Unlike a four-stroke engine, whose crankcase is closed except for its ventilation system, a two-stroke engine uses 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 premix or 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 can be petroleum, castor oil, semi-synthetic or synthetic oil and is mixed (or metered by injection) with petrol/gasoline at a volumetric 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. Additionally a non-2T-specific oil can turn to gum in a matter of days if mixed with gasoline and not immediately consumed. Another important factor is that 4-stroke engines have a different requirement for 'stickiness' than 2-strokes do. 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. As a rule of thumb, most containers of oil commercially offered will have somewhere on the label printed that it is compatible with 'Autolube' or injector pumps. Those bottles tend to have the consistency of liquid dish soap if shaken. A more viscous oil cannot reliably be passed through an injection system, although a premix machine can be run on either type.
"Racing" oil or castor-based does offer excellent lubricity - at the expense of premature coking. For the average moped/scooter/trail rider it will not garner an appreciable increase in performance and will require very frequent teardowns.
Additives for two-stroke oils fall into several general categories: Detergent/Dispersants, Antiwear agents, Biodegradability components and antioxidants (Zinc compounds). Some of the higher quality include a fuel stabilizer as well.
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