Mercedes-Benz has ruled out three-cylinder petrol and diesel engines for its main range of rear-drive saloons, despite rivals like BMW adopting this fuel-saving technology.

"We have looked at three-cylinder engines, but there are too many compromises, such as refinement and the savings aren’t that significant," Bernhard Heil, head of engine development said at the recent Geneva motor show.

Heil says Mercedes has experimented with three-cylinder engines, but the vibration and loss of refinement, particularly at low speeds when fuel savings could be maximised, are too big a penalty to pay.

Mercedes is gearing its engines to operate at as low a speed as practical, a strategy Heil calls ‘Down-Speeding’. To boost refinement, three-cylinder engines need balancer shafts, which add weight and cost and wipe out many of the advantages of the smaller swept volume.

Mercedes is, however, likely to develop a second family of slightly smaller-capacity diesels with a swept volume less than today’s ubiquitous 2.1-litre OM651 unit. The OM651 is increasingly popular, powering around 70 percent of all rear-drive Mercedes models. Heil didn’t specify a capacity, but 1.9 or 2.0-litre capacities are possible alternatives.

Front-wheel drive models, like the new A-class and B-class, could however, be powered with three-cylinder units. Although these engines would need balancer shafts, Heil says the packaging advantages would allow Mercedes to add a motor or generator unit between the front wheels for future hybrid applications.