advantage is that the motor should make a slightly more efficient generator and also that rectifying three phase (dead easy with just six diodes) will give a smoother output to the charging circuit, I think.
Some sort of control system will be needed, not just to start/stop the generator as required, but also to regulate the throttle with varying demand from the generator. It may be that some form of simple speed governor might be good enough, but I think I'd be inclined to use a model aircraft throttle control servo, driven by the control system. That way it should be possible to create a system that works pretty much the same way as a hybrid car.
I've got too many projects on the go to spend too much time on this now, but I may well start collecting bits as they come along to make a possible addition to my electric motorcycle. It'd be neat to have a proper hybrid two wheeler.
Jeremy
