NSDI-3: A Small Bore GDI Engine

Gasoline Direct Injection (GDI) is today more regarded as a suitable technology for relatively high displacement engines. The literature shows that the R&D effort on GDI engines is generally made for bores larger than 80 mm. But because GDI appears to be the most relevant way to improve fuel efficiency of S.I. engines, it should also be considered for small bore engines (bore below 75 mm). Nevertheless, locating an injector in already congested cylinder heads, with ultra lean stratified combustion capability while maintaining high engine specific power and proper cylinder head cooling is a real challenge. For such an engine, IFP “narrow spacing” proposal is a 3-valve per cylinder layout or NSDI-3 concept, with a spark-plug-close-to-the-injector design and a suitable piston to confine the fuel spray within the vicinity of the ignition location. This paper describes stage by stage the prototype engine realization using this novel concept. In the early stage, computer aided design and three-dimensional computational fluid dynamics tools were used in a step-by-step iteration cycle to characterize and validate the concept at low cost. One-dimensional calculations were carried out to design the entire intake system and valve timing as well as three-dimensional calculations were conducted to optimize the cylinder head water cooling. In-cylinder flow, injection timing and combustion characteristics were examined with the help of three-dimensional calculations for the stratified charge operation in order to evaluate the criteria required to optimize this crucial process. Special emphasis was given to the mixture formation, to spray and in-cylinder flow interaction as well as to combustion and pollutant formation process. Finally, corresponding engine tests confirmed the potential of the concept for stratified combustion.