French automobile maker PSA, responsible for the Peugeot and Citroen brands, has just sold the 50,000th example of a drivetrain we've not yet seen in the U.S.--diesel hybrids.

PSA launched its diesel-hybrid system at the beginning of 2012. It's used in four different models on sale throughout Europe--Citroen's DS5 Hybrid4, Peugeot's 3008 and 508 Hybrid4, and the Peugeot RXH, a diesel-hybrid wagon.

The system is a through-the-road hybrid. In contrast to a traditional hybrid where gasoline and electric power are combined and sent through the same set of wheels, through-the-road hybrids use a different powertrain at each axle.

In PSA's case, that means a 2.0-liter turbocharged diesel engine at the front axle (with power sent through an automated manual transmission), and an electric motor setup at the back.

With 167 horsepower to the front wheels and 37 bhp from the electric motor, combined power is 200 horsepower--making the diesel hybrids some of the more rapid options in their respective ranges.

The benefits for European drivers are mostly tax-related. Diesel engines ensure strong highway economy, while the hybrid drivetrain allows greater city mileage, for a high combined figure and corresponding low CO2 output--under 100 grams per kilometer, in most cases.

As a result, sales as a proportion to the models they're based on are high. In the Citroen DS5's case, one in every three models sold is the hybrid.

We've tested both the 3008 Hybrid4 and the DS5 hybrid. We rate both models highly--though each uses an automated manual transmission on its diesel engine that lacks the smoothness of traditional automatic or continuously variable transmissions.

It's unlikely we'll see such a drivetrain in the U.S. for some time though, let alone 50,000 of them--and not just because PSA has no plans for U.S. sales.

Other companies, including Mercedes-Benz and Volvo, also offer diesel hybrid models--the latter a plug-in diesel hybrid. But the costs of producing them are high and the U.S. lacks the strict tax limits and fuel costs that make such models popular with European consumers.

In effect, the models wouldn't be shown in their best light by EPA ratings, and the high prices would deter consumers--payback would be slim to non-existent for most.

Throw in diesel's limited nascency in the market and the resulting sales simply wouldn't make it worth the manufacturers' time and effort. For the consumer, they simply don't make as much sense as you'd think.

Unless Land Rover changes its mind and brings its Range Rover Hybrid stateside, U.S. buyers are just going to have to stick with regular gasoline hybrids, or non-hybrid diesels, for the foreseeable future.


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