Combustion patterns in Common rail d.i. engines inferred by experiments and CFD computations

Some particular features of combustion evolution and emission performance of new direct injection diesel engines for passenger cars are discussed. Two engines were employed for the experiments: a single-cylinder optical research engine and a real four-cylinder engine. The optical engine combustion system design was similar to the four-cylinder engine one. Both engines were equipped with a common rail injection system. Single-cylinder findings were verified with the four-cylinder tests and reproduced with the CFD simulation by KIVA3V code. High-speed visualisation of spray and combustion processes indicated a strong sensitivity to the combustion system design and the chosen injection strategy. The analysis showed that the mechanism involved in the increase of the net soot mass emitted at the tailpipe, when pilot injection is active, is very complex, accountig for both chemical and physical effects of pilot products on main combustion evolution.