Investigation of the Acoustic Performance of After Treatment Devices
Elnady, T., Elsaadany, S., and Herrin, D., “Investigation of the Acoustic Performance of After Treatment Devices,” SAE Int. J. Passeng. Cars – Mech.
Mufflers for internal combustion engines should be carefully designed. The main objective of a muffler is to reduce the engine noise while maintaining the back pressure below a certain limit. A specific target acoustic performance has to be met under space constraints and allowable engine back pressure limit. Usually, the insertion loss of the exhaust system is required to satisfy a certain target performance curve. The insertion loss is most appropriate to describe the exhaust system acoustic performance since it is dependent on the engine acoustic impedance, which varies with the engine loading and rotational speed. In this paper, a muffler optimization problem is formulated so that several shape parameters are optimized under some space constraints with flow. Any combination of linear space constraints can be imposed. The allowable engine back pressure is introduced as a non-linear constraint so that the optimum shape design will meet the engine back pressure specifications. The interior point optimization algorithm, which is available as a built-in MATLAB function “fmincon”, is used in this paper. The formulated problem is applied to a real case study, where a truck exhaust system consists of a diesel engine, two mufflers, intermediate pipes, and a tailpipe. The first muffler is a typical EU-regulation compliant. The dimensions and location of the second muffler are to be optimized. A limit for the system back pressure is imposed by the engine manufacturer. An optimum design was investigated for different engine speeds and loadings. It was found that using the suggested formulation in this paper; one can obtain an applicable design of a muffler to meet both the acoustic regulations and the engine specifications.
Protection of the environment is a concern of global interest. It is universally acknowledged that the steady rise in all kinds of pollution cannot continue. The pollution from internal combustion and diesel engine emissions is especially alarming, but exhaust emissions are not the only concern. Diesel engines are also notorious for their noise levels. Currently, trucks and heavy equipment must pass noise certifications in order to be admitted in many markets. Moreover, noise is a major factor influencing marketability and competitiveness.
One way of mitigating both exhaust emissions and noise are via the use of after treatment devices like catalytic converters and diesel particulate filters (DPF). The research proposed herein will focus on DPF units. DPF units are devices which remove from 50 to over 90 percent of diesel particulate matter from exhaust gases. Their use in both the United States and Europe will likely be mandated by 2013. DPF units are similar to catalytic converters but differ by introducing a less direct path through the filter as shown in Figure 1. Instead of a straight-through path, exhaust gases must penetrate through a porous cell wall before exiting the filter. A number of different filter materials have been used including ceramic and silicon carbide materials, fiber wound cartridges, knitted silica fiber coils, ceramic foam, wire mesh, and sintered metal structures. DPF units not only filter out particulate matter but are also excellent noise filters due to the indirect air path plus the filter itself consists of sound absorbing materials.
The objective of this paper is to investigate several after treatment devices. The passive acoustic properties of three different units are measured and compared to the 1D simulations using two-port techniques. The acoustic two-port is measured at no flow in addition to 3 different flow conditions. The permeability of the DPF walls was estimated from the measurement of the linear pressure drop across the filter.
Three units were tested in this paper:
The first is a DPF unit which consists of a Diesel Oxidation Catalyst Filter Type A and a Diesel Particulate Filter, both enclosed between side inlet and outlet end caps (Figure 1).
The second unit is the same but with a different Diesel Oxidation Catalyst Filter Type B.
The third is an SCR unit which consists of an Ammonia Oxidation Catalyst (AMOX) filter, and a Selective Catalytic Reducer (SCR) filter, both enclosed between side inlet and outlet end caps (Figure 2).
The inlet diameter of all units is 4 inch and the cavity diameter is 12 inch. These three units are divided into a number of two ports for the simulation in SIDLAB as shown in Figure 3.
The green dot is the inlet node, whereas the red dot is the outlet node. The side inlet and outlet end caps are divided into 3 pipes as shown in the figure. The blue dot indicates a closed pipe to simulate the quarter-wavelength resonator from both sides. Between the inlet and outlet end caps, there are two filters installed with a short empty pipe in-between.
Measurement Test Rig
The passive characteristics (transfer matrix) can be measured using the decomposition method. The method is based on decomposition theory, which was originally used to measure acoustic properties in ducts (such as the absorption coefficient and surface impedance of absorbing materials). If a two-microphone random-excitation technique is used, the sound pressure may be decomposed into its incident and reflected waves. After the wave is decomposed, the sound power of the input wave may be calculated. The major drawback of the decomposition method is that an anechoic termination is required for measuring the transmission loss (TL). In practice, an anechoic termination could be constructed using a long exhaust tube, high absorbing materials, horn shaped pipes or an active sound anechoic termination. However, a “fully” anechoic termination is difficult to build, particularly one that is effective at low frequencies. The determination of two-ports or four poles has been investigated by many researchers. Åbom reviewed these methods and demonstrated that the best method, which always creates independent test cases and gives the smallest error sensitivity, is the so called two-source method originally proposed by Munjal and Doige. This method is used in this paper but extended to include flow and also to allow different pipe sizes on each side of the element under test.
A measurement platform was developed which is based on LABVIEW and National Instruments Data Acquisition cards, to automate the measurement accounting for different theoretical and practical considerations. This is called SIDLAB Acquisition module. The procedure used in SIDLAB Measurement also takes advantage of the analysis of the error sensitivity in the two-microphone technique. To cover a sufficiently large frequency range three microphones are used on both sides of the two-port tested so that we can utilize two different microphone separations.
The test rig consists of PVC pipes 100 mm in diameter and has a wall thickness of 4.7 mm. Each pipe length is 2.5 meters as shown in Figure 6, which shows the pipe assembly and the locations of the loudspeakers and microphones. The loudspeakers are mounted at equal distances from the upstream and downstream, the distances between the loudspeakers were chosen to avoid any pressure minima at the source position. Six microphones flush mounted in the duct wall, three upstream and three downstream of the test objects, were used to cover the plane wave range in the test duct. The flow speed was measured upstream of the test section using a pitot-tube connected to a digital manometer.
The measurement System consists of six ¼ inch B&K 4944A microphones. Signals from the loudspeaker and from the microphones are fed into PXI data acquisition system as input signals and output signals. The signal from the loudspeaker is used as the reference signal. Both input and output signals are converted into digital signals by PXI acquisition system and then processed by SIDLAB acquisition (a LabView Program used to determine the Transmission Loss by the decomposition method). The signal from the loudspeaker is used as reference signal. The start frequency is 30 Hz and the measurements were carried out up to 2 kHz. The pressure drop across the filter unit was measured using a digital manometer.
Figure 6, Figure 7 and Figure 8 show a comparison between the transmission loss simulated with SIDLAB and the measured transmission loss. The vertical dotted line indicates the plane wave limit inside the filter chamber. This is the valid range of the 1D simulations in SIDLAB. The figures show good agreement between the measured and simulated transmission loss in the low frequency range. When higher order modes cut-on inside the filter chamber, the agreement is less. However, the 1D simulations still captures well the overall behavior of the units. The vertical red dotted line indicates the plane wave limit for the unit, which is determined by the diameter of the cavity. Note that the model correctly predicts the quarter-wavelength resonant frequencies close to 1800 Hz. For the SCR unit, the lengths of the resonators in the inlet and outlet caps are different; therefore, there are two resonances.