Knock (and pre-ignition) damage on engine components: Case studies

Due to the stringent regulations in terms of CO2 emissions, in order to increase engine efficiency, design strategies are oriented towards both increased spark advance and higher compression ratio, determining a higher probability to induce knock and abnormal combustions. Since slight knock does not lead to compromising engine damage, it should be tolerated in order to pursue the maximum efficiency. In the present study, knocking damages on several combustion chamber components after industrial durability tests are shown, aiming to underline the main effects of knock and to guide materials and coatings selection. Some hints are also made to the deleterious effects caused by persistent knocking combustions turning into pre-ignition. Piston Al alloys are usually more sensitive to knock induced thermo-mechanical stresses, but also cylinder heads, liners and spark plugs might be affected.