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Motorcycles have been classified by engine displacement from commercial and legal demands. If this restriction on displacement were removed, what could be done to improve fuel economy? “Down sizing” would be an obvious idea. However this paper proposes a quite inverse concept, i.e. a large displacement lower speed engine. Mechanical friction in four-stroke engines generally decreases in proportion to the square of engine speed. Thus mechanical efficiency should be better at lower operation speeds. According to this concept, developing a 200cm3 experimental engine was successful in improving fuel economy by 79% in the ECE40 mode and by 97% at 60km/h cruise condition, compared to the 100cm3 original engine.

A low pressure pneumatic direct injection engine by AR Combustion concept has been developed. In this engine, the scavenging process is performed by air only, prior to the fuel feed process, which minimizes the fuel short-circuiting accompanying the scavenging flow. The wide spray form generated by the rotary type injector facilitates homogeneous combustion even in the high speed high load range, which promises higher maximum power output. Meanwhile, in the light load range, AR Combustion effectively solves the irregular combustion. The bench test results show a remarkable reduction in the exhaust emissions and fuel consumption while successfully maintaining high power output of original two-stroke engine. Moreover, actual vehicle tests using a motorcycle reveal its good potential as a vehicle engine.

The one-way clutch mechanism seen on bicycles, etc. greatly ease the load of riders at times of coasting or descending downhill. Their use on motor vehicles are restricted to some automatic drive four-wheeled vehicles with torque converters. There are neither any example of mass-produced motorcycle with one-way clutch between the engine and the drive system nor any reports of quantitative study on its impact on the performance of the vehicle including fuel economy. The present paper reports the results of experimental research on one-way clutch employed in the drive system, obtained for motorcycles having five kinds of two-cycle engines of different displacements. The test on fuel economy showed an improvement of 4 - 9%.