A Study On The Acoustic Characteristics Of The Out-of-phase And Split-gas Rushing Muffler Unit

The exhaust noise, which falls into low-frequency noise, is the dominant noise source of a diesel engines and tractors. It is a direct and effective way to reduce the exhaust noise by using an exhaust muffler. However, the traditional exhaust silencers, which are normally constructed by combination of expansion chamber, and perforated pipe or perforated board, are with high exhaust resistance, but poor noise reduction especially for the low-frequency band noise. Normally, there is a contradiction between the noise reduction and the exhaust resistance for an exhaust silencer, so it has a great significance in studying the theory or principle of the exhaust muffler with high-efficiency noise reduction and energy saving. For this reason, a new theory of exhaust muffler of diesel engine based on counter-phase counteract and split-gas rushing has been proposed. The mathematical model and the corresponding experimental validation for the new exhaust muffler unit based on this theory were performed, and following conclusions are obtained:1) In order to solve the problems of traditional exhaust silencer with poor characteristics of noise reduction in low-frequency range and high exhaust resistance, a new theory of exhaust silencer of diesel engine based on counter-phase counteract and split-gas rushing has been proposed. The theory is:the dominant low-frequency noise components are controlled by counteract of two sound waves with counter-phases (180°phase difference); the other frequency-band noise is reduced by lowering the exhaust gas flow rate thus lowering exhaust noise. The exhaust resistance based on new theory is low because of the structure without no perforated pipe or perforated board of small holes used, and the no reduction of gas-flow sections in all the way of gas flow.2) A physical model for the counter-phase and split-gas rushing exhaust muffler unit was established. Based on acoustic radiation theory, its mathematical model was established and analyzed, and the theoretical formula for transmission loss of the new type muffler unit was derived. The relationship between its performance and structural parameters was obtained, which provides a theoretical basis for the design and optimization of the new mufflers.3) Taking the single-cylinder diesel engine CG25 as the experimental engine, the exhaust noise was measured and its spectrum was analyzed. In order to accurately extract the exhaust noise as the control target, the singular value decomposition technique was utilized for decomposition and reconstruction of the signal. Based on this, three new mufflers with different parameters were designed and manufactured. The sound level pressure dB(A), octave-band sound pressure level as well as detailed spectrum of exhaust noise from the tested diesel engine with three new mufflers were measured and analyzed respectively at five different speeds. By comparing the results with that without muffler and with the original muffler of the engine, the new theory of muffler has been verified.The tested results show that the insertion loss of three new mufflers has all reached 7dB (A), more than 4dB (A) improved compared to original muffler. The original muffler can only reduce the high-frequency noise components, it cannot reduce, even strengthen the noise of frequency below 500 Hz, proved conventional muffler with poor capacity of lowering the low-frequency noise again. The new exhaust mufflers were obviously effective in controlling the low-frequency exhaust noise, which proved correctness of the new theory. In addition, it has been found that the new mufflers have a good effect of noise control in the whole frequency domain especially in the range of under 500 Hz. This not only proved that the new mufflers have very good performance for low-frequency noise reduction, but also proved that using split-gas rushing can lower the air flow speed thus lowering the air-regeneration noise. The counter-phase is only effective to the exhaust noise of the target frequency and its odd multiple harmonics instead of the whole frequency range, but the split-gas rushing is working to the noise in all frequency range. By comparing the results of three new mufflers, it was noticed that the new muffler typeⅡwas the best, especially at the rated speed the effect is more pronounced, which is consistent with the theoretical model analysis, further proved the correctness of the theory.4) The theory of counter-phase and split-gas rushing muffler unit was proposed for the diesel engine, but it can be applied to all the internal combustion engines, such as gasoline engine, considering that the generation mechanism of exhaust noise is the same.