Analysis of SI and HCCI Combustion in a Two-Stroke Opposed-Piston Free-Piston Engine

The German Aerospace Center (DLR) is developing a free-piston engine as an innovative internal combustion engine for the generation of electrical power. The arrangement of the Free Piston Linear Generator (FPLG) in opposed-piston design consists of two piston units oscillating freely, thereby alternately compressing the common combustion chamber in the center of the unit and gas springs on either side. Linear alternators convert the kinetic energy of the moving pistons into electric energy. Since the pistons are not mechanically coupled to a crank train, the bottom and top dead centers of the piston movement can be varied during operation e.g. to adjust the compression ratio. Utilizing these degrees of freedom, the present paper deals with the analysis of different combustion processes in a port scavenged opposed-piston combustion chamber prototype. This contains the experimental investigation of spark ignition (SI) as well as the transition to homogeneous charge compression ignition (HCCI). Based on test bench results, a 3D-CFD model, which has been used to develop the first prototype, is calibrated by actual measurements. The simulation results enable a deeper analysis of scavenging and in-cylinder processes exceeding the measurement possibilities on a test bench.