In a free piston engine the piston is neither connected to a crankshaft mechanism nor to any other kinematic system. Instead the piston movement is determined by the free forces that act upon it. This difference between the kinematic principle of the crankshaft engine and the free piston principle has a significant influence on the combustion process.In this paper the combustion process in a free piston engine is described on the basis of experiments. The experimental data were obtained from measurements on the free piston engine that has been developed by the Dutch company Innas.This article discusses the influence of the free piston principle on cold start, ignition delay, heat release, heat transfer, indicated efficiency and emissions.In the optimum point the engine has an indicated efficiency of 51%, a NOx emission of 6 gr/kWhi and a soot emission corresponding to a Bosch Filter Number of less than 0.5. The combustion process of the free piston engine is furthermore characterised by a nearly constant volume combustion process.