Simulation And Experimental Investigation On Performance Of Marine Diesel Engine With Compressor-Air-Injection

During the accelerating and loading process of the marine turbocharged diesel engine,the intake air flow is insufficient for the fuel injection due to the mechanical inertia of turbocharger and the volume inertia of the intake and exhaust system,which leads to the significant increase of particulate matter(PM)emissions and the poor acceleration performance.Compressor-air-injection is an efficient method to increase the intake flow by injecting the air into the engine,and could extend the surge margin by the reasonable use of air injection velocity.However,influence mechanism of compressor-air-injection for the compressor performance is unclear,and modeling approach of the relevance model has not been studied.This paper researched the effect of Compressor-air-injection on the the compressor characteristic and diesel engine performance by the way of simulation and experiment,then proposed the modeling methods for Compressor-air-injection model,combustion model and PM model.Based on the three models,the diesel engine model was establish and used to predict the effect of air injection and calculate the comprehensive energy consumption during the different transient processes.The main research content of this paper was listed as follow.The vaned diffuser compressor and vaneless diffuser compressor were simulated by CFX Software to study the influence mechanism of compressor-air-injection.A selection method was put forward to decide a preferred plan for each diffuser form from three Compressor-air-Injection methods.The injection positions of three methods were at the intake,the casing structure and the outlet of the target compressor.For the vaneless diffuser compressor,Outlet-air-injection was the preferred plan.For the vaned diffuser compressor,Outlet-air-injection and Casing-air-injection were the alternative solutions,the preffered plan should be decided by combination operation point of diesel engine and compressor.When the combination operation point was at large mass flow region of compressor characteristic diagram,the plan of Outlet-air-injection should be selected.When the combination operation point was at high efficiency region and small mass flow region,Casing-air-injection was the preffered plan.For the Casing-air-injection,the injection angle should match with the installation angle of the diffuser vane.In order to study the effect of air injection on the compressor characteristics,the application of Outlet-air-injection was adopting on a vaneless compressor and an experiment was carried on the thermal power test bed.Contrast analysis of the frequency domain characteristics of the pressure pulsation in the compressor with or without air injection,the result showed the air injection would not influence the instability character.According to the characteristic data of compressor,the surge margin and transient performance could be improved by air injection.Based on the test data,Partial Least Squares(PLS)was applied to establish the Compressor-air-Injection model by reconstructing the compressor characteristics,and the model was used to increase the density of compressor characteristic data by predicting the unknown operating points.In the modeling process,the effect of maximum fitting power of the polynomial and cross validation method on the model accuracy had been discussed widely,and the numerical oscillation at the off-calibration points had been avoided.A marine diesel engine test bed with two kind of turbocharger system(Conventional Turbocharging and Sequential TurboCharging)was set up to study the improvement of engine performance by Outlet-air-injection.By analyzing the data from the steady state and transient state experiment,adopting air injection could increase the intake mass flow,accelerate the burning velocity during the middle and later periods of combustion process,make CA90 move forward,shorten combustion duration,decrease the fraction of post-combustion,reduce the PM emission and exhaust temperature.By studying the effect of air injection on the diesel engine performance with two kinds of turbocharger system,combination operation point influenced the effect of air injection.When the combination operation point was set at large mass flow region,air injection could make the combination operation point move to the high efficiency region,and more intake mass flow could be achieved.When the combination operation point was set at high efficiency region and small mass flow region,air injection could make the combination operation point move to the lower efficiency region,the increment of intake mass flow would be small.The experiment data of diesel engine were used to establish the combustion model and emission model.A novel method was proposed to decide the initial values of the diffusion combustion start point by calculating the corresponding angle of the maximum value of the second derivative of the heat release rate,and the use of this method could effectively avoid the occurrence of impossible solutions or multiple solutions.The effect of data preprocessing and residual gas fraction on the model accuracy was studied and used to improve the model accuracy.Comparisons of the combustion model developed by PLS and Least Square were carried on to illustrate the superiority of PLS.When the pressure difference of intake and exhaust significantly changed,the effect of residual gas fraction on the combustion model should taken into account.A detailed error analysis for combustion model was carrid on,which showed it was the effective means to improve the model accuracy by increasing the number of sub-Wiebe function.The diesel engine simulation model adopting Outlet-air-injection was established by the combination of GT-power software and Matlab/Simulink software.The accuracy of the simulation model during the transient process was validated by comparing with the experimental data.Control strategy for air injection was proposed based on the target PM emission.The diesel engine model was used to study the engine performance during the switching process of STC system in the steady state,the acceleration process in the propeller characteristics,the load step process for the tugboat operation and the generator operation process.The results showed adopting the air inejction method would slightly increase the amount of the energy consumption,but significantly reduce the PM emission.Consideration of the percentage of the delivered power during the transient process for oceangoing freighter and tugboat in one year,adopting air injection could achieve 36.8% and 33% reduction of PM emission at the expense of additional 0.004% and 0.05% energy consumption.From the comprehensive energy consumption analysis,the air injetion method is a higher cost-effective scheme for improving the diesel engine transient performance.