The 21 st International Congress on Sound and Vibration 13-17 July, 2014, Beijing/China CAR ENGINE CATALYTIC COLLECTOR NOISE REDUCTION Georgy M. Makaryants, Kirill A. Kryuchkov, Artur I. Safin and Alexander N. Kryuchkov Department of Automatic Systems of Power Plants, Samara State Aerospace University (National Research University), Samara, Russia 443086 Mikhail I. Fesina and Ilya V. Malkin Department of Management of Industrial and Environmental Safety, Togliatti State University, Togliatti, Russia 445667 e-mail: georgy.makaryants@gmail.com The article presents the solution for the problem of the excessive catalytic collector noise in the car engine exhaust system. The high level noise has been observed under the idle state engine condition. In order to understand the reasons of the high catalytic collector noise emission the investigation of its vibroacoustic properties has been carried out. An experimental setup for the acoustic and vibration response on broadband dynamic excitation tests has been created. The power of the acoustic noise, emitted by the catalytic collector and vibration distribution of its surface have been defined. It was found that the catalytic collector surface has the low acoustic impedance in the range of 2.5 to 4.0 khz. The surface vibration damper design has been developed. A special material, so called metal rubber, has been used to damp the vibration. The main benefit of this material is the temperature tolerance, quite useful for the engine exhaust system unit. The use of the damper has allowed to reduce the structure-born noise of the catalytic collector. 1. Introduction The catalytic collector is one of the expensive elements of the car exhaust system. It is installed directly at the engine outlet (Fig. 1). The collector is typically used for two reasons. First of all, the collector is used to commutate the engine exhaust pipes with the car exhaust line. In the second place, the collector reduces the harmful substance atmospheric emissions. The general collector layout is presented on Fig. 1. Basically, there is a branched pipe composed of separate sleeves. The number of the sleeves corresponds to the engine cylinder count. One side of each sleeve is fixed to the engine, and the other one unites in a common diffuser-shaped gas receiving chamber, connected to the collector hull. Inside this hull there is a block of catalytic material (Fig. 2), connected by means of the thermal insulating gasket. The block of catalytic material is a gas-permeable porous structure. At the outlet of the collector hull there is a gas confusor-shaped mixing chamber. The outlet of this chamber is attached to the exhaust line pipe. A special thermal shield is used to protect the engine compartment from the high temperature of the collector surface. A new manufacturing technique has been developed to reduce the net cost of the catalytic collector. ICSV21, Beijing, China, 13-17 July 2014 1
An expensive technological operation of casting the collector hull, gas receiving chamber and gas mixing chamber has been replaced by the cheaper stamping process. Figure 1. Photo of catalytic collector and its location in the exhaust system. 1 Inlet flange, 2 sleeves, 3 gas receiving chamber, 4 collector hull with thermal shield, 5 gas mixing chamber, 6 outlet flange. Figure 2. Photo of the catalytic material block. 1 Catalytic material block, 2 thermal insulating gasket, 3 collector hull. 2. Preliminary tests During the acoustic tests of the car, equipped with the new catalytic collector, we detected an increase of the noise level for all engine operating modes. As shown by the preliminary experimental study of the noise in the engine compartment and in car interior (Fig. 3 and 4) the noise increases in the frequency band from 2.5 to 4.0 khz regardless of the engine operating conditions. The increase of the noise has the resonant character in the said frequency band and is presumably related to the weak soundproofing of the thin-walled stamped collector hull at the modal frequency of its surface. The investigation of the exhaust system modal characteristics has been considered by many authors. 1,2,3,4 The main goal of these research works was to reduce the resonant vibrations, that take place due to the dynamic excitation, exerted by the road and the engine. Therefore, the modal characteristics were determined in the frequency range, limited to 200 Hz. At the same time, the catalytic collector was excluded from consideration. ICSV21, Beijing, China, 13-17 July 2014 2
In this case, the study of the catalytic collector dynamic properties at high frequencies is an actual problem. 3. The catalytic collector acoustic analysis In order to verify the catalytic collector acoustic characteristics we have carried out the acoustic power measurements for the engine running conditions. For this purpose G.R.A.S. Intensity Probe type 50AI-B was used. The acoustic power was determined by the ISO 9614, using the 5x5 mesh with 0.08 m cell size. The results analysis showed a resonant increase of the noise in the frequency band of 2.5 to 4.0 khz (Fig. 5). This conclusion correlates well with the preliminary tests data. It confirms, that the catalytic collector sound radiation is precisely the main reason of the excessive noise at these frequencies. Figure 3. The engine compartment noise. Figure 4. The car interior noise. ICSV21, Beijing, China, 13-17 July 2014 3
Figure 5. Collector acoustic power spectrum. 4. Catalytic collector modal analysis As discussed above, we have presumably detected the structural noise in the frequency band of 2.5 to 4.0 khz, caused by the collector surface vibration. In order to confirm this fact, the catalytic collector vibration investigation was performed. Since the structural noise is the reason of the vibration on the frequencies, related to the membrane mode shapes, we have researched the response of the collector surface on broadband dynamic excitation in the frequency band of 5 to 5 000 Hz. It is commonly known, that the greatest accuracy in the membrane mode shapes identification is given by the noncontact measurement methods. The 3-D scanning laser vibrometer Polytec PSV-400-3D was used in our investigation (Fig. 6). Figure 6. Scanning laser vibrometer Polytec PSV-400-3D. The investigation was conducted on a so called chilly setup (Fig. 7) in view of the collector surface vibration measurement difficulty in the cramped conditions of the engine compartment. The catalytic collector has been installed on the car engine. In its turn the engine has been installed using the anti-vibration mounts on the supporting frame. The dynamic excitation was produced by means ICSV21, Beijing, China, 13-17 July 2014 4
