Optimum Compression Ratio Variation of a 4-Stroke, Direct Injection Diesel Engine for Minimum bsfc

A thermodynamic simulation model for the performance of a 4-stroke, direct-injection (DI), variable compression ratio (r_c), diesel engine is presented. The model investigates the effect of varying r_c on engine performance over whole engine speed range. Simulation sub-models are fuel burning rate, combustion products, thermodynamic properties of working fluid, heat transfer, fluid flow, friction, and soot and NO_x formation mechanisms. Comparison of model predications with some other published experimental works with constant r_c under different operating conditions results in good agreement. A comprehensive optimization analysis is conducted to an engine with specifications similar to HELWAAN M114 under normal operating conditions for seeking an optimum variation of r_c to achieve a constant minimum bsfc over whole engine speed range. The study shows that, for constant minimum bsfc, optimum variation of r_c lies in the range of 16.4 to 18.6 giving the following advantages over the conventional constant r_c engine of 16.4: a reduction in bsfc and soot emission by about 11.9 % and 37.5 % respectively and an increase in brake power by about 12.8 %. However, that variation results in an increase in NO_x, P_max and T_max by about 77.8 %, 16.8 % and 6.4 % respectively.