Are Turbodiesel I-4 Engines Refined Enough for Luxury Cars Like the 2014 Mercedes E250?

While German automakers have offered diesel engines in luxury cars for quite some time, most of those engines have been six-cylinders in U.S.-spec variants. We recently sampled Mercedes new 2.1-liter turbodiesel four-cylinder during our 2014 Mercedes-Benz E-Class First Drive, and despite the four-cylinder engine making just 15 hp and 31 lb-ft of torque less than the current E350’s 3.0-liter turbodiesel V-6, the new engine is expected to offer a noticeable improvement in fuel mileage. But we wonder if the downsized diesel engine is right for a midsize luxury sedan.

During our drive, we managed 44 mpg at a steady highway speeds over hilly terrain; an impressive number for a midsize luxury sedan. But turbodiesel engines aren’t without flaws. Due to compression ignition, diesel engines can be rougher than comparable gas engines. Combine that trait with the inherently less balanced nature of inline-four cylinder engines compared to inline-six and V-6 engines and there is potential for more NVH issues inside the cabin of what’s supposed to be a quiet luxury car.

BMW’s 2.0-liter turbodiesel I-4 has also been confirmed for the U.S., though the automaker hasn’t announced which models it will be available. The automaker currently offers a turbocharged gas 2.0-liter I-4 in the BMW 528i sedan. Do modern turbodiesel four-cylinder engines have the refinement needed for midsize luxury sedans? Tell us what you think below.

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