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In this paper the development approach and the results of numerical and experimental investigations on homogeneous charge compression ignition in a free piston engine are presented. The Free Piston Linear Generator (FPLG) is a new type of internal combustion engine designed for the application in a hybrid electric vehicle. The highly integrated system consists of a two-stroke combustion unit, a linear generator, and a mass-variable gas spring. These three subsystems are arranged longitudinally in a double piston configuration. The system oscillates linearly between the combustion chamber and the gas spring, while electrical energy is extracted by the centrally arranged linear generator. The mass-variable gas spring is used as intermediate energy storage between the downstroke and upstroke. Due to this arrangement piston stroke and compression ratio are no longer determined by a mechanical system.

In this paper experimental measurements and simulations of the gas flow inside the combustion chamber of a free piston engine are presented. This combustion unit is integrated into a power train concept, named free-piston linear generator (FPLG), which is designed as a new type of gasoline engine for hybrid electric vehicles. By combining a two stroke combustion chamber, a linear alternator and an adjustable gas spring the engine design aims at a highly efficient conversion of chemical energy into electrical energy. In this context a high system efficiency can only be reached if a two stroke combustion cycle is applied. Efficiency advantages are expected due to the missing mechanical link to a crank which enables a new flexibility in terms of stroke and compression ratio. Instead of scavenging ports the presented FPLG combustion unit has poppet valves which are actuated by a variable electro-magnetic valve train.