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Piston Loading Under Knocking Combustion (Detonation)
Published January 01, 1982 by SAE Australasia in Australia
The improvement of the thermal efficiency of gasoline engines by means of increased expansion ratio is limited by the occurrence of knock and the resultant damage to the piston and other combustion chamber components. Piston loading during engine operation with knock has been examined by Karl Schmidt GmbH, with the aim of finding the predominant requirments of the surface and of testing protective coatings which can withstand operation with knocking combustion.
Cumulative frequency distributions for knock amplitude were established as a measure of the combustion chamber loading by means of an analysis of pressure traces from each of the 4 cylinders of the test engine (capacity = 1.05 dm3, compression ratio = 12:1). The knock intensity was increased by adjusting the ignition timing in steps up to a critical ignition advance at which damage occurred after a few seconds' operation. The tests were carried out at full load and in an engine speed range between 1000 and 6000 rpm. The piston temperature in the region of the knock damage was monitored by a thermistor and contactlessly transmitted to the measuring equipment.
The tests demonstrate that the formation of the typical erosion-like surface damage is not necessarily associated with a significant piston temperature rise. Thin coatings of iron or nickel, which cover the piston crown and top land down to the top ring groove with a dense protective layer, prevent surface destruction during operation with heavy, sustained knock