An internal combustion engine runs more efficiently at constant speed. If it was run to charge the batteries when needed, via a generator, it seems to me it would be much more efficient than today's dual mode hybrids. This is essentially what diesel-electric locomotives do, and they get much better gas mileage than cars.
When there are changes in the electrical load on a generator then the power required to drive the generator also changes. So the gas engine wouldn't run efficiently at a constant speed unless the car runs at a constant speed. Diesel electric locomotives usually run at a constant speed. They don't stop and go as often as a car does so the diesel electric configuration works. Listen to a train that is accelerating and you will hear the change in engine pitch due to the higher energy demand.
The next generation of Hybrid vehicles are being designed to do exactly this. They are referred to as REEVs, or Range Extended Electric Vehicles. Essentially, they are a fully electric vehicle, like a Tesla roadster. Electric vehicles have greater low end torque than internal combustion engines, and are inherently more energy efficient (more energy used goes toward locomotion rather than waste heat) To extend the range of these electric vehicles beyond just plugging them in to charge the batteries, they have an on board generator that can be set up to run on gasoline, diesel, bio diesel, or ethanol. They also take advantage of the same regenerative braking systems that are already being used in current hybrid vehicles. REEVs are pretty much awesome. If you're looking at getting a new ride, I'd recommend waiting a few years and getting one of these.
Currently Volvo is experimenting with this idea, using the additional caveat that the car use rim motors to ditch the heavy and complicated transmissions. The problem is that you are limited by your motor as to power. Generally an hybrid is a large electric motor(say 50 hp) and a small gasoline engine(also about 50 hp(ease of calculations)). When you want to accelerate faster than 50 hp can provide, both engine and motor work to drive the wheels(combined 100 hp). The motor then can be used as a generator, as mentioned by other answers. There is limited space in a car, and you would then need a more expensive, larger, heavier (100 hp) electric motor if it cannot pair with the gasoline engine. This means more weight for the car, though some would be conserved due to simpler transmission design. As you probably know more weight in the vehicle means less efficiency, all things equal. I am thinking of trying a similar design to what you mention with a motorcycle due to it's smaller power requirements.
Truly it has to do with type of fuel to create energy. So using whatever type of fuel you want to create a specific amount of energy, say to move a vehicle, you end up not considering the type of fuel but the cost of fuel. Seriously, if gas was a couple of cents a gallon who would care about alternate fuels. So as a race we seem to be more satisfied to leave our ability to travel around, not only our countries but also our planet, to the money Barons rather than placing our fuel needs in the hands of our Governments where our tax dollars could be efficiently used to develop human efficient fuels that are FREE. Sunlight comes to mind, seriously how hard can it be with our technology today to develop ways of utilizing this never ending source of vast power. Then there is water made up of H2O and yet we are all aware of how much energy we get from splitting an atom, dah! So how about splitting water into hydrogen and oxygen and utilizing that energy and God hasn't hinted he/she/gay/partner/bi/lesbian etc ( to make sure I am politically correct here, will be stopping the rain anytime soon. Oh rain and sunlight are FREE, hmm, I wonder, is there someone not wanting us all to have free energy. WHY would that be possible?
My understanding is that this IS how some hybrids work. At low levels of acceleration, the battery provides most of the work. It is when the car accelerates hard that the switch is made to use the gasoline engine. It is depends on whether the hybrid has a series or parallel configuration. The hybrids key nitch is stop-and-go traffic. Without it, there are other compact cars that get better gas mileage. The electro-resistive braking is one of the best features of hybrids, because it takes kinetic energy and converts it to electro-potential energy. That means in light acceleration and light breaking traffic, a driver would use far less energy. The engine in Toyota and some Honda hybrids are essentially high-end, high-efficiency generator engines. Engine acceleration is smooth (efficient) as the battery provides a good chunk of the energy in the beginning of the acceleration. Then the computer evaluates current draw, reserves, and current acceleration to set the engine speed. If you suddenly take your foot off the accelerator, the engine slowly revs down, sending the suddenly excess power to the battery. Because of the large battery bank and computer management, hybrids are actually far more efficient for variable demand situations than a standard generator. I don't know much about diesel-electric trains, but diesel is almost always the better bet for mostly constant-rate driving, and gasoline is the best bet for highly variable-rate driving. I can see a line of logic for a bio-diesel hybrid car with a significant battery pack to further smooth out the highs and lows. The link below might help you out.
