Effect of initial temperature and fuel properties on knock characteristics in a rapid compression and expansion machine

In this study, the effect of initial temperature and fuel properties (such as ignition delay time variation with temperature) on knock characteristics was investigated for methane and propane dual fuels via a rapid compression and expansion machine, which can emulate one compression and expansion stroke in a real engine. Numerical calculations using CHEMKIN-PRO with AramcoMech1.3 were carried out to obtain fuel properties. The correlation between flame propagation velocity at the moment of autoignition and knock intensity was confirmed. The findings revealed that unburned mass fraction and initial temperature did not directly affect knock intensity. Additionally, experimental results were analyzed based on theories proposed by Zeldovich and Bradley. The results of the analysis indicated that smaller gradients of autoignition delay time and temperature caused the higher flame propagation velocity and the resultant higher knock intensity. It was concluded that initial temperature affected both the gradient of autoignition delay time and that of temperature, which in turn indirectly influenced the knock intensities.