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The design parameters of a turbo-compound engine system affecting the thermal efficiency and engine output have been analyzed, considering the allowable limit of the maximum cylinder pressure, exhaust temperature and other factors. By adjusting the values of the engine compression ratio to 20, the cross sectional area ratio of the turbine inlet to 0.4, and the compressor pressure ratio to 2.2 at full load and to 1.7 at part load, the thermal efficiencies of approximately 40% at full load and 25% at part load have been obtained in turbo-compound engine. Furthermore, the changes the optimum values are comparatively small, even if the engine compression ratio is greatly changed from 22 to 14.

A turbo-compound engine system is considered to be an effective way to improve the thermal efficiency of diesel engine. In this study, the sensitivity of the design parameters of turbo-compound system, such as the compressor pressure ratio and the turbine expansion ratio, has been analyzed using a numerical analysis. And the potential of the improving effects of the thermal efficiency by turbo-compound system has been also examined. As the results, it was found that there are the optimum values of compressor pressure ratio and turbine expansion ratio to maximize the thermal efficiency of turbo-compound engine.