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The basic Semi-Direct-Injection System (SDIS) which is already in production for PWC and applied to small 2-wheeler engines features a low-pressure fuel injection system injecting through the rear scavenge port window in the cylinder symmetry plane onto the piston crown. The patented new SDIS Mk.II System [1] injects through an (additional) scavenge port window that is positioned above the scavenge ports and is controlled by a window in the piston skirt. This new arrangement allows longer injection duration and also other injector positions and directions. A CFD simulation by AVL's FIRE-CFD-code with moving piston and exhaust gas dynamics compares the different injector positions and directions for WOT and rated speed and for a part throttle low speed case. The SDIS Mk.II injection system consists of mass-produced automotive parts thus giving a low cost approach for present 2-stroke engines requiring only moderate engine modifications.

CFD (computational fluid dynamics) theory such as equations of conservation and of the turbulent kinetic energy are explained along with their numerical solution and the FIRE solution algorithm. The techniques are applied to different areas of four-stroke and two-stroke engines such as to a small 4-valve engine with moving piston and moving valves and different intake ports to show the in-cylinder charge movement, followed by the flow calculation and heat transfer in the water jacket and applied also to detect the velocity pattern on the front face of the catalyst, all in the same 150cc engine. One of the CFD applications to different two-stroke engines deals with the variation of manifold-diffuser-catalyst layouts to obtain an evenly distributed velocity in the catalyst. Second a fan cooling system with rotating fan indicates the weaknesses of the original layout in terms of cylinder temperature distribution and improvement steps are shown.