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An oil-cooled engine has been developing to achieve better warm-up performance. The oil-cooled engine has an oil jacket that pass through around the exhaust port and the cylinder liner. Fins were installed inside the oil jacket to enhance cooling performance. The result of a bench test shows that the fins enhance the cooling performance with slight loss of warm-up performance. The aim of this study is to clarify effects of the fins. This study conducted two simulations. One is a cooling simulation that was conducted to clarify the reason why the fins enhanced the cooling performance. The other is a warm-up simulation that was conducted to clarify the reason why the fins almost maintained the warm-up performance. The cooling simulation was conducted by steady flow simulation. It simulated a full-load operation of the bench test. It compared converged temperature between the engines with/without the fins. The warm-up simulation was conducted by unsteady flow simulation.

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.