Investigation of Knock Damage Mechanisms on a GDI TC Engine

The recent search for extremely efficient spark-ignition engines has implied a great increase of in-cylinder pressure and temperature levels, and knocking combustion mode has become one of the most relevant limiting factors. This paper reports the main results of a specific project carried out as part of a wider research activity, aimed at modelling and real-time controlling knock-induced damage on aluminum forged pistons. The paper shows how the main damage mechanisms (erosion, plastic deformation, surface roughness, hardness reduction) have been identified and isolated, and how the corresponding symptoms may be measured and quantified. The second part of the work then concentrates on understanding how knocking combustion characteristics affect the level of induced damage, and which parameters are mainly responsible for piston failure. For this purpose, steady-state tests have been conducted controlling different and constant levels of knock intensity (i.e., pressure waves oscillation amplitude) and thermal load (i.e., average temperature and pressure levels inside the combustion chamber). Since these parameters are strictly interrelated for a given engine operating condition and for a given fuel, fuels with different knock resistance (i.e., RON number) have been employed, to allow a clearer understanding of the damage distribution in the knock intensity-thermal load domain.