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.
In the first generation hybrid car, the internal combustion engine only serves as an on-board generator to supply power to the electric motor which provide the sole driving force to the wheels. In the second generation, the internal combustion engine drives the wheels directly with the electric motor serves as a power assist when extra power is needed and to recapture the kinetic energy into electic energy during braking. The extra power from the electric motor enables the manufactorers to reduce the engine size to achieve fuel economy. Either approach has its limitations. Starting from 2004 model year, the Toyota Prius uses the third generation hybrid design. In this new design, the wheels can be driven by either the internal combustion engine or the electric motor using a planetary gear system to draw power from either source. The on-board computer optimizes the fuel usage by shutting off the internal combustion engine when the electric motor is sufficient to provide the power. The internal combustion engine starts up whenever extra power is needed or the battery needs recharging. The electric motor serves as the main driving force and a generator. The more efficient new design enabled Toyota to build the new Prius as a mid-size car without sacraficing fuel economy.
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. (In 2004, they introduced the world's first mass-market hybird SUV, the Lexus RX 400h). It appears many European companies, where diesel is much more common, will follow Toyota's lead and move 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%.
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Switched-mode power supply; SMPS - low voltage