A plug-in hybrid is a car which can run off a combination of electricity (stored in batteries) charged from a wall outlet, and some other fuel, such as gasoline.
This vehicle uses ONLY the battery-electric system to start with, and has manual control over when the other energy source is used, in this case, a propane generator.
Using only electric power, and THEN switching to a hydrocarbon fuel only when needed, is sometimes also called a Range-Extended Electric Vehicle, or EREV.
Think of this car as a poor-man's Chevy Volt or Plug-In Prius.
I already converted this Geo Metro to run on electricity. You can read all the details about that at my ELECTRIC CAR CONVERSION INSTRUCTABLE.
This Instructable will detail adding a second power source to the vehicle to extend its range.
I've been driving this car for about two years now as electric. The only thing I don't like about it is that I find that there are a few too many times where my destination is just a little outside my range, or there is no access to electricity at the destination to be able to recharge while I was there.
Adding the second power system to the car allows me to make trips that I would otherwise need to use a gasoline vehicle for.
In a nutshell - the generator makes AC electricity that powers the traction pack battery charger. The charger passes DC electricity into the batteries. By constantly "topping-off" the batteries, they are kept more full and allow the car to drive farther. The generator is NOT powerful enough to drive the car directly from. However, the generator can continue to run while the car isn't using any battery power, such as rolling downhill, or at a stop light.
So, the smaller, but steady power of the generator provides additional range, while the torque and energy of the electric motor and battery pack give the car good power for acceleration and hill climbing that the small generator engine would NOT be able to provide.
Also, engines running at a steady speed at fairly wide open throttle are MUCH more efficient than one running lightly loaded or at varying speeds (such as in a standard gasoline car.)
See more of my clean transportation projects at 300MPG.org