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Rapid Compression Machine Lab

Intro block State Key Laboratory of Automotive Safety and Energy | Center For Combustion Energy

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Introduction to the laboratory

To achieve the goals of energy saving and emission reduction, enhancing compression ratio and better combustion organization are required. The main obstacle to further improving the power and efficiency of internal combustion engines (ICEs) is knock or deto-knock.  The suppression of detonation consists of many scientific issues, including chemical kinetics, shockwave, turbulence, and the interaction of those factors.

The Rapid Compression Machine (RCM) laboratory is affiliated to the State Key Laboratory of Automotive Safety and Energy at Tsinghua University. The THU-RCM is able to decouple the complex factors in ICEs, i.e. turbulence and inlet temperature, thus providing resolution to practical problems under engine-relevant conditions. Meanwhile, THU-RCM can adapt to wide temperature and pressure range. Apart from multiple existing measurement methods, including pressure trace, visualization access, and intermediate species sampling, it provides even broader test range with the connection to one-dimensional detonation cavity and time of flight mass spectrometer (TOF-MS). Main fundamental studies in our lab are

         1) Development and validation of chemical kinetics mechanisms;

         2) Dynamics of deflagration-detonation transition inside confined space;

         3) Evaporation and combustion of a single droplet.

Besides, research focused on practical problems include:

         1) 3-D CFD simulation coupled with detailed chemical kinetics;

         2) Prediction and suppression of engine knock;

         3) Mechanism of pre-ignition that originates from particles and oil droplets;

         4) Fuel design and novel combustion modes development.

Since established in 2013, many projects have been assigned and completed in THU-RCM, including National Natural Science Foundation programs, 863 and 973 projects. International cooperation was also carried out with famous enterprises like GM, Commins and Afton. Up to now, about 20s SCI and 10s EI papers have been published. As recognized, THU-RCM attributes much to the studies about engine knock and reveals the essence of deto-knock in ICEs as detonation for the first time. The principal investigator of our lab, Professor Zhi Wang, was invited by Progress of Energy and Combustion Science to write a review named ''Knocking combustion in spark-ignition engines'', published in 2017. With such efforts, the works of RCM Lab provide solutions to engine knock in practical applications and give more insights into the mechanism of detonation initiation.

 

• Deto-Knock

Occurrence of sporadic super-knock is the main obstacle in the development of advanced gasoline engines. By utilizing a rapid compression machine, events of pre-ignition and super-knock in a closed system under high temperature and high pressure were captured by synchronous high-speed direct photography and pressure measurement. Two different types of engine super-knock could exist. The first type is the super-knock induced by pre-ignition followed by deflagration of the end-gas. This type of super-knock is quite similar to conventional knock and usually causes moderate pressure oscillation. The second type of super-knock exhibits significantly higher magnitude of pressure oscillation than that of the first type due to the detonation of the end-gas. The second type of super-knock is designated as ‘‘Deto-knock’’.

Three conditions must coexist for deto-knock to occur. First, pre-ignition triggers the combustion. Second, end-gas pressure and temperature are high enough to cause detonation. Third, local hot-spot exists in the end-gas that triggers the detonation of the end-gas. The mechanism of deto-knock could be described as hotspot-induced deflagration followed by hot-spot-induced detonation in the end-gas.

The above mechanism obtained in the RCM can also be used to explain the phenomena of super-knock in boosted gasoline engines, as shown in the following Figure. First, pre-ignition occurs before TDC due to a local hot-spot (oil, deposit, oil-gasoline, etc.) in the combustion chamber during the compression stroke. A pre-ignition-triggered flame propagates from the hot-spot to the rest of the mixture. Then, the spark ignition occurs, and the 2nd flame front may propagate if the spark ignition is in an unburned zone. The rapid expansion of the burned gas rapidly compresses the unburned mixture to higher temperature and pressure (about 1000 K, 10 MPa). Finally, a second hotspot (or multiple hot-spots) in the end gas induces the detonation of the un-burned mixture at high temperature and high pressure.

As the timing of the “hot spot” combustion in the unburned mixture is crucial to detonation, this mechanism also helps to explain why an earlier pre-ignition does not always lead to a higher knock intensity. If the “hot spot” appears too early, the in-cylinder pressure and temperature are relatively low. It may turn out to be a deflagration, similar to the combustion processes in the 1st stage. If the “hot spot” appears too late, the majority of the mixture has already been consumed by the deflagration, and pressure tends to decrease with the downward movement of the piston. As a result, the pressure rise and pressure oscillation will be smaller. If the “hot spot” starts near TDC, it is likely to trigger detonation under high pressure and high temperature conditions.

