Experimental Study on Diesel Spray Combustion and Wall Heat Transfer with Multiple Fuel Injection Strategies - Results of Rapid Compression and Expansion Machine Experiment

The rapid compression expansion machine (RCEM) was used to investigate the temporal variations of the spray flame and wall heat flux in the diesel engine combustion process by using 120 MPa and 180 MPa common rail pressure. A stepped cavity was applied to investigate spray and flame behavior under the pilot, pre and main multiple injection strategy. Wall heat flux sensors were installed in the piston cavity and the cylinder side. The injector has 3 holes with the neighboring angle in the left direction and another 3 holes in the right direction to simulate the spray interaction in the 10-hole injector combustion system in the actual diesel engine. The spray and flame behavior were taken by a high-speed video camera with direct photograph. A two-color analysis was applied to investigate gas temperature and KL factor distribution. The effect of locations and common rail pressure on heat transfer was investigated. The result shows that multiple injections improve better atomization and air fuel mixture formation which reduces combustion duration in the combustion chamber. Pilot injection and pre-injection have no significant effect on the wall heat flux due to the low in-cylinder pressure and ambient temperature before TDC. The wall heat flux in cylinder head is the highest of all locations due to the intense combustion flame occurs in this region. The increasing common rail pressure tends to increase the peak value of the wall heat flux and decrease the combustion duration. To investigate the heat transfer phenomena in more, the correlation between Nusselt and Reynold numbers is presented in this study. The direct flame image, heat flux waveforms, and quasi-steady state are applied to obtain the characteristic velocity. Two-color analysis is used to obtain the gas temperature. The result shows that the heat transfer phenomena can be expressed by the correlation between Nusselt and Reynold numbers.