In the previous papers, the relationship between the power exerting the combustion chamber wall and the combustion noise power radiated from the outer wall of engine was studied with both a single combustion excitation and running operation. This relationship was denoted by the transmission-radiation coefficient, and was compaired with some different types of engine. In this paper, the vibration transmission coefficient and the noise radiation coefficient were separately obtained as two parts of the transmission-radiation coefficient of the engine structure. The generation mechanisms of combustion induced engine noise were investigated in detail calculating these coefficients for each engine wall and their contributions to the total.