 

The energy density of the unburned end-gas mixture at the onset of knock was identified as a criterion for super-knock. For gasoline fuel in the test engine, when the energy density of the unburned end-gas mixture exceeded 30 MJ/m3, super-knock was always observed. For lower energy densities, knock or non-knock was observed.

REPRSENTATIVE PAPERS

• Detonation Initiation

       高温高压封闭体系内的起爆和传统的爆燃转爆轰(DDT)机理有所不同,由于约束壁面的影响,原本在自由空间中会熄灭的爆轰能够经由反射聚焦而强化,得以传播。而传统的火焰加速机理同样也适用,如下图所示,火核发展过程逐渐形成皱褶火焰面,逐渐加速抵达壁面,接近壁面处的未燃气体收到压缩,发生自燃,产生的激波在壁面附近马赫反射,最终形成爆轰波。

      本课题组通过可视化快速压缩机实验,已经探明高温高压封闭体系内的两种起爆模式:激波壁面反射起爆(shock wave reflection induced detonation,SWRID),激波与火焰面交互起爆(shock wave and flame front induced detonation, SWFID),二者都是由于末端自燃产生的激波到达界面,发生了形态转变,形成压力温度骤升的局部热点,使得激波与反应面耦合,发展成为爆轰,两种起爆模式的示意图如下。

SWRID:

SWFID:

1.SWRID

       由边缘点火产生的火焰面向下传播的过程中,同时产生了弱激波,入射激波在壁面处发生马赫反射,随着夹角不断减小,三波点贴近壁面,滑移线与壁面之间急剧压缩形成高温区,三波点的激波汇聚导致局部热爆炸,化学反应面得以和激波面耦合产生爆轰。温度(左)、压力(右)。

2.SWFID

       中心点火时,我们还观察到火焰面附近的起爆现象,SWFID。首先,火焰传播过程引发了末端自燃,上方的自燃产生了向下传播的反应面和激波,激波在抵达中心火焰面时,会产生折射,和三波点有着类似效果,形成了界面上的局部热点,最后起爆,同样在后续的传播中,在近壁处产生了更强的二次起爆,即SWRID。第一列:温度(左)、压力(右),第二列:HCO(左)、OH(右),第三列为同步实验图片。

代表性论文: 

 

 

• Droplet Evaporation and Ignition

机油液滴是发动机超级爆震的主要早燃源,对单个液滴的蒸发与着火过程的研究有助于探明液滴诱发早燃的机理,提供消除早燃源的方法。因此,本课题组建立了基于快速压缩机研究液滴蒸发与着火过程的实验方法。

1.常压高温条件下液滴蒸发与着火过程

常压高温条件下,机油液滴将经历较长的吸热时间,同时,其中的轻组分会率先蒸发并引发液滴表面微爆、形成燃油蒸气的周期性喷发。机油液滴火焰面形成于较远位置,这是由于轻组分喷发并扩散至周围环境导致的。

2.高温高压条件下液滴蒸发与着火过程

在快速压缩机中使用微米级石英丝挂滴方法,借助高速相机和长工作距离显微镜,能够很好地捕捉液滴蒸发与着火过程。在着火过程中,由于温度上升,机油表面张力及粘性下降,出现了液滴分离的现象,液滴着火后,进而引燃周围可燃混合物气。

3.大气环境中异辛烷液滴蒸发过程

本课题还开展了常温常压下,燃料(异辛烷)液滴蒸发过程的研究。实验结果可供验证液滴蒸发数值模型的准确性,同时提供与机油液滴的对照。

 

针对机油液滴蒸发过程建立了多组分瞬态一维液滴蒸发数学模型,描述液相、气相与相界面热力学、动力学过程,可分析液滴内部温度、组分浓度、液滴半径与寿命等随时间及环境条件的变化过程。下图显示高温高压环境中液滴温度随半径的分布,同时随时间的变化。

       模拟计算所用数学模型如下:

       气相过程:

        

       液相过程:

       相平衡:

       下列结果为液滴内部温度分布:

 

 

 

 

 

 

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