Spark effects on compression ignition of PRF95 direct injection spray in a constant volume combustion chamber

In this study, spark assisted compression ignition (SACI) combustion was investigated using PRF95 (a low reactivity high octane reference fuel of common commercial gasoline), a mixture of 95% isooctane (by volume) and 5% n-heptane (by volume) with a RON of 95 and compared with pure compression-ignition without spark. Characteristics like the cumulative heat release (CHR) and the heat release rates (HRR) were studied using pressure data from experiments conducted in a constant volume combustion chamber (CVCC). The combustion process was visualized with a high speed imaging technique. Tests were conducted to understand the effects of ambient oxygen concentration and temperatures on the heat release and combustion flame developments while controlling other factors. From the experiments it is evident that the peak heat release rate increases initially and then decreases with the increase in the ambient temperature and the highest peak heat release rate appears around 650-700 K. The peak heat release rate timing decreases with the increase of the ambient temperature or oxygen level. Under a low ambient temperature, the oxygen level plays a major role in affecting the peak heat release rate. Under lower oxygen levels, the flame becomes weaker, the ignition delay becomes longer, and the combustion process takes more time to complete. Proper spark timing can help advance the peak HRR and shorten ignition delay, but this effect becomes minor when the ambient temperature increases. For the current high octane low reactivity fuel, auto-ignition can be achieved for all the investigated ambient temperature and oxygen levels. But it is noticed that a spark is necessary for low ambient oxygen and low ambient temperature to achieve proper combustion.