Effects of Octane Number and Sensitivity on Combustion of Jet Ignition Engine

Octane number (ON) and octane sensitivity (S), the fuel anti-knock indices, are critical for the design of advanced jet ignition engines. In this study, ten fuels with different research octane number (RON) and varying S were formulated based on ethanol reference fuels (ERFs) to investigate the effect of S on combustion of jet ignition engine. To fully understand S effects, the combustion characteristics under EGR dilution and lean burn were further investigated. The results indicated that increasing S resulted in higher reactivity with shorter ignition delay and combustion duration. The increase of reactivity led to heavier knocking intensity. The competition between the flame speed and the reactivity of the mixture determined the auto-ignition fraction of mixture and the knocking onset crank angle as S varied. Medium S (S=3) was helpful to improve the combustion speed, reduce the auto-ignition fraction of mixture and retard the knocking onset crank angle. For high RON (RON=100) and medium RON (RON=93) fuel, medium S (S=3) could better balance the combustion and emission, achieving higher indicated thermal efficiency (ITE). Whereas, low S (S=0) was more favorable for low RON fuel (RON=85). For the same MON fuel, increasing S reduced the knocking intensity and benefited ITE. The increase of reactivity remained for higher S fuel under EGR dilution and lean burn conditions. More stable engine operation was observed for high S fuel (S>7) allowing for higher dilution tolerance and promising to achieve higher ITE. Through active jet ignition under lean burn, the combustion was improved with lower knocking intensity. Benefited from faster combustion speed and lower combustion temperature, higher ITE was achieved by high S fuel under lean burn.