Jaguar engineers are currently testing a new version of the XK in Germany that has some very peculiar traits, leading to suggestions that the car could be a prototype for a new diesel model or possibly even a hybrid. A number of XK prototypes have been spotted in recent months but most of these were for the 2009 facelift model expected to be revealed for the first time at next month’s Paris Motor Show.

The car seen here is something very different. For starters, it has a different bumper to the one seen on the recent facelift prototypes. This one appears to have additional air intakes and no fog lights. The car also sports a vented XK-R bonnet but at the back there is only a pair of standard exhaust pipes. Onlookers also claimed that it ran much quieter than any other Jaguar XK.

There are two schools of thought as to what is lying beneath the car’s long hood. The mostly likely option is that Jaguar engineers are testing a new diesel powertrain that is expected to be offered in the XK next year. Several reports have speculated that it will feature a newly-developed 3.0L turbodiesel V6 based on the current 2.7L unit.

The other option is that this particular XK is a development test-mule for a new hybrid powertrain planned for the next-generation XJ saloon due in 2010. Jaguar has formed a partnership with Land Rover as well as the British government to speed up the development of emissions reducing technologies, and one of the main avenues for research is a new hybrid powertrain.