Application of Perforated Muffler in Automotive Intake Noise Control

The application of turbochargers in engines can improve the power performance of automobiles economy But it will also seriously affect the NVH performance of automobiles. Exhaust gas turbochargers were more commonly used in diesel vehicles in the past. At present, gasoline cars are also widely used, especially passenger cars, which makes turbochargers more popular. NVH major automotive companies attach great importance to performance optimization.

The typical operating speed range of an exhaust gas turbocharger is 1 million revolutions per minute. The airflow noise generated by the high-speed rotation of the compressor side impeller can mainly be concentrated in the [1.5-3] kHz frequency band [1]. Under acceleration conditions, when the impeller rotates at high speed to 100000 revolutions per minute, the airflow noise generated is radiated through the intake pipe and perceived by the driver. As the engine speed continues to increase, the engine noise is masked. Compared with non turbocharged engines, vehicles equipped with turbochargers need to solve the problem of [1-3] kHz intake flow noise under [1] acceleration conditions.

Many scholars at home and abroad have conducted in-depth research on the production principle of turbocharger airflow noise [2] - [4]. According to some research papers, currently, resonance mufflers with perforated plate structures are used to suppress the airflow noise of turbochargers. Compared to simple expansion chambers, this muffler has the advantage of a smaller structure, making it easier to arrange near the intake system. At the same time, the fluid pressure loss is small and has little impact on power.

The effective bandwidth of the muffler is within the range of 1800-3000Hz, but the valley value within the frequency band is relatively low, averaging around 20dB, and about 17dB near 2500Hz. The main reason is that the muffler designed in the paper only has three resonant cavities, distributed at 1200Hz, which appear sparse in the bandwidth at each resonant frequency, resulting in a low valley value.

The transmission loss of the muffler designed in reference [4] is ideal. This article uses theoretical formulas to optimize parameters without considering the mutual influence between cavities, which has certain limitations. When the internal pipe diameter is large and close to the limit of the plane wave cutoff frequency [5], high-order waves in the pipeline need to be considered. Virtual. Lab can solve this problem very well. When approaching but not reaching the cutoff frequency, its AML attribute can be assigned to the export end. When analyzing beyond the cutoff frequency, it is important to consider higher-order waves, and adding modes of (0,1) or more can be analyzed through simulation settings.

Problem Description

For a turbocharged SUV model, some throttle slowly accelerates between 1800-200 revolutions per minute, and the product generated by the intake system can be clearly perceived as a "hiss" sound inside the car. After the speed continued to rise to 2500rpm, the abnormal noise was gradually masked by other noises.

2 Problem Analysis

2.1 Noise source determination

According to the technical parameters of the turbocharger, when the noise appears, the turbocharger has already intervened and its main working speed is [80000-180000] rpm. It disappears with the sound of the throttle loosening. I initially suspected that the turbocharger was a source of noise. To verify this speculation, the turbocharger pressure reducing valve was in a normal open state. After subjective driving evaluation and testing, it was found that the abnormal noise disappeared, thus determining the turbocharger as the noise source.

2. Develop a solution

In the initial stage, considering the poor sound insulation ability of the intake system pipeline, the aerodynamic noise generated by the turbocharger is radiated into the engine compartment through the pipeline and then enters the car. Therefore, in the intake air filter After wrapping the inlet and outlet pipelines and intake pipelines of the compressor with high-energy soundproof materials, the abnormal noise after driving evaluation is suppressed.

Considering that the cost of increasing the sound insulation capacity of pipelines will significantly increase, and due to spatial layout limitations, this solution was ultimately not considered. Based on references [1] - [4], it is finally considered to install appropriate pipe mufflers on the intake system pipes to suppress the airflow noise during compressor acceleration. The outer wall of the muffler needs to have a certain sound insulation ability, otherwise it cannot achieve the expected noise reduction effect.

Test the noise between the driver's right ear and the air intake, as shown in Figure 1 and Figure 2. The LMS software continuously played back filtering analysis and found that after the noise of the driver's right ear filter [13203000] Hz, the abnormal sound disappeared. Therefore, the designed pipeline muffler should have a sound transmission loss capacity of over 20dB at [13203000] Hz. 􀀿􀀛􀅙􀅢􀆑􀅹􀆉􀀉􀀰

Due to the radiation of airflow noise from the inlet and outlet pipelines of the turbocharger, a pipeline muffler was designed in the inlet and outlet pipelines of the compressor to suppress airflow noise.

3 Muffler Design

3.1 Selection of noise reduction indicators

There are many evaluation indicators for the acoustic performance of mufflers, and the commonly used one is insertion loss Transmission loss and reduction of end noise, etc. Due to the fact that transmission loss reflects the acoustic transmission characteristics of the muffler itself, and its value is only related to the structure of the muffler and the internal medium properties of the muffler, it is an inherent property of the muffler. Therefore, transmission loss is usually used as the main performance indicator for the design and performance evaluation of mufflers [8].

The transmission loss represents the degree of attenuation of the sound volume after the silencing element, that is, the difference TL between the incident sound power Wi, the sound power transmission Wt, and the transmission loss is expressed as 􀃕􀅠􀁷

3. Parameter design of muffler

The preliminary calculation of the perforation diameter is 5mm, and the finite element method can be used for left and right analysis. This article adopts the finite element method. Virtual. Lab acoustic simulation calculation, defining AML attributes for export pipes.

Considering the limit of layout space, the compressor outlet muffler is designed with 6 resonance chambers, and the compressor inlet muffler is designed with 4 resonance chambers. The two mufflers are designed as a circular tubular structure as a whole. Refer to Figures 3 and 4 for details. The preliminary determination is that the resonance frequencies of the six chambers of the outlet muffler are 1550.1700.1900.2200.2500.2800Hz. The resonance frequencies of the four chambers of the inlet muffler are 1700.2000.2300.2700Hz.