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. Hence the gas exchange components are independent from the piston trajectory.
The resulting scavenging flow has a complex three dimensional flow field, which is strongly affected by the location and pressure level of the inlet and the outlet ports. In order to improve the overall efficiency of the two-stroke operating mode fundamental knowledge of the scavenging process is required. Therefore computational fluid dynamic simulations as well as laser diagnostic measurements have been conducted. Using the software package ANSYS CFX 13, a three dimensional mesh of the combustion chamber with valves and pipes was modeled including dynamic mesh movement and automatic re-meshing. The experimental measurements were realized by means of the Particle Image Velocimetry (PIV) techniques. The existing cylinder has been replaced by an optically accessible combustion chamber dummy, whose geometry is equal to that of the combustion chamber during the scavenging process. The two dimensional instantaneous velocity fields were measured in three different section planes which allowed for detailed validation of the CFD simulations.
Through the combination of experimental and numerical methods, an in-depth knowledge of in-cylinder processes during scavenging of a poppet valve two stroke engine was attained. In terms of the global flow field the CFD results show a good accordance with the measured PIV results. Whereas the positions of local velocity peaks are predicted very well, absolute values of velocity deviate to some extent. The authors assume that this discrepancy can be explained by the standard turbulence model and the spatial discretization. Nevertheless the capability of the developed CFD Model to predict in-cylinder flow structures was demonstrated. As a consequence this numerical tool can be used for future simulations in order to investigate new cylinder head geometries and gasoline direct injection.