排气消声器消声性能仿真分析/

2019-04-21 18:16:40

消声器 loss noise Hz muffler



排气噪声是汽车最主要的噪声源之一。消声器常被用来控制汽车排气噪声,并且它是切实有效的降噪措施。
在市场竞争日益激烈的今天,对任何企业来说,尽可能的缩短产品研发周期以达到尽快占领市场是非常重要的。所以这就要求研发人员,不能再依靠以前的修改产品进行试验,这种一改一试的方法。这样做有时会花费太长的设计周期,并且成本太高。随着近些年计算机应用水平的不断提高,在产品样机制做之前,计算机辅助工程软件经常被工程师用来进行快速的产品的设计、评估。随着发动机性能和消声器仿真软件的不断涌现,使得消声器与发动机相互影响的问题得到了综合解决,更加顾及全局的效果,减少了问题孤立化带来的困扰。本文从消声器的传递损失、插入损失、尾管声压级三个方面对消声器的消声性能进行了综合的评价。
传递损失和插入损失常被用来评价汽车排气消声器的消声性能。消声器的传递损失通常可以从试验和计算两个方面得到,它主要是在消声器设计的初级阶段起决定作用。而插入损失则是作为最终的消声性能指标。
传递损失之所以常被用来评价消声器的消声性能,主要是因为只要有消声器的几何参数,传递损失就可以获得,它不受声源的阻抗和辐射阻抗的影响,是消声器本身固有的一种特性。传递损失测试平台和消声器模型建立好之后,就可以对任何一个声学元件进行传递损失的测试通过延长的阻抗管。待测试的声学元件通过两端很短的直管连上白噪声发生器和消声末端。不一定非要加上额外的两根管子,但用在这里经过实践证明有很好大效果。 试验中经常用这两个额外的管子来布置麦克风。 这两个外加管子一定要和消声器的进口和出口有相同的直径,否则会有额外的压力损失。待测的声学元件可以是简单的组件也可以是复杂的排气系统。对本文用到的消声器进行了传递损失计算,发现改消声器有很宽的消声频带。
在进行消声器插入损失计算前要建立发动机模型。该发动机为一款四冲程单缸机,它的模型的建立主要由外界环境、进气道、进气管、化油器、进气门、气缸、发动机、排气门、排气道、排气管组成,最后在每个模块里相应的把试验数据输入进去。然后按照气流流动的方向从左向右把整个模型连接起来。当发动
机模型全部建立后再与消声器模型相连。在距离尾管0.5米处布置麦克风。在整个计算过程中,在发动机全负荷的工况下,发动机转速由4000r/min加速到8000r/min。在低转速范围内,计算结果和试验结果都基本比较吻合。随着转速的升高计算结果在高频范围内远离试验结果,这是因为在低转速阶段气流速度也比较慢,空气动力噪声决定尾管噪声。在高转速阶段,气流速度也比较高,这时气流摩擦噪声决定尾管噪声。但GT-POWER应用的是一维流体假设的动力学模型,所以不能模拟气流摩擦噪声。
插入损失定义为排气系统中的与排气口同一测距测定装上消声器前与后的声压级,两者之差即为插入损失。因此为了计算插入损失,常用一与消声器等长的直管代替消声器。计算结果与实测结果在各个转速下都吻合的很好。在高频范围内,计算结果与试验结果不是很吻合,因为该软件为一维模拟软件。通过直管噪声与带消声器的尾管声压级的对比,发现在低频段有一峰值。从计算的传递损失来看,在发动机转速4500r/min时,出现两个消声高峰,一个是在397Hz,消声量为35.5dB,另一个在793.7Hz,消声量为38.6dB。同时在500Hz出现一消声低谷,这与试验测得的通过噪声在500Hz出现最大噪声峰值基本吻合。所以降低500Hz左右的低频范围内的噪声将作为改进的主要目标。
穿孔管在低频范围内有很好的消声效果。国外的Sullivan 和 Crocker已推导出穿孔管的穿孔率在5-10%范围内会起到共振腔的效果。所以在尾管处增加了一段穿孔率为5.29%的穿孔管。计算的结果显示在200Hz和500Hz处噪声降低2-6dB左右。从传递损失、插入损失的计算结果都是在500Hz左右的中低频范围内起到了很好的效果。同时排气背压与原机相比基本相同,说明这样的改进没有对原发动机的性能有很大的影响。



The exhaust noise was one of the most important noise sources of the automobile. Muffler was used to control the exhaust noise. And it was the effective method to reduce the noise.

