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A thermodynamic simulation model for the performance of a 4-stroke, direct-injection (DI), variable compression ratio (rc), diesel engine is presented. The model investigates the effect of varying rc 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 NOx formation mechanisms. Comparison of model predications with some other published experimental works with constant rc 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 rc to achieve a constant minimum bsfc over whole engine speed range.

The thermodynamic simulation model for the performance of a 4-stroke, direct-injection (DI), variable compression ratio (rc), diesel engine, previously developed by the authors [1], is used to investigate the effect of varying compression ratio (rc) on engine brake power and torque and engine soot and NOx emissions. An optimization analysis is conducted to an engine with specifications similar to HELWAAN M114 under normal operating conditions to seek optimum rc 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 rc engine reaching about 19.8 MPa compared to 10.7 MPa for constant rc engine, which means difficult practical applicability. For constant maximum brake torque, the optimized variation of rc lies within the range of 16.4 to 19.3.