Diesel Engine Lubricants: Their Selection and Utilization with Particular Reference to Oil Alkalinity

Abstract

The main functions of heavy-duty lubricating-oil additives are to control engine fouling, bearing corrosion, and wear of liners and piston rings. Alkalinity is desirable for the control of wear and is one of the major requirements for the avoidance of piston fouling when conventional organo-metallic additives are used. In a well-balanced oil, sufficient dispersive power and oxidation stability may be incorporated to ensure adequate piston cleanliness and freedom from bearing corrosion provided that the alkalinity level is satisfactory. The alkalinity level falls during service, and for satisfactory performance with certain types of additive in common use it must be kept above a minimum value. For these additives equations are given which enable the variation of the alkalinity level with time in given circumstances to be predicted approximately. The most satisfactory arrangement is to use an oil containing sufficient alkalinity so that the concentration never falls below the critical value. The oil-change period is then determined by other considerations, for example, contamination with abrasives. If an oil of lower alkalinity-concentration is used, then the equations developed permit an approximate estimate of the oil-change period, determined solely from the aspect of additive effectiveness. The application of these results to engines with separate cylinder lubrication is discussed. Thus oils, and, where appropriate, oil-change periods, may be selected on a rational basis instead of by trial and error.