A numerical study of the effect of changing compression ratio on the auto-ignition of iso-octane in an RCM

The impacts of several rapid compression machine (RCM) parameters on ignition delay time were explored with 0D CHEMKIN simulations that consider the volume profile when accounting for the heat losses that occur during the compression and post-compression periods. Five compression ratios were considered, each with a unique combination of initial conditions, t50 and post-compression heat loss. The models were used to simulate the autoignition of iso-octane from low to high temperatures at an equivalence ratio of phi = 1.3 and a compressed pressure of Pc = 20 bar. The results show that the method used to obtain the compressed condition, particularly the use of different compression ratios and diluent gas compositions, can strongly influence the ignition delay time, especially under two-stage ignition conditions. Comparison with the altered volume profiles shows that a reduction in post-compression heat losses leads to shorter ignition delay times. Furthermore, for conditions at which the ignition delay time is short (< 15 ms), longer t50 initiates reactivity prior to the end of the compression stroke, thus reducing the ignition delay time. The influence of t50 and post-compression heat loss at different equivalence ratios and on different diluent gases is also studied. This range of simulation conditions helps provide a diverse array of auto-ignition results, involving single-stage and two-stage auto-ignition.