CONTINUOUS increase in the power output of aircraft engines introduces from time to time lubricating problems including excessive wear and scuffing, excessive oxidation of the oil, and ring sticking. The one problem of ring sticking was chosen and the discussion is limited to the testing of lubricating oils to compare their abilities to prevent this type of failure. Although the best answer as to the ring-sticking tendencies of a lubricant rests with the full-scale engine in service, a simple test is needed during the development period.
The development work which led up to the selection of an L-head CFR engine for a ring-sticking test is discussed. Various criteria used for detecting incipient ring sticking are mentioned and a method for direct measurement of incipient ring sticking is described. This direct method utilizes an opening cut in the cylinder wall so that a tool can be inserted into a notch in a pinned ring to determine the tangential force required to move the ring at any time during the test. The tangential force required to move the ring was found to agree well with the degree of ring sticking.
The engine operating conditions necessary to give ring sticking in a reasonable length of time on an average oil are enumerated.
Data are also given concerning the effect of air-fuel ratio, spark timing, and jacket and oil temperatures on piston-ring-land temperatures. Ring-sticking ratings on a number of oils are given and satisfactory separation and relative performance of the lubricants are evident.