Your average 24v 20AH battery pack can recharge at 0.5c (that's 50% charge in 1 hour, so 2 hours = full charge). That's 24v @ 10A. Grab a pair and wire in series to get 48v 20AH, still recharges at 0.5c (so 48v 10A, or 480 watts). a 500 watt motor will use about 11A. a 48v 1000w generator will recharge at about 22A. 10A for the 500 watt motor, 10A for the batteries, full charge in 2 hours.
A 48cc engine gets roughly 2 hours runtime per gallon at top speed, but geared with a jackshaft (because the 1000w generator will ask for about 2200 RPM, and if you gear 4:1 that's 8800 RPM, don't want to redline it for so long). Port and build a pipe to get your max power at 4400 RPM. 1/2 Redline on a standard carb should net you more than twice the runtime (because gas intake at 1/2 throttle is less per stroke [thanks to the needle] than intake at full throttle per stroke). But we'll assume 2x for easier maths. so 4 hours per gallon and 2 hours to recharge the batteries fully, giving you 2 full recharges. 2 recharges while running means 4 hours with the engine on, 6 hours (initial charge + 2 full charges) with the engine off. 10 hours runtime total. I think the 48cc tuned properly should be able to produce 1.3hp easily.
So 10 hours runtime on a 500 watt motor @ top speed (500 watt output is full throttle, full stop acceleration, so worst case scenario, they typically run at 350 watts). is usually around 30mph (45-50kph). So 30 mph for 10 hours is 300 miles per gallon. That's pretty good, but again thats full throttle all the time, a more realistic version would be about 22mph for about 1/2 the power (250w), as to double your speed, you use 4x the power. That ups your runtime to 4 hours per charge, but full recharges still take 2 hours as the batteries can't take 1.0c, but instead of using 1000w to recharge and run at the same time, you're using 750w, which is 1hp. 48cc's can easily make that.
So still 2 full recharges, at 4 hours runtime per charge, gives you 12 hours per charge plus 4 hours during charging. 16 hours total runtime @ 22.5mph = 360 mpg.
But wait, we're not using as much power now. That means you can run the engine a little slower. 3/4 the power output means 1/2 the rotational speed of the generator (most follow a curve where maximum output is where the curve flattens out). So that means we only need to spin the generator 1100 RPM, or the engine at 2200 RPM (remember we're 1:2, not 1:4). again doubling runtime, but again that needle says "use less gas" and the engine says "okay". So 1/2 the strokes at 1/2 the gas means 4x runtime. 4x the runtime of 16 hours? 64 hours. 64 hours @ 22.5mph? 1440 mpg.
However if you plan on using the generator to directly drive a generator for the wheel motors, you're lookin at much, MUCH less power usage on that engine. You wouldn't need to tune the engine, you wouldn't need to drive such a big generator, or as fast. 500 watts is 2/3hp. but again the average use will be much less than 300 watts. If you can control yourself, thats 250 watts, or 1/3hp. But its going to be hard to drive that engine at less than 1200 RPM (rough idle) and still make usable power, as the engines don't come tuned that low from the factory. Yet if you drive the motor at 2200 RPM at 2:1 for the generator to make 1100 RPM, that's still a significant amount of power, a lot of it will be going to waste. Unless you plan on running 2x 1KW motors or 2 500w motors, but the most efficient method would be to run the gas to charge a set of batteries, and use the batteries to power the motor.
But of course real world numbers (I'm just guesstimating) and real-world results could vary these calculations greatly, but I plan on such a hybrid to put the theory to the test
