Rapid Compression Machine Lab

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

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.


• Detonation Initiation


      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。










• Droplet Evaporation and Ignition