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Conventionally, the engine oils for passenger cars have been used as four stroke cycle gasoline engine oils for motorcycle from old-time. However, the engine oils for passenger cars have been required to have fuel economy performance, and these engine oils are not fit in the requirement of motorcycles because of low friction and low viscosity. For this reason the four stroke cycle gasoline engine oils for motorcycles have been classified into two categories of ‘MA’ and ‘MB’ based on differences in friction characteristics according to standard of Japan Automobile Standards Organization (hereinafter referred to as ‘JASO’) -JASO T 903:1998. In currently, some engine oils for passenger cars can fit in JASO standard, but the range of friction coefficient in MA category is becoming wider.

Just as CO2 reduction is required of four wheeled vehicles for environmental protection, similar environmental concerns drive the development of motorcycle oil technology. Zinc dialkyldithiophosphate (Zn-DTP) type additives are widely used for engine oil formulations. However, phosphorus compounds are environmental load materials. The reduction of the quantity of phosphorus compounds in engine oils is required to reduce poisoning of three-way catalysts used to purify exhaust gases from internal combustion engines. Mr. Ito and his co-authors1) reported that they developed a sulfur-based additive as a substitute for Zn-DTP. Their non-phosphorus engine oil formulation for four-wheeled vehicles with a sulfur-based additive was examined to evaluate its anti-wear performance using the following test methods:JASO M328 for gasoline engines (KA24E) and JASO M354 for Diesel engine (4D34T4).

Honda developed a 750cm3 V-4 engine adopting an elliptical piston, and began selling the “NR” motorcycle with the engine installed in 1992. The adoption of an elliptical piston and cylinder achieved a compact layout of eight valves, which consists of four intake valves and four exhaust valves per cylinder. This paper explains the features of an engine with such a layout, focusing on the following: 1) Multiple valves and short-stroke enable the 750cm3 engine to achieve 15,000rpm. 2) The engine is more compact and lightweight than an engine having the same displacement, and more powerful than one with twice as many cylinders (8 cylinders). Also, this paper describes the techniques giving improved blowby gas and oil consumption characteristics as related to the sealing property of the piston, cylinder and piston ring and achieving performance equivalent to a conventional motorcycle engine.

As well as a four-wheeled vehicle, in the field of motorcycle, development of the CO2 reduction technology and practical use are required for global environment. Zinc dialkyldithiophosphate (Zn-DTP) type additives are widely used for engine oil formulations. However, phosphorus compounds are environmental load materials. The reduction of the quantity of phosphorus compounds in engine oils is required to reduce poisoning of three-way catalysts used to purify exhaust gases from internal combustion engines. Ito1) reported that they developed a sulfur-based additive as a substitute for Zn-DTP. Their non-phosphorus engine oil formulation for four-wheeled vehicles with a sulfur-based additive was examined to evaluate its anti-wear performance using the following test methods: JASO M328 for gasoline engines (KA24E) and JASO M354 for Diesel engine (4D34T4).

With environmental issues at the forefront of the automotive industries, there is a clear need to improve the fuel economy and longer drain performance of four-cycle motorcycle engine oils. Evaluation of oil consumption in actual engines was used to clarify an optimal volatility for engine oils, and the feasibility of using zinc dialkyl phosphate, a sulfur-free additive, as an alternative to ZDTP for improved of fuel economy performance was studied. Engine oils with optimized additives demonstrated excellent friction characteristics and wet clutch performance.