Due to the future application of combustion engines in small and hybrid vehicles, the demand for high efficiency with low mass and compact engine design is of prime importance. The diesel engine, with its outstanding thermal efficiency, is a well suited candidate for such applications. In order to realize these targets, future diesel engines will need to have increasingly higher specific output combined with increased power to weight ratios. This is therefore driving the need for new designs of 3 and/or 4 cylinder, small bore engines of low displacement, sub 1.5l.Recent work on combustion development, has shown that combustion systems, ports, valves and injector sizes are available for bore sizes down to 65 mm. This paper concentrates therefore on the discussion of the downsized engine design and mechanical relationships: the vibration levels are higher for the 3-cylinder versus the 4-cylinder, without additional balance shaft(s). the friction losses show benefits for the 3-cylinder but in the case of added balance shafts those benefits will only be marginal casting demands for a given wall thickness valve train components and injection nozzle cannot be “downsized” below a certain level valve train arrangement does influence the valve train drive system … The aspects are discussed by the aid of a variety of examples from today's engine development programs, so that well-founded conclusions can be derived.The discussion of these different aspects shows, that the 3-cylinder architecture is the simplist, the most compact and the highest fuel efficient engine, however, fuel consumption benefits are more or less marginal, when a first order mass balance system is employed to achieve acceptable NVH-behavior.