A car that can go 80mpg on renewable fuel such as soy and canola would seem like an ideal solution to oil prices bumping around historic highs of $50 a barrel.
In fact, the technology already exists in the form of so-called diesel hybrid vehicles, which yoke a conventional diesel engine to an electric motor and battery to store unused energy for clean and quiet driving at lower speeds.
But car firms say such cars are unlikely to move out of the research lab any time soon, even as fuel efficiency becomes a must for more and more customers appalled by prices at the pump.
The main problem is that diesel hybrid cars cost too much to produce - thousands of euros more than petrol-electric hybrids such as Toyota's Prius, which has proven a huge hit with motorists.
Standard diesel engines burn less fuel than petrol engines. Hybrid technology makes them even more frugal by letting cars run on stored electricity captured during braking and coasting.
But a diesel engine typically costs around 10 per cent more than its petrol-driven cousin of similar power, even without the cost of adding an electric motor, batteries and the electronics to run them.
Whether consumers are ready to pay a big premium for super-efficient cars remains open. Toyota which produced a diesel-hybrid truck last year doesn't produce a passenger-car version.
A Bernstein Research study estimated that hybrid cars able to go 20 miles on electric power alone would still cost 20 per cent more than conventional cars, even if volumes rose to 100,000 a year by 2010.
Fuel savings would recoup the price premium only after 300,000 miles - more than twice the life expectancy of most cars.
Toyota has questioned whether it is viable, practical or sensible to sell diesel hybrid passenger vehicles in the foreseeable future. Yet many carmakers are pressing on. VW will enter an experimental diesel hybrid at the Michelin Challenge Bibendum in Shanghai next month, a contest for alternative powertrain vehicles.
"The technology is there and has been well known for years," said a VW spokesman. "The problem with hybrid is the battery technology." He pointed out that nobody yet has come up with a hybrid battery that lasts as long as the car it powers.
Reuters