Matching method of electric turbo compound for two-stroke low-speed marine diesel engine

Two-stroke low-speed diesel engine is widely employed in marine industry thanks to its evident advantages on fuel economy and reliability. Normally, turbocharger together with an auxiliary air blower are employed in the two-stroke engine to ensure the smooth scavenging over whole propeller characteristics, but fuel economy and layout compactness of the engine are scarified due to the employment of the auxiliary device. This paper investigates influences of electric turbo compound, which integrates a conventional turbocharger with electric motor/generator (EMG) and capable of replacing the auxiliary blower, on the performance of a two-stroke low-speed marine engine as well as the matching method of the turbocharging system. Firstly, influences of EMG power and compressor/turbine matching on the engine nominal efficiency are discussed in details. The analytical model together with simulation results show that the efficiency of the turbocharger is the most important factor influencing variation behaviors of engine nominal efficiency. In particular, the peak engine nominal efficiency can be achieved when the operational points of the compressor and turbine are pushed to their peak efficiency islands by either the control of EMG power or compressor/turbine matching. Therefore, a matching guideline of the electric turbo compound is proposed based on the study and then applied on a two-stroke low-speed marine engine. Compared with the conventional turbocharger, a smaller compressor is matched at high loads while a smaller turbine is matched at low loads for the optimal electric turbo compound turbocharger. Specifically, brake specific fuel consumption (BSFC) of the engine is reduced by about 2-3% at three load profiles by the new technology.