Cycle-to-cycle variation (CCV) of engine output not only results in fluctuations of the engine torque, but also prohibits the engine from operating at its optimal design condition. Considering that it is difficult to control the sources of variation, designing a combustion system that is more robust to the root causes of variation is therefore a more viable option. This paper assesses the influence of flash boiling and swirl ratio on the characteristics and CCV of spray structure in an optical single-cylinder spark-ignition direct-injection (SIDI) engine. The experiment was carried out under cold-start condition. The spray conditions studied include liquid, transitional flash boiling and flare flash boiling. Crank angle resolved spray images were recorded in a tumble plane for 50 consecutive cycles using high-speed laser scattering technique. Results show that the flash boiling spray is strongly influenced by air flow with the penetration decreasing and the spray angle increasing with swirl ratio. Also the penetration of flash boiling spray is shorter than that of the liquid spray, which is beneficial to reduce impingement with piston. Results also show that high swirl increases the CCV of spray, but flash boiling help to mitigate the cyclic variation induced by high swirl.

发动机输出的逐周期变化（CCV）不仅会导致发动机扭矩波动，而且会禁止发动机在其最佳设计条件下运行。考虑到难以控制变化的来源，因此设计对变化的根本原因更鲁棒的燃烧系统是更可行的选择。本文评估了闪蒸和旋流比对光学单缸火花点火直接喷射（SIDI）发动机喷雾结构的特性和CCV的影响。实验是在冷启动条件下进行的。研究的喷雾条件包括液体，过渡闪蒸和火炬闪蒸。使用高速激光散射技术将曲柄角分辨的喷雾图像连续50个循环记录在一个翻转平面中。结果表明，快速沸腾喷雾受到气流的强烈影响，渗透率降低，喷雾角度随涡流比增大。而且，闪蒸沸腾喷雾的渗透比液体喷雾的渗透短，这有利于减少与活塞的碰撞。结果还表明，高旋流增加了喷雾的CCV，但闪蒸有助于减轻高旋流引起的循环变化。