Optimum Compression Ratio Variation of a 4-Stroke, Direct Injection Diesel Engine for Maximum Brake Power and Torque and Minimum Soot and NO _x Emissions 1999-01-2728

The thermodynamic simulation model for the performance of a 4-stroke, direct-injection (DI), variable compression ratio (r_c), diesel engine, previously developed by the authors [1], is used to investigate the effect of varying compression ratio (r_c) on engine brake power and torque and engine soot and NO_x emissions. An optimization analysis is conducted to an engine with specifications similar to HELWAAN M114 under normal operating conditions to seek optimum r_c variation to achieve some specific targets. For constant maximum brake power or constant minimum soot emission, the analysis shows an enormous increase in maximum cylinder pressure inside the variable r_c engine reaching about 19.8 MPa compared to 10.7 MPa for constant r_c engine, which means difficult practical applicability. For constant maximum brake torque, the optimized variation of r_c lies within the range of 16.4 to 19.3. This results in advantages over constant r_c engine: a reduction in bsfc and soot emission by about 15.7 % and 46.9 % respectively, and an increase in brake power by about 17.7 %. Some drawbacks include an increase in NO_x, P_max and T_max by about 95 %, 24.4 % and 9 % respectively. For constant minimum NO_x, the optimization results in reduction of brake power by about 5.8 %, and an increase in bsfc, and soot emission by about 1.5 % and 30.8 % respectively.

A direct-injection diesel engine can be safely operated with variable compression ratio to achieve constant maximum brake torque or constant minimum NO_x over the whole engine speed range (1500-2800 rpm) but, can not be safely operated with r_c to achieve neither constant maximum brake power (beyond engine speed 2250 rpm) nor minimum soot emission over any speed range.