IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:
THE LUBRICATION SYSTEM
THE FRICTION-TYPE LUBRICATION SYSTEM
ENGINE BEARINGS AND BUSHINGS
HYDRODYNAMIC AND BOUNDARY LUBRICATION
TWO-STROKE ENGINE LUBRICATION
THE LUBRICATION SYSTEM
Adequate lubrication is key to engine performance. Oil helps to: 1)reduce friction between moving engine parts, keeping them cool; 2)clean internal components as an engine operates; and 3)form a tight seal between the piston rings and cylinder wall, enhancing compression, and ultimately engine power.
Some small four-strokes employ splash lubrication: this is a system in which engine bearings and cylinder walls are splashed with oil by an oil flinger or dipper, or by an oil slinger. The crankcase is used as an oil reservoir. The oil-spreading device resides at the connecting-rod bottom. When the crankshaft rotates, the oil slinger reaches into the crankcase like a cupped palm, and, as the piston rises, splashes lubricant onto bearings and over cylinder walls.
Other four stroke engines utilize pressure lubrication: in which a small oil pump is driven by the crankshaft via timing gears(the same way a camshaft is rotated). The pump draws oil from the crankcase, and pumps it onto bearings and through oil passages in the connecting rod to the cylinder walls.
Two-stroke engines are usually lubricated by a gasoline-oil mixture: when such a mixture is used, the oil and gasoline must be pre-mixed. The carburetor then combines air with the gasoline and oil, and the three substances circulate through the motor, and are ultimately burned together in the combustion chamber. Since the gasoline-oil mixture travels through all parts of the engine before burning, efficient lubrication is achieved.
THE FRICTION-TYPE LUBRICATION SYSTEM
The two primary foes of a small engine are friction and heat. There are countless internal areas where friction occurs; here are just a few of them:
1)Between the cylinder wall and piston rings
2)Within the intermeshed crankshaft and camshaft gears
3)Between the valve guides and valve stems
4)In the wristpin area where piston and connecting rod attach
5)Between the cam lobes and valve lifters, pushrods, or rocker arms
6)In the crankpin area, where the crankshaft and connecting rod join
Friction is defined as “the opposition to movement that occurs when two contacting surfaces are moving against one another. The goal of lubrication is to lessen or eliminate this friction. How much friction exists will depend on: 1)the finish on the two surfaces that are in contact; 2)the pressure being exerted on one or both of those surfaces.
Contacting components in a small engine will either be highly polished, or surrounded by bearings, to limit friction. The surface of a bearing or bushing, or any moving component in a small engine, will appear shiny and smooth to the naked eye; however these same surfaces under a microscope will be scarred and irregular, ripe with hills and valleys which can interlock when the surfaces meet. This interlocking effect is what procreates friction.
Such friction will lessen significantly if a lubricant is placed between the contacting surfaces. A layer of grease or oil will generate enough separation between such mated surfaces so that interlocking between surface imperfections does not transpire.
Sliding friction: is what was described above, the rubbing together of two contacting surfaces. You will often find sliding friction where the connecting rod attaches to the crankpin, and where its opposite end connects to the wristpin in the piston head. This is not the only kind of friction found in a small engine.
Rolling friction: occurs in ball, roller, and needle bearings. In such a rolling-element bearing, a ball, roller, or needle is suspended between two polished races. These race surfaces, while they appear smooth to the naked eye, possess the same defects, the same hills and valleys, that other smooth-appearing engine components do.
Friction occurs when a ball, roller, or needle strikes an area of deformity, a hill or a valley, on one of the race surfaces; however, just a thin covering of lubricant will segregate the rolling element from these potentially-troublesome sections, limiting friction. If you ever have the chance, try spinning side by side a dry ball or roller bearing and one that has been lightly lubricated. What you will notice is that: 1)the bearing absent lubrication will not spin as long; and 2)it will feel rougher and make greater racket.
Many small engines operate at 3,600 rpm. It is not atypical for an individual ball, roller, or needle inside a bearing to rotate at 10 times the speed of the crankshaft; in other words, if an engine is operating at 3,600 rpm, and there is a needle bearing between the connecting rod end and the crankpin, its individual needles would spin at 36,000 rpm, give or take. In dinky two-stroke motors which are designed to rev to levels exceeding 3,600 rpm, individual needles can rotate at rates 60% higher than they would in a four-stroke engine!
Friction generates heat: this can be particularly problematic if friction is excessive. Internal engine components, bearings included, are designed to expand as the engine heats up; expansion will progress only until the motor achieves its normal operating temperature. At normal operating temperatures, the rings should be sealing the cylinder, the crankshaft and camshaft gears should be meshing perfectly, there should be ideal amounts of clearance between the connecting rod ends and the crankpin and wristpin. If friction increases engine temperatures to above-normal levels, internal components can expand past their intended limits, begetting still greater friction and more heat in a vicious and damaging cycle. If such a cycle continues long enough, internal components will seize up, and stop rotating.
