点燃式发动机燃烧稳定性的非线性动力学分析

为了揭示点燃式发动机燃用汽油、液化石油气(liquefied petroleum gas,LPG)和压缩天然气(compressed natural gas,CNG)时,循环变动的动力学本质。采用非线性动力学的方法,围绕燃烧过程的稳定性开展了研究。分析了最大爆发压力的循环变动系数,对缸内压力的时间序列进行了3维相空间的重构,采用庞加莱截面法简化了重构的相空间轨迹,分析了平均有效压力(indicated mean effective pressure,IMEP)的循环变动规律,围绕返回映射开展了研究。结果表明,缸内压力的相空间轨迹具有一定的混沌特性,燃烧初始阶段结构相对紧密;燃用汽油时,轨迹分布密集,燃用LPG和CNG时,轨迹逐渐发散,燃烧稳定性下降;庞加莱映射降低了相空间结构的复杂性,LPG和CNG燃烧时散点发散,燃烧参数随机性增强;汽油燃烧时,平均指示压力的循环变动较小,LPG和CNG燃烧时,循环变动逐渐增大,燃烧随机性增强。发动机燃烧过程具有混沌特性,燃用LPG和CNG时稳定性下降。通过论文的研究揭示了点燃式发动机燃烧过程循环变动的产生原因,为改善发动机的燃烧稳定性提供了理论依据。

Nonlinear dynamics analysis for combustion stability of spark-ignition engine

In order to reveal the dynamic essence of combustion cyclic fluctuations, gasoline, liquefied petroleum gas (LPG) and compressed natural gas (CNG) were burned in the ignition engine, and the cyclical fluctuations of combustion pressure were investigated using nonlinear dynamics method. The pressure data in 200 cycles were collected to discuss the regulation of in-cylinder pressure which varied with the time and to analyze cyclical fluctuation factors of the maximum break pressure. In addition, time sequences of pressure in cylinder were rebuilt three-dimensionally combined with differential coordinates, and the principles and distribution features of phase space structure changing during combustion period were studied. Phase space tracks were simplified and rebuilt by the Poincare section method to probe into the Poincare mapping points distribution of in-cylinder pressure in cross sections on whose the second derivative was zero (The second derivative of pressure was zero, Z=0) and the pressure rise rate was zero (The first derivative of pressure was zero, Y=0). The cyclical fluctuations of indicative mean effective pressures (IMEPs) were analyzed while burning 3 fuels separately. Results indicated that the cyclic fluctuation coefficient of in-cylinder pressure was 7.5% while the spark ignition engine burnt gasoline, and the coefficient increased to 7.8% and 13.8% respectively while burning LPG and CNG. The phase space track of in-cylinder pressure was a closed curve with chaos properties showing the intake and exhausting process in sharp curve and the combustion process in arc curve when the initial period structure of combustion was relatively compact and the diffusion was most obvious in the maximum of Y direction, and then it duplicated quickly after reaching the maximum at X axis direction(X axis represents pressure. Compact track distributions were observed while burning gasoline, however, there were partial combustion and lost fire cycles when LPG and CNG were employed in which the reconstructed phase space trajectory spread gradually, the degree of divergence maximized at the X axis maximum and the combustion stability declined. Poincare mapping decrease the complexity of the phase space structure effectively. Specifically, Poincare mapping points on Z=0 and Y=0 plane were distributed in the shape of ribbon. On the plane of Z=0, the slope of scatter distribution was 0.076 and the correlation between combustion parameters was strong. It showed that scatters were close together when gasoline was used, however, and mapping points diffused gradually with combustion parameters improved casually. Besides, cyclical fluctuation coefficient of the mean indicated pressure was 2.16%, cyclical fluctuations of IMEPs were slighter and the IMEPs before and after were dramatically relevant to each other, the working process was also stable when gasoline was used. On the other hand, cyclical fluctuations were increased gradually, when the slope was 0.089 and 0.085, the activation energy of the fuel was improved, the combustion speed was declined, the combustion cyclic fluctuations gradually increased to 11.1% and 15.1% respectively, the combustion stability decreased and the randomness of combustion enhanced and a certain chaotic characteristic was shown when LPG and CNG were burned. In summary, the combustion process of engine has chaotic characteristic, and the stability of combustion is declined when the engine is fueled with LPG and CNG.