Abstract:To investigate the transfer characteristics of combustion noise in a diesel engine, the one-third octave band spectrum was analyzed under the same operating conditions for four different directions with the help of combustion and motoring tests. The pressure level of combustion noise was separated and the transfer functions were calculated. FFT was used to get the frequency spectrum of cylinder pressure level on the basis of combustion indicator diagram in a diesel engine. At the same time,the attenuation curves of structure were calculated in two different operating conditions. The results showed that the relative increasing rate in the noise level of operating condition was over 4.5 percent in contrast with that of motoring condition in four different directions. The maximum noise levels in different directions all occur in the medium and high frequency range. The transfer functions also increase with the rise of frequency in four different directions. The trend of attenuation curves are almost the same.