Technology Research On Knock Detection And Control Of Marine Natural Gas Engine

In order to improve the power and economy,marine engines are developing in the direction of high compression ratio and high supercharge.The accompanying knocking problem will necessarily become the biggest obstacle.When marine natural gas engines work in medium and high load conditions,the working window is narrow,the allowable range of air-fuel ratio is small,and knocking is prone to occur.Controlling the engine to work under light knocking conditions can improve the operating efficiency and fuel economy of the engine.The difficulty in the realization of the control process lies in the detection of the light knocking condition.Firstly,based on the acceleration sensor installed at the position of a cylinder block,the vibration signals at different spark advance angles are measured.Through the analysis of the test data,the study found that the engine knock characteristic frequency on the test bench is in the range of 4.2～12.2 k Hz,traditional knock detection methods such as direct filtering can extract knock information under strong knocking condition,but the ability to recognize light knocking condition is insufficient.Secondly,in order to improve the ability of knock detection,this paper proposes a new knock index:based on the average energy value after discrete wavelet transform.Use the camshaft signal to select the knock calculation window,perform discrete wavelet transform on the data in the window,select the db2 of Daubechies wavelet,analyze the detail component signal D₄[4096Hz,8192Hz]to calculate the average energy value,set the knock index threshold AE under no-knocking and strong knocking conditions to 30 and 45respectively,that is,when AE ≦30,it is judged as no-knock condition,and when AE﹥45,it is judged as strong knocking condition;Combined with the occupation ratio of knock cycle,it realizes the discrimination between light knock and moderate knock conditions,that is,the knock index(AE∈（30 45,]and knock cycle occupancy rate<8%）is judged as a light knocking condition,otherwise it is judged as a moderate knocking condition.Experiments have proved that this method can effectively extract a light knocking signal from the vibration signal of the cylinder block,and realize the graded determination of knock intensity.Finally,a real-time knock analysis system was developed based on the Lab VIEW platform and NI USB-4432 data acquisition card,and the knock level was communicated with ECU using CAN bus.For marine natural gas engines,a knock hierarchical control based on the spark advance angle is proposed,combined with a torque-based speed and air-fuel ratio double closed-loop control strategy,and then the control strategy is tested and verified.The analysis result of the vibration signal is feedback to the ECU,by controlling the spark advance angle,the ECU maintains the knock index AE within the range of[30,45]in real time,realizing the hierarchical closed-loop control of knock control system.When the engine is running in no-knock conditions,slowly increase the spark advance angle to bring the engine close to the light knocking condition to improve the operating efficiency of the engine;in the moderate knocking condition,immediately reduce the spark advance angle to make the engine out of moderate knocking condition to avoid further strong knocking and protect the safe operation of the engine.