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Lubrication for Diesel Engines Effect of Moisture in Compressors—Deposits—Proper Oil—Bearing Lubrication — Results of Oil Breakdown — Oil Characteristics OMPRESSOR troubles arising C from moisture are found in the final stages. The rush of wet Lubrication Air Compressor Cylinders high pressure air into the tube-like cylinder for final compression has a severe washing effect upon the lubri- cating oil film, interfering also with film removal, so that during compres- sion it is difficult to maintain effective seals between piston rings and cyl- inder wall. Lack of a complete film results in blowby and excessive wear, especially toward the end of the com- pression stroke when pressure behind piston rings is greatest. Moreover, moisture and air cause rusting of valves, piston rings and cylinder walls —a condition frequently noticed by operators during inspection of high pressure cylinders. This rust forma- tion causes pitting of valves; it also forms a grinding compound between piston rings and cylinder walls, which results in additional wear. For these reasons, it is advisable to use an oil which, notwithstanding ex- cessive moisture, will adhere to cyl- inder walls and valves, and supply a complete film that will prevent rust during idle periods. Formation of Deposits The formation of deposits is the most usual difficulty encountered in the operation of multi-stage high pres- sure air compressors. The most com- mon causes are the use of too much lubricating oil, the use of unsuitable oil, improper distribution of oil be- tween the several stages, excess due to the throw of bearing oil from mov- ing parts, and impurities in the in- take air. Air compressor cylinders require very little lubricating oil to provide and maintain a complete and efficient film. The correct feed for any given unit is determined by frequent exam- ination when the engine is first in- stalled. Oils too heavy in body will deposit more carbon than oils of correct body, even though of the same high quality. Moreover, oils too heavy in body dis- tribute very slowly and require exces- sive feed in order to assure the main- This is the second and concluding installment of the authoritative article on lubrication for diesel engines prepared for MARINE REVIEW by the Technical Department of the Vacuum Oil Co. The first part appeared in the November issue. Part II tenance of a complete lubricating film. Since excessive feed breeds carbon in compressor cylinders, troubles are minimized, and explosions avoided, by a reduced feed of suitable oil, and not by the liberal use of very heavy oil. Correct Oil for Compressors To meet necessary requirements and to assure reliable, continuous opera- tion, cylinder oil for diesel air com- pressors must possess exceptionally Section through cylinder of trunk piston type Diesel engine showing method of lubrication high lubricating value; it must pro- vide film strength; it must resist wear and maintain efficient piston seal even though used in very small quantities; it should have moderate body and sufficient fluidity to give rapid and complete distribution; it should be of such a nature that any carbon formed will be soft, flaky, and easily removed, and will not interfere with valve operation; it must be able to resist washing effect even under most severe moisture cond'tions, thus assuring complete lubrication and _ preventing MARINE REVIEW—December, 1929 rust during periods when not in use. Where it is difficult to maintain correct injection-air pressure, this trouble has been traced to deposits on compressor valves which hold them open and greatly reduce volumetric efficiency. Generally this condition will be noted first by an increase of discharge temperature. An additional cause of high dis- charge temperature is the accumula- tion of sludge and scale in water jackets. When this happens air dis- charge temperatures will rise, even though compressor valves are tight. Until this accumulation has been re- moved there can be no efficient cool- ing. The cooling of air between stages results in the precipitation of consid- erable quantities of water in the intercoolers. If this water is allowed to accumulate for too long a period it will eventually be carried to suc- ceeding compression stages where it will seriously affect lubrication. Fre- quent draining is necessary. Even under favorable operating conditions, deposits will gradually ac- cumulate in the inter- and after-cool- ers, and high pressure air line. The cleaning of these parts, at least an- nually, will often prevent future trouble. How to Lubricate Bearings In circulation oiling systems oil es- capes from the ends of bearings and is whipped about the crankcase until it is broken up into fine spray. Con- tact of this spray with hot air in a crankcase is favorable to oxidation, especially in the presence of water and other impurities. Constant churning accelerates this oxidation, promotes emulsion, and altogether has a very detrimental effect on lubricating oil. Depending upon design, impurities from other sources may place further serious burden on _ circulating oil. Leakage from water cooling systems, deposits from cylinder drip, dust from intake air, fuel—any or all of these may contaminate the oil in a crank- case, Where pistons are oil cooled, part of the bearing oil circulating through the pistons comes into contact with the hot piston heads. Exposed to this high temperature, the oil must resist to the greatest extent any 43