A hybrid car is an automobile that uses more than one power source, almost always an internal-combustion engine driving a generator to provide power to an electric motor. In the hybrid design the engine replaces batteries that would normally be used in an all-electric car, thereby avoiding problems with range that many complain about.
Early hybrid designs tended to use the electric motor for all power, due to simplicity. The engine would charge batteries from which the motor drew power, running only when needed to charge them back up, and at its "best power" speed when doing so.
More modern designs reverse this to some degree, using the gasoline engine for primary power, but using one that is smaller than would otherwise be needed. The motor is essentially a very large starter motor, which operates not only when the engine needs to be turned over, but also when the driver "steps on the gas" and requires extra power. Instead of the engine solely charging the batteries, the motor acts as a generator during braking, using the momentum of the car to generate electricity. Thus the energy that would normally be lost when stopping is used to speed the car back up. Since the amount of electrical power needed is much smaller, the need for large battery systems is eliminated. Such designs were released in the late 1990s in the Honda Insight and Toyota Prius, but both were impractical, small designs that didn't see wide consumer acceptance. Newer designs are considerably more conventional, often appearing and performing identically to their non-hybrid counterparts while delivering 50% better gas mileage. The Honda Civic Hybrid appears identical to the non-hybrid version, for instance, but delivers about 50 mpg (US gallons).
One particularly interesting combination uses the newer technique, with a diesel engine. Diesels are excellent engines for delivering constant power for long periods of time, suffering less wear while delivering higher efficiency. However, the engines also suffer from poor acceleration due to having a limited RPM range. This poor acceleration can be addressed with the hybrid technique, and such designs appear to offer normal performance in a car delivering over 100 mpg.
It appears that battery technology will not evolve to address the needs of fully-electric cars in the near future. All companies involved in such research have since given up, and moved to fuel cells and hybrids. Toyota announced in 2002 that their entire lineup would be hybrid in under ten years, and it appears many European companies, where diesel is much more common, are moving in the same direction.
Perhaps surprisingly, the same is true for fuel cell designs. This is because the hydrogen needed to fuel them is typically extracted from natural gas, resulting in the burning of even more of this fossil fuel. However if the fuel cell does catch on the demand for hydrogen would require newer sources, which can be delivered in a number of ways. In addition, the fuel cell itself is considerably more efficient than any sort of internal-combustion engine, typically 60% or better compared to 15 to 20%.
Switched-mode power supply; SMPS - low voltage