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In this paper, the temperature of coupling system including cylinder, head, inlet/exhaust valve, and the cooling jacket of a 400cc engine is investigated by computational fluid dynamic (CFD) method. Firstly, the total pressure loss of water jacket, radiator, and thermostat is calculated first, and then the mass flow rate inside the cooling system can be determined by fitting the water pump’s performance curve (P-Q curve). The thermal boundary conditions for analysis of conjugate heat transfer of cooling system, such as combusting gas temperature and heat transfer coefficient are utilizing the results of 1-D engine simulation software (Ricardo WAVE). The current approach is that the heat transfer coefficients of valve while opening are calculated by considering the intake and exhaust stroke using FLUENT to overcome the difficulty of these values that are not modeled in such 1-D software. Finally, the finite element method (FEM) is used for the valve stress calculation.

In this study, the re-starting characteristics of a motorcycle engine with idle-stop were investigated. Generally when turning off the engine, there is; or when restarting engine, the air-fuel mixture will become rich to cause the incomplete combustion. When the restarting period is shortened, the aforementioned phenomena would be improved. The aim of this study was to shorten the engine re-starting time during start-up. In the initial stage of the study, the gear ratio of the starter was changed, and the parameters of the engine speed and cylinder pressure were measured and analyzed. The results showed that supplying the additional fuel injection duration of 3 milliseconds into the combustion chamber before the engine was stopped would give the quicker restarting characteristics.