The Gasoline Motor

by Harold Whiting Slauson

A lubricant acts as a sort of pacifier between two surfaces that would otherwise move in contact with each other. No surface can move in direct contact with another of the same or a different material without the generation of heat; but the amount of heat generated, or resistance met with, is determined by the nature of these two rubbing surfaces. The oil, or grease, or whatever suave, slippery substance is to be used as a lubricant, interposes itself in a thin film between the two rubbing surfaces and smooths matters over, as it were. If a sufficient amount of this mechanical soothing syrup is not fed to the rubbing surfaces, the temper and temperature of each will be raised to the point where they will "clinch," and much time and effort may be required before harmony can again be restored.

Thus it is actually upon a film of lubricant that a shaft rests, rather than upon the bearing, or "box," in which it turns. If the bearing is set so tight that there is no room for the interposition of an oil film, the shaft and journal will at once heat. The greater the pressure of the shaft in its box, the thicker, or heavier, should be the lubricant used, for a light oil would be squeezed out or "broken down" more easily than would one that possesses greater viscosity.

The "coefficient of friction" may be termed the mechanical "amount of irritability" generated when two surfaces are rubbed together. Thus if two metals are rubbed together, this figure is high, and a large amount of friction, or heat, will be generated. A metal rubbing over oil, however—as is the case with a well-lubricated bearing—will arouse but little resentment and its pathway will be made smooth and easy, for the coefficient of friction of these two materials is low. The lower this figure can be kept, the more easily can the surfaces be rubbed over each other and the higher will be the efficiency of the bearing.

Apply this to every bearing or rubbing surface of a motor, and we see that proper lubrication affects not only the length of life of the moving parts, but the ease with which the engine can be run and the consequent power development. Thus, a lubricant that will prevent wear between the moving parts may be supplied to the bearings and pistons of a motor, and under this condition the engine might "last" indefinitely; but this oil might be so viscous or possess so high a coefficient of friction that each bearing would turn with difficulty and much effort would be required to run the motor before it could begin to develop power.

But the introduction of oil to a bearing not only reduces the friction between the surfaces that would otherwise move in contact with each other, but it serves another very important purpose. Every properly-lubricated portion of a motor either moves in a bath of oil or is connected with an oil reservoir so that a certain amount will be fed regularly to the rubbing surfaces. There is always some heat generated in a bearing, no matter how well it may be lubricated, and the continuous flow or circulation of the oil serves to carry off this heat that would otherwise tend to dry the lubricant if there were no fresh supply.

The proper lubrication of the motor is even more necessary than is the adjustment of the carburetor or the condition of the ignition system. To be sure, if either the carburetor or the ignition system is out of order, the motor will not run, but no actual harm to the mechanism will result from this fact. On the other hand, a motor may be run indefinitely with a defective lubricating system, and no apparent harm will result—until the end of that indefinite time arrives and it is found that the machine is a fit subject for a junk heap.

Let us see how many parts of the motor are reached by the gallon or so of oil that we pour into the tank. A six-cylinder motor may have seven crank shaft bearings; it will certainly possess six connecting rods, each of which will be provided with a bearing at both its large and small ends—or twelve in all; there may be two cam shafts, each with five bearings and half a dozen cams; these will require, together with the magneto and pump shafts, five or six gears in the forward train; and the six pistons will demand their share of attention from the lubricating system. Here is a grand total of over fifty rubbing surfaces on a large motor, and the oil must be thoroughly and constantly distributed to each. Of course, many smaller motors, provided with but a single cam shaft and a three-bearing crank shaft, may possess but one-half of this number of lubricated parts, but at the least, the oil must reach with unfailing certainty two dozen vital places of the engine.

At some of these portions, the movement is comparatively slow and the pressure is not great. Therefore such surfaces as the cams or valve stem rollers will demand less oil than will the bearings revolving at higher speed and carrying heavier loads. But it is the hardest-worked bearings that form the majority of the friction surfaces of a motor, as will be realized when it is remembered that all points on the circumference of a three-inch crank shaft bearing will travel at the approximate rate of 1,000 feet per minute—and these are the portions that also carry the heaviest load.

But while the pistons can hardly be called bearings in the generally-accepted layman's definition of the term, they require the lion's share of the lubricant, and are the first portions of the motor to feel—and show—the effect of any failure of the oiling system. While in terms of miles per hour, the movement of the pistons may not seem very rapid, the thousand feet per minute at which each ordinarily travels is rather a high rate of speed when it is considered that it is entirely a rubbing or a sliding motion, and that the direction is reversed more than two thousand times during each sixty-second period. This means that each piston slides or rubs within the cylinder walls for a distance of between two and three thousand miles during an ordinary season. And remember that this is not a rolling motion, but a continuous rubbing! In addition to this high-speed rubbing, the pistons are pressed firmly against the side of the cylinders on each explosion stroke throughout a portion of their travel. This corresponds to a heavy pressure carried by the rubbing surfaces, and is caused by the side thrust induced by the angularity of the connecting rod as it overcomes the resistance of the load through the crank shaft.

But this is only a small portion of the difficulties that must be overcome in cylinder lubrication. Not only must the oil pacify the rubbing surfaces and keep them well separated, but it must remain within a restricted territory of the cylinder walls. Whatever oil reaches the upper portion of the cylinder walls will be burned and will contribute to the formation of the carbon that is the mortal enemy of efficient running. Large quantities of oil burned in the cylinder will also form the dense clouds of choking blue smoke that the health authorities of many cities have been investigating, which have led to the enactment of city ordinances making the driving of a smoking automobile a misdemeanor.