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

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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.