Bi-fuel cars are now available from many manufacturers, including Ford, Volvo and Vauxhall. Specialist firms can convert other makes to bi-fuel operation. Such vehicles are not true hybrids, operating as they do in 'either/or' mode.
Similarly, prototype vehicles that use high efficiency internal combustion engines driving on-board generators to charge batteries which then drive electric motors are not true hybrids either.
The rationale for this set-up is the elimination of lengthy charging from the mains, unlimited range, greater efficiency from the constant speed internal combustion engine and the recovery of energy during braking. Drawbacks include the need for a large battery pack if reasonable performance is required. A potentially more efficient design is the true hybrid.
As developed by Toyota for its (heavily-subsidised) Prius model launched in Japan last year, hybrid power comprises an internal combustion engine linked to both a generator and the car's wheels via a planetary power splitter.
This allows the engine either to charge the relatively small battery pack or to help the electric motor drive the wheels when extra performance is required - accelerating away from traffic lights, for example - or both, as power demands vary. Toyota claims the Prius can out-accelerate a conventional car while using half as much fuel in city driving and with 80-90% less emissions.
In the longer run, experts believe even higher economies could be achieved by replacing the current piston engines with high speed gas turbines powering high speed generators. These gas turbines, based on current turbocharger technology, could well be running on gas, even hydrogen.