Thermal Stresses Induced by Knocking Combustion in Spark-Ignition Engines

Knocking combustion severely limits the performance and life of spark-ignition engines. Knock damage typically involves surface erosion of pistons and heads, which is thought to result from a combination of shockwave pressure and thermal stresses. This work investigates the contribution of thermal stresses to knock-induced erosion damage by modelling the temperature profile within the walls, using existing thin-film thermocouple data and 1-dimensional transient heat transfer relationships. The temperature profile is then used to establish the thermal stresses through the wall (of the piston or cylinder head). Results demonstrate that knocking combustion causes thermal stresses between 2 to 3 times greater than that of non-knocking combustion. For knocking conditions, tensile and compressive stresses exceed 20 MPa and 90 MPa respectively, with the maximum tensile stresses occurring about 1mm below the hot surface and the maximum compressive stresses at the hot surface. Due to combination of material properties, cast iron surfaces are shown to produce significantly lower thermal stresses than aluminium alloys. A shear stress cycle reversal is also identified which would play a significant role in the surface destruction process. This research sheds new light on the knock-erosion process by revealing the importance of thermal stresses on the surface damage process.