可变湍流和高能点火对混合稀燃稳定性的协同影响机理研究

Lean-burn in gasoline engine could break the limit of traditional stoichiometric ratio combustion and realize low temperature combustion. It will improve thermal efficiency and lower emission. However, lean-burn has great limitation in both operating load range and combustion stability. This is also the main reason why lean-burn cannot be extended to engineering applications. To overcome this disadvantage of existing lean-burn, in this project, a mechanism and regularity research, which is about effects on SI-HCCI stability by high energy ignition and variable reducing intake, will be proposed. Based on visualization and multi-parameter collaborative measurement test, the effects on ignition, flame propagation and auto-ignition with ignition energy and turbulence intensity in different concentration lean-burn, will be investigated in an optical engine. According to the test, a mixed lean-burn model will be developed. The chemical process of lean-burn and controlling mechanism of flame propagation and auto-ignition will be studied. On the basis of test and model, a proportion estimation model of mixed combustion modes will be developed to estimate the proportions of flame propagation combustion and auto-ignition in mixed lean-burn.Based on the model, a method, which is about controlling the proportions of flame propagation combustion and auto-ignition with ignition energy and turbulence intensity, will be purposed to improve operating load range and combustion stability of lean-burn.

汽油机稀薄燃烧能够突破传统当量空燃比附近的燃烧控制局限，并能实现低温燃烧，进一步提高汽油机热效率，减少排放产物。但是稀薄燃烧无论在运行负荷范围上还是燃烧稳定性上都存在较大的局限性，这也是稀薄燃烧无法推广到工程应用的主要原因。为此，本课题拟利用高能点火和可变湍流强度渐缩气道进行点火能量和湍流强度对混合稀燃稳定性影响机理研究。在光学单缸机上利用可视化与多参数协同测量试验研究点火能量和湍流强度对不同浓度稀薄混合气着火、火焰传播和自燃着火过程的作用机制。并利用在试验基础上建立的稀薄混合燃烧计算模型，研究稀薄混合燃烧的化学反应过程以及控制火焰传播和自燃的机理。根据试验和计算结果，建立以混合燃烧模式比例判断模型，利用燃烧过程中的关键参数指示混合燃烧中火焰传播燃烧和自燃的比例。基于该模型提出不同空燃比浓度下以可变点火能量和湍流强度来控制火焰传播和自燃比例的方法，从而提升稀燃极限和燃烧稳定性。

稀薄燃烧能大幅提高汽油机的热效率，部分燃烧甚至失火使稀燃的应用范围受到限制，提高点火能量和滚流强度能有效解决这一问题。为了清晰的观察缸内的状态，采用配备光学玻璃缸套的单缸发动机。粒子图像测速技术（PIV）用于对进气道高滚流改造进行量化评价，相比原始进气道，滚流比为原来的1.67倍。点火能量从65mJ增加到300mJ。在无油喷射状态下，高能点火的电弧更长，但长电弧有被短路的可能。长电弧持续时间更长，峰值面积约为普通点火的4倍。试验研究了点火能量和滚流强度在不同过量空气系数下对燃烧稳定性，稀燃极限忽然燃烧过程的影响。结果表明，高能点火能使稀燃极限的过量空气系数扩大0.2，而高滚流能使燃烧循环变动降低50%。在微观层面上，火焰变动存在显著差异。高能点火较大的火焰变动有助于初始火核的形成和火焰的传播。MFB（Mass Fraction Burned）50-90这段时间内稳定的火焰对整体燃烧稳定性起主导作用，是高能点火能扩大稀燃极限的重要原因。.此外，本项目进行了废气再循环(EGR)缸内稀释燃烧技术､空气缸内稀释燃烧技术与原机燃烧的经济性､排放特性对比试验研究｡研究了不同缸内稀释技术对发动机性能和排放影响的变动规律,并对比分析了相同稀释率下､采用不同稀释技术时发动机的性能变化｡结果表明:空气稀释率在49.5%时比油耗相比原机下降6.2%,而EGR稀释率在20.5%时经济性改善4.2%,在相同稀释率时,EGR稀释可采用更为提前的点火角实现更优的燃烧相位,但空气稀释所带来的多变指数提升使其经济性优于EGR稀释,且发动机燃烧系统对空气稀释程度具有更强的容忍性;NOx排放在空气稀释率为11.0%时达到峰值水平,随后随着稀释率的提高不断下降,而EGR稀释的NOx排放随着稀释率的提高持续大幅下降;空气稀释的CO排放水平远低于原机,EGR稀释的CO排放随着稀释率的增加而略有下降;对于HC排放,空气稀释的排放量低于EGR稀释,而当空气稀释率由49.5%增加为68%时,HC排放出现较大幅度上升｡