The bottom line in avoiding excess friction, and friction-induced heat, is to keep an engine properly lubricated via regular oil changes; and to use a good-quality motor oil, preferably the type recommended in a given engine service manual.
ENGINE BEARINGS AND BUSHINGS
Bearings and bushings are used to absorb wear that would otherwise arise when internal engine components were turning or sliding against each other. By reducing friction, bushings and bearings can also increase engine performance.
There are many varieties of bearings that can be utilized in small-engine applications. Which specific bearings are employed will depend on engine type, its rpm limits, and its peak horsepower. Generally speaking, small-engine bearings fall into two categories: 1)plain or friction bearings; and 2)rolling-element or frictionless bearings.
Plain bearings: will usually be used where engine forces are at their worst, at the connecting rod and camshaft ends for instance. These bearings are composed of soft metals such as aluminum or bronze, and will be either: 1)sleeve-like in appearance; or 2)a hole reamed into a soft-metal engine component.
The sleeve-like plain bearing: is designed in two pieces – like a set of split connecting rod bearings. Since both connecting rod and crankshaft are composed of hard steel, they cannot effectively rotate against each other absent a soft-metal intermediary to curb the friction. To solve the friction dilemma, halves of an aluminum sleeve are pressed into the connecting rod ends; when the connecting rod ends are bolted around the crankshaft crankpin, the crankpin journal rotates against the two-piece aluminum sleeve instead of against the hard steel of the connecting rod.
The hole-style plain bearing: is a polished hole – like, for example, the hole in an aluminum crankcase in which the steel crankshaft end spins. The crankcase itself functions as the bearing surface. The hole drilled into it is painstakingly exact in size, simply large enough to accommodate the crankshaft end diameter plus operating-temperature expansion. Small two-stroke engines often employ aluminum instead of steel connecting rods; the connecting rod end will then have a polished hole reamed into it, and be fitted around the steel crankshaft crankpin in the same way.
The bronze plain bearing: like the hole-style variety described above, begins with a hole drilled into a crankcase or other engine-component surface. The hole itself, in this case, does not serve as the bearing surface; instead, a bronze insert is press-fitted into the hole, and the steel crankshaft end rotates against this insert surface.
Long life self-lubricating bearings: are composed of bronze and Teflon-like lead inside a steel case. These monsters work by depositing a thin layer of their bronze and lead lubricating substance onto the steel surface they support. In the event of excessive friction, or especially friction-related heat, the bronze-lead substance actually expands to prevent wear on whatever steel engine component is spinning inside the bearing.
Precision insert bearings: are steel shells coated with anti-friction material like babbit, aluminum, or copper-lead alloy. They work much like standard sleeve-type plain bearings.
The split bearing: locks onto the surface it means to protect, so that the bearing interior remains immobile. Its outside, meanwhile, spins against the metal of the hole or component into which it is fitted.
The bushing: is a sleeve-type plain bearing made of soft low-friction metal which rotates against a harder steel component. Frequently there will be a bushing placed at the top of the connecting rod where it turns against the wrist or piston pin. Some small engines will also employ bushings to sustain the camshaft and crankshaft ends.
What you should be aware of: sometimes a plain bearing or bushing contains a channel in its surface to permit oil from the crankcase to enter and lubricate it during engine operation. A bearing or bushing so furnished may need to be press-fitted into its hole with this oil channel in a particular position; consult the engine service manual for instructions.
There are several types of rolling-element bearings:
Ball bearings: are sometimes used to support the crankshaft ends. The balls of the bearing are held inside two grooved rings called races. The bearing is press-fitted so that the exterior of its outer race wedges into the crankcase hole. The crankshaft end, meanwhile, slides tightly into the inner race. The outer race remains stationary; the inner race and crankshaft end spin together, using the series of balls between the races like so many wheels. This reduces friction to a far greater extent than could be achieved with the best plain bearing.
Balls in a ball bearing: are precisely machined and polished, hardened, and then chromed to provide them with the smoothest rolling surfaces possible. A cage or train keeps the balls separate from one another within the races, so that they cannot touch as they spin. The last part of a ball bearing is an end shield composed of rubber, metal, or a combination of the two; the end shield: 1)prevents crankcase oil from seeping past the balls and out of the engine; and 2)keeps wayward dirt from coming in.
Roller bearings and tapered roller bearings: work much like ball bearings; the rollers employed sit between polished inner and outer races. They differ from balls only in their cylindrical appearance.
