Simulation And Experimental Study On Knock Combustion In Two-stroke Kerosene Engine

Kerosene has some characteristics, such as high flash point, weak evaporation, high security, etc. It has a great advantage as a single fuel. Research on spark-ignition piston engine using kerosene as fuel has an important significance to carry the single fuel. Kerosene has a poor anti-knock performance. Knock is a unique abnormal combustion phenomenon in the spark-ignition piston engine. Knock constrains to improve power and economy of the spark-ignition piston engine, and even damage the engine block. For the spark ignition engines, the ignition system is an important component. Ignition timing, ignition energy and ignition asynchronous phase of dual-spark plug are the key parameters to the ignition system, which will have a major impact on the performance of knock combustion in engine. At present study on knock combustion characteristics of two-stroke kerosene engine, especially for the key parameters of ignition system such as ignition timing, ignition energy and ignition asynchronous phase of dual-spark plug affecting knock combustion is less at home and abroad. Therefore, research on the key parameters of ignition system affecting the knock combustion in two-stroke kerosene engine has important theoretical significance and practical value.Taking a Defense Advanced Research Project as the research background, this paper studied the related issues which were the key parameters of ignition system such as ignition timing, ignition energy and ignition asynchronous phase of dual-spark plug affecting knock combustion in two-stroke kerosene engine. The content was to open out around numerical simulation of knock combustion, slight knock signal feature extraction, knock intensity evaluation and experimental study on knock combustion. The major study work and innovation were as follows:(1) Three-dimensional CFD numerical simulation method was used to study the knock combustion characteristics of a spark ignition two-stroke piston engine which burning kerosene. The combustor model of the two-stroke kerosene engine was established. The impact of ignition system key parameters for temperature of kerosene homogeneous mixture, flame propagation speed ignition and heat release of mixture combustion was calculated. Spontaneous combustion of the mixture was described using the Shell knock model. Concentration of the reaction intermediate Q was used to evaluate the knock intensity. And the influence of the key ignition system parameters on knock intensity was analyzed.(2) A slight knock feature extraction method based on Intrinsic Mode Characteristic Energy Method was proposed in order to improve the reliability and accuracy of the slight knock feature ex-traction. In this method,the cylinder pressure signals of the two-stroke kerosene engine were decomposed into each order intrinsic mode function through the empirical mode decomposition method. Then the intrinsic mode function which contained the main intrinsic mode function feature energy was obtained used the intrinsic mode function feature energy method,and it was selected as the dominant mode analysis object for the knock signal. The knock characteristic frequency and knock detection frequency of the electronic controlled two-stroke kerosene engine was obtained through analyzing the power spectral density of this intrinsic mode function. The slight knock feature extraction method based on Intrinsic Mode Characteristic Energy Method can effectively extract the knock characteristic frequency of engine cylinder pressure signals and body vibration acceleration signals at lower SNR. And it laid the foundation for studies such as knock intensity evaluation, design of knock on-line monitoring system and experimental study on knock combustion.(3) Cylinder pressure signal knock intensity evaluation method based on intrinsic mode high-frequency component integration method and body vibration acceleration signal knock intensity evaluation method based on dominant intrinsic mode peak method were proposed. Using the cylinder pressure signal knock intensity evaluation method based on intrinsic mode high-frequency component integration method, the two-stroke kerosene engine cylinder pressure signal was decomposed by the empirical mode decomposition. The knock high frequency signal component and the ideal combustion curve were acquired. The crank angle corresponding to the peak of the ideal combustion curve was chosen as the starting angle of the knock window and the end angle of the reference window. By analyzing the width of the window, the reasonable window width was obtained. High-frequency integrated energy ratio inside the knock window and the reference window had been calculated, that is, knock factor. The knock factor was defined as the knock evaluation index to evaluate the knock intensity for the cylinder pressure signal. Using the body vibration acceleration signal knock intensity evaluation method based on dominant intrinsic mode peak method, the two-stroke kerosene engine body vibration acceleration signal was decomposed. The dominant intrinsic mode function component was acquired and defined as the knock evaluation index to evaluate the knock intensity for the body vibration acceleration signal.(4) The two-stroke kerosene engine knock on-line monitoring system was designed and developed based on dominant intrinsic mode peak method to monitor the knock intensity on line. The design of the two-stroke kerosene engine on-line monitoring system was determined. Hardware and software of the two-stroke kerosene engine on-line monitoring system were designed. The software included a signal acquisition module, a signal processing module, and a data storage module knocking judgment module. The test bench of the two-stroke kerosene engine knock on-line monitoring system was built to carry out the knock on-line monitoring test. Validity of the two-stroke kerosene engine knock on-line monitoring system was verified.(5) The two-stroke kerosene engine knocking combustion test system was built to perform the experimental research on knock combustion in the two-stroke kerosene engine. The key parameters of ignition system such as such as ignition timing, ignition energy and ignition asynchronous phase of dual-spark plug were changed to get different cylinder pressure signals of different key parameters of ignition system. Effect of key parameters of ignition system on knocking combustion characteristics was analyzed using the cylinder pressure signal knock intensity evaluation method based on intrinsic mode high-frequency component integration method. Comparative study was carried out between the three-dimensional two-stroke kerosene detonation combustion engine numerical simulation results with the two-stroke kerosene engine knocking combustion test results. The results showed that knocking combustion simulation results were consistent with the experimental results. Correctness of the three-dimensional two-stroke kerosene engine knocking combustion model was verified.