Powertrain Surface Radiated Noise Analysis using Passive Patches

Abstract

Recently, the level of exciting forces from automotive powertrains has been increased and their frequency spectra become complicated due to the weight reduction and the efficiency development. However, conventional NVH solutions have limited performances since they are focused on structural modification in order to avoid the operating range of the powertrains. In this research, the effect of passive patches on the powertrain surface will be observed via computational analyses and experimental tests. From the normal mode analysis, the location of patches will be determined. Results show that the acceleration level on the Nu engine has been decreased by 10 dB with the passive patches applied.

1. Introduction

Recently, the excitation force of engines has been increased and their frequency spectra become complicated caused by powertrain weight reduction and high efficiency. However, conventional NVH solutions are mostly limited to structural modifications in order to avoid the operating range. Many research efforts on the vibration attenuation by attaching viscoelastic passive patches on the surface of vibrating structures. [1-4]

Figure 1 compares the surface radiated noise on a squared metal plate with constrained layer damping or free layer damping. It shows how much the level of noise and vibration could be attenuated with passive patches. The design procedure of such damping systems has commonly been empirical and little research has been performed on predicting the relationship between the size, properties, and location of damping materials and automotive NVH performance, especially noise.

Fig. 1 Vibration attenuation effect of passive patches

In this research, the effect of passive patches on the reduction of surface vibration and radiated noise from powertrain timing chain covers will be investigated through computational analyses and tests. In addition, it is validated via driving tests for Hyundai i40 with the Nu 2.0 GDI engine.

2. FEM Modeling and the Selection of Weak Parts

Figure 2 shows the finite element model of the Nu 2.0 GDI engine, which is the subject of this research. This model consists of block, head, ladder frame, head cover, and timing chain cover, not including auxiliary parts such as intake/exhaust manifolds, alternator, starter, etc. CAE analysis for the normal modes and the velocity distribution on the surface of timing chain cover has been carried out by exciting unit forces of x, y, z directions simulateously on the main bearing cap #3. MSC.Nastran is utilized for the vibration analysis software.

Fig. 2 Finite element model of Nu 2.0 GDI engine

From the result of mode analysis, vibrational modes of the timing chain cover could be observed from around 1700Hz and three weak parts are identified as shown in Figure 3. In order to simulate the effect of attaching passive patches on the identified weak parts, shell elements are added as described in Figure 4 and a representative property, such as structural damping coefficient, is employed. After setting the material properties and the thickness of passive patches (asphalt pad, a type of viscoelastic damping materials), same analyses have been performed.

Fig. 3 Vibrational modes of timing chain cover

Fig. 4 Modeling of passive patches

3. Analysis and Test Results of Engine Surface Vibration

Based on the analysis results of mode shapes, three locations have been selected for attaching passive patches and then computational analysis and engine tests are performed. Passive patches with 6mm thickness are attached to two upper locations and one mid location of the timing chain cover and then the effect of passive patches are validated by vibration analysis and engine tests.

Figure 5 shows the analysis results of surface vibrations. The acceleration on the upper middle point of the timing chain cover is measured after a unit excitation force has been applied to the main bearing cap #3. When the passive patches are employed, vibration on the range of 1∼2kHz, including two chain cover modes, has been attenuated by 5-7dB. It is verified that passive patches do not have any effect in relatively low frequency range and can reduce surface vibrations in the frequency range of 1kHz and over.

Fig. 5 Surface vibration analysis result

In addition, it is observed that vibration level could be attenuated by 3-5dB with passive patches at about 2kHz and above 2.5kHz, from the result of experimental modal analysis (EMA). The reason why the shapes of frequency response functions are different is that the engine used for the test consists of all the auxiliary parts, different from the FE model, and their boundary conditions are also different, free-free condition for the FE model and fixed-free condition on the ground for the real engine.

Fig. 6 Experimental modal analysis results

4. Analysis Result of Radiated Noise

Radiated noise analysis has been performed using surface velocity distribution of the Nu engine timing chain cover with and without passive patches. Noise analysis software named LMS SYSNOISE is utilized for the analysis and the sound pressure levels at the front 50cm point of the timing chain cover are compared and analyzed. Moreover, the radiation patterns at the front field points and the sound pressure level have been compared.

Figure 7 describes the analysis results of radiated noise at each modes of the timing chain cover. When the passive patches are applied, it is observed that the sound pressure levels are significantly reduced compared to the case of no passive patches. Additionally, Figure 8 compares the sound pressure level of two cases with and without the passive patches. It is validated that the sound pressure level at two vibrational modes of the timing chain cover has been attenuated. In the aspect of the surface radiated noise, the sound pressure level in front of the engine is reduced by approximately 3dB in mid-frequency region (highlighted) when the passive patches are attached, compared to the case of no passive patches on the surface.

Fig. 7 Radiated noise analysis for each modes

Fig. 8 Comparison of sound pressure level

5. Driving Test Results of Hyundai i40 with the Nu Engine 

The change in vibrational characteristics of the timing chain cover with and without passive patches is validated through vehicle driving tests. On the surface of the timing chain cover from the Nu 2.0 GDI engine installed in Hyundai i40, passive patches are applied and the surface vibration has been measured. There are three test conditions, idle with N gear, idle with D gear, and RPM sweep condition (1500 to 5500rpm), which is conducted by pressing the accelerator pedal slowly with N gear engaged. Figure 9 shows the test vehicle and the engine surface with passive patches and accelerometers attached.

Fig. 9 Test vehicle

From the test results, it is observed that the acceleration level is reduced by 10dB maximum under the condition of idle and rpm sweep. Also, the acceleration level on the upper parts and the mid parts of the timing chain cover has been attenuated by approximately 10dB in the range of about 2kHz, and by 5dB in the range of about 3kHz, respectively.

Fig. 10 Comparison of acceleration level

Fig. 11 Comparison of acceleration level (RPM)

However, as expressed in Figure 12, the noise levels in the cabin appear to be almost the same. Since the contribution level of the radiated noise in front of the timing chain cover to the overall cabin noise level is relatively low, the noise reduction effect of the passive patches are masked, so called “masking effect”.

Fig. 12 Cabin noise measurement

6. Conclusion

In this research, the effect of passive patches on the powertrain surface vibration and radiated noise have been observed via computational analyses and experimental tests. From the normal m ode analysis, the optimized location of patches are determined and the change of noise and vibration characteristics are observed with passive patches attached. From the results of analyses and engine tests, surface vibration has been reduced by approximately 5dB and radiated noise has been attenuated by approximately 3dB. Vehicle driving test results show that the acceleration level on the Nu engine has been decreased by 10dB with the passive patches applied.