of the shaker (Fig. 8) that has been placed on the engine cylinder head. We have used the periodic chirp as the excitation signal. Figure 7. Setup for the catalytic collector modal analysis. Figure 8. The dynamic excitation shaker. The vibration velocity spectrum of the amplitude, averaged over the collector surface is shown on the Fig. 9. Here, the mode shapes are correlated to each frequency or frequency groups. The obtained collector mode shapes (Fig. 10-13) can be subdivided into the low frequency beam mode shapes (50-1 000 Hz), membrane mode shapes of the thermal shield on middle and high frequencies (1.5-2.5 khz) and mixed mode shapes of the thermal shield and the gas receiving chamber sleeves (2.5-4.0 khz). Figure 9. The collector surface vibration velocity spectrum. The obtained spectrum analysis of the collector vibration velocity has shown the density increase of the high amplitude modal membrane shapes in the frequency band of 2.5 to 4.0 khz. The oscillations amplification on modal membrane frequencies is associated with the coincidence of the sleeves bending resonances and the thermal shield resonances. 5. The catalytic collector noise reducing measures We have developed several collector hull designs, however, none of them have significantly affected the separation of sleeves and thermal shield modal frequencies. That is to say, we were unable to weaken the mutual influence of the shapes on each other. ICSV21, Beijing, China, 13-17 July 2014 5
Figure 10. Collector mode shape (81.3 Hz). Figure 11. Collector mode shape (607.8 Hz). Figure 12. Collector mode shape (2573.4 Hz). Figure 13. Collector mode shape (3333.0 Hz). Therefore, the vibro-absorbing gasket design has been developed (Fig. 14). The main task of this gasket was the collector hull vibration amplitude decrease. 5,6 The vibro-absorbing gasket looks like the rectangular briquette of a compressed twisted wire. This kind of material has been called metal rubber. 7,8 It is widely used for the vibration damping of units, operating at high temperatures. Figure 14. Design of the vibro-absorbing gasket. ICSV21, Beijing, China, 13-17 July 2014 6
6. Results First of all, we have derived the vibration velocity spectrum averaged over the collector surface for both designs. Then we have analyzed the differences between the spectra, the results are shown on Fig. 15. The vibro-absorber implementation allowed to reduce the vibration in the range of 2.5 to 4.0 khz by more than 20 db, thus reducing the noise by 4.1 dba. As shown on the Fig. 16 the noise reduction was caused by the high frequency resonant vibration suppression in the range of 2.5 to 4.0 khz. Figure 15. Effectiveness of the developed damper. 7. Conclusion Figure 16. Collector noise in far field. The high level noise problem of the car exhaust system for the frequency range of 2.5 to 4.0 khz was investigated. It was shown that the noise is caused by the resonant vibration of the catalyt- ICSV21, Beijing, China, 13-17 July 2014 7
ic collector surface. We used the 3-D scanning laser vibrometer in order to investigate the collector surface vibration response on broadband dynamic excitation. It was found out that the thermal screen membrane mode shapes amplification is caused by the collector sleeves resonant vibration. In order to decrease the resonant vibration amplitude, the vibro-absorber was designed. It is represented by rectangular metal rubber gasket, placed between the collector hull and the thermal shield. The vibro-absorber implementation allowed to reduce the vibration in the range of 2.5 to 4.0 khz by more than 20 db. Thus the noise was reduced to the acceptable level by 4.1 dba. The noise reduction was caused by the high frequency resonant vibration suppression in the range of 2.5 to 4.0 khz. The damper application allowed to reduce the catalytic collector resonant vibration and solve the problem of its high noise level. 8. Acknowledgement The research was supported by the Russian Federation President's grant (project code NSH- 1855.2014.8). REFERENCES 1 2 3 4 5 6 7 8 Viswanathan, A., Perumal, E. Deciding isolator and mounting points of a truck's exhaust system based on numerical and experimental modal analysis, Proceedings of the 16 th International Congress on Sound and Vibration, Krakow, Poland, 5 9 July, (2009). Verboven, P., Valgaeren, R. Some comments on modal analysis applied to an automotive exhaust system, Proceedings of the International Modal Analysis Conference, Santa Barbara, USA, (1998). Belingardi, G., Leonti, S. Modal analysis in the design of an automotive exhaust pipe, International Journal of Vehicle Design, 8 (4/5/6), 475-484, (1987). Englund, T. Dynamic characteristics of automobile exhaust system components, Licentiate thesis, Department of Mechanical Engineering, Blekinge Institute of Technology, Karlskrona, Sweden (2003). Igolkin, A.A. Vibroacoustic loads reduction in pipe systems of gas distribution stations, Journal of Dynamics and Vibroacoustics. 1 (1), (2014), [Online.] available: http://www.dynvibro.ru/paper/1/5.pdf Rodionov, L.V., Gafurov, S.A., Melentjev, V.S., Gvozdev, A.S. Noise and vibration protection of roof boiler equipped house, Journal of Dynamics and Vibroacoustics. 1 (1), (2014), [Online.] available: http://www.dynvibro.ru/paper/1/6.pdf Igolkin, A.A., Izzheurov, E.A., Hongyuan, J. and Shakhmatov, E.V. Acoustic performances of metal rubber, Proceedings of the 18 th International Congress on Sound and Vibration, Rio de Janeiro, Brazil, 10 14 July, (2011). Khaletskiy, Y., Igolkin, A., Pochkin, Y. Acoustic response of a fan duct liner including porous material, Proceedings of the 20 th International Congress on Sound and Vibration, Bangkok, Thailand, 7 11 July, (2013). ICSV21, Beijing, China, 13-17 July 2014 8