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利用锥形腔或者分束器直接对高能激光进行分束存在一些应用方面的挑战。首先,所用的光路系统比较复杂[31]。其次,高能量密度的激光辐射和热效应会破坏吸收介质和光学涂层,减少激光点火系统的寿命[31]。
另一个实现多点激光诱导火花点火的方法是利用多光束输出的微片激光器实现多点点火。微片激光器体积小、结构简单,可产生兆瓦量级峰值功率的激光,广泛用于激光诱导火花点火[32-34]。为缩小系统体积,提高能量转换效率,微片激光器通常采用半导体激光器作为抽运源[35-36]。为提高脉冲峰值功率,微片激光器通常采用调Q技术缩短脉宽并提高脉冲能量[37-38]。国内学者YANG[39]和MA[40]等人对用于激光火花点火的激光光源的研究状况进行了总结。用于激光等离子体诱导点火的微片激光器通常采用Nd:YAG或Yb:YAG晶体作为增益介质[39]。此外,可用于激光点火的基于复合材料的被动调Q固体激光器亦受到研究人员的广泛重视[41]。为了产生多束激光,多点点火用微片激光器通常采用多束抽运光抽运同一个增益晶体[31]。这种方法可降低光路的复杂性并提高激光多点点火系统的寿命。
多束抽运光可来自多个独立的抽运源。2011年,NICOLAIE等人[42]开发了一种两光束输出的峰值功率为几个兆瓦的被动调Q Nd:YAG/Cr:YAG微片激光器。图 11和图 12分别为双光束输出的被动调Q Nd:YAG/Cr:YAG微片激光器示意图和实物图。研究人员采用多个抽运线路和一个复合陶瓷的Nd:YAG/Cr4+:YAG微片结构实现了两光束输出,这种方法减少了单位体积的热积累,同时简化了光路。与之前多采用分离的Nd:YAG和Cr:YAG晶体的微片激光器不同,这台激光器中的Nd:YAG和Cr:YAG均为陶瓷且通过一定工艺将其键合在一起。Cr:YAG饱和吸收体的初始透过率为30%。抽运能量为26.2mJ时,输出脉宽为800ps,脉冲能量为2.5mJ,产生了空气击穿现象。将抽运光的重复频率升高到100Hz时,仍可实现稳定的脉冲输出。
同年,NICOLAIE等人[43]搭建了一个三光束输出的被动调Q Nd:YAG/Cr:YAG微片激光器。该微片激光器可用于内燃机多点激光诱导火花点火。图 13和图 14为三光束输出的被动调Q Nd:YAG/Cr:YAG微片激光器示意图和实物图。研究人员用3个独立的抽运源抽运一个复合全陶瓷Nd:YAG/Cr:YAG谐振腔以产生多光束输出。Cr:YAG饱和吸收体的初始透过率为30%。输出激光的脉冲能量为2.4mJ,峰值功率2.8MW,重复频率5Hz。如图 14所示,激光器成功实现空气击穿。与高能激光分束法相比,这种方法避免了高能激光对光学元件的损害且降低了光路的复杂性。但是需要用到多个独立的抽运源,因而增加了系统的成本。
多束抽运光亦可来自同一个抽运源。2014年,WANG等人[44]将激光二极管抽运的单片固体激光器与达曼光栅组合,开发出一种阵列抽运多光束输出的被动调Q Nd:YVO4激光器。如图 15所示,使用一个达曼光栅将808nm激光二极管发出的抽运光衍射为一个2×2的抽运光束阵列,然后用抽运光抽运一个单片Nd:YVO4激光晶体,最终产生2×2阵列的多光束输出。Cr:YAG饱和晶体对1064nm激光的初始透过率为95%。研究发现,当抽运功率为7.44W时,微片激光器的最大输出功率为366mW。激光器脉宽为155ns~175ns,重复频率为80kHz~300kHz,脉冲能量范围为0.2μJ~0.5μJ,脉冲峰值功率1W~2W。此前学者的研究发现在空气中产生击穿现象的激光能量密度需达到100GW/cm2数量级。由于没有足够的峰值功率,此台激光器尚且不能用于内燃机的激光诱导点火。但是他们的研究为小体积低成本激光多点点火点火系统的研究提供了新的思路:达曼光栅可被用于对抽运光进行分束,从而减少抽运源数目,进一步缩小系统体积降低成本。
2015年,MA等人[45]开发出一种用2×2微透镜阵列进行抽运的4光束多脉冲输出的Nd:YAG陶瓷激光器。与Nd:YAG单晶相比,Nd:YAG陶瓷具有成本低、尺寸大和易于加工的优点。如图 16所示,研究人员用波长为808nm的光纤耦合脉冲激光二极管作为抽运源,然后用一个2×2微透镜阵列将抽运光分为4束进行抽运。Cr:YAG饱和吸收体对1064nm激光的初始透过率为80%。4束激光包含的脉冲数目分别为5个、3个、2个和3个。4束激光的总脉冲能量分别为0.70mJ,0.45mJ,0.31mJ和0.55mJ。对应的频率分别为10.8kHz,7.2kHz,6.8kHz和5.2kHz。单个脉冲能量为0.12mJ~0.22mJ,脉宽宽度为10.5ns~11.5ns。这套微片激光器激光点火系统体积小、成本低,非常适合用于激光诱导火花点火。
