Spray and Combustion Development in a Four-Valve Optical DI Diesel Engine

An optical single-cylinder four-valve high speed DI Diesel engine equipped with a high-pressure electronic fuel injection system has been used to obtain information about the spray development, combustion and exhaust emissions (NOx and smoke levels) for a range of operating conditions corresponding to engine speeds between 600 and 1800 rpm, injection pressures up to 1200 bars and fuel injection quantities from idle to full load. Two six-hole vertical mini-sac type injection nozzles with different hole sizes have been employed in order to investigate the effect of nozzle hole diameter on spray formation, combustion and exhaust emissions. Parallel to the experimental programme, a computational investigation of the fuel flow distribution inside the injection system and of the subsequent spray characteristics has been performed in order to assist in the interpretation of the results.

Both experiments and calculations have confirmed that spray wall impingement is not only unavoidable in small size diesel engines equipped with high pressure injection systems but also the main factor controlling the fuel vapor distribution within the piston bowl of the engine. CFD spray calculations revealed that spray vaporization resulted after the spray impingement on the wall which explains why combustion was observed to start almost uniformly in the vicinity of the piston wall. The emission tests, on the other hand, showed that the NOx-smoke trade-off curve was shifted towards smaller values only when injection pressure was increasing and hole size was decreasing at the same time.