CFD Modelling of a Free-Piston Engine Using Detailed Chemistry

Licentiate thesis, 2006

The Free Piston Engine, FPE, which is capable of operating in a variable compression ratio mode, is highly suitable for the promising Homogeneous Charge Compression Ignition (HCCI) concept, allowing flexible auto-ignition control on a cycle-to-cycle basis without any hardware modifications. In this study, the effects of varying the start of injection on the emission formation and efficiency of such an engine have been investigated, using simulations based on the KIVA-3V CFD code. In order to simulate an FPE, the code was modified by replacing the conventional crankshaft-controlled piston motion by a motion profile, calculated from the balance of forces acting on the piston, according to Newton’s second law. The combustion was simulated using a detailed chemical kinetics mechanism of a Diesel oil surrogate represented by a blend of aliphatic (70% n-heptane) and aromatic (30% toluene) components. Dimethyl ether, DME, which is a very promising fuel for the future, has also been investigated. To help analyze the formation of emissions resulting from HCCI/Conventional Diesel combustion modes in the engine, an approach based on temperature-equivalence ratio maps has been used. The simulations show that the combustion process can be optimized by changing injection timing, leading to high indicated efficiencies.