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2014-36-0773

Analysis of Vibro-Acoustic Propagation Generated by an Automotive Refrigeration System

Rodrigo Santos, João Paulo Sabioni PSA Peugeot Citroën do Brasil Automóveis Ltda.

César Abrahão, Alexandre Ramos LMS, A Siemens Business

Copyright © 2014 SAE International

Abstract

Air conditioning systems are being increasingly applied in the Brazilian automotive industry becoming indispensable rather than an optional item for car buyers, who are constantly seeking out more comfort. Conversely, the application of these systems in motor vehicles involve the use of compressors which operate at high pressures, making them important sources of vibration and unwanted noise in the cabin of the vehicle, thus causing vibratory discomfort and noise annoyance. Despite the major technological developments in automotive air conditioning systems, there is still a shortage of engineering research conducted in the field of vibration and noise originated in these particular systems. The present study is aimed at investigating and characterizing experimentally the vibration and acoustic propagation in the interior of a vehicle, generated by the compressor of the air conditioning system. In order to allow correlation between noise and vibration, measurements were taken over time using the acquisition system LMS SCADAS, in two conditions: increasing rotation in the engine and stabilized rotation at the highest noise level. The data collected was processed and analyzed using different methodologies in time and frequency domain using the LMS software Test.Lab v.12A. The results show that the noise emitted by the air conditioning system is related to the operating rotational speed of the compressor and the H6 and H12 harmonics are the most emissive inside the cabin. Moreover, the results indicate that the vibration propagation happens primarily via the condenser. Therefore, a quieter system can be produced by a close cooperation between car vehicle and compressor.

Introduction

Noise and vibration play important roles in passenger comfort and influence a client's perception of vehicle overall quality. Thus, in designing a vehicle, the effects of noise and vibration should be limited in compliance with the appropriate legislation and kept under levels accepted by customers.

The noise of a vehicle is produced mainly by:

(A) Structure’s vibrations which are caused by direct transmission from the engine, efforts resulting of road surface excitation and aerodynamics, which cause the surface of the vehicle to vibrate and produce noise.

(B) Direct radiation from various individual sources of noise such as gases intake and exhaust, the engine itself, the transmission system, the ventilation and air conditioning system, the wheels, and accessory belts. Many of these sources radiate noise through openings on the vehicle’s structure. Table 1 shows the primary source of noise, along with the importance of their contribution in the car interior and exterior [1].

Table1: Noise source and contribution in the vehicle [1].

Source Internal noise

levels External noise levels

Engine vibration Main source of

noise in low frequencies

Unimportant

Engine and transmission aerial noise

Main source of high-frequency

noise

Main source of high-frequency noise

Exhaust system Unimportant Main source of low-

frequency noise

Admission system

Unimportant Main source of low-

frequency noise followed by exhaust system

Fan Perceptible Significant in the low and medium frequency bands

Vibration due to road surface

excitation

Main source of low-frequency

noise Insignificant

Tire noise due to road surface

excitation Insignificant Significant

The intensity of noise in the interior of vehicles has been decreasing significantly over the years due to the reduction of

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noise levels from the engine, tires and aerodynamics. The noise level of the engine can be reduced by using better exhaust systems and mufflers. Yasuda [3] presented a muffler with a noise attenuation behavior similar to the Helmholtz resonator, capable of reducing both low and medium frequency noise simultaneously.

The dynamic characteristic of the tires is a crucial factor in noise generation inside the vehicle, due to efforts exerted by the track. Thus, in recent years the acoustics of the tire air chamber has become one of the main subjects of studies and of great concern to the industry. Molisani [6] proposed an efficient model for investigating the mechanisms of physical coupling between the acoustic cavity and tire structure. The results show that the model captures the key mechanisms of the enclosed air acoustic effects over tire dynamics.

Systems that use belts, when poorly designed, can lead to unwanted noise that substantially increases of the vehicle warranty cost. Sheng [2] introduced an experimental characterization and analysis of friction and vibro-acoustic behavior of automotive rubber belts in wet conditions. The study demonstrated that extreme overload conditions can lead to excessive slippage on the interface between pulley and accessory belt, generating noise.

As a result of the technological advances in engines, in regards to the lowering of noise levels, the contribution made by the most important noise sources in the vehicle’s interior has been changing. Consequently, large numbers of secondary noise sources previously camouflaged, became noticeable to passengers and may have a significant impact on their well being. One such source is the heating, ventilating and air conditioning system, HVAC, responsible for the heating, ventilation, purification and air cooling in the cabin. This system can operate at different temperature and air flow settings, which are automatically or manually set by the user according to the thermal conditions inside the cabin. In hot weather conditions, the HVAC assembly operates at high ventilation speeds with the air conditioning compressor on. In these conditions, the HVAC noise impact on passenger acoustic comfort is very significant and has been recognized as a source of dissatisfaction amongst car buyers.

