Carlos Ghosn and Dieter Zetsche buddied up for an alliance between Renault-Nissan and Daimler just over two years ago, and while the duo has been cooperating in the background, today's groundbreaking in Tennessee for a new plant that will build engines for both Mercedes-Benz and Infiniti is a much more concrete step forward.

Earlier this year, the alliance announced the plant, in Decherd, Tennessee, would build four-cylinder engines for use in cars of both brands starting from 2014.

Exactly which engines will be built there hasn't yet been disclosed, but the frontrunner is likely Mercedes-Benz's new 201-horsepower 1.8-liter turbocharged four-cylinder found in the C250. It's rated at 21 mpg city and 31 mpg highway in the C250 sedan, making it a fairly fuel-efficient, fairly powerful choice for either brand. Nissan says the plant will have capacity to build up to 250,000 engines per year.

In the future, however, more engines could be added to the mix, including three- and four-cylinder engines from Renault for use in Mercedes-Benz cars, plus six-cylinder engines and diesels from Mercedes-Benz for use by Infiniti.

Beyond engines and other powertrain elements, however, it's not clear what Mercedes-Benz and Infiniti may have planned. Previous rumors have hinted at a shared compact sedan, or at least its architecture, as well as a small crossover based on the same platform.

With two years remaining before the plant is finished and operational, even Renault-Nissan/Daimler may not have completely finalized their plans. All we know for sure is that four-cylinders will be built in the U.S. for use in both Infiniti and Mercedes-Benz vehicles. Beyond that, we have speculation, but we'll keep you apprised of the latest as it develops.