Research On Electric Turbo-compound Matching And Control Strategy For Two-stroke Low-speed Marine Diesel Engine

With the promulgation and implementation of regulation such as Energy Efficiency Design Index(EEDI)and IMO-III,the requirements of energy-saving and emission-reduction for marine engines have become more stringent in recent years.Two-stroke low-speed diesel engines are widely used due to their significant advantages in fuel economy and reliability.Turbocharger is the key component in the two-stroke engine,which ensures scavenging pressure at most of loads and hence smooth working of the engine.However,the turbocharged two-stroke engines usually suffer from lacking the exhaust energy in the low load and is prone to over-boost in the high load.Electric turbo-Compound(ETC)is a technology which integrates the turbocharger with electric motor/generator(EMG),thus is capable of power-generation via the turbine and assisted boosting via motor-driving compressor.Therefore,the research on the matching method and control strategy of electric turbo-Compound for twostroke low-speed marine diesel engines should be carried out,which mainly include the following aspects:Firstly,the influences of key factors of the ETC on two-stroke lowspeed marine engine is studied.The one-dimensional numerical simulation method is used to establish the steady-state performance prediction model of ETC marine engine,which is validated by the experimental data.The influences of EMG power are analyzed in details,the result of which shows that turbocharger efficiency is a key factor that affects the overall efficiency.By optimizing the EMG power to push the turbine and compressor operating points near the highest efficiency circle,turbocharger efficiency could be increased,so is the overall efficiency of the low speed machine.And the optimal EMG power increases rapidly as the engine load increases.Based on the influence mechanism of EMG,the EMG power switching and control strategy for achieving the highest fuel economy is obtained.Secondly,the influence of turbine and compressor matching on the performance of low speed machine is studied.The results show that the turbine needs to be as small as possible,and the compressor should become smaller as the engine load increases.According to the influence and sensitivity of EMG power,turbine and compressor matching on engine performance,a matching method of ETC for two-stroke low-speed diesel engine is proposed,which then is applied to the 6EX340 engine.The results show that brake specific fuel consumption(BSFC)can be reduced by about 2%~3% at three load profiles.Finally,based on the obtained optimal matching results,the research on the ETC control method for the two-stroke low-speed marine engine is carried out.The transient performance prediction engine model is established via modifying the steady-state model.And then the variable geometry turbine performance prediction model and motor/generator model are built which are embedded in the transient engine model for real-time simulation prediction.The sensitivity analysis of key parameters of transient ETC engine model is carried out.And the results show that the sensitivity on the engine speed of the three key control parameters are: fuel mass,EMG power and equivalent area of variable geometry turbine.Based on the sensitivity analysis,a feedback control method of the ETC engine is proposed and modified.The results show that good tracking performance for engine speed is obtained.And the transient performance of loading procedure in the low load is studied based on the feedback control method.It is shown that the ETC engine has obvious advantages on BSFC over the conventional turbocharged marine engine in the loading procedure,and notable fuel economy up to 4.8% is demonstrated.The research in this paper can provide theoretical guidance and direction for the application of electric turbo-Compound technology in the two-stroke low-speed marine engine.