The Control Strategy Research On Marine Medium Speed Natural Gas Fuel Engine

Stringent emission regulations from International Maritime Organization(IMO)drive marine engine technology to the direction of high efficiency and low exhaust emissions.Under this circumstance,the natural gas fuel has an advantage of low fuel price and low exhaust emissions when burned inside an engine,so that it naturally makes the natural gas fuel engine with high efficiency and low exhaust emissions become one of the most important research areas.The control system of a natural gas fuel engine is one of the main components to assure the engine work with low exhaust emissions and high safety.It directly controls the natural gas fuel injection process and excess air coefficient.However,the key technology inside a control system is the control strategy.Thus,the research on the control strategy inside a control system is of great theoretical importance and engineering application value for our country to develop an engine control system for a mediumspeed marine diesel engine.In this paper,an engine control system was developed for a medium-speed marine natural gas fuel engine.It was based on a rapid prototyping technology according to the control function requirements.The control strategy of the engine dynamic process was regulated,and control strategy of engine start with gas mode was researched.Besides,the gas injection balance control was also studied.The main conclusions are as below:(1)The control functions and signal features were analyzed;the overall structure of the control system was built;the hardware system for the control system of a natural gas fuel engine was designed based on a rapid prototyping technology,the hardware system is the integration of injection unit,speed governor unit,monitoring unit and safety unit.(2)The features of torque change,speed change and response delay during a transient process of a natural gas engine were analyzed.The variable increment PID dynamic control method was proposed and based on which the control strategies of engine transient process,fuel mode switch,detonation and misfire were planned.These control logic and strategies were validated on a hardware-in-loop platform in order to verify their rationality and validity.(3)A series of experiments including engine transient processes,fuel mode switch,cylinder deactivation under gas mode,optimizations of natural gas injection parameters,pilot injection parameters and excess air coefficient were carried out to validate the control strategy.Results indicate that the fuel consumption rate and HC emissions were reduced during cylinder deactivation under low load and gas mode,but the instantaneous speed fluctuations and NOx emissions were increased.The gas injection advance angle was increased with the increase in load and was optimized with different loads.The pilot injection quantity and injection angle were also increased with the increase in load,they were also optimized,as well as the excess air coefficient.(4)The interactions of excess air coefficient between the pre-combustion chamber and the main chamber of a spark ignition engine were analyzed,as well as the main parameters which affect the pilot fuel ignition.A 3D engine model with two different ignition methods was built in AVL FIRE software.These ignition methods are the pre-combustion chamber ignition method and the micro-injection ignition method.After then,the normal start conditions were studied in those two ignition methods and based on which different control strategies were regulated.Experimental results indicate that the air intake in the pre-combustion chamber needs to be closed at the initial start period for an engine with pre-combustion spark ignition.The excess air coefficient needs to be between 1.5-1.75 and the micro ignition quantity needs to be larger than 180 mg/cyc in order to normally start a micro ignition engine in gas mode.(5)The reasons why there were imbalances between cylinders were analyzed.A gas injection quantity modification method was proposed based on the intake manifold pressure fluctuation and a gas injection balance control method was also proposed based on the burst pressure.Both methods were validated on a test rig.Results indicate that significant burst pressure fluctuation rate reductions were achieved in both balance control methods.More specifically,the burst pressure fluctuation rate without balance control is 12% and it is reduced to 4.7% under gas injection modification method and to 4.2% under gas injection balance control method,respectively.