Developments in the line of motor lubrication as a matter of course follow on the heels of progress in the art of motor design and construction, for the increase of efficiency which marks the refinement of the motor and its various parts can only be reached if the refinements are paralleled by those essentials which permits the units to perform their required duty. In commercial car motors, the two features, outside of actual mechanical construction and the essential details of ignition and carburet ion, which help to the attainment of the highest results with the least expenditure are the cooling and lubricating systems.
The functions of the cooling system are different from those of the lubricating system and will be considered in a separate article following the present one.
All parts which rub together under pressure, such as the pistons moving upward and downward within the cylinders, the connecting rods on the piston and crank pins, the crank shaft in its bearings and all reciprocating and revolving parts must be efficiently lubricated. Whenever two surfaces are in contact and one or other is free to move when a force is applied, there is present a certain amount of friction, which develops instantly when these surfaces are not properly lubricated.
Friction is a resistance to motion of two bodies in contact and is dependent upon certain laws. It will vary in proportion to the pressure applied when the rubbing velocity remains constant. All wearing surfaces, no matter how carefully prepared, are known to consist of minute lumps and hollows. This is true, even of the smoothest surfaces that can be made, although, of course, the height of the depressions and hollows varies with different materials and the finish. The condition of bearing surfaces in an exaggerated degree may be likened to the nap of woolen cloth and the pile of velvet.
When the two surfaces are held in contact by an appreciable force these minute parts of the surfaces interlock and resist relative motion. This force is called the force of friction. This fact can easily be demonstrated by placing a book upon a table or smooth board and moving it across the latter, comparing the difference in energy required to move the book across the surface by placing a weight upon it. If this experiment were conducted with two metallic objects and the surfaces were lubricated, considerable less energy would be required.
Friction in reality is heat, which in a short period of time will reach an intense degree and in time the surfaces will be broken down and if the rubbing continues they will be completely destroyed. The action and effect of this friction upon two surfaces not properly lubricated may be explained as follows: At first small particles of metal are torn from the surfaces and these cut and abrade the bearing. As the surfaces continue to roughen, the friction increases enormously until it reaches the fusing point of the metal, when the two surfaces will weld into one solid mass.
A lubricant should possess certain qualities, since its object is to flow between the surface of a bearing, reduce its friction and to radiate a certain amount of heat generated by the bearing. These properties are termed adhesion, cohesion or viscosity, a high flash point and a comparatively low cold test point. It should have considerable adhesion so that the molecules of oil will cling together, considerable cohesion or viscosity, as all bearings are subjected to a pressure, created by the force acting upon them and the effect of this pressure is a tendency to separate these molecules forcing sufficient lubricant out of the bearing to allow the metals to come into contact.
The advantage of a high flash point is that the oil will not give off an inflammable vapor at ordinary temperature, while it should have a low cold test point, so that it will stay in fluid state when cold temperature are encountered. The high flash point is very desirable; for in lubricating the cylinders, the oil comes in contact with the walls of the combustion chamber and the heat in the cylinder on the power stroke. If the temperature is raised sufficiently to vaporize the oil, the viscosity is entirely overcome and the various residue products of the lubricant are deposited on the walls of the combustion chamber. This residue product is termed carbon and in time will harden, become incandescent and fire the charge prematurely, causing excessive strains on the various parts of the motor. The final result is that the motor will begin to pound, overheat very easily, the valves will pit and the spark plugs become fouled.
The advantage of a low cold test point is that it will flow freely in cold weather, preventing the oil leads and pump from clogging up.
There are various methods of lubricating the working parts of commercial car engines, and it is a difficult matter to classify them. We may distinguish, however, between the following: Plain splash; splash from constant level troughs with pump circulation; part force feed and part splash; force feed without splash; either from an external source or built into the motor.
The simple splash system was much used on the earlier models of commercial cars. The crank case contained a supply of. oil into which the connecting rods dipped at each revolution of the crank pin, thereby splashing oil over the interior parts. As the oil worked out through the bearings or past the pistons, the oil level in the crank case fell and the splash became weaker. When the operator considered that the oil level had become too low, he would replenish the supply by either pouring oil into the crank case through a filling hole or transferring it from a supply tank to the crank chamber by means of a hand pump provided for the purpose.