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A recently developed Laser Raman Scattering system was applied to measure the in-cylinder air-fuel ratio and the residual gas content (via the water content) of the charge simultaneously in a firing spark-ignition engine during cold start and warm-up. It is the main objective of this work to elucidate the origin of misfires and the necessity to over-fuel at cool ambient temperatures. It turns out that the overall air-fuel ratio and residual gas content (in particular the residual water content) of the charge appear to be the most important parameters for the occurrence of misfires (without appropriate fuel enrichment), i.e., the engine behaviour from cycle to cycle becomes rather predictable on the basis of these data. An alternative explanation for the necessity to over-fuel is given.

It is demonstrated that direct air injection leads to substantial improvement of the cold starting performance of an SI engine. Raw pollutant formation is considerably reduced and fuel conversion efficiency increased. Air is directly injected into the cylinder through a small orifice during the compression stroke. Optical measurement techniques, in particular Spontaneous Raman Scattering, are applied to elucidate the in-cylinder processes that lead to improved cold starting perfomance. It is demonstrated that air injection causes enhanced combustion via increased turbulence and this leads to rapid warm-up of the combustion chamber walls. Thus the quality of the combustion process after the first few cycles is comparable to warmed-up engine operating conditions. It turned out to be possible to avoid fuel-enrichment and start the engine with lean fuel/air mixture (at 20°C).