Micro direct-injection (DI) strategy is often used to extend the operation range of the reactivity controlled compression ignition (RCCI) to high engine load, but its combustion process has not been well understood. In this study, the ignition and flame development of the micro-DI RCCI strategy were investigated on a light-duty optical engine using formaldehyde planar laser-induced fluorescence (PLIF) and high-speed natural flame luminosity imaging techniques. The premixed fuel was iso-octane and an oxygenated fuel of polyoxymethylene dimethyl ethers (PODE) was employed for DI. The fuel-air equivalence ratio of DI was kept at 0.09 and the premixed equivalence ratio was varied from 0 to 1. RCCI strategies with early and late DI timing at –25° and –5° crank angle after top dead center were studied, respectively. Results indicate that the early micro-DI RCCI features a single-stage high-temperature heat release (HTHR). The combustion in the low-reactivity region shows a combination of flame front propagation and auto-ignition. The late micro-DI RCCI presents a two-stage HTHR. The second-stage HTHR is owing to the combustion in the low-reactivity region that is dominated by flame front propagation when the premixed equivalence ratio approaches 1. For both early and late micro-DI RCCI, the intermediate-temperature heat release (ITHR) of iso-octane, indicated by formaldehyde, takes place in the low-reactivity region before the arrival of the flame front. This is quite different from the flame front propagation in spark-ignition (SI) engine that shows no ITHR in the unburned region. The DI fuel mass is a key factor that affects the combustion in the low-reactivity region. If the DI fuel mass is quite low, there is more possibility of flame front propagation; otherwise, sequential auto-ignition dominates. The emergence of the flame front propagation in micro-DI RCCI strategy reduces its combustion rate and peak pressure rise rate.

微型直接喷射（DI）策略通常用于将反应性受控压燃（RCCI）的操作范围扩展到高发动机负载，但是其燃烧过程尚未广为人知。在这项研究中，使用甲醛平面激光诱导荧光（PLIF）和高速自然火焰发光度成像技术，在轻型光学引擎上研究了微型DI RCCI策略的点火和火焰发展。预混合的燃料是异辛烷，DI使用聚甲醛二甲基醚（PODE）的含氧燃料。DI的燃油-空气当量比保持在0.09，预混合当量比在0到1之间变化。研究了上止点后–25°和–5°曲柄角下DI提前和滞后的RCCI策略。 。结果表明，早期的micro-DI RCCI具有单阶段高温放热（HTHR）的特征。低反应性区域的燃烧显示了火焰前传播和自燃的结合。后期的micro-DI RCCI提出了两阶段的HTHR。第二阶段的HTHR是由于在低反应性区域中的燃烧，当预混合当量比接近1时，燃烧以火焰前传播为主。对于早期和晚期的micro-DI RCCI，中温放热（ITHR）由甲醛指示的异辛烷的缩合反应发生在火焰前沿到达之前的低反应性区域。这与火花点火（SI）发动机的火焰前沿传播有很大不同，后者在未燃烧区域没有显示ITHR。DI燃料质量是影响低反应性区域燃烧的关键因素。如果DI燃料质量很低，则火焰前传播的可能性更大；反之，否则，顺序自动点火起主导作用。micro-DI RCCI策略中火焰前沿传播的出现降低了燃烧速率和峰值压力上升速率。