Since the pistons are water cooled, at. times water mixes with the oil in the crankcase. If in any considerable amount, there pis danger of bearing trouble. A water separator along the lines of Fig. 178 will remove any entrained water: It should be connected between the crankcase and the oil filter. Being provided with a drain, any water caught in the separator can be drained off each day. A sight glass at the side enables the operator to observe the water level. It is evident that, as long as the velocity of the oil is low, any water entering the separator will settle at the bottom due to its greater weight.
Splash Oiling Systems.—The splash oiling system is almost abandoned in Diesel engine practice. It was, however, the method employed on the first American Diesels for lubricating the bearings and connecting-rod brasses as well as the piston. The American Diesel Co. used a mechanical oil pump for the purpose of lubricating the cylinder walls but, in fact, depended on the splash from the crankcase to meet most of the cylinder lubrication demands.
The great objection to the splash system is the difficulty of preventing excessive oil deposits forming on the cylinder walls; the oil burns and gums the piston rings. Since the shaft bear ings are covered with oil emulsion, it is impossible to detect loose bearing shells. It is generally conceded that, due to the air-tight crankcase, the heat passing down the cylinders vaporizes a considerable portion of the oil. Experience proves that the lubrication demands are high with the splash system. On many old engines the splash has been abandoned in favor of a stream lubrication system.
Lubricant Oil Requirements.—The qualities which a lubri cant must possess to enable it to be used successfully in the cyl inder of a Diesel are few in number. First, it must possess ' lubricating qualities; in other words, it must be able to form a film over the rubbing surfaces that will prevent the piston and cylinder from touching. Furthermore, it must possess a low coefficient of friction. The oil must possess sufficient body to enable it to seal the clearances between piston rings and grooves, thereby preventing loss of compression. Finally, the lubricant must have a fire test high enough so that it will not be burned at the temperature in the cylinder during the suction and exhaust strokes and during the final part of the power stroke. In burn ing, during the period of high cylinder temperatures, the oil should leave no carbon or tarry deposits. Many engineers are of the opinion that the lubricating oil should not be burned. In this they are in error since the temperature of the flame produced by the combustion of the fuel oil is far above the burning point of any lubricating oil. If too great an amount of lubricant is fed into the cylinder, only the top surface is consumed; the part next to the cylinder wall does not burn but merely breaks up into its several constituents, thus leaving a residue of heavy carbon on the walls. The best oil will show these deposits when
fed in excessive amounts.
Much of the trouble of carbon deposits on the cylinder and piston is primarily traceable to poor filtering facilities. If the oil circulating through the bearings and other parts is filtered and used in the engine, including the piston, this filtered oil will contain fine particles of carbon held in suspension. This free carbon settles on the cylinder walls in a hard flint-like coat. The remedy is the purchase of a filter that will remove all but the very finest of these particles. Probably the most advisable method of operation is to use only new oil in the cylinder lubri cating oil pumps and to place the refiltered oil in the bearing oiling system. • The ordinary plant has no equipment for analyzing the oil purchased. A rough method of ascertaining if the lubiicant is free from a gummy residue is to place a smear of the oil on a clean pane of glass. After the glass has been placed in the sun for some time, the oil, with the exception of the gummy remain der, will evaporate. The amount remaining indicates, in a fair manner, the gumming characteristics of the oil.
It has been frequently claimed that a lubricant from an asphal tum base oil is more desirable in the Diesel than paraffine base oil. The experience of many operators indicates that this is a matter dependent on the particular engine tested. Frequently an oil is purchased with the understanding that it has a paraffine base. If a small amount is placed on a piece of paper, the paper will become translucent. Upon evaporating, if the oil is of a paraffine base, the paper returns to its former color; if the oil has an asphaltum base, the paper will have a darker shade.
The most important detail in the manufacture of lubricating oils is the process of filtering. All Diesel oils should be filtered through fuller's earth. However, the cheapest oils have been treated with sulphuric acid in place of the fuller's earth process. The acid-treated oils may be detected by suspending a polished brass strip in the oil. If left for several days, the acid-treated oil will cause the surface of the strip to become somewhat mottled in appearance due to the formation of minute pits.
Lubrication Oil Specifications.—While oils of widely varying characteristics give excellent service in many plants, the following specifications, if adhered to, will guarantee the procurement of an oil that will give satisfactory results. Practically all refineries have a lubricant that will meet these requirements.
If the lubricating oil is to be used on the compressor cylinders, the flash point must be above 450°.
Amount of Lubrication.—The lubrication consumption varies over wide limits in various makes of engines. Table III gives values that follow very closely the amounts necessary on engines from 200 to 600 h.p. capacity.