The Kia Optima Hybrid has been the Korean brand's green-car mainstay for four years now, but now Kia is taking its hybrid technology in a new direction--at least for some parts of the world.

Kia is preparing a mild-hybrid version of the Optima, fitted with a diesel engine, most likely aimed at the European market.

The Kia Optima Mild Hybrid concept will debut this powertrain at the upcoming 2014 Paris Motor Show, although Kia hasn't discussed production plans.

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The concept will feature a 1.7-liter turbodiesel four-cylinder engine, teamed with an electric motor and a lead-carbon battery pack.

However, because this is a mild-hybrid system, the electric motor function more as a booster for the internal-combustion engine--it can't move the car under its own power alone.

Kia will likely wait until the Optima Mild Hybrid's Paris debut to release efficiency figures, and those will be quoted using the more-optimistic European testing cycle.

2013 Kia Optima Hybrid

2013 Kia Optima Hybrid

The new concept picks up where Kia's previous diesel-hybrid concepts left off. Unveiled at the 2009 Frankfurt Motor Show, they were also based on production vehicles--in this case a Sorento crossover and Cee'd compact hatchback.

Those vehicles never advanced beyond the concept stage, but if the Optima Mild Hybrid makes it to production, it will join a very small group of diesel hybrids.

Peugeot and Citroen currently offer a handful of diesel-electric "through-the-road hybrid" models in Europe, where the Mercedes-Benz E 200 BlueTEC Hybrid is also sold.

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A Mazda 3 hybrid is also planned, with the company's "Skyactiv-D" four-cylinder diesel combined with a hybrid system from Toyota.

Diesel hybrids will probably remain a small niche, though.

While the marriage of more-efficient diesel power with an electric motor might seem ideal, several factors make such powertrains less impressive in practice than they are in theory.

Among the challenges: the higher cost of the diesel engine, and the similar torque curves of diesel engines and electric motors--versus the complementary curves of high-revving Atkinson-Cycle gasoline engines and electric motors.


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