Effect of changing compression ratio on ignition delay times of iso-octane in a rapid compression machine 2020-01-0338
Previous studies have shown that several facility dependent factors can influence ignition delay times measured in a rapid compression machine. Compression ratio variation can have a major impact on measured delay times due to the change in surface-area-to-volume ratio even when the same compressed conditions are maintained. In this study, iso-octane, which exhibits two stage ignition delay and has a pronounced negative temperature coefficient (NTC) region, is used to investigate the effects of several facility dependent factors on ignition delay. Resulting trends are also compared to previous results obtained with ethanol, which has very different combustion properties. In addition to the experimental results, numerical analysis is also used to gain a deeper understanding of the complex processes occurring inside the combustion chamber at a variety of different compression ratio conditions. Experiments and simulations were carried out for stoichiometric mixtures of iso-octane and air over a compressed temperature range of 700–900 K at 20 bar compressed pressure. The compression ratio was varied from 9.58 to 17.07 which also led to changes in initial conditions, compression time and surface area-to-volume ratio, which together affected the ignition delay time at a given compressed condition.