Research On The Modeling Method Of Marine Large-scale Two-stroke Diesel Engine Oriented To Marine Engine Room Simulator

In this paper,to develop a mathematical model for marine large-scale low speed two-stroke diesel engine that meets the requirement of marine engine room simulator on both the simulation speed and simulation accuracy is set as the key of this project.The emphasis of this paper is put on the modeling approaches of the mass flow rate and isentropic efficiency for marine large-scale compressor as well as solving the problem that the traditional mean value engine model is unable to predict the in-cylinder pressure.Combined with years of experience in developing actual projects,the development process and implementation plan for the main engine simulation system is introduced and summarized comprehensively,and the related key techniques are also discussed.As a result,the transformation from the theoretical research to pratical application is achieved.The compressor mathematical model has significant influence on both the steady-state simulation accuracy and transient response capability for the turbocharged engine model,however,it is found that there is no relavant comprehensive applicable and comparative study of compressor mass flow rate and isentropic efficiency models in marine large-scale compressors.To reveal their predictive accuracy and extrapolative capability in the different operating area of the marine large-scale compressor and also to better contribute to the development of main engine simulation system for the marine engine room simulator,two marine large-scale compressors with different size,flow rate range,and speed range are selected as the research objects,and a comprehensive applicable and comparative study is carried out to compare and analyze the predictive and extrapolative ability of several classical and recently proposed compressor mass flow rate and isentropic efficiency models.The comparison range includes both the predictive accuracy for the available sample data points in the design operating area but also the extrapolative capability to the off-design operating areas.Based on the comparative and analysis results,the advantages and disadvantages of these surveyed models are summarized and several guidelines are provided,which can be followed for researchers in the same research field.In addition,an estimation method for the compressor blade diameter based on the Euler equation for turbomachinery is proposed,which only needs the compressor performance map as the input data.With three marine large-scale compressors of different sizes including A270-L59,TCA88-25070 and TCA55 as the test objects,the relative error of the estimation result is less than 1%,presenting satisfactory estimation accuracy.Aiming at the problem that the extrapolative result of the look-up table method is unreliable and that the predictive and extrapolative accuracy is inconsistent in the different operating area with a single curve-fittting method for the compressor,a zonal compressor mass flow rate modeling approach is proposed.Based on the compressor performance map,firstly by defining zone division standard the whole compressor operating area is divided into design,low speed,high speed and low pressure ratio area,respectively,and then the compressor mass flow rate model that achieves the best predictive accuracy is selected for each area.To avoid the possible discontinuity point during the simulation process when the operating point enters into the low pressure ratio area from other areas,a curve blending method is adopted to guarantee the smooth transition of the iso-speed line.The proposed zonal compressor mass flow rate model takes full advantages of existing models,which is able to not only accurately predict the available sample data points in the design operating area but also extrapolate to the off-design operating areas reasonably and robustly.The Hadef isentropic efficiency model is improved by firstly dividing the "mass flow rate-actual consumed enthalpy change" plane into several zones by using the available iso-speed lines in the performance map and then parameterizing the model for each zone separately,as a result,it can capture the working characteristic of the compressor in different rotational speed range more accurately.In comparison to the original model,the improved Hadef isentropic efficiency model presents higher predictive accuracy for the available sample data points in the performance map,meanwhile,it also presents satisfactory extrapolative capability.The main engine working cycle simulation model is developed under the MATLAB/Simulink simulation environment with the MAN B&W 7S80ME-C9.2 marine large-scale slow speed two-stroke diesel engine as the research object.A model parameter tuning procedure is proposed,which can effectively balance the model simulation accuracy at each engine loading condtion.By carrying out steady and transient simulation experiments,the correctness and rationality of the developed main engine working cycle mathematical model is validated.The mean value engine model is simplified by removing the differential equations about the working medium temperature in the scavenging box and assuming that the scavenging air temperature is always equal to that of the air at the outlet of air cooler.By carrying out simulation experiments,it is found that the simplification method has no obvious influence on both the steady simulation accuracy and transient response for each engine operating parameter.This model simplification method can accelerate the running speed of the main engine simulation model to some extend and provide the refining space for the math models of other electromechanical equipment in the marine engine room simulator.According to the characteristic of the in-cylinder pressure trace during the gas-exchange phase for the two-stroke diesel engine,a cylinder pressure analytic model for the four-stroke spark-ignition engine is amended by using two linear functions to approximately calculate the in-cylinder pressure during the gas-exchange phase,as a result it can be applied to the marine large-scale two-stroke diesel engine as well.To achieve satisfactory predictive accuracy,the model parameters in the cylinder pressure analytic model is tuned elaborately,including the polytropic index of the compression and expansion polytropic process,the temperature and pressure at the reference point of the compression polytropic process,the combustion efficiency factor and model parameters in the Wiebe function.By coupling the tuned cylinder pressure analytic model and the mean value engine model,the problem that the traditional mean value engine model is unable to predict the in-cylinder pressure traced is solved.In comparison to the measured indicator diagram,it is found that the coupled engine model is capable of capturing the changing trend of in-cylinder pressure during each phase of the working cycle for marine large-scale two-stroke diesel engine,in addition,the compression pressure,explosion pressure and its crank angle position can be predicted accurately as well.By adjusting the calculation frequency of the cylinder pressure analytic model and the mean value engine model,the problem of different calculation speed for the two models is solved and they are synchronized.In comparison to the "control oriented-mean value" hybrid engine model,the developed"cylinder pressure analytical model-mean value" hybrid engine model is capable of capturing the transient response of the in-cylinder pressure more accurately under the condition of similar simulation speed.Finally,based on the developed and validated marine large-scale low speed two-stroke diesel engine working cycle mathematical model,the main engine simulation system of the very large crude carrier marine engine room simulator is developed.The relevant 2D simulation interfaces are designed and developed based on the WPF technology,in addition,two utilitarian functions including the resolution self-adaption and the local zoom are developed for the 2D simulation interface.In addition,the running and refreshing mechanism of the simulation interface program and simulation model program are optimized,which improves the running smoothness and real-time performance of the simulation system.