Effects of engine operation on the lubricating oil used in it determine to a large extent the ability of the oil to maintain continuous lubrication and, consequently, of the engine to function efficiently. Engine operation has three major effects on the oil: (a) complete destruction of part of the oil, (b) physical and chemical changes in the oil and (c) contamination of the oil by foreign matter.
Oil is not worn out by friction but is destroyed by burning or decomposition caused by exposure to the intense heat of fuel combustion in the cylinders or the metallic parts of the combustion-chamber. The quantity so destroyed depends upon (a) fuel-combustion temperatures, (b) temperatures of the metallic parts, (c) quantity of oil exposed to these temperatures, (d) length of time of such exposure, and (e) volatility of the oil.
The quantity of oil that is exposed to the destructive temperatures, and thus consumed, depends upon the mechanical condition of the engine, the operating conditions and the viscosity of the oil. With the splash system of cylinder lubrication an excess of oil is supplied, some of which passes above the piston-rings and spreads over the tops of the pistons, the combustion-chamber walls and the valve heads, where it is constantly exposed to the flame of combustion and is destroyed. Oil on the cylinder-walls is covered by the piston-skirts part of the time and is renewed at every piston-stroke, hence less oil destruction occurs there. The common practice of using an oil of high viscosity to reduce leakage past the piston-rings, thereby decreasing the oil consumption, may easily be carried too far and result in inadequate lubrication of the upper cylinder-walls and consequent excessive wear there. With an engine running at 1000 r.p.m., the duration of the power-stroke is approximately 1/2000 min., or 1/33 sec., during which brief interval only a small portion of the oil on the cylinder-walls can be destroyed.
Lubricating oil must be converted into a gas before it can burn, hence its volatility is important. The flash-test, however, is of little value and may be misleading in determining volatility, as it does not indicate the volatility of the entire mass. Straight-run oils composed of a narrow range of fractions from crude petroleum and having a straight distillation-curve may show a slightly lower flash-point than a blended oil, yet contain a smaller total quantity of the more volatile fractions than an oil having a higher flash-point and hence will have greater ability to resist heat.
Ordinary temperature changes do not permanently alter the viscosity of an oil but the specific viscosity is changed by relatively high temperature and by contamination. Distribution of oil to the bearing surfaces, ability of the oil to maintain complete separation of the surfaces, internal friction or resistance of the oil to motion and effectiveness of the oil as a piston seal are all functions of its viscosity; therefore changes in viscosity are of importance. These are caused by gradual consumption of the lighter fractions by oxidation and cracking and by the admixture of water, unburned fuel, carbon, dust and metallic particles.
The excessive quantity of fuel used when starting and warming-up a cold engine is the principal cause of dilution by fuel, water contamination is due to cold surfaces in the crankcase that condense the water vapor of combustion, dust enters the engine through the carbureter and breather-pipe and metallic particles wear off of the bearing surfaces most rapidly when wearing-in a new engine. Contamination by fuel reduces the viscosity of the oil, water forms an emulsion and, with carbon, dust and metallic particles, forms a sludge. All of these conditions are likely to have deleterious effects on the engine.