LUBRICATION, FRICTION AND WEAR STUDIES WITH HIGH-OUTPUT AIRCRAFT ENGINES 490232
The paper discusses the general lubrication problems associated with operation of high-output aircraft engines. Since the paper is concerned with two types of aircraft engines, namely, turbine and reciprocating, a natural division into two parts is made. Part I deals with the problems of turbine engines, and part II deals with the problems of reciprocating engines.
In part I it is indicated that the choice of a lubricant is very difficult for the turbine engine particularly, because of the wide temperature range (from -67°F to approximately 400° F). Two solutions to the problem of proper choice of a lubricant are discussed, namely (1) the use of supplemental lubricants, and (2) the use of additive lubricants. Data are presented on supplemental lubricants including the various oxides of iron, molybdenum disulfide and graphite. Data are also presented on free sulfur as an additive lubricant and it was indicated that, at the high sliding speeds, critical sliding velocities might be reached where the sliding was so rapid that not enough time was available for effective chemical reaction to occur between the surfaces and the additive. In consequence, at velocities above the critical, the additive could not adequately perform its function as an “anti-weld” agent and the result was surface welding and high friction. This result indicated that the additives should not be expected to be “cure-alls” but should be used with some caution.
Part II deals with the problems of reciprocating engines. In the reciprocating engine the present trend toward higher compression ratios, higher bmep's, higher speeds, and higher temperatures has imposed even greater loads on the piston ring and cylinder surfaces. Materials for these surfaces must be found such that the performance characteristics of wear, oil consumption, and reliability are satisfactory. Theoretical considerations of friction under boundary conditions (metal-to-metal contact) indicated that the required material properties were those of high hardness and low shear strength. In accordance, hard materials were chosen and an investigation of run-in was carried on to determine optimum conditions for natural formation of low-shear-strength coatings on the hard surfaces.
Because of the high temperature conditions in high-output engines, a temperature investigation of the resistance to loss of piston-ring diametral tension was made in order to eliminate those materials which show the least ability to maintain strength at elevated temperatures. On the basis of these results, nitrided rings were chosen and high-output runs were made at imep's approaching 300. These runs indicated excellent performance with respect to low-wear rate, oil consumption blowby, life, etc. A correlation was also indicated between oil-consumption rate and wear rate. Dust tests were also made to measure the relative resistance to abrasion of the nitrided rings and, again, excellent performance was obtained. In all cases, the nitrided rings would operate successfully under conditions which were too extreme for the cast-iron rings.
An investigation of run-in was performed both on the nitrided rings after the high-output runs and on specimens from a reciprocating slider. It was observed that surface physical and chemical changes occur which may provide the low-shear-strength property which was indicated by the theoretical equations, to be necessary for minimum friction and low wear. There was also some evidence of critical load points in the run-in process; at these loads the maximum benefit due to run-in may be derived.