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.

In view of the difficulty which has been experienced by many drivers in sufficiently lubricating the pistons without causing the car to emit clouds of smoke, it may well be asked, "Why cannot an unburnable oil be used and thereby eliminate this trouble?" This is out of the question, for the mineral oils now used are obtained from petroleum and are cousins of kerosene, gasoline, benzine, and many of the other highly-inflammable liquids that need but the touch of a match to burn almost with the rapidity of an explosion. But notwithstanding the excitable family to which the mineral oils belong, the modern motor car lubricants are removed a sufficient distance from their more inflammable relatives to enable them to withstand a temperature of between 400 and 500 degrees, Fahrenheit. This is sufficient heat-resisting ability to enable the oil to stay on the cylinder walls near the bottom of the stroke, where it is most needed; but even though its burning point could be raised to a degree double its present amount, it could not withstand the high temperature generated in the top of the cylinder at the time of the explosion. The temperature here reaches a point well above the 2000-degree mark, and were it not for the cooling system, parts of the interior of the cylinder would probably be melted by the continued application of this excessive heat.

Any oil, consequently, would find but small opportunity to remain in its normal state after it once reached a point at which it would be exposed to the heat of the explosions, and we must look for a preventive measure other than that of increasing the flash-point or burning-point of the lubricant. But this high temperature does not exist throughout the stroke, for as the piston descends and the gas expands, heat is given off until the oil on the lower portions of the cylinder uncovered by the piston is sometimes able to remain in comparative peace. And even though this oil remaining on the cylinder walls at the bottom of the stroke should be burned, it would not be present in sufficient volume to create the dense clouds of objectionable smoke. Consequently it is the endeavor of engineers so to design the pistons and lubricating system that excess oil will not be fed to the pistons and allowed to remain on the walls after the former have descended.

But an excess amount of oil fed to the cylinders will result in so much less harm than will an insufficient supply, that we are treading on rather dangerous ground when we warn the amateur to cut down his lubricant to the point where there will be no smoke. As there are no ordinances that absolutely prohibit the slightest appearance of smoke at the exhaust, and as a faint blue trail is an excellent indication that the motor is receiving sufficient lubrication in the cylinders, it forms a satisfactory test by which the novice can determine the condition of the oiling system.

By the time that the exhaust gases have passed through the pipes and have expanded in the muffler, some of the blue smoke may have disappeared, and consequently the fact that a car does not give a trace of vapor at its exhaust should not necessarily be taken as an indication that the motor is not well lubricated. If the owner would satisfy himself that the cylinders are receiving a sufficient amount of oil, he may open the individual pet cock on each, and if he finds there a faint blue trail of smoke at each explosion in that cylinder, he may rest assured that harmony exists between the rubbing surfaces of the piston and the cylinder walls.

With the increase in the size and power of the automobile motors and the proportionately greater number of parts demanding lubrication, the attention required from the driver by the oiling system has been greatly lessened. Instead of the necessity of turning on individual oil cups whenever the motor is started, the modern driver merely twirls the starting crank or presses the button of the self-starter, secure in the knowledge that whenever the motor runs, the lubricating system operates—provided, of course, the reservoir is filled and there is no stoppage in the pipes. The oiling system of the modern motor is absolutely automatic, and if supplied with a sufficient quantity of a good lubricant, it will perform its work with an absence of trouble that places it among the greatest improvements of the engine of recent years.

Individual oil cups such as were used formerly, have been eliminated from the cylinders, and whatever sight-feeds there may be are placed on the dash in plain view of the driver. Instead of relying upon the suction of the cylinders for the positive feed to the piston, mechanically-operated pumps are used to force the oil to the various portions of the motor. In some systems, there is a separate pump for each oil lead. This is known as a mechanical oiler, and generally consists of an oil tank located on the dashboard of the car—either in front of the driver, or under the motor hood—and connected by means of a belt or gear with some shaft of the motor. The belt or gear drives a shaft to which is connected the plungers of the various oil pumps that force the oil to the different parts of the motor. Before passing to the individual pipe, however, the oil drops through a sight-feed connected with that lead, and as all of these sight-feeds are mounted in a row within plain view of the driver, the condition of the lubricating system in part or in whole may be determined at a glance.

The parts of the motor that are lubricated by an independent feed line in this manner may vary with different motors. In general, however, it may be said that it is seldom that the oil is fed directly to the piston, but that the lubricant is first distributed to the oil wells in the crank case. Here, the splash of the cranks as they revolve in the oil is depended upon to throw the lubricant upon the exposed portion of the piston as it reciprocates below the cylinder walls. The sides of the piston thus covered carry the oil to the cylinder walls.

It is evident that if an excess amount of oil is continually carried up by the piston to the cylinder walls, a certain proportion of this lubricant will reach the open space in which the charge is ignited, and will there be burned—with the attendant formation of clouds of objectionable smoke. This trouble is overcome to a certain extent in some motors by the use of a type of ring set in the piston that prevents the lubricant from passing to the upper portion of the cylinder; but all the oil cannot thus be retained, and it therefore behooves the driver not to allow too great a quantity to be fed to the crank case if the "splash" system is used.

