Flame Characteristics and Abnormal Combustion of Methane Port Injection and Isooctane Direct Injection with Injection Timings Considered

Abstract

Natural gas (NG) is a low-carbon fuel that can achieve low carbon emissions and high efficiencies. However, the low mean molecular mass and low energy density of methane result in low engine performance, and blending methane with gasoline is an effective way of improving the performance of NG engines. In this study, flame characteristics (including abnormal combustion) and engine performance are optically studied under methane port injection and isooctane direct injection conditions. The results indicate that the addition of isooctane can significantly increase the power output of NG engines. Under partial-load conditions, the minimum ignition advance for best IMEP is delayed with the addition of isooctane because of the higher flame speed of isooctane, and isooctane addition primarily improves the turbulent flame propagation rather than the initial flame formation. As for the isooctane injection timing, the indicated mean effective pressure (IMEP) first increases and then decreases with the delay in isooctane injection. The hot spots in the flame images confirm that late injection results in isooctane inhomogeneity, which reduces the engine’s thermal efficiency. At high loads, the low energy density of methane can lower the knocking intensity, even in the presence of auto-ignition at a speed of 120.7 m/s. The knocking pressure can lead to the ejection of oil droplets, which can increase the flame speed in the subsequent cycle.