9 MTZ worldwide 3/2003 Volume 64 MATERIALS Titanium Knocking combustion in spark-ignition (SI) engines is a phenomenon that has always accompanied the development of internal combustion engines. In addition to unwanted noises, this uncontrolled form of combustion can damage the engine severely. The pre- sent article reports on knock characteristics, modern knock control systems and IAV GmbH's methodology for the adaptation of these systems to production-vehicle engines. 1 Introduction A number of development projects were carried out in the 20 th century, resulting in the optimisation of fuel characteristics, the development of combustion chambers with higher knock resistance and the intro- duction of knock control systems. The in- creasing use of more complex engine com- ponents calls for sophisticated knock signal detection systems. 2 Thermodynamics Extensive research projects [1] have helped to determine the most important influenc- ing factors and processes that cause knock- ing combustion. The term “knocking com- bustion” describes self-ignition processes of end-gas zones in the combustion chamber before the actual combustion flame. These self-ignition centres are often located in the direct vicinity of the combustion chamber walls, i.e. at a distance from the flame front, Figure 1, [2, 3]. The spontaneous propagation of the re- action fronts is very often inhomogeneous with sequential, apparently uncontrolled ignition of adjacent mixture pockets and with a shock wave reaching propagation velocities of up to 600 m/s. This means the reaction fronts reach the range of end-gas sound velocity and can cause thermal ex- plosions which damage the engine's valves, cylinder head gasket, pistons or piston rings, especially at high engine speeds. At lower engine speeds, knocking combustion primarily results in unwanted engine nois- es and low engine torque when driving off. The knock events that take the form of detonations lead to high-frequency cylin- der pressure vibrations that decay expo- nentially. The excited natural frequencies of the gas column enclosed in the hollow cylinder can be computed on the basis of the general wave equation [4,5]. Figure 2 shows the frequencies computed for five vi- bration modes in relation to the cylinder di- ameter at constant sound velocity. The occurrence of knocking combustion is strongly influenced by the cyclical cylin- der pressure variations that are typical of SI engines as well as by the inhomogeneous end-gas temperature spread [6]. If the end gas burns out as a consequence of heat transfer and diffusion processes, a number of isolated self-ignition centres distributed over the whole end-gas zone is produced, By Michael Fischer, Michael Günther, Karsten Röpke, Michael Lindemann and Rolf Placzek Klopferkennung im Ottomotor – Neue Tools und Methoden in der Serienentwicklung You will find the figures mentioned in this article in the German issue of MTZ 3/2003 beginning on page 186. Knock Detection in Spark-Ignition Engines New Tools and Methods in Production-Vehicle Development