表 1中总结了过去20年中出现的多点激光诱导火花点火方法。由表中可以看出,研究人员普遍采用调Q Nd:YAG激光器作为系统的光源。这种激光器具有较高的脉冲能量和纳秒级的脉宽,能够击穿空气,产生等离子体并点燃可燃气体混合物。甲烷空气混合物和氢气空气混合物是点火实验中常见的两种可燃气体混合物。使用不同的方法,研究人员实现了激光单点、两点和多点点火。实现激光多点点火的方法包括利用锥形腔收集剩余激光能量,利用喷射孔实现二次点火,利用分束器、光栅、衍射透镜、空间光调制器对单束点火激光进行分束以实现多点点火。利用锥形腔和喷射孔实现的激光多点点火系统,光路简单,但需要改变现有发动机的燃烧室结构。利用对点火激光进行分束的方法能够实现多点激光点火,尤其是基于空间光调制器实现的多点激光点火系统已在实际的发动机中经过了测试并获得了较为理想的实验结果,但是这种方法需要较为复杂的光路系统。
表 1 基于调Q Nd:YAG激光器的多点激光诱导火花点火方法
年份 光源 脉宽/ns 脉冲能量/mJ 混合气体 方法 激光点火 点火距离/mm 参考文献 1999 调Q Nd:YAG激光器 7 80 甲烷-空气 铝锥形腔 单点 — [25] 2000 调Q Nd:YAG激光器 5.5 18.5 甲烷-空气,氢气-空气 两个光源 两点 37.5 [24] 2001 调Q Nd:YAG激光器 7 80 甲烷-空气 锥形腔 两点 10 [26] 2003 调Q Nd:YAG激光器 7 80 氢气-空气 锥形腔 三点 — [27] 2005 调Q Nd:YAG激光器 5 24 氢气-空气 50%分束器 两点 184 [29] 2005 调Q Nd:YAG激光器 5 24 氢气-空气 衍射透镜 三点 5 [29] 2009 调Q Nd:YAG激光器 7 80 甲烷-空气 喷射孔和锥形腔 五点 20 [28] 2014 调Q Nd:YAG激光器 10 — — SLM 七点 5 [15] 2016 调Q Nd:YAG激光器 6~10 100 甲烷-空气 分束器 两点 5 [17] 2017 调Q Nd:YAG激光器 10 60 汽油-空气 SLM 两点 4 [30] 表 2中总结了可用于多点激光诱导火花点火的多光束输出微片激光器的研究进展。T0为可饱和吸收体Cr:YAG的初始透过率。由表 2可以看出,多光束输出微片激光器多采用Nd:YAG陶瓷材料作为增益介质。较低的Cr:YAG可饱和吸收体初始透过率可以实现高脉冲能量短脉宽的激光输出。实现微片激光器多光束输出的方法包括利用多条抽运线、利用达曼光栅和利用微透镜阵列。其中利用多条抽运线实现的多光束微片激光器输出的激光脉冲,脉冲能量达到几个毫焦,脉宽为纳秒量级,达到了击穿空气并产生等离子体的要求。利用达曼光栅和微透镜阵列实现的多光束输出微片激光器由于采用了较高初始透过率的可饱和吸收体,输出脉冲的峰值功率较低。
表 2 可用于多点激光诱导火花点火的微片激光器
内燃机激光多点点火技术研究进展
Review of multi-point laser ignition for internal combustion engines
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摘要: 稀薄燃烧能够提高内燃机的热效率并降低污染物的排放, 但稀薄燃烧的火焰传播速度慢且在高压下易出现局部淬火现象。激光诱导火花点火能够有效解决燃料在低当量比和高压下燃烧遇到的问题,此外激光点火能够实现多点点火从而缩短燃烧时间并增大燃烧室压力, 相较于传统的电火花塞点火技术具有很大优势。锥形腔、衍射透镜、空间光调制器和达曼光栅均已被用于实现多点激光诱导火花点火。归纳了多点激光诱导火花点火的几种技术途径,讨论了内燃机多点激光诱导火花点火的研究状况和最新成果。对实现多点激光诱导火花点火的几种方法进行了评价,并指出了每种方法在多点激光诱导火花点火中的优势和需要解决的问题。在此基础上,对内燃机激光多点点火技术的研究前景进行了展望。