In the interest of fuel efficiency, luxury manufacturers are replacing six-cylinder engines with forced-induction fours. BMW has already made the switch on its new 3-Series cars, and the 2012 Mercedes-Benz C250 models sport a turbocharged 1.8-liter four.

If luxury buyers are willing to downsize gasoline engines, it’s likely that they’re willing to give small diesels another look, too. Mercedes-Benz will reintroduce its first four-cylinder diesel to the U.S. market in nearly three decades with the 2013 GLK, which offers up a 2.1-liter turbodiesel as an available engine.

The GLK may just be the start of Mercedes-Benz models with an available four-cylinder  diesel engine. Gunter Fischer, project manager for the Mercedes SL roadster, tells Wards Auto that four-cylinder turbodiesels could also be used to power future C Class sedans, E Class sedans and even the SLK roadster.

In Europe, roughly 50-percent of new vehicles sold are diesels, largely because diesel fuel is less expensive than gasoline. In the U.S., diesel is on parity with premium gasoline in most markets, negating much of the (proportional) cost savings enjoyed by Europeans.

Still, diesels offer up significantly better fuel economy that their gasoline counterparts, and fuel economy is first and foremost on manufacturers’ minds these days. The fact that both the U.S. and the E.U. will have common standards for diesel emissions in 2014 eliminates the excuse that “it’s too costly to certify an engine just for U.S. sales.”

Audi has done relatively well with its 2.0-liter TDI turbodiesel on these shores, as has Volkswagen with their version of the same engine. BMW currently has six-cylinder turbodiesels available in the 3-Series and the X5, and is said to be considering a four-cylinder version for the new 3 Series.

Even Cadillac is on board, as GM officials have committed to giving the new Cadillac ATS a diesel option, though it’s unclear if that will be offered worldwide.

That seems to be a strong enough argument for the proliferation of small diesels in the luxury segment, and it seems more a matter of “when,” not “if” Mercedes-Benz will increase its four-cylinder diesel engine offerings in the U.S. market.