Effects of Partial Oxidation and Octane Sensitivity on Flame Stretch Rate at Extinction under EGR Conditions

Potential of Renewable Fuels toward Higher Thermal Efficiency of Spark-Ignition

抄録

The regulation of fuel economy of the transport sector has become strict to prevent global warming, and hence improvements of thermal efficiency of spark-ignition (SI) engines has an urgent necessity. Major solutions to increase thermal efficiency of SI engines are diluted combustion by exhaust gas recirculation (EGR) and an increase of compression ratio. On the other hand, a highly diluted mixture makes flame susceptible to stretch, which leads to deterioration of burning velocity and thermal efficiency. In addition, a high compression ratio oxidizes the air-fuel mixture before the arrival of propagating flame, and hence changes in chemical species and reactivity in the mixture due to the partial oxidation could affect dilution tolerance. In recent years, bio-fuel and synthetic fuel made from “green-hydrogen” have also received attention for the carbon neutralization of existing vehicles. These fuels have the capability to optimize their composition via their production path to increase octane number and octane sensitivity, the difference between research octane number (RON) and motor octane number (MON), to prevent knocking, whereas an increase in octane sensitivity suppresses the partial oxidation in a mixture, which could also influence dilution tolerance. Based on the background, effects of the partial oxidation and octane sensitivity on stretched flames under diluted conditions should be clarified for extension of dilution limit and thus improvement of thermal efficiency of SI engines with these renewable fuels. The paper presents a discussion of the effects of octane sensitivity on dilution tolerance for SI engines with high compression ratios. For this purpose, stretched flames with fuels that have various octane sensitivities are simulated using detailed chemical kinetics with an opposed-flow flame reactor model. The inlet mixtures of the opposed-flow flame reactor have different reaction progress variables to consider effects of the partial oxidation in unburned mixtures which are pronounced in high compression ratios. The mixtures are also diluted by complete combustion products to simulate EGR. Fuels with high and low octane sensitivities are examined with changing blend ratio of iso-octane, n-heptane, and ethanol. The simulation result shows that the fuel with the high octane sensitivity has a lower flame extinction limit under the diluted condition. For the partially oxidized condition, the extinction limits of both fuels extend and the values reach maximums at certain reaction progress variables. Furthermore, the high octane sensitivity fuel has the higher reaction progress variable at the maximum extinction limit than the low octane sensitivity fuel. This result suggests that the high octane sensitivity fuel requires a higher compression ratio to enhance dilution tolerance. For further understanding of the mechanism of the relation between the reaction progress variable at the maximum extinction limit and octane sensitivity, the results of sensitivity analysis and reaction path analysis are also discussed.