I made a generator about two years ago using a 6HP lawnmower engine, it works like this: Lawnmower engine drives GM 94 amp alternator, alternator connects to 12v deep cycle battery, 5000 watt peak/2500 watt continuous inverter connects to same battery.
Stew - that's an awesome science project
A few months ago I hooked up a 2200 watt generator to my truck using a crude 16 amp charging system. My truck at the time had eleven 12v deep cycle batteries. With a drained battery pack the generator would limp my truck home at 17mph on flat ground. Rich, the charger Guy, took a look at my charger circuit diagram and told me it was 50% inefficient and that with an SCR and choke coils of the right size it would be much closer to 100%. Let's say I did this and it made my truck go at least 25mph on flat ground. Now take the same system and put it in a vehicle that weighs close to 1/3rd the weight of the truck and is much more aerodynamic, (a GEO Metro with very few batteries(120lbs) and a 60lb generator). I HAVE to build it and find out if it really works. If anything I think I could hook up the 6hp engine to the drivetrain and do 60mph easy, but then it wouldn't be a "magical" hybrid.