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Motorcycle has broad spectrum of developments, such as excellent engine performance, low fuel consumption, emission and noise reduction. As global warming become a serious issue internationally, reduction of fuel consumption is especially of importance. In this study, an evaluation method for the WMTC mode fuel consumption using a one-dimensional engine simulation is investigated. The fuel consumption for the WMTC mode can be predicted in a short time without a complicated vehicle model to simulate transient behavior. The proposed method mostly showed good agreement with measured data for middle-class motorcycle using a chassis dynamometer.

For the prediction of pumping loss in the engine crankcase at the early stage of engine design, a similarity law of the pumping loss on parallel cylinders with the phase difference of 180 degrees has been derived from the sum of the power loss due to the drag force of airflow through the cylinder bulkhead holes and the inertia force of fluid. It has been found that the mean effective pressure in the crankcase is in proportion to the square of the mean flow velocity at the bulkhead holes. Then, in order to validate the similarity law, by using a prototype engine with inline four cylinders, the pumping loss was estimated by subtracting the frictional loss with special pistons that had air holes, from the power loss in which the area of the bulkhead holes and engine speed were changed. The experimental results of the pumping loss were normalized by the similarity law. As a consequence, it has been shown that the pumping loss obeys the law.

To attain high performance and low emission at the same time, computational fluid dynamics was incorporated into a design optimization stage of the muffler. Splitters were equipped at the short bend just upstream of the catalyst to suppress flow separation and mal-distribution within the catalyst region. Design screening had been done for the splitters based on result from the steady states CFD. Then the unsteady CFD for the selected splitter design showed uniform flow field within the catalyst region even when the flow rate was abruptly changed due to intermittent exhaust process.