Lubrication & Cooling System
How does the lubrication and cooling system function?
Exhaust gases and radiant heat emitted from engine components carry off much of the heat produced by a small engine, but not enough to keep an engine running reliably. The lubrication and cooling system is designed to handle that task. As a lubricant, oil not only carries away heat, it reduces a major source of heat - friction between engine components. Air flow also serves a secondary function on some engines, triggering the air vane in a pneumatic governor (see "Governor System").
Reducing Friction With Oil
Viscosity is the most important quality of engine oil. It is a measure of an oil's ability to resist motion. This quality is critical to oil's performance, since moving parts constantly try to push oil, much as a plow pushes snow. Oil must resist this force so it can maintain a continuous film that keeps the parts themselves from touching.
While viscosity allows oil to cling to surfaces and resist the snow-plow effect, it also reduces the ability to flow at low temperatures or within tight clearances. A more viscous oil also takes longer to reach its optimal temperature.
Oil grades are generally a compromise that tries to anticipate typical operating conditions. A common recommendation for small engine oil is SAE (Society of Automotive Engineers) 30 four-cycle oil.
Some oils are altered to make them less viscous during the winter. These multi-viscosity oils have ratings such as SAE 10W-30. The 10W indicates a lower winter viscosity. At normal operating temperatures, the oil acts like SAE 30 oil. Always follow the recommendation of your when selecting the proper oil for your engine.
Getting Oil To Circulate
Most small engines rely on the splashing motion of a dipper or slinger in the crankcase to distribute oil. On a horizontal crankshaft engine, a dipper is attached to the connecting rod. It picks up oil in the oil reservoir located in the crankcase and spreads it across bearing surfaces as the piston travels through the cylinder.
A slinger is used on many vertical crankshaft engines. It consists of a spinning gear with paddles cast into the plastic gear body. Part of the slinger is submerged in the oil. As the crankshaft turns, the slinger disperses oil throughout the crankcase.
Keeping Oil Clean
Small engines designed for tractors and other heavy-duty equipment may include an oil filter. The pleated paper inside removes dirt, metal particles and other foreign matter that accumulates in the oil. If the paper becomes clogged, oil is rerouted through a spring-loaded bypass valve to ensure lubrication even when oil is very dirty. Even if your engine has an oil filter, you need to inspect the oil every time the engine is run to make sure the level is correct and the oil still has its clean amber color.
Cooling With Air
An engine relies on air circulating around engine parts to maintain an acceptable engine temperature. Fins on the outside of the cylinder block and cylinder head improve the engine's cooling ability the way pipes do on a car radiator - by increasing the surface area that radiates heat and is exposed to cool air. Although air is not the most efficient way to transfer heat, it is plentiful and usually offers a substantial cooling effect.
A different set of fins - the flywheel fins - are also an important cooling feature; as they spin they distribute air to many engine parts. The blower housing and air guides route air to the flywheel fins. On some models, a rotating screen over the flywheel prevents grass and other debris from clogging the flywheel fins. The screen blocks debris from entering or cuts it into smaller, less harmful particles.
Additional Cooling Components
Some engines require additional air cooling and contain a cooling air plenum, a duct that provides a separate means for outside air to enter the engine. Some contain an air discharge that directs hot air away from the engine.
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