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Study On Matching And Performance Optimization Of A Marine Two-stroke Engine Equipped With Exhaust Gas Recirculation System

Posted on:2023-03-31Degree:DoctorType:Dissertation
Country:ChinaCandidate:D Y LuFull Text:PDF
GTID:1521307040483944Subject:Marine Engineering
Abstract/Summary:PDF Full Text Request
Although the EGR technology has been proven effective to reduce the marine engines NOx emissions,several challenges emerge for the engine-turbocharging system matching considering the contradictory requirements of the engine and its subsystems operation.Such challenges become more pronounced in complex engine configurations that include parallel turbochargers and the EGR system along with cut out and bypass branches.Furthermore,engine fuel consumption increase pronounced with the use of the EGR system,thereby affecting the associated environmental carbon footprint.This study aims at the numerical simulation of a large marine two-stroke diesel engine equipped with the typical exhaust gas recirculation systems and two parallel turbochargers,targeting to propose the matching procedure of the engine and the turbocharging system,and explore the optimization methods to minimize the engine fuel consumption whilst ensuring compliance with the respective NOx emissions limits and satisfying imposed constraints for the compressors operation.The investigation results will providing support to the designer for turbocharging system configuration,parameter optimization,operation management,and control strategy design of marine engines employing the EGR technology.The zero/one-dimensional steady-state simulation models of the 7G80 ME marine two-stroke diesel engine with the exhaust gas bypass(EGR-BP)and the turbocharger cut-off(EGR-TC)exhaust gas recirculation system are established by adopting the GT-SUITE software.The simulation models are calibrated and verified using the diesel engine bench test data,and the percentage error between the predicted data and the measured data is less than3%.Simulation runs are performed to investigate the engine with four different turbocharger configurations of varying capacity ratio and under various operating conditions in terms of the EGR rate and engine load.The simulation results are analyzed to reveal the impact of the turbocharger selection of the engine performance and emissions parameters.Furthermore,modulation schemes with EGR blower speed control,exhaust gas bypass and cylinder bypass are investigated to overcome the mismatch on the engine components flow rates and avoid turbocharger operational issues.The derived results demonstrate that the lowest weighted BSFC is achieved for the case of 70:30 capacity ratio between the large and small turbochargers,whilst the engine operation with the EGR is associated with a 2.6% penalty in the weighted BSFC.The EGR blower speed control is found sufficient to avoid the compressor overspeed at high engine loads exhibiting the lower BSFC penalty,whereas the cylinder bypass control is appropriate for controlling the compressor speed at low engine loads.This study contributes on delineating the underlying parameters and interactions between the engine components for the investigated marine two-stroke engine and provides recommendations for the engine-turbocharging system matching procedure.Based on the established zero/one-dimensional engine steady-state simulation model,parametric simulation runs are performed considering the engine operating both without and with the EGR system considering EGR rates up to 40% in conjunction with cylinder bypass rates up to 50%.The results demonstrate that EGR rate above 30% is needed,so that both engine configurations comply with the Tier III limits.For the EGR-BP engine configuration,the excessive EGR rate results in the considerable BSFC increase and the deterioration of the turbocharger condition,which is improved by opening the cylinder bypass.Combinations of the optimal EGR rate and the cylinder bypass rate for each engine load are identified.For the EGR-TC engine configuration,the Tier II and Tier III operating modes are optimized respectively,and the optimization scheme to minimize the engine fuel consumption is given;the optimized Tier II mode reduces the engine fuel consumption by 2.9g/k Wh and 0.7g/k Wh at 25% and 50% loads,respectively;optimized Tier III mode reduces engine fuel consumption by 2.1g/k Wh and 1.7g/k Wh at 25% and 50% load,respectively,while balancing the capacity ratio of the large/small turbochargers at high/medium/low loads,which makes its matching easier.After a comprehensive comparative evaluation of the two exhaust gas recirculation systems,the recommended configuration and operation modes are proposed.In order to investigate the dynamic response performance of marine engines equipped with the exhaust gas recirculation system,on the basis of the established EGR-TC engine steady-state simulation model,the simulation range of the working condition was further expanded and the dynamic simulation model of the engine under all working conditions was established.The control system model of the engine and exhaust gas recirculation system are established by adopting the MATLAB/SIMULINK software,and coupled with the dynamic simulation model of the engine.For the engine dynamic coupling model,the steady-state and dynamic simulation verification are carried out using the 7G80 ME engine bench test data and the experimental data of the reference model.Based on this,the dynamic simulation research is carried out on the dynamic process from Tier II to Tier III mode,from Tier III to Tier II mode,and the acceleration/deceleration process of the engine in Tier II and Tier III modes to analyze and evaluate the transient response performance of the EGR-TC engine.The simulation results show that the deactivating and activation of the exhaust gas recirculation branch and the small turbocharger will have a significant impact on the operation of the engine and the large turbocharger.The deactivating of the exhaust gas recirculation branch in Tier III mode and the deactivating of the small turbocharger in Tier II mode will cause the engine fuel consumption and exhaust temperature to rise to a certain peak momentarily.This phenomenon is more pronounced under high load.In addition,when the engine is running at high load,either deactivating the small turbocharger in Tier II mode or deactivating the exhaust gas recirculation branch in Tier III mode can result in excessive exhaust gas temperatures and cylinder maximum pressures.The derived results present that the 50% load as the upper allowable load during the above operation can control the exhaust temperature and the cylinder maximum pressure within a safe range.In addition,extending the during time of the exhaust gas recirculation system blower and the small turbocharger input and removal process can effectively reduce the impact of related operations on the engine and the large turbocharger.
Keywords/Search Tags:Marine two-stroke diesel engine, Exhaust gas recirculation, Parallel turbochargers, Numerical simulation, Performance optimization
PDF Full Text Request
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