Effect of Changing Compression Ratio on Ignition Delay Times of Iso-Octane in a Rapid Compression Machine

Previous studies have shown that several facility dependent factors can influence ignition delay times measured in a rapid compression machine. Compression ratio variation represents one such aspect of many facility-to facility differences in RCMs, and can have a major impact on measured ignition delay times due to changes in surface-area-to-volume ratio, initial conditions and compression duration 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 changing compression ratio on ignition delay. Resulting trends are also compared to previous results obtained with ethanol, which has very different combustion properties. Experiments were carried out for rich mixtures (ϕ = 1.3) of iso-octane and air over a compressed temperature range of 675-900 K at 20 bar compressed pressure. Two compression ratios are considered for each case with initial temperature and pressure adjusted in conjunction to achieve the identical compressed condition. The compression ratio was varied from 6.8 to 17.1 which also led to changes in initial conditions, compression time and surface area-to-volume ratio, all of which together affected the ignition delay time at a given compressed condition.