Pulse train ignition with passively Q-switched laser spark plugs under engine conditions

除了稀薄燃烧和小型化外，还采用了新的燃烧工艺来改进内燃机。如今，汽油直接喷射是最先进的技术。这一概念使非常稀薄的燃烧没有大的循环波动，并增加了发动机的功率密度。然而，汽油的直接喷射会导致更高的碳烟颗粒排放。因此，排气后处理系统的成本和复杂性显著增加。压缩天然气(CNG)是一种很有前途的直接燃油喷射替代燃料。CNG的高爆震极限可以实现更高的压缩比，从而提高发动机的热效率。此外，研究表明，CNG的应用减少了排放。与汽油相比，天然气的碳氢比更低，二氧化碳排放量最多可减少25%。在这个项目中，到目前为止已经研究了激光脉冲串点火对点火过程的影响。研究了激光到等离子体的能量传递以及火核的发展和传播。在此基础上，研究了脉冲串点火后流量和升压对点火过程的影响。实验研究表明，在加压定容燃烧室中，用激光火花塞替代传统的火花塞可以改善流动的甲烷/空气混合物的着火性能。可以扩大稀薄极限，提高甲烷/空气流动的速度。此外，与单脉冲激光点火和火花点火相比，激光脉冲串点火具有更高的燃烧效率。在本项目的实施过程中，对甲烷/空气混合物点火的研究将扩展到分层装药的被动调Q直接注入甲烷激光点火。压缩冲程中的直接喷射可以实现更高的倾斜极限，而不会显著增加循环变化。在此背景下，被动调Q激光火花塞脉冲串点火具有很大的发展潜力。气体射流旁边的脉冲串分布可以扩大火焰核心，从而支持可靠的点火。此外，通过激光点火，可以选择距离燃烧室壁较远的最佳点火位置。

Beside lean combustion and downsizing, new combustion processesare applied to improve internal combustion engines. Nowadays, directinjection of gasoline is state of the art. This concept enablesextremely lean combustion without large cyclic variations and theincrease of engine power density. However, the direct injection ofgasoline leads to a higher emission of soot particles. Thus, the costsand the complexity of the exhaust after-treatment system increasesignificantly. Compressed natural gas (CNG) is a promisingalternative fuel regarding direct fuel injection. The high knock limit ofCNG enables higher compression ratios and therefore a higherthermal efficiency of the engine. In addition, studies indicate that theapplication of CNG decreases the emissions. The lower C/H-ratio ofCNG compared to gasoline can lead to a reduction of CO2 emissionof up to 25 %. In this project, the effect of a laser pulse train ignitionon the ignition process has been examined so far. The energy transferfrom the laser to the plasma as well as the flame kernel developmentand propagation were investigated. Based on these results, theinfluence of a flow and of an elevated pressure on the ignition processafter pulse train ignition were studied. The experiments wereperformed with a premixed and homogeneous methane/air mixture.The studies have shown that the ignitability of a flowing methane/airmixture in a pressurized constant volume combustion chamber can beimproved by using a laser spark plug instead of a conventional sparkplug. The lean limit can be extended and the flow velocity of themethane/air flow can be increased. Furthermore, the application ofthe laser pulse train ignition leads to a more efficient combustion incomparison to single pulse laser ignition and to spark ignition. In thecontinuation of this project, the research of the ignition of premixedmethane/air mixtures shall be extended to the passively q-switchedlaser ignition of direct injected methane with stratified chargeoperation. The direct injection during the compression stroke enablesa higher lean limit without a significant increase of cyclic variations. Inthis context, the pulse train ignition with passively q-switched laserspark plugs possesses a great potential. The distribution of pulsetrains next to the gas jet can enlarge the flame kernel and cantherefore support a reliable ignition. Furthermore, with laser ignitionan optimum location for ignition can be chosen which may be at largedistance to the combustion chamber wall.