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. The gas exchange is carried out via an electromagnetic valve train, which is mounted on the cylinder head. This combination of variable compression ratio, piston trajectory and fully variable valve train provides the ability for a fundamental optimization of the combustion process.
Homogeneous charge compression ignition (HCCI) is a promising combustion process leading to high combustion efficiencies and low raw emissions. Thereby a challenge is the active control of the ignition and combustion process. The present paper summarizes the approach for a first demonstration of HCCI combustion using the high variability of the FPLG engine concept. 3D-CFD and chemical kinetics simulations of the in-cylinder processes are the basic steps towards the implementation of HCCI combustion in the FPLG engine. By means of these simulations new engine components and a HCCI specific operating strategy were developed. A key aspect in this context is the internal exhaust gas recirculation (EGR), which is realized by an early intake valve opening and intermediate storage of combustion gases in the intake ports. The numerical results were confirmed by experimental measurements and show a promising potential for HCCI combustion in the FPLG engine.