The biggest, and perhaps only problem beside cost with electric vehicles at the present time is range. Though 90 percent of all commutes are less than a hundred miles, or a run of the mill electric vehicle's range, sometimes you just want to go on a road trip, or to visit the in laws or something like that.
Enter the hybridizing trailer.
What this trailer does is add a small engine with a generator to the electric vehicle to make it in essence a series hybrid. The greatest part about it is that when it is not needed, it can be removed and stored. The Chevy Volt on the other hand includes the engine inside the envelope of the car which adds several hundred pounds of weight that needs to be dragged around when it is not in use. It would be better to use a trailer and to use that weight for more batteries to raise the car's electric only range to something a little more impressive than 40 miles. On the other hand, the Volt is very convenient in the fact that it is a stand-alone turn key machine.
Here, you can see a number of important parts. To the left toward the rear is the engine. From what I can gather at http://www.evnut.com/rav_longranger.htm and other sources the engine is a 500cc Kawasaki Ninja engine which is very convenient because it is water cooled and produces good power in a small package. Toward the right rear is the radiator with electric fan. This is most likely powered by the engine's native 12 volt alternator which charges a battery to power its own starter and electronics. On the right side is the electrical box and controller, and in the front is the 9 gallon fuel tank. A very handy feature of this little trailer is its electronically controlled steering feature which steers the trailer for you when backing up since it is so small, you can't really see it in the mirrors.
This trailer has an output of around 20 kW DC which is enough to power the car at highway speeds as long as the trailer has gas. Because the engine runs at peak efficiency and constant load, it gets about the same gas mileage as the Toyota Rav4 gas version would. Look up the Tzero, it used the same kind of trailer, but was much more sporty.
If I were working on a project like this (and I certainly hope to some day,) I would do a few small things different for the sake of utility. Firstly, I would use a small diesel or turbo diesel engine specifically designed for generator duty to maximize efficiency and fuel economy and to be able to use biodiesel, the easiest and most sustainable biofuel to make. Secondly, I would make the fuel tank a bit larger, though how large, I do not know. On one hand, it would be great to have like a 20 gallon tank because I do make cross country trips from time to time, but there might be the possibility of the fuel going stale because not all of it gets cycled through between trips. Third, I'd build the trailer with some cargo space, or make the generator a unit that can be switched between trailers so as to be able to use the cargo space of a larger trailer. It would also be nice to be able to use the same generator as a back up generator for a home renewable energy system by simply plugging it in to your battery bank and hooking into the battery charge controller. I guess that would actually be a benefit for the large fuel tank as well.
These types of systems are how I see liquid fuel being used in the future. For most people, the trailer is only used for long trips, and it could be shared between families like sometimes I've seen done with a boat. You might go to your friend's house and say "Hey, I'm goin' to visit the family next month, can I borrow the Long Ranger?" "Sure bro, just bring it back with a full tank!" Or perhaps you could rent one from Uhaul.
Steps to electric cardom.