Auto-ignition and knocking combustion characteristics of iso-octane-ammonia fuel blends in a rapid compression machine

Ammonia has attracted much attention because of its carbon-free nature. Ammonia also has a high research octane number (RON), making it a promising anti-knock additive for conventional engine fuels. In this study, to investigate the effects of ammonia addition on the knocking combustion of iso-octane, a series of spark ignition experiments were conducted in an optical rapid compression machine. And compression ignition experiments were also carried out to measure the ignition delay times. The molar fractions of ammonia in iso-octane-ammonia blends were set as 80% (A80), 40% (A40), and 0% (A0), and the initial thermodynamic conditions were 630-840 K and 8-25 bar. Chemical kinetics analysis of the end-gas was also performed with a blended reaction mechanism obtained by directly merging the LLNL iso-octane mechanism and the Glarborg ammonia mechanism. The experimental results showed that the ignition delay time increased with increasing ammonia fraction, but this trend could only be qualitatively predicted by the blended mechanism. At the same initial pressure and temperature conditions, the knock intensity (KI) decreased with increasing ammonia fraction, but the inhibitory effect was less pronounced at high initial temperature conditions. At the same initial energy density and high -temperature conditions, the KI of A40 and A80 was higher than A0, which was ascribed to the higher pres-sure requirement for the blended fuels with higher ammonia fractions to achieve the same energy density as those with lower ammonia fractions. At low temperature and pressure conditions, the auto-ignition showed a two-stage characteristic. With the increasing fraction of ammonia, the first-stage auto-ignition shifted from su-personic to subsonic, and the second-stage detonative auto-ignition gradually disappeared. The weakened auto -ignition with increasing ammonia fraction was attributed to the less reactive end-gas, as evidenced by the lower mole fraction of OH radical and lower heat release rate in the end-gas at the auto-ignition timing. When using the & epsilon;-& xi; diagram to examine the auto-ignition mode, the experimental detonation cases were well distributed in the detonation peninsula, but the non-detonation and critical detonation cases were scattered irregularly.