Leite [5] using psychoacoustic techniques demonstrated that the annoyance caused by HVAC noise can be described by the static model of Zwicker and a maximum acceptable level can be determined.

Mavuri [4] presented a comparative measurement of noise levels of various HVAC systems inside the cabin of vehicles of different makes and models. Tests were conducted with the engine off, at idle (vehicle stationary) and some measurements were taken in a moving car with the HVAC system operating in different modes. This study can be a valuable tool for the industry.

One of the HVAC system’s major components is the compressor. When actuated it is responsible for producing a pressure differential in the refrigerant gas, generating gas flow and causing the refrigerant to circle through the cooling circuit. Compressors operate at high pressure peaks and are therefore important sources of unwanted noise and vibrations inside the cabin, provoking acoustic and vibration discomfort to

passengers. However, there is still a shortage of engineering work in the field of vibration and noise from HVAC systems related to the air conditioning compressor. Thus, the objective of this work is to investigate and experimentally characterize vibration and sound propagation into the interior of a vehicle, originated in the air conditioning compressor.

The vehicle used in this study was a hatchback with a six-cylinder wobble plate compressor.

HVAC System

The HVAC system consists of five main components: evaporator, compressor, condenser, receiver-dryer/accumulator and the expansion device, as shown in Figure 1.

The most important component is the compressor. It increases the pressure of the refrigerant and promotes the circulation of the fluid in the system. The high pressure, then overheated refrigerant is led from the compressor to the condenser where it is liquefied upon heat exchange with the external environment. The cycle is completed in the evaporator, in which the evaporation of the refrigerant takes place, absorbing heat from the inside of the vehicle cabin.

An essential component of an air conditioning assembly is the expansion device. This component regulates the passage of refrigerant to the evaporator. Expansion valves and orifice tubes are the most commonly used regulatory devices. To ensure the overheating of the refrigerant after the evaporator, the HVAC system uses an accumulator or receiver-dryer, capable of retaining particles [8].

Figure 1: Basic components of HVAC system [7].

Variable-displacement compressors have been the most used currently, due to its continuous operation, ability to provide superior thermal comfort inside the car and lesser fuel consumption [9]. This type of compressor is manufactured from a plate set at a skewed angle to the axis of the compressor, and pistons positioned axially and parallel to the power axis. Currently there are two models: wobble plate and swash plate. The difference between them is in the way the disc and the pistons are interconnected. In the case of the wobble plate model, Figure 2, the plate and the pistons are connected by a rod, while in the swash plate model the connection is made directly.

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Figure 2: Wobble plate model compressor [9].

Order tracking

In the present study, the methodology used to identify the main excitation frequencies of the air conditioning system was the harmonics analysis, obtained by calculating the Hi orders, of

the system:

(1)

Where i=1,2,...,n is a multiple of the rotation frequency of the engine.

In order to visualize the orders, a colormap was used, showing the distribution of energy levels of the system over the frequency spectrum.

In the studied system, the speed ratio between the compressor and the engine is of 1.2. The engine speed was measured via the CAN central monitoring of the vehicle. This allowed the calculation of the harmonics of the compressor.

Experimental assembly

For the date recording, the acquisition module LMS SCADAS Mobile, model SCM05, was used as shown in Figure 3. Data were processed and analyzed using the LMS software Test.Lab v.12A.

Figure 3: Data acquisition module LMS SCADAS Mobile.

Triaxial accelerometers, Brüel & Kjaer model 4506, were installed on the compressor, high pressure pipeline attached to the body, condenser and expansion valve, points 1, 2, 3 and 4 as shown in Figure 4.

Figure 4: Cooling circuit and data assessment points

Two microphones, Brüel & Kjaer model 4189-A-021, were installed in positions close to the ears of the driver and front passenger of the vehicle, to record the level of the cabin interior noise.

A pressure sensor was installed between the compressor and pipes to record the discharge pressure of the compressor.

Data acquisition procedures

Measurements were performed in air conditioning set to maximum cooling, the first stage of ventilation and external aeration.

Vibratory acquisitions were performed at a bandwidth of 2,048 Hz. For the acoustic acquisitions a bandwidth of 25,600 Hz was used.

Tests were carried out under two conditions: sweep up mode, in the 1,000-4,000 rpm range, and stabilized rotation at the highest noise level, 2,150 rpm. In all test conditions, the noise level was measured over ninety seconds, being the first thirty seconds the air conditioning switched off, the next thirty seconds with air conditioning on and the final thirty seconds off.

Results

Vehicle cabin noise

Figure 5 shows the colormap of the internal noise level of the vehicle in the sweep up mode, from 1,000 to 4,000 rpm, with the air conditioner turned off. Results indicate that the level of internal noise is mainly due to the second and fourth harmonics of the engine, H2 and H4.

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6000 100 200 300 400 50050 150 250 350 450 550

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AVDD (CH20)

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CH

36) 55

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2.00 4.00

Figure 5: Noise measured in sweep up mode, 1,000 to 4,000 rpm, air conditioning switched off, front passenger seat.

