Introduction to knocking combustion
The desired combustion process in a spark-ignition engine is sketched in Figure 1. The fuel-air mixture is ignited at the spark plug which leads to a flame propagation through the cylinder and consumption of the whole mixture. The slope of the measured in-cylinder pressure for such a regular combustion is smooth.
Figure 1: Temporal evolution of regular combustion inside the cylinder of an internal combustion engine. The corresponding pressure trace over crank angle degree is shown on the right.
In contrast to this, knocking is seen as an abnormal combustion phenomenon. The initial phase of the combustion is similar to the regular combustion. Nevertheless, in the later phase the thermodynamic conditions in the end gas lead to auto ignition events and the development of a second reaction front (s. Figure 2). The caused pressure fluctuations vary and depending on their amplitude are referred to as moderate or strong knock. While for the latter the propagation of pressure wave and reaction front couple and lead to high amplitudes of oscillating pressure, for moderate knock those propagations are not coupled – resulting in a lower level of oscillation.
Figure 2: Temporal evolution of moderate knocking combustion inside the cylinder of an internal combustion engine – prior to complete consumption of the mixture auto ignition can be detected. The auto ignition triggers the propagation of a pressure wave and a reaction front. For moderate knock those are not coupled – the corresponding pressure trace over crank angle degree is shown on the right. Pressure oscillations are visible, which characteristic frequency is audible as a knocking sound.
When operating a spark-ignition engine the efficiency is highest close to the knock limit. Therefore, modern engines usually apply a knock control detecting moderate knock. Nevertheless, this approach is sub-optimal being a trade-off between efficiency and preventing severe damage by high pressure oscillations. With a better understanding of what primary causes lead to the occurrence of knock, improved control strategies could be developed leading to a tremendous increase in efficiency.
Introduction to the work of STFS in TP7
In this project (TP7) the STFS will address understanding of moderate knocking combustion by simulation. While most available methods aim for statistical information such as knocking probability it is the project’s goal to extend existing models to enable spatial and temporally highly-resolved simulations. In connection with experiments (TP6) those allow a joint cause-and-effect chain analysis of knocking combustion.
For this goal high-resolution simulations need to be performed. This results in three main goals:
- Development of Auto Ignition Model for LES
The high temporal resolution of the LES requires the knock model to capture transient effects. Extension of existing models is needed to account for such details in auto ignition processes. Model adaptions as well as their verification and validation are based on 2D DNS data of early flame kernels (provided by TP2 – ITV, RWTH Aachen).
- LES and backward Analysis of Knocking Cycles with External Mixture Formation
A sufficient large number of knocking cycles will be investigated by highly resolved simulations to face the statistical phenomenon of knock. The simulations are accompanied by external experiments (TP6 – VKA, RWTH Aachen) to gain fundamental knowledge. The joint cause-and-effect chain analysis will give an insight how knocking combustion in an engine with external mixture formation is initiated, develops and finally leads to the detectable effects of pressure oscillations.
- Mixture Inhomogeneities due to Internal Mixture Formation
The transition from a port fuel to a direct injection system increases the complexity of the problem dramatically. It becomes a multiphase problem which involves the interaction of fluid (fuel) and air in processes such as heat, mass and momentum transfer. The investigations of mixture inhomogeneities in a direct injection engine will be the first step towards the cause-and-effect chain analysis of knocking in such environment.
This project is only possible due to collaboration on both the numerical as well as the experimental side. The strong interconnection to the DNS expertise of ITV, RWTH Aachen (TP 2) will allow the extension of existing knock modeling. The engine experiments performed by VKA, RWTH Aachen (TP 6) will be the base for the simulation setup as well as the validation case for the use of the extended knock model in LES of knocking combustion inside an internal combustion engine. The close collaboration will allow to perform a joint experimental and numerical cause-and-effect chain analysis.
Figure 3: Collaboration plan for knock model development and Large Eddy Simulation (LES) of multiple knocking cycles performed by STFS, TU Darmstadt. Inter-connection to other research facilities as shown is crucial to this project.