The growth of the two-cycle market has been steady and rapid during rhe last 12 years. This is illustrated with data based on outboard motor sales.
Because the lubricant for these engines is mixed with the fuel and a large portion is burned in the combustion chamber, certain operating difficulties are often encountered. Chemical additives offer a means of reducing or eliminating these difficulties. These additives should not be classified into categories, such as ash-containing additives and ashless additives, in an effort to define performance. A wide range in performance has been observed within both groups of additives. Data indicate that the chemical structure of the organic portion of these additives is of more significance than the metallic elements from which the additives are made.
Various base stocks have been studied to determine their effect on two-cycle performance and their response to additive treatment. Most of the Coastal and Pennyslvania solvent extracted untreated oils gave better performance than Mid-Continent solvent extracted untreated oils. Generally speaking, all oils responded favorably to additive treatment; but the response of the Mid-Continent solvent extracted oils was greater than that of the Coastal or Pennsylvania oils, resulting in finished products of comparable quality.
Exceptions to these generalities were found with some base stocks. Because of these exceptions and because of the wide variety of both design and application of two-cycle equipment, the final selection of a base stock and additive treatment should be determined from performance tests conducted in a variety of two-cycle engines and under various operating conditions.