Direct Air Injection for Substantial Improvement of SI Engine Cold Start Performance

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).