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In recent times, due to the improvement of internal cylinder flow analysis technology with Computational Fluid Dynamics (CFD), the prediction accuracy of fuel consumption and emission has improved. However, small motorcycles often have complex intake ports which restrict the layout of injectors. Therefore optimization of injection spray to achieve high combustion efficiency and less wall wetting is a challenge. In this study, we predicted fuel consumption and emission performance by the simulation result of air fuel distribution and wall wetting amount with an actual motorcycle engine model. We optimized injector nozzle length, spray angle and spray tip penetration. After the optimization, we evaluated the emission performance and fuel consumption with an actual engine. As a result, we were able to confirm the improvement of fuel consumption and emission performance.

A compact, low-cost fuel pump module has been developed for use in motorcycles with a small-displacement engine. Various considerations are given to make the module as compact as possible. The pump motor, which is one of the major component parts, is down-sized specifically for applications to small-displacement engines. The pressure regulator uses a simple construction consisting only of a ball and a spring without a diaphragm. Especially noteworthy is that with the volume reduced by approximately 40% from the conventional pressure regulator while using the construction that reduces self-excited vibrations caused by fuel pressure pulsations, the pressure regulator contributes significantly to the down-sizing and cost reduction of the module. Furthermore, the down-sized module remarkably reduces the size of fuel pump mount surface, allowing a modification from the flat-surface sealing to the radial sealing.

With the interest in global environmental issues growing in recent years, the demand for the reduction of exhaust gas emission and improvement in fuel consumption for small motorcycles has increased greatly. Recently, small motorcycles have been marketed equipped with an electronically controlled fuel injection system effective in reducing emissions and enhancing fuel consumption by accurately controlling the air-fuel ratio. The small motorcycles' market comprises mainly ASEAN countries, and the majority of the motorcycles consist of reasonably priced models with air-cooled engines. Fuel injection systems have already been adopted for motorcycles equipped with water-cooled engines in the markets of advanced countries, mostly in EU. Given the above situation, two issues must be addressed to adopt a successful fuel injection system for air-cooled, low-priced small motorcycles.

Amidst the growing concerns for the global environment and the escalating demands for low exhaust emissions and high fuel economy small motorcycles, the electronically controlled fuel injection system (hereafter referred to as FI system) is making its appearance as an effective device to reduce emissions while improving fuel economy by precisely controlling the air-to-fuel ratio. Fuel injectors and fuel pumps currently used in small motorcycle FI systems were originally derived from multi-cylinder engine automobile and large motorcycle FI system components, and therefore have not been necessarily suitable for small motorcycle application.

This emission reduction technique has been researched on motorcycles equipped with a four-cylinder, liquid-cooled engine of 1100cm3 displacement in order to comply with the EURO-3 emission regulation. The EURO-3 emission regulation will be enforced in Europe beginning in year 2006. Compared to the EURO-2 regulation, reduction of cold-start HC and reduction of NOx from the extra-urban driving cycle are the main issues for EURO-3 compliance. The hydrocarbon reduction during cold-starting was achieved by the method of early catalyst activation using a combination of an Idle Air Control Valve system (IACV), ignition-retard, and atomization of fuel spray. In the extra-urban driving cycle, the fine controlled air-fuel ratio reduced. In addition, optimization of the number of three-way catalyst cells and their capacity also reduced NOx. Moreover, power loss decline caused by increased exhaust resistance due to increasing the catalyst size was avoided by optimizing the catalyst location.

In conventional electronically controlled fuel injection systems, when the battery is inadequately charged, the small amount of electric power generated from the alternator by the kick starter operation is consumed by all electrical loads including the battery. This causes a voltage drop, hence the fuel injection system does not function due to a power shortage. To eliminate the power shortage, an installed relay circuit opens all electric loads other than the fuel injection system. This allows the fuel injection system to use all the electric power generated by the kick starter operation aided through using an additionally incorporated condenser. This type of electric power control system has been incorporated into the ECU. Thus, the control system has been realized that permits starting of an engine by using the kick-starter even when the battery is completely discharged.

The authors developed a simple simulation method of motorcycle emission mode, using a popular chart calculation software for a personal computer, to calculate from the motorcycle's various specifications and the engine bench test data. The precision or verification by a mass production model was of a sufficiently good level. This simple simulation method was easy to use and effective in reducing the research period. Moreover, the experimental motorcycle (Scooter, Two-stroke, 50cm3) that adopted the stratified combustion system for a fuel economy improvement and CO2 reduction, was made and experimented on the ECE40 mode as a sample case. Approximately 20 % of fuel economy improvement and CO2 reduction were achieved and the estimation was effectively proved.