Laser Ignition of Multi-Injection Gasoline Sprays 2011-01-0659
Laser plasma ignition has been pursued by engine researchers as an alternative to electric spark-ignition systems, potentially offering benefits by avoiding quenching surfaces and extending breakdown limits at higher boost pressure and lower equivalence ratio. For this study, we demonstrate another potential benefit: the ability to control the timing of ignition with short, nanosecond pulses, thereby optimizing the type of mixture that burns in rapidly changing, stratified fuel-air mixtures. We study laser ignition at various timings during single and double injections at simulated gasoline engine conditions within a controlled, high-temperature, high-pressure vessel. Laser ignition is accomplished with a single low-energy (10 mJ), short duration (8 ns) Nd:YAG laser beam that is tightly focused (0.015 mm average measured 1/e₂ diameter) at a typical GDI spark plug location. Ignition timing is varied during, after, and between injections of a rapid, 0.4-ms/0.35-ms dwell/0.4-ms injection schedule. Results show success in igniting a single injection after the end of injection, but with poor combustion efficiency because the flame does not move downstream to earlier-injected charge. Findings are similar when igniting after the end of a double injection. Best results are observed when igniting between injections. The tail of the first injection ignites, and the second injection acts to pull the flame downstream to the first-injection charge, causing high combustion efficiency. However, the timing of ignition between pulses is critical. If ignited too soon after the end of the first injection, ignition may fail or, if ignition succeeds, the flame grows such that it immediately ignites the second injection, forming fuel-rich combustion and significant soot generation. The optimal timing produces no soot formation, but still maintains high combustion efficiency. However, accomplishment of this timing requires ignition timing control on the order of 0.1 ms, which is much shorter than current electric spark ignition systems that have spark durations on the order of 1.0 ms. Therefore, the benefits of this double-injection ignition strategy are only realized with the use of a short-pulse laser ignition system.