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Unscavenged prechamber ignition has recently been developed at LENI-EPFL to reduce the emissions from cogeneration spark ignition gas engines (natural gas and biogas) within the Swiss Motor project supported by the Federal Office of Energy. Prechambers fitted with conventional spark plug allow a 40% reduction of the CO emissions and a reduction of the NOx emissions below the Swiss limits of the OPair law (Swiss emissions legislation), and a reduction of 55 % of the HC emissions. Unfortunately, spark plugs are weak components in therms of lifetime, reliability and maintenance. There consequently exists a strong motivation to develop new ignition systems which include: pilot injection of fuel, direct injection of fuel with a glow plug or hot gas jets from a prechamber where the comustion is initiated by auto-ignition, controlled mainly by pressure and temperature conditions. In this mode, the prechamber auto-ignition replaces the spark plug and the combustion mode is similar to a HCCI (Homogeneous charge compression ignition) one. The first step in the development of the system is the simulation of the auto-ignition in the prechamber in order to understand the phenomena and later to control them. As HCCI combustion is strongly dependent on chemical kinetics and fluid dynamics, a CFD code and a chemical reactions solver have to be numerically linked. First the 3D fluid flows in the prechamber and in the engine cylinder has been modelled with the well known KIVA 3-V code. Those results will be taken as reference for further calculations and initial conditions for the chemical reaction calculations. To elaborate and validate an auto-ignition reaction mechanism for natural gas, auto-ignition delays for various gas mixtures have been measured in the rapid compression machine of Lille [1[. The data identification will be performed using the GRI mechanism as a base for the auto-ignition delays simulation with Chemkin-II. The next step will be the development of an interface between the Chemkin-II solver and a CFD code NSMB (Navier Stokes Multi Block solver [2,3]). NSMB is an in-house code developed to deal with large meshes on parallel computers and to be coupled with other applications. It has been modified to support dynamic meshes. The coupling is under development.