Some Notes on the Mechanical Factors Affecting Ring Gumming in Petrol Engines with Particular Reference to the Rating of Lubricating Oils

The phenomenon of piston ring gumming in petrol engines is well known. It is of importance not only because it is the cause of considerable trouble in service, but it has also formed the chief criterion in the majority of attempts to rate or classify lubricating oils according to their deterioration characteristics in an engine cylinder.

Existing standard chemical tests on lubricating oils form no guide to the behaviour of oils from the point of view of ring gumming in a petrol engine, and the only reliable method of grading oils at present is by tests in an actual engine. The next logical step is to develop a simple chemical test or series of tests which will give the same grading as that obtained in an actual engine. Before this can be done it is necessary to have a knowledge of the nature of ring groove deposits, the mechanism of ring gumming, and the relative effects of the various factors which affect the rate and degree of ring gumming. Mechanical factors can be of sufficient magnitude not only to affect, but possibly reverse, the relative rating of oils on a ring gumming basis. Various mechanical factors which affect the degree of ring gumming obtained are discussed in detail in the paper. Those mechanical variables which cannot be eliminated must either (1) be taken into account when sifting available engine data, or (2) must be a consideration when formulating a laboratory test or series of tests simulating engine conditions.

Correlation between engine tests and laboratory tests is best based on the time to give incipient gumming in the engine. To do this it is necessary to have a reliable indicator to show when ring gumming commences.

Preliminary experiments have indicated that piston-crown temperature is probably the best control temperature in oil rating tests; and the temperature at the back of the ring groove, taken in conjunction with the piston crown temperature, is probably the best indicator of incipient ring sticking capable of application to any engine.