Abstract: Lean combustion can improve the thermal efficiency of an engine and reduce the emission of pollutants. But lean combustion is challenged by the low flame propagation and the possible local quenching of the initial flame kernel near the lean limit at high pressures. Laser-induced spark ignition can effectively solve these problems residing in the combustion process of the fuel at low equivalence ratio and high pressures. In addition, the laser-induced spark ignition can achieve multi-point ignition easily and can reduce the combustion time and increase the combustion pressure significantly. Therefore, laser induced spark ignition has many advantages over the traditional electric spark plug ignition technique. Conical cavity, diffractive lens, spatial light modulator and Dammam grating have been used to achieve the multi-point laser-induced spark ignition. Several technical approaches of multi-point laser induced spark ignition are summarized. The research status and the latest achievements of multi-point laser induced spark ignition for internal combustion engines are discussed. Several methods to achieve multi-point ignition of laser-induced spark are evaluated. Furthermore, the advantages and problems to be solved for each method in the multi-point ignition of laser induced spark are pointed out. On the basis of the above examinations, some suggestions on the future work are also proposed.
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表 1 基于调Q Nd:YAG激光器的多点激光诱导火花点火方法
年份 光源 脉宽/ns 脉冲能量/mJ 混合气体 方法 激光点火 点火距离/mm 参考文献 1999 调Q Nd:YAG激光器 7 80 甲烷-空气 铝锥形腔 单点 — [25] 2000 调Q Nd:YAG激光器 5.5 18.5 甲烷-空气,氢气-空气 两个光源 两点 37.5 [24] 2001 调Q Nd:YAG激光器 7 80 甲烷-空气 锥形腔 两点 10 [26] 2003 调Q Nd:YAG激光器 7 80 氢气-空气 锥形腔 三点 — [27] 2005 调Q Nd:YAG激光器 5 24 氢气-空气 50%分束器 两点 184 [29] 2005 调Q Nd:YAG激光器 5 24 氢气-空气 衍射透镜 三点 5 [29] 2009 调Q Nd:YAG激光器 7 80 甲烷-空气 喷射孔和锥形腔 五点 20 [28] 2014 调Q Nd:YAG激光器 10 — — SLM 七点 5 [15] 2016 调Q Nd:YAG激光器 6~10 100 甲烷-空气 分束器 两点 5 [17] 2017 调Q Nd:YAG激光器 10 60 汽油-空气 SLM 两点 4 [30] 表 2 可用于多点激光诱导火花点火的微片激光器
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