Recently, the emission regulations for small motorcycles have become more and more severe in many countries each year. Therefore, an emission reduction system for a small motorcycle, with fuel injection which accurately controls the air-fuel ratio, was investigated based on a four-stroke engine of 100cc displacement. The amount of fuel injected is precisely controlled by two maps according to the throttle openings, the engine speed, and the intake pressure. In the control mode, the peak value of intake pressure during the opening of the inlet valve is maintained to avoid the influence of the big pressure changes of a single-cylinder engine. The peak value is also used as a control index. Furthermore, an O2 feedback control and a three-way catalyst are used to purify exhaust emissions, and a small fuel pump of a power saving type was developed to improve fuel economy. In addition, a control system to obtain excellent start characteristics with a kick-starter was developed. By developing these systems, hydrocarbon emissions were decreased to about 1/30 of the motorcycle emission regulation limit "Directive 97/24/EC chapter 5" (EURO-1) in Europe compared to the base engine, and the fuel economy has improved by about 20%.
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