Additives are substances included in engine oils and have the following effects:
■ Antioxidation: Because engines have a hot portion called the "combustion chamber," oil that lubricates components nearby the combustion chamber, such as the piston rings, cylinder wall, and valve stems, becomes hot when it is in a thin film state. This, coupled with contact with chemically activated hot gases results in significant acceleration of oxidation reaction.
This oil is also exposed to extreme pressure due the high temperatures generated from the frictional heat that results from the rotation of lubricated parts such as the connecting rods, crankshaft, and bearings. This pressure is a factor that promotes oxidation.
Furthermore, catalysis caused by metal powders generated by wear is another factor that promotes oxidation. It is to prevent these types of oxidation that antioxidants are added.
■ Anticorrosion: The various acids that are generated when fuel burns corrode metal and cause more severe wear in the case of parts subject to friction. This is particularly true of sulfur content, which is produced by the burning of sulfur in diesel engines.
For these reasons, additives capable of neutralizing invasive acids and providing adequate protection from acid corrosion and wear are added.
■ Rust prevention: When an engine is stopped after running, the inside of the engine contains an acidic atmosphere due to exhaust gases and its temperature drops to an ambient temperature. Moisture in the exhaust gases (blow-by gases) condenses and adheres to the pistons and cylinders in the form of water droplets. Acid gas then begins to dissolve in those water droplets, after which the oil film that covers the metal surfaces is gradually replaced by a thin film of acidic moisture, which then oxidizes and causes rust to form.
Agricultural engines are often left outside for long periods of time without being used, which is a condition under which rust is likely to form.
■ Foaming prevention: When oil is supplied by being circulated from the oil pan using a pump, it may become agitated during operation and begin to foam. When this occurs, the oil pump will take in the resulting bubbles, causing a temporary interruption in supply.
Because this will prevent lubrication from being performed smoothly, additives that prevent foaming are added.

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