Toyota's award-winning Prius petrol-electric hybrid is unveiled at Auto Africa this month for purchase early next year, but what are the chances of diesel-electric hybrids becoming mainstream in the near future?

Toyota's award-winning Prius petrol-electric hybrid is unveiled at Auto Africa this month for purchase early next year, but what are the chances of diesel-electric hybrids becoming mainstream in the near future?

While the technology for these diesel hybrids already exists, major manufacturers maintain that these vehicles are unlikely to move beyond the research lab soon.

As international oil prices continue to surge, fuel efficiency is becoming more important to consumers. The main concern for manufacturers is that the diesel hybrid vehicles would simply be too expensive to produce for the mainstream market.

"Diesel hybrid is one possible propulsion system that we are researching and testing, but no one can say whether this is the path to the future," a DaimlerChrysler AG spokesperson said.

"It is certainly the most efficient method among the conventional powertrains - but also the most expensive."

A diesel engine usually costs more that a petrol engine of similar power, though they do consume less fuel than conventional petrol engines. Hybrid technology makes them even more frugal, capitalising on energy captured while braking.

However, one study estimated that hybrid cars would still cost about 20 per cent more than conventionally powered vehicles even if manufacturing volumes increased to 100 000 units a year by 2010.

A Toyota diesel hybrid truck has been available in some markets since 2003 despite costing about R70 000 more than the diesel version.

Toyota has no immediate plans to produce a passenger car version of the diesel hybrid and questions the viability of producing these vehicles.

"Before adding a hybrid system to a diesel engine, we still think there's room for improvement in the diesel-only and gasoline-only engines in terms of fuel efficiency," a Toyota spokesman said.

Honda also cited costs and the prospects of launching a large-scale production of a diesel hybrid car as its main deterrents.

"It is more difficult to hybridise diesel," Jean-Martin Folz, chief executive of PSA, said recently, "but we are going to show that it is possible." A PSA spokesman said development of a diesel hybrid version of the Citroën Berlingo was underway. The company had not yet decided whether to make it commercially available.

Volkswagen will enter an experimental diesel hybrid car at a Chinese competition for vehicles using alternative powertrains next month. A VW spokesman said the technology has been well known for years. The real problem, he said, was the battery technology as no company has come up with a battery able to last the car's lifespan.