Direct fuel injection combined with charge stratification represents an important upgrading for S.I. engines. In the case of two-stroke engines, it prevents fuel loss from the exhaust port and incomplete combustion or misfire at light loads. In the case of four-stroke engines, it increases power and, especially at light loads, fuel economy.
To obtain the best results, stratification should be kept stable when engine operating conditions change, fuel dilution and spreading in consequence of burnt gas expansion should be prevented especially at light loads and fuel impingement on combustion chamber walls should be avoided.
In the case of two-stroke engine, air-and-wall guided spray represents a suitable solution owing to its affordable cost. For four-stroke engine, self-guided spray is preferable, since it avoids fuel impingement on piston surface.
This paper especially deals with reduction of fuel spreading during combustion in four-stroke engines, thanks to a concept previously intended for two-stroke engines.