Next year, European automakers Peugeot and Mercedes-Benz will introduce the first diesel hybrid cars, which will get about 60 miles per gallon. Peugeot expects to be the first to market with its 3008Hybrid4 in the spring. The Mercedes E 300 Blue Tec hybrid is due out by the end of 2011.

Diesel engines use about 30 percent less fuel than gasoline engines, but automakers have been reluctant to use them in hybrids because they cost more, often adding $1,500 or more to the price of a car. Adding that cost to the premium for hybrid technology, which can run in the thousands of dollars, could make the cars unattractive to many consumers, says Sasha Simon, a product manager for Mercedes. "You're basically combining two price premiums," he says.

The first diesel hybrids will appear in Europe, where diesels are already popular, because fuel prices are high enough for drivers to quickly recoup the added cost. Fuel prices are too low in the United States diesel hybrids to make as much sense for drivers there.

Neither Peugeot nor Mercedes have announced how much the cars will cost. The Peugeot is based on a crossover called the 3008 that costs about $28,000 with a diesel engine. The Mercedes diesel hybrid is based on the Mercedes E250 sedan, which costs about $46,000. It's difficult to predict how much adding the hybrid technology will increase the price of each, since automakers sometimes take a loss on new technology.

The Peugeot diesel hybrid will use 3.8 liters of fuel per 100 kilometers, based on standard European drive test conditions. That's roughly equivalent to 62 miles per gallon. The Mercedes will use 4.1 liters per 100 kilometers (about 57 miles per gallon). These figures are comparable to the Toyota Prius, which uses 3.9 liters per 100 kilometers in European tests. But the European cars are considerably more powerful. The Prius can put out about 100 kilowatts (134 horsepower), while the Peugeot hits 147 kilowatts (197 horsepower), and the Mercedes 165 kilowatts (221 horsepower). Peugeot says the new hybrid will be about 35 percent more efficient than a diesel-powered car with the same performance.

Only minimal changes are needed to incorporate a diesel engine, rather than a gasoline one, in a hybrid. Diesel engines mainly require a more powerful starter and extra equipment for reducing emissions. The starter technology is particularly important for hybrid vehicles, since the engine is turned off whenever the car stops, or when the power demands can be handled by the electric motor alone, and then quickly restarted when the driver hits the accelerator. The new Peugeot will use a starter that the company developed for its upcoming "microhybrids" (cars that use oversize starters and starter batteries to get some of the efficiency benefits of hybrids). The companies can use emissions controls from their other diesel vehicles.