Abstract: Abstract: 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.