In today’s competitive world market, it is important for a company to shorten product development cycle time in order to be successful in the target markets in which they compete. Engineers can no longer rely on just the “build and test” method to design products because sometimes it simply takes too long and is too expensive. Along with the improvement of the computer application level in recent years, computer aided engineering tools are often used by engineers to evaluate different designs quickly before building prototypes. With the computer simulation of exhaust mufflers and engines, the influence of mufflers on engine performance as well as exhaust noise can be predicted. In this paper, we discuss the acoustic performance of the exhaust muffler in transmission loss, Insertion loss, and tailpipe noise.

Transmission loss and insertion loss are the most frequently used acoustic performance criteria of automotive exhaust mufflers. Transmission loss of a muffler is usually determined and analyzed computationally and experimentally in the development stage of an exhaust system, while insertion loss is the final acoustic performance indicator of the system.

Transmission loss (TL) is one of the most frequently used criteria of muffler performance because it can be predicted very easily from the known physical parameters of a muffler and don’t have effect on it for source impedance and the radiation impedance, TL is a property of the muffler only. Transmission Loss Test and muffler model was built; transmission loss of an acoustic element is measured by using an extended impedance tube setup. The test piece is connected to the acoustic driver and an anechoic termination through two straight pipes. It is not absolutely necessary to have extra pipe lengths added to the muffler, but is used here to demonstrate good practice. Experiments often use these extra lengths for placement of the four microphones. These extra lengths must have the same diameter of the connecting pipes to prevent additional pressure loss. The test acoustic element could be a simple component or a complex exhaust system. The TL is calculated for the muffler, the muffler exhibits a broad attenuation band.

The engine model must be built before the predicted of insertion loss.
The model is a basic single-cylinder engine, consist of environment, intake port, intake pipe, injector, intake valve, cylinder, engine, exhaust valve, exhaust port, exhaust pipe, entering the data for these object, then connect all the components together from left to right in the general flow direction. Once the engine model is fully built, the exhaust muffler is attached to the engine. A microphone is located at 12 inches away from the tailpipe. During the test, the engine runs from 4500 to 8000 r/min under the wide-open throttle condition. It is seen that the predicted and measured tailpipe noise match very well in low engine speed. With the speed increased, the prediction differs from the measured result. This is due to that at low r/min, flow speed is slow, the air flow noise dominates tailpipe noise. At high r/min, flow speed is high, the friction noise dominates tailpipe noise. But GT-POWER is 1-D software, it can not predicted the friction noise.

Insertion loss (IL) is defined to be the difference in the acoustic power at one point in space with and without the muffler inserted between that point and source. For comparison and calculated insertion loss, the muffler is replaced with one straight pipe with the same length as the corresponding components. The predicted and measured tailpipe noises match very well for the straight pipe. At higher frequencies, the prediction differs from the measured result due to the simulation software is a one-dimensional simulation software. We make a comparison in tailpipe noise between straight pipe and muffler, it is seen that there is a sharp peak in low frequent. From the predicted IL, it is seen that there are two peaks at 4500r/min, one is 35.5dB at 397Hz, and the other is 38.6dB at 793.7Hz. There is a valley at 500 Hz that match very well with the measured which has a peak at 500 Hz for the pass noise. So the attenuation of low frequency noise about 500 Hz is the main objectives when designing the new muffler.
Perforated pipe has good acoustic performance at low frequent. The work of Sullivan and Crocker suggests porosity in the range of 5 to 10% to achieve the expansion chamber. So we add a perforate tube in tailpipe, take a 5.29% porosity. The predicted result has a good effect in low frequent. At 200 Hz, the tailpipe noise attenuates 2-6dB. At 500Hz, the tailpipe noise attenuate 1.5dB.From the other acoustic performance, transmission loss, insertion loss have the same results. The back pressure is almost same with the old muffler, so adding perforated tube doesn’t change the engine performance.