Roller bearings are preferred to ball bearings anytime that heavy forces are pressing on the side of the bearing. You could compare these forces to the weight of a car pushing down on the tread-surface of its tire.
Tapered roller bearings are utilized if there is both force against the side of the bearing(known as radial force), and perpendicular axial force, which presses against the bearing face. Car axles apply this, axial force, to the wheels they are connected to. To accommodate both directional forces, wheel bearings, whether on a car or piece of power equipment, will often be of the tapered roller sort.
What makes a tapered roller bearing different: from a standard roller bearing is that it harbors a larger internal diameter on one side than the other. The exterior of the shaft which slides through the tapered roller bearing must possess the same degree of taper so that the two surfaces fit together seamlessly.
Needle bearings: will frequently be used at the ends of the connecting rod in a two-stroke engine; in other words, you will find them where the connecting rod attaches to the crankshaft crankpin, and where it attaches to the wristpin inside the piston head. Needle bearings are ideal for high-revving applications where internal forces on the face and side of the bearing will be relatively insignificant – such as in the two-stroke motors of string-headed weed trimmers and chain saws.
A needle bearing will usually not have an inner and an outer race the way a ball bearing does. Consider a needle bearing being used on the wristpin in the piston head; the wristpin will actually serve as the inner race, while the connecting rod end which slips over the wristpin functions as the outer bearing race. These surfaces will hold the needles in position. A cage or train keeps the needles from making contact with each other as they spin.
HYDRODYNAMIC AND BOUNDARY LUBRICATION
Moving parts within a small engine can see two kinds of lubrication; these are: 1)hydrodynamic; and 2)boundary lubrication.
Hydrodynamic lubrication: happens when there is a constant film of oil in between moving metal parts during engine operation – say at the crankpin bearing and camshaft bearing.
Boundary lubrication: happens whenever there is not a complete film of oil around moving metal parts, leaving some metal to metal contact between them. Boundary lubrication is common to its internal components when an engine is started, and crankcase oil has yet to be distributed through the engine.
Some internal engine components, like the crankpin and crankshaft bearings, will never lose contact with crankcase oil for long enough to suffer boundary lubrication. The cylinder walls and piston rings, and the camshaft lobes, should endure boundary lubrication just briefly, when a cold engine is started. In most engines, the top compression ring on the piston is meant to have constant and unvarying boundary lubrication.
The motor oils of today harbor additives that lessen the adverse effects of boundary lubrication. These additives ensure that at least a thin coat of oil remains on metal surfaces when an engine is shut down, and the majority of the lubricant drips back into the crankcase.
TWO STROKE ENGINE LUBRICATION
Most two strokes are lubricated with a pre-mix of gasoline and engine oil. Remember that: pre-mixes do not utilize the standard motor oil that would fill the crankcase of a four-stroke engine. You should always use special two-stroke oil in such a fuel-oil pre-mix.
Standard four-stroke motor oil should not be used in a pre-mix: unless a service manual specifies it. Why? For one thing, standard four-stroke oil, unlike two-stroke motor oil, is not specially formulated to change form.
When the fuel-oil pre-mix goes through the two-stroke carburetor, it combines with air, the same as fuel would in a four-stroke engine. Once in the carburetor, however, the air and fuel in a two-stroke transform into vapor, and within the vapor, the special two-stroke oil forms pinpoint droplets. These droplets cling to internal components as the three-substance mixture is ushered through the engine, supplying the entirety of lubrication garnered by cylinder walls and piston rings, the crankpin, connecting rod, and wristpin, and their bearings.
Two-stroke oils also possess fewer detergent additives: than four-stroke oils, entailing that they will burn inside a combustion chamber much cleaner. This is significant since, in a two-stroke engine, oil burns along with air and fuel in the combustion process.
What you should be aware of: the proper ratios of fuel to oil in the pre-mix will vary between engines, and oils; the recommended ratio is generally in the range of from 16 parts fuel to one part oil, to around 40 parts fuel to one part oil. Some synthetic two-stroke oils can have a pre-mix ratio of roughly 100 parts fuel to one part oil.
Always try to use the correct ratio when mixing oil and fuel. Consult the engine service manual, and the directions on the oil bottle. Using the correct ratio in the pre-mix will help ensure that: 1)the engine will garner sufficient lubrication; 2)the carburetor does not become clogged by excess oil; 3)that oil cannot displace fuel in the critical fuel-air mixture. Displacement of fuel can procreate a lean mixture that harbors way too much air for the amount of fuel it contains. A lean fuel-air mixture can afflict internal damage on a high-revving two-stroke motor because it generates much-hotter operating temperatures than intended.