The main bearings on which the crank shaft revolves are generally supplied with oil by independent leads from the oiler, and when the above-described system is used they may be regulated independently of the splash feed lubricating pipes. Excess oil at the bearings will cause no damage, but each crank shaft journal does not demand as great an amount as that supplied to a piston and connecting rod bearing.

Many lubricating systems that are now in popular use employ but one pump to force the oil to the various bearings and rubbing surfaces, and regulate the supply by the size of the pipe leading to each. A satisfactory method of overcoming the possibility of excess oil in the cylinder has been adopted by some manufacturers. This consists in placing a channel, or trough, directly under the lower sweep of each connecting rod bearing. Each channel is kept filled to overflowing by a separate pipe connected with the main lead from the pump, and a constant level is consequently maintained at all speeds of the motor. An elaboration of this method consists in attaching one end of each trough to a rod operated in conjunction with the throttle, so that as the speed of the motor increases, the end of the channels may be tilted, with the result that the connecting rod scoop will dip deeper into the lubricant.

After the proper level in each trough has been reached the excess oil overflows into the bottom of the crank case. From here, it is again started on its way by the pump and is distributed to the various bearings and troughs through the different pipes leading from the pump. As a further precaution against a smoking exhaust, some designers have added a baffle plate above each crank case compartment that serves to reduce the size of the opening through which the oil may be splashed. With this combination of troughs and baffle plates the possibility of a smoking motor is practically eliminated.

All motors are not so equipped, however, and in the case of those provided with the bona-fide splash system, care must be taken to keep the separate crank case compartments filled to the proper level. Too high a level in the crank cases will cause the motor to smoke; while the supply should not be allowed to become so low that when the angle of the crank case is changed—as in ascending a hill—the lubricant will run toward the rear and will not be reached by the scoop on the connecting rod bearing. This latter danger makes it advisable to give this system plenty of oil when any touring is to be done through a hilly district.

In some lubricating systems, the oil is supplied as it is used, and either is discharged with the exhaust, or collects in the bottom of the crank case, from which it should be drained occasionally. In the circulating systems, however, which are now used on a majority of the cars, the same oil is used continuously until it becomes "worn" or filled with sediment and particles of dirt and other foreign matter. The pump used for maintaining this circulation may be either of the plunger, centrifugal, or gear type, and is generally housed in a portion of the crank case. A strainer is usually placed in the suction end of this pump for the purpose of removing all the free foreign matter from the oil before it is again started on its mission of lubrication. In these systems, the oil well is generally located in a "secondary" bottom of the crank case. From here it may be drained when the supply is to be renewed.

Another successful system by which all the bearings of the crank shaft are positively lubricated is used on many of the best cars. In this system, a continuous oil hole passes throughout the length of the crank shaft, including its "arms" and connecting rod bearings. At each bearing, one or two small oil holes connect with this main artery and extend radially to the surface. Oil is forced into the longitudinal oil hole by means of a small pump, and naturally finds its way through every radial opening to all the bearings. The excess may overflow into the individual oil wells, from which it will be splashed upon the exposed portions of the pistons as they descend.

It will be seen that, no matter what modern oiling system is used, the same kind of lubricant is supplied to all parts of the motor. This feature makes matters much simpler than was the case when one oil was used for the cylinders, another, of a different thickness, supplied to the crank case, and still a third required for the gears. By the old gravity systems, the flow of oil depended largely upon its viscosity, or thickness. Therefore, in winter, a thinner oil was required than in summer, for the more a lubricant is warmed, the thinner does it become—and vice versa. With the mechanical force systems now in use, however, practically the same kind of oil may be used throughout the year—although many motorists believe that better results will be obtained if a heavier oil is used in summer than in winter. The oil will be warmed by the motor and it will not require many minutes of operation before a lubricant made thick by a low temperature will flow freely and do its work as efficiently as a thinner oil.

But no matter how reliable a lubricating system may be in its operation, the driver must do his share and make certain that fresh oil of the proper quality is supplied when needed, and assure himself that all the passages are free from obstructions. Negligence on the driver's part may result in one or more "stuck" pistons that will either seriously injure the motor, or will put it out of commission until the trouble can be remedied. If a sufficient supply of oil is not fed to the rubbing surfaces between the piston and the cylinder walls, a high degree of heat is generated which will tend to expand the piston until it grips the cylinder so closely that the former cannot be moved. In this event the motor will stop "dead," and cannot be started again until the piston has cooled and contracted to its normal size. Even then, however, the motor should not be run under its own power until the burned and gummed oil has been removed and the scored surfaces have been cleaned. While this may best be done by removing the piston—at which time an examination for any badly burned rings may be made—this is not always possible, and it may be necessary to run the car home or to the nearest repair shop before the proper repairs can be made.

In this case, the motor should be turned by hand until it is certain that the piston is again free in its cylinder. Liberal quantities of kerosene oil should be poured in through the spark plug opening, and if possible, the motor should be "rocked" back and forth by the flywheel to give the kerosene an opportunity to reach all parts of the piston and rings. The kerosene will serve to cut and remove much of the carbon and gummed oil and to make the way free for the fresh lubricant, which should be poured in liberal quantities into the cylinder head. The flywheel should again be moved back and forth so that the oil will reach all parts of the piston surface, and after this—if the damage has not been too great—the motor should be ready for operation.