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Improvement of warming performance is required for engine cooling system, because such improvement has an important role for lower emissions. However, the cooling performance and the warming performance of engine are related to the transactions. To improve such trade-off relation between the two performances, oil-cooled system was developed which components are simpler than conventional water-cooled system with a single cylinder, 339cc engine. The oil-cooled system has an “oil jacket” that closely encircles the combustion chamber to cool the cylinder head and cylinder sufficiently. This system adopts the engine lubrication oil to cool combustion chamber, thus the average temperature of the combustion chamber is higher than usual water-cooled engine that enables better warming performance.

Reduction of flow resistance in an intake system is essential for increasing the output of a four-stroke engine. Evaluation method regardless engine displacement or number of valves or cylinder must be required in intake system design. This study proposes intake loss coefficient as total evaluation method from flow in an intake port to charging flow into a cylinder. A three-dimensional, general-purpose Computational Fluid Dynamics (CFD) code was used to calculate an intake loss coefficient. A correlation was confirmed between an intake loss coefficient and the engine power output. Intake loss coefficients and the CFD technique may be used for efficient optimization of the shape of an intake system.