封闭空间内烈性爆震及其破坏机制

Knock is a considerable obstacle of some new combustion technologies and it also prevents the further increase of compression ratio thus hinder thermal efficiency of engines from improving further. The origin and effects of conventional knock have been well studied before, however, extensive attention has been aroused again these years by severe knock which is caused by the down-sized technology and the introduction of some new technologies in recent years. Such severe knock is different from the conventional knock. The formation mechanism and destruction mechanism of severe knock haven’t been revealed sufficiently and deeply. Our research will be conducted from following aspects. 1) Reveal the detonation formation mechanism in combustion chamber when severe knock occurs. 2) Explore the behavior of non-linear waves occurring in the limited closed spaces of combustion chamber. 3) Find the convergence effect of shock waves on the heat and force transmission in engine parts. 4) Research the destruction mechanism of the oscillation force and heat on engine parts material. According to above work, on one hand, the corresponding method can be found to cut off the formation condition of detonation inhibiting the occurrence of severe knock, on the other hand, reasonable combustion chamber shapes and engine parts material constructions can be obtained to eliminate the convergence of shock waves inhibiting the destruction of engine parts under severe knock. It can provide theoretical support for the further improvement of thermal efficiency and for the application of new combustion technologies. Such work can also provide a guidance to improve engine parts material avoiding damage.

爆震是内燃机中新型燃烧方式应用以及通过增加压缩比提高热效率的重要障碍。有关常规爆震发生的成因及其影响已经开展了众多的研究，但是近年来小型强化技术及新型燃烧方式引起的烈性爆震再次引起了广泛关注。该烈性爆震不同于常规爆震，其形成机理及高烈度破坏机制还未得到充分、深入的研究。本课题将从以下方面开展研究，1）烈性爆震发生时，在封闭空间的燃烧室内爆轰形成机理的揭示。2）探索有限封闭空间内，非线性波的行为特性。3）探索激波汇聚作用对活塞等组件的受力特性的影响规律。4）研究爆轰波和热协同作用下对组件材料的破坏机制。通过上述研究，一方面找到相应措施，阻断爆轰形成的条件，从而抑制烈性爆震的发生；另一方面提出合理的燃烧室形状及组件材料构型，消除汇聚点，增强组件抗爆性以避免其在爆震下被迅速破坏。以上研究可为内燃机进一步提高热效率以及新型燃烧方式在全工况下的应用提供理论支持，也为改进组件材料以避免破坏提供指导。

围绕课题任务书中提出的研究内容和目标，主要开展以下三方面的工作，一是设计并制造了可以重现爆震现象的定容燃烧弹，采用乙炔气体作燃料和高达12焦耳的点火能量诱发爆震，研究封闭空间中爆轰燃烧的行为。二是采用FLUENT结合CHEMKIN软件，对发生的爆轰波瞬态过程进行数值模拟，详细研究封闭空间中爆轰现象及其作用。三是在一台四缸增压发动机上，研究发动机失火对超级爆震可能的影响。研究结果发现了（1）封闭空间发生爆轰后会发生波的汇聚效应。在棚顶型燃烧内，该爆轰波会有径向和轴向两个方向传播，如果一旦汇聚可能会对活塞产生破坏作用。另外， 普通内燃机的缸压传感器和测量方法无法测出封闭空间的爆轰波行为；（2）爆震在封闭空间的狭窄处会聚集并诱导出烈性爆震。活塞顶面有凹坑以及减小压缩余隙有助于减小爆震的危害；（3）爆轰波的连续作用会在活塞内继续热量。该热量无法通过自身传导到外部而致自身烧融；（4）内燃机某气缸的失火会诱发相邻气缸发生爆震。发动机表明，在低速大负荷条件下，某气缸失火其混合气排出气缸后会在高热排气道中产生爆轰以诱发其它气缸发生爆震。本研究结果阐明了封闭空间中烈性爆震破坏破坏发动机活塞的原因，并揭示了超级爆震发生的机理，为发动机避免发生爆震的结构设计提供了重要的指导。