Tribopolymerization as an approach to two-stroke engine lubrication

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Virginia Tech


A literature review was performed to investigate the current status of two-cycle gasoline engines from the standpoint of design, performance, benefits, lubrication and emissions. The two-stroke engine offers several significant advantages over its four-stroke counterpart, which include: simple design, low production cost, higher power to weight ratio, smaller size, lower peak operating temperatures, and lower NO𝗑 emissions. The main drawbacks of two-stroke engines are the high unburned hydrocarbon and particulate exhaust emissions which are attributed to the lubricating oil and the scavenge process. Technologies to reduce exhaust emissions from two-stroke engines are strongly contingent on the ability to develop improved, environmentally friendly, lubricants which can be used at high fuel:oil ratios.

An experimental study of lubrication of an air-cooled, two-stroke, gasoline engine by tribopolymerization is presented. Tribopolymerization is defined as the planned or intentional formation of protective polymeric films directly and continuously on rubbing surfaces to reduce damage and wear by the use of minor concentrations of selected compounds capable of forming polymeric films in situ[12]. Two monomers were investigated as potential anti-wear compounds, namely (a) Cas dimer acid/ethylene glycol monoester, a condensation monomer, and (b) diallyl phthalate, an addition monomer.

The condensation monomer was found to provide sufficient wear protection under a range of unloaded test conditions. Under loaded test conditions, however, piston scuffing occurred well before the planned test period of 20 hours had expired with all of the lubricants tested. Photomacrograph and SEM photographs reveal rather thick deposits on the monoester lubricated piston using a mass concentration of 0.5% monoester in gasoline. The heavy deposits were not noted using a mass concentration of 0.5% diallyl phthalate or with a 0.1% mass concentration of the monoester in bis (2-ethylhexyl) sebacate, a synthetic base oil.

As a secondary topic which parallels this study, other existing and rising fuel lubricity problems have been investigated. The poor friction and wear characteristics of diesel, jet, and alcohol fuels have led to tribological problems in various engine fuel system components. The outstanding anti-wear performance of monomer additives in several similar tribological systems in the past shows good potential for the solution to a variety of current fuel lubricity problems.