If you want to maximize you fuel efficiency in the city, your best bet is probably a hybrid. On the flip side of the coin, if you do most of your driving on the open road, diesel engines tend to be much more economical than their gasoline-powered siblings. What if you could combine the two, though, and end up with the best of both worlds?

While it would hardly be the first automaker to embark down that path, Autocar is reporting that General Motors is working on a diesel hybrid powertrain that could see production in various Opel and Vauxhall models (like the Astra and Insignia) sometime after 2012. Sounds good, right? Of course, there are problems to work out.

One thing that's holding all major automakers back from diesel hybrids are both development and production costs, which can be substantial since both diesels and hybrids already carry a premium over gasoline-burning engines. Maurizio Cisternino, GM of Europe's advanced technology chief engineer, has reportedly told the UK publication that the automaker is aiming for a premium of 1,000 euros (£880 or about $1,400 U.S. at current exchange rates) for the diesel hybrid over traditional hybrids, something that would be quite an achievement.

If the hybrid powertrain does come to fruition, expect it to be paired with GM's upcoming 1.6-liter turbodiesel four-cylinder, a special version of which is said to be prepared with computer controls that are compatible with The General's hybrid systems. It's also possible that a diesel engine will at some point be coupled to the automaker's Voltec powertrain as seen in the Chevrolet Volt and Opel Ampera.

[Source: Autocar]