Abstract: The cam, by which the intake and the exhaust mechanisms in the CG motorcycle engine are both driven, can be divided into two types—the equidirectional and the reverse in the design perspective. Under the same parameter of the two types, the cam profiles designed in different ways are different. The paper aims to apply the basic theory of the cam, in the project design, to analyze the intrinsic relation of design parameters, calculate the optimum structure synthesis of valve train in the CG motorcycle engine and reveal the matching intension and the design principle of the parameters. The calculating method has been tested in the design of the CG150 engine and the results show that the maximal engine power is improved from 8.51 kW (liter power is 56.7 kW) to 9.7 kW (liter power is 64.7 kW), and the noise level is reduced by approximately 5 dB, reducing the impact noise while the engine running at a high speed, and better characteristics is achieved.

Key words: CG series engine, Motorcycle, Optimum dimension synthesis, Valve train