How to size a generator needed for an electric motor

[pengyou]

I will buy an electric bicycle in a couple of months and want to modify it. I will install a motor that has a max rating of 6,000 watts - common use is about 3k watts. I am also looking for a small, lightweight gasoline engine to connect to another motor - to make a small generator - so that I can use the generator to charge the batteries when they are low, or simply to power the electric motor on the ebike. You can think of this bike as being a tri-brid, being able to pedal it, use the battery/motor combination or use the generator/motor combination. The motor I will use on the bike was designed to be used in r/c aircraft and is often used now in ebikes. In the ebike it turns at about 4,800 rpms at 48v. I saw a gasoline powered 35cc 4 stroke motor that put out 1.4 hp at about 8,000 rpms. It weighs about 9 pounds. I would love to find a small rotary motor, weighing about 5 pounds :)

I would like to be able to find a gasoline engine + electric motor combination to make this generator so that

electricity produced by gasoline engine + electric motor = (1.1 or 1.2) times the electricity needed for the motor on my ebike.

Can someone point me to the equations and principles I would need? also any advice that you might have?

Thanks in advance

 

[MisterX]

It seems wasteful to have a separate generator and electric motor for a "tri-brid" bike.

http://en.wikipedia.org/wiki/Alternator#Hybrid_automobiles"

 

[pengyou]

Yes, in a way it wasteful but I am doing it to stay legal with the local laws. In Shanghai, it is almost impossible for a foreigner to get a license plate for a motor driven vehicle. If I made a hybrid ala prius I would simply use the gas engine to turn the wheels at times, but then it becomes a "motor drive cycle" and cannot legally be licensed and used on the streets. With my idea, the bike is always either an electric motor driven cycle or a pedal driven cycle so I am within the law.