Auto-ignition characteristics of coal-based naphtha

Coal-based naphtha features a low Octane number and is a promising fuel as potential energy supplements for advanced compression ignition engines where the auto-ignition dominates the combustion process. However, its auto-ignition characteristics including ignition delay times (IDTs), knocking propensity have not been well studied yet. In the present work, the Leeds optical Rapid Compression Machine (RCM) equipped with simultaneous pressure acquisition and high-speed photography was implemented to study the IDTs and knock properties of coal-based naphtha over a wide range of temperatures (640–900 K), pressures (1.0–2.0 MPa) and equivalence ratios (0.5–1.5). A three-component surrogate model with detailed oxidation kinetics has been developed to predict the IDTs, excitation time and heat release rate for coal-based naphtha. Both pressure traces and combustion luminosity images indicated that the onsets of knock and super-knock are related to the initiation of ‘detonation-like hot-spot’ before the arrival of reaction wave. A knock map derived from the measured pressure traces showed the knock and super-knock tendency to occur at high temperature and rich equivalence ratio regime. Moreover, the Bradley's ξ-ε diagram was adopted to classify and interpret the evolution of ignition modes of coal-based naphtha in RCM. In this context, ξ represents ratio of acoustic velocity, a, into autoignitive velocity, ua. The ε quantifies the rapidity of heat release in a hot spot and is defined as r0aτe. The combination of super-knock pressure oscillation, detonation images and ξ-ε parameters all provided the evidence that the super-knock is related to the transition of detonation.