| Environmental degradation and the energy crisis are forcing the ship power industry to look for efficient,low-emission,stable and reliable alternative solutions of ship power system in response to increasingly stringent emission regulations and depleting oil resources.Natural gas has become one of the ideal alternative fuels due to its high efficiency,cleanliness and abundant reserves.A marine micro-pilot-ignition(MPI)dual-fuel engine can achieve the same level of power output as marine diesel engines,and is capable of meeting the IMO Tier III emission requirements without any after-treatment.However,the domestic marine MPI dual-fuel engine is still in the early stage of research and development,so it is of great practical significance to carry out research on its control technology for its engineering development and promotion in China.In this paper,the key technology research of marine MPI engine control system is carried out on the ACD320 DF engine for the technical difficulties and problems in engineering application,mainly including the analysis of combustion and emission performance of the MPI dual-fuel engine,the optimization of the control logic and strategy under the gas-mode,the simulation and experimental research of gas-mode start control strategy and performance optimization under low-load conditions,fuel mode switchover control method,cylinder balance control method and engine knock monitoring and control method.The main research contents and conclusions are as follows.(1)An engine measurement and control system and a data acquisition system were developed;A 1D simulation model of the engine and a 3D CFD simulation model of the in-cylinder process were established to accurately simulate the natural gas injection process,wastegate valve adjustment process,pilot fuel injection process,combustion process,etc.The effects of the engine intake and exhaust,gas injection process on the multi-cylinder uniformity were simulated and analyzed based on the 1D simulation model.Based on the 3D CFD simulation model,the combustion and emission characteristics of the engine,including natural gas escape,combustion and heat release law,emission generation law,flame propagation law,etc.,are simulated and analyzed.Through the cross-validation of simulation results and test results,the basic law of the in-cylinder working process of the engine is revealed.The fuel substitution rate reaches 99.5%,the thermal efficiency reaches 44.03%,the steady-state speed fluctuation rate is less than 1.51%,and the weighted NOx emission is 2.25g//k W·h,which meet the requirements of CCS type approval and the IMO Tier III NOx emission limit.(2)The analysis of the engine control system requirements was completed,the rapid control prototype of the engine controller was developed based on NI hardware and software,and the basic control logic and control strategy of the engine were formulated.The feasibility of the full operating range in gas-mode of the engine was analyzed,and the control strategy of the engine gas-mode start was proposed and verified by the real engine.The control strategy of the engine gas-mode low-load performance optimization and cylinder deactivation was explored,and the full range of engine operating conditions in gas-mode is realized.(3)To address the problem of large speed fluctuation in the engine fuel mode switchover process,the combustion performance of the engine under different fuel substitution rates was analyzed,and the engine fuel mode switchover control strategy was proposed accordingly.The effects of switchover duration and main fuel cut-off position on the fuel to gas mode switchover process were studied,and the effects of fast switchover duration on the gas to fuel mode fast switchover process were investigated,and finally verified in the engine.The results show that the maximum speed fluctuation is about 5.7% when switching from fuel to gas mode in the range of 20%-80% of rated power of the engine,and the maximum speed fluctuation is about 10.8% when switching from gas to fuel mode in any working condition with a switching duration of 0.2s.(4)For the multi-cylinder unbalance problem of the engine,firstly,the uneven gas inlet phenomenon and the unbalanced state of each cylinder caused by singleend inlet and low-pressure gas injection are simulated and analyzed based on a one-dimensional simulation model,and a closed-loop control method based on cylinder pressure and exhaust temperature feedback is proposed.The effectiveness of the control method is verified on the real engine,and the optimized cylinder balance control strategy for different engine loads is obtained by combining the test results.It is confirmed that the developed multicylinder balancing control method reduces the unbalance of each cylinder.(5)In response to the problem that knocking will occur in gas-mode,the mechanism of engine knocking occur is analyzed,and the interconnection between misfire and knocking is described.The knocking characteristics of the engine are analyzed based on cylinder pressure and cylinder head vibration signals.The knocking characteristic parameters are extracted,and he correlation of each parameter was analyzed.Finally,the knocking criterion and knocking level discrimination method were proposed,which confirmed the diagnosability of engine knocking and its intensity judging. |
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