Theoretical and Experimental Investigation of Knock Induced Surface Destruction

Engine knock causes severe damage to the surfaces of the combustion chamber in I.C. engines. Since the detailed damaging mechanisms are still unknown, we performed a theoretical and experimental study in order to identify potential erosion processes.

To circumvent the considerable cyclic variations of combustion in I.C. engines the experiments use an optically accessible bomb. It is shaped to resemble a typical geometry which is known to be particularly susceptible for knock damage. In this way we establish well defined conditions for knock simulations. By means of very high speed Schlieren diagnostics we measure the propagation speeds of detonation waves in the duct and use the data to estimate the wall loading due to instantaneous pressure peaks and sudden large temperature increases. Qualitative agreement between numerical simulations and experimental observations is achieved in the calculations. The surface damage generated in the simulator agrees well with knock induced surface erosion in real engines. The experimental data indicate that erosion may be caused by excessive surface stresses due to large local heat fluxes and/or by high peak pressures in positively interfering reflected shocks which are driven by the very rapid chemical heat release.

In the two-dimensional numerical simulations we consider shock induced combustion in an L-shaped duct. This idealized geometry represents the top-land region in a combustion chamber and resembles the experimental configuration. The results reveal an important role of the geometry in triggering the onset of detonation waves. The algorithm integrates the reactive Euler equations employing a higher order Godunov type scheme extended to cover the chemical source terms.

In the paper we describe in detail the theoretical model, the experimental set-up and results from erosion studies on aluminum alloys in the knock damage simulator.