THE LUBRICATION SYSTEM

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

OIL INJECTION SYSTEMS
FOUR STROKE ENGINE LUBRICATION
THE SPLASH TYPE SYSTEM
THE SLINGER SYSTEM
THE CONSTANT-LEVEL SPLASH SYSTEM
THE INJECTION PUMP AND BARREL PUMP SYSTEMS
THE POSITIVE PRESSURE SYSTEM
GEAR PUMPS
ROTOR PUMPS
THE PRESSURIZED OIL SYSTEMS IN GARDEN TRACTORS

OIL INJECTION SYSTEMS

Some large two-stroke engines use an injection system rather than an oil and fuel premix. Oil is injected into the crankcase, where it mixes with the fuel-air mixture obtained from the carburetor. A small pump, driven by the crankshaft, is used to pressurize the oil for injection.

The most basic type of oil injection system uses a fixed displacement pump, one that produces the same amount of oil at the same pressure for each rotation of the pump. Other more complex systems employ a variable displacement pump which is controlled by throttle linkage; these pumps will change pressure based on engine rpm.

Advantages of oil injection: 1)there is no need to premix oil with gasoline; rather, gasoline is poured into one tank, and two-stroke oil into another; 2)by introducing the ideal amount of lubrication based on engine speed, oil injection insures that an engine is always adequately lubricated.

What you should be aware of: in an oil-injection system, oil pumped into the engine is burned with the fuel; it does not return to the crankcase for reuse. This means that the oil level in the tank must be meticulously monitored as the engine runs.

FOUR STROKE ENGINE LUBRICATION

In most small four stroke engines, oil from the crankcase is either splashed onto moving engine parts, or drawn into a pump, pressurized, and driven into components through oil passages.

THE SPLASH SYSTEM

The splash system employs an oil flinger or dipper, attached to the bottom of the connecting rod in a horizontal-crankshaft engine, or to a crankshaft counterweight in a vertical-crankshaft model. Each time the crankshaft rotates or the piston moves down in the cylinder, the flinger or dipper reaches into oil at the bottom of the crankcase, and splashes it over internal engine parts. This is a reliable yet inexpensive system; for these reasons, it is common in small four stroke engines.

What you should be aware of: 1)to ensure that the system works, the crankcase oil level must be at capacity; 2)the engine must remain generally level as it is being operated so that the dipper or flinger will not lose contact with the crankcase oil; any time this happens, internal engine lubrication effectively stops.

THE SLINGER SYSTEM

Every time the crankshaft turns in a splash system, the flinger or dipper casts oil across the internal engine components. Some more-expensive small motors utilize a paddle wheel or slinger to splash around oil instead. Such a device will be driven via intermeshing gears by the crankshaft, exactly the way a camshaft is moved.

Advantages of the slinger system: because its wheel possesses multiple paddles to spread oil, the slinger provides more-continuous lubrication to internal engine parts than the splash method described above.

What you should be aware of: just as with the splash system, 1)the crankcase oil level must be kept at capacity; 2)the engine must remain level or near level during operation so that the paddles do not lose contact with crankcase oil.

THE CONSTANT-LEVEL SPLASH SYSTEM

This system works exactly like the simple splash system described above; however, instead of dipping into and spreading oil straight from the crankcase, the flinger or dipper draws its oil from a smaller reservoir within the crankcase. A plunger type pump driven by a camshaft lobe is used to keep this reservoir filled.

Advantages of the constant-level system: because the pump is continuously replenishing the oil supply inside the reservoir, the dipper rarely loses contact with oil, even when an engine is being operated at an angle.

What you should be aware of: the constant-level splash system will still not afford adequate lubrication to an engine which is to be operated at: 1)extremely steep angles; or 2)gentler angles for an extended period of time.

THE INJECTION PUMP AND BARREL PUMP SYSTEMS

The injection pump system: uses an oil pump to spray oil onto moving engine parts, which then distribute or splash the oil over remaining internal components. The pump used is a plunger type; its plunger is moved up and down by a rotating camshaft lobe. Pump spray will saturate the connecting rod end, the camshaft, and sometimes the flinger or dipper attached to the lower connecting rod.

The barrel pump system: employs a plunger to open and close a cylindrical pump case. When the plunger is drawn back, a pair of holes, one in the plunger and one in the pump case, align and oil is drawn into the pump. It will remain trapped there in the pump until the plunger compresses it into the closed end of the pump casing. This is when two different holes, one in the plunger and one in the pump casing, align and release the pressurized oil into an oil passage in the camshaft; from there, the oil proceeds to the main crankshaft bearing and connecting rod end, which together distribute it across other internal engine components.

THE POSITIVE PRESSURE SYSTEM

The positive pressure system is used on large, multi-cylinder engines like those found in some riding mowers. Oil is pumped from the crankcase through lines straight to bearing surfaces. Both a screen and an oil filter sift debris from the oil as it is drawn from the crankcase to prevent oil line clogs which could stop lubrication. An engine may possess one or two major oil lines in a positive pressure system. If two lines are present, one will feed lubricating oil to the crankshaft, while the other feeds oil to the camshaft assembly. It is the crankshaft which subsequently feeds oil to the pistons and cylinder walls.

GEAR PUMPS

Gear pumps are normally composed of two intermeshed gears and a protective housing. One of these gears will be connected to either the crankshaft or camshaft by a drive shaft. Oil is drawn into the intake side of the pump, and the spinning gears transfer it to the pump’s discharge side. The low amount of clearance between the pump housing and gears ensures high pressure, and helps prohibit seepage between the discharge and intake areas.

Gear pumps also harbor a pressure relief valve to help maintain consistent pressure within the pump as its internal components wear. A spring keeps the relief valve firmly closed until internal pump pressure reaches hazardous levels; then the valve opens, and releases oil back into the crankcase until the pressure equalizes. This valve and spring combination is necessary to limit oil pressure within the pump when it and the engine are new. As the gear pump ages, wear on its parts will reduce internal pressures and make the relief valve progressively less critical.

ROTOR PUMPS

The rotor pump consists of an inner rotor and an outer rotor. These rotors mesh and spin together, the inner rotor turning slightly faster; it is this modest difference in rotational speed between the two rotors that creates the pumping activity.

As the two rotors turn, oil is drawn into the pump through intake or suction ports on one side; spokes of the inside rotor transfer this oil to the discharge side of the pump where it leaves via a discharge port. A spring-controlled relief valve helps maintain a safe level of oil pressure within the pump during operation.

THE PRESSURIZED OIL SYSTEMS IN GARDEN TRACTORS

Most multi-cylinder tractor engines use pressurized oil systems for lubrication purposes. An oil pump is mounted on the engine crankcase cover. Lubricant is sucked from the crankcase, through a screen and an oil filter to eradicate impurities, and is pressurized and distributed throughout the engine via oil passages. Oil moves through the crankshaft to reach the connecting rods and crankpin bearings. It travels through each connecting rod to reach the piston and cylinder walls above. Other engine bearings are lubricated by an oil mist.

The system is equipped with a relief valve, ball, and spring that together maintain a minimum oil pressure, and guard against aggressive pressures that could damage the system. Complex systems like these will almost always contain one or more oil pressure sensors to alert the machine operator if system pressures descend below a preset level.