Three-cylinder engines are all the rage right now.

Once the preserve of basic economy cars, they're now used in everything from the latest generation of economy vehicles, through sporty hatchbacks like the 2015 MINI Cooper and Ford Fiesta 1.0 EcoBoost, to BMW's i8 plug-in hybrid sports car.

Mercedes-Benz though will not be joining the three-pot party, at least for its more traditional offerings like the C-Class and the models above it.

The automaker told Autocar that three-cylinder units have been considered, but rejected as they have "too many compromises".

Head of engine development Bernhard Heil believes that the fuel savings aren't that great, and refinement can be an issue--particularly at the lower engine speeds most beneficial for fuel efficiency.

Part of Mercedes' current fuel-saving strategy is designing engines that can operate at as low a speed as is practical--a term dubbed "down-speeding".

While balancer shafts can be used to smooth out the uneven pulses generated by a three-pot engine, these add weight and cost, negating many of the advantages of using a smaller, lighter engine in the first place.

As cost and weight are pushed up, the benefits of using three-cylinder engines to power executive sedans like the C-Class are fairly slim--and many customers may be better served by diesel powerplants or downsized four-cylinder units.

Instead, Mercedes may reserve smaller three-cylinder engines for its A- and B-Class front-wheel drive ranges. There, says Heil, packaging advantages could allow Mercedes to add an electric motor/generator as part of a future hybrid application.

A 3-cylinder B-Class would compete head-on with BMW's recently-launched 2-Series Active Tourer, itself a front-wheel drive, raised hatchback with a turbocharged inline-three engine option.


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