Unlike four-stroke engine whose crankcase is closed except for its ventilation system, two-stroke engines use the crankcase as part of the induction tract, and therefore, oil must be mixed with gasoline to be distributed throughout the engine for lubrication. The resultant mix is referred to as petroil. This oil is ultimately burned along with the fuel as a total-loss oiling system. This results in increased exhaust emissions, sometimes with excess smoke and/or a distinctive odor.
The oil-base stock is either petroleum, castor oil, semi-synthetic or synthetic oil and is mixed (or metered by injection) with petrol/gasoline at a fuel-to-oil ratio ranging from 16:1 to as low as 100:1. To avoid the high emissions and oily deposits on spark plugs, modern two-strokes, especially for small engines such as garden equipment and chainsaws, may now demand a synthetic oil and can suffer from oiling problems otherwise.
Engine original equipment manufacturers (OEMs) introduced pre-injection systems (sometimes known as "auto-lube") to engines to operate from a 32:1 to 100:1 ratio. Oils must meet or exceed the following typical specifications: TC-W3TM, NMMA, [API] TC, JASO FC, ISO-L-EGC.
Comparing regular lubricating oil with two-stroke oil, the relevant difference is that two-stroke oil must have a much lower ash content. This is required to minimize deposits that tend to form if ash is present in the oil which is burned in the engine's combustion chamber. Since the 1980s different types of two-stroke oil have been developed for specialized uses such as outboard motor two-strokes, premix two-stroke oil, as well as the more standard auto lube (motorcycle) two-stroke oil.
Additives for two-stroke oils fall into several general categories: Detergent/Dispersants, Antiwear agents, Biodegradability components and antioxidants (Zinc compounds).
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