The pursuit of flexibility is a recurring theme in engine design and development. Engines that are able to switch between the two-stroke operating cycle and four-stroke operation promise a great leap in flexibility. Such 2S-4S engines could then continuously select the optimum operating mode - including HCCI/CAI combustion - for fuel efficiency, emissions or specific output. With recent developments in valvetrain technology, advanced boosting devices, direct fuel injection and engine control, the 2S-4S engine is an increasingly real prospect. The authors have undertaken a comprehensive feasibility study for 2S-4S gasoline engines. This study has encompassed concept and detailed design, design analysis, one-dimensional gas dynamics simulation, three-dimensional computational fluid dynamics, and vehicle simulation. The resulting 2/4SIGHT concept engine is a 1.04 l in-line three-cylinder engine producing 230 Nm and 85 kW. When simulated in the Ford Focus, a European C-class car, the powertrain recorded a fuel consumption of 5.93 l/100km over the NEDC drive-cycle, without the need to use lean combustion. This represents an improvement in fuel efficiency of 24% compared with the series production Focus vehicle equipped with an equivalent conventional gasoline engine. The project has been undertaken with funding from the UK Department of Trade and Industry, as part of the Foresight Vehicle initiative.