Effect of octane number and thermodynamic conditions on combustion process of spark ignition to compression ignition through a rapid compression machine

Spark assistance in homogeneous charge compression ignition (HCCI) is a promising method to improve combustion stability. Fundamental experiments were carried out in a rapid compression machine along with chemical kinetics analysis to investigate the complete combustion process of spark ignition to compression ignition (SICI) using ethanol-blended fuels. Five fuels, consisting of n-heptane, iso-octane and ethanol with different fractions, are divided into two groups. The fuels with different research octane number (RON) and motor octane number (MON) but identical octane sensitivity (S) are in the same group. The equivalence ratio is fixed at 0.5, and the experimental pressure covers the engine-relevant conditions (10-35 bar) while the target temperature ranges from 735 K to 860 K, overlapping most regions with negative temperature coefficient (NTC) of n-heptane and iso-octane. Results show that octane sensitivity with low RON has poor ability to evaluate fuel reactivity especially in the vicinity of beyond MON area due to low-temperature oxidation acceleration of ethanol. The influence of fuel reactivity, auto-ignition heat release amount and flame compression effect on knock intensity enhancement decreases in turn. The lower transition temperature and pressure at the time of auto-ignition is observed in the fuel with lower RON regardless of S, resulted from stronger LTHR and greater temperature rise from cool flame. The fuel with medium RON and S based on ethanol blending is more suitable for SICI combustion since it can make a better balance among knock intensity, dilution tolerance and control authority from flame in the conditions studied, which gives an insight into the effect of ethanol blends on combustion process and provides a reference for fuel design aimed at lean SICI combustion.