The Influence of Fuel Injection Pressure and Intake Pressure on Conventional and Low Temperature Diesel Combustion 2012-01-1721

The influence of fuel injection pressure and intake pressure on conventional and low temperature diesel combustion was investigated in a light duty diesel engine. The in-cylinder pressure and exhaust emissions were measured and analyzed in each operating condition. The two combustion regimes were classified in terms of intake oxygen concentrations, which were adjusted by varying the amount of exhaust gas recirculation. The fuel injection quantity and injection timing were fixed in order to minimize the influencing factors.

Fuel injection pressures of 40 MPa and 120 MPa were used to verify the effect of the fuel injection pressure in both combustion regimes. The injection pressure significantly affected the combustion phase in the low temperature diesel combustion regime due to the longer premixing time relative to the conventional diesel combustion regime. In the low temperature diesel combustion regime, the ignition delay period and combustion duration with a higher injection pressure were shorter due to the improved atomization and enhanced evaporation of the fuel droplets. With regard to exhaust emissions, a higher injection pressure resulted in lower carbon monoxide and hydrocarbon emissions in both combustion regimes. This was due to the reduced fuel-rich mixture in the low temperature diesel combustion and higher combustion temperature in the conventional diesel combustion. The reduction of soot emissions with a higher injection pressure was predominant in the conventional combustion regime where the soot formation rate is high. Higher injection pressure was also advantageous in terms of combustion efficiency and fuel conversion efficiency in both combustion regimes.

The intake pressure was varied from 0.08 MPa to 0.15 MPa in both combustion regimes. The exhaust emission trends according to the intake pressure were quite similar to those observed in the test involving injection pressure. However, the emission reduction mechanisms were somewhat different. The increased intake pressure not only improved the air-fuel mixing process but also provided more oxygen and higher ambient density inside the cylinder, leading to enhanced combustion process and reduced exhaust emissions, such as carbon monoxide and hydrocarbon.