Figure 6 shows the colormap of the noise level for when the air conditioning is turned on. In this condition, it can be clearly noticed that between 1,500 and 2,200 rpm, the H6 harmonic originated in the compressor becomes evident, consequently increasing the noise level inside the vehicle.

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Figure 6: Noise measured in sweep up mode, 1,000 to 4,000 rpm, air conditioning switched on, passenger seat.

Since the noise level due to compressor operation is between 1,500 to 2,200 rpm, the following vibro-acoustic analyzes were performed from measurements in the stabilized engine speed at around 2,150 rpm.

6000 100 200 300 400 50050 150 250 350 450 550

Hz

AVDD (CH20)

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Figure 7: Noise measured inside the cabin, front passenger seat.

Figure 7 shows the noise level in the passenger side for the air conditioning on and off conditions, at stabilized engine speed. It is observed that the H6 harmonic from the compressor is the most emissive, around 258 Hz, thus being is the most important contributor to the noise level inside the cabin.

Vibration due to compressor discharge pressure pulsation

Figure 8 shows the pulsation of the discharge pressure of the compressor. It can be observed that when the compressor is actuated, between seconds thirty to sixty, the H6 and H12 harmonics are the ones with the highest amplitudes. Thus, pulsation of discharge pressure is a source of noise correlated to the internal noise.

6000 100 200 300 400 50050 150 250 350 450 550

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DPP (CH21)

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(B)

Pa

Figure 8: Compressor pressure discharge pulsation

Compressor, pipes and mounts vibration

The figures 9, 10, 11 and 12 show the results of the harmonics of the four measured points in the directions of the highest vibration levels.

6000 100 200 300 400 50050 150 250 350 450 550

Hz

CompScrew :+Z (CH3)

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200e-3

400e-3

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Log

m/s

2

H6 H12 OFF

ON

OFF

H6

Figure 9: Compressor colormap, Z axis.

H6 OFF

ON

OFF

H6 OFF

ON

OFF

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6000 100 200 300 400 50050 150 250 350 450 550

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Pendard:+Y (CH17)

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According to the results, the H6 harmonic presents the highest vibration levels at all points evaluated. Therefore, it is evident that the noise inside the cabin, due to compressor operation is closely related to the vibration of the cooling system.

Figure 13 shows the vibration levels due to the H6 harmonic, on the four points assessed, on air conditioning turned off and on conditions. It can be observed that the compressor is the one with the highest level of vibration, this being the main source of vibration of the air conditioning system. Furthermore, it can be seen that the condenser is an important path of vibration transmission to the body, since it presents high vibration amplitudes. The expansion valve and the attachment point of the high-pressure pipe in the body vibrate at lower

levels. The level of internal cabin noise is increases in about 15 dB(A) when the air conditioning is turned on.

900 10 20 30 40 50 60 70 805 15 25 35 45 55 65 75 85

s

Time

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Peak)

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Figure 13: Noise and vibration levels in the H6 harmonic from the compressor in each assessment point.

Conclusions

The internal noise in the vehicle cabin when the air conditioning is turned off in the sweep up mode, in the 1,000-4,000 rpm range, is mainly due to the harmonics H2 and H4 emanated by the engine. When air conditioning is turned on, it can be clearly observed the emergence of H6, the main harmonic of a six piston compressor, which reaches its highest noise levels between 1,500 and 2,200 rpm. Therefore, the compressor contributes to the level of noise inside the vehicle.

Vibration results show that the H6 harmonic, close to 258 Hz, presents the highest vibration levels at all points evaluated. Therefore, it is evident that the noise inside the cabin, due to compressor operation is closely correlated with the vibration of the cooling system.

Furthermore, it was determined that the compressor presents the highest level of vibration, followed by the condenser which is an important path for vibration transmitted to the body. The expansion valve and the attachment point of the high-pressure pipe vibrate at lower levels. However, these components should also be taken into account in vehicle vibratory comfort design.

The discharge pressure of the compressor and its vibration are the observed noise sources and therefore, items that need to be improved to achieve acceptable noise levels inside the cabin. Additionally, from the viewpoint of throughput, isolation on the fixing points of the pipes and condenser should be improved in order to reduce body overall vibration.

References

[1] Priede T., 1971 J. Sound and Vibration 61-73, “Origins of Automotive Vehicle Noise”.

[2] Sheng G., Lee J. H., Narravula V., Song D., 2011,Tribology International 258–265, “Experimental characterization and analysis of wet belt friction and the vibro-acoustic behavior”.

OFF ON OFF

Condenser

Expansion valve

HP pipeline attachment

Figure 10: High pressure pipeline attachment colormap, Y axis.

Figure 11: Condenser colormap, X axis.

Figure 12: Expansion valve colormap, Z axis.

H6

H6

H6

OFF

ON

OFF

OFF

ON

OFF

OFF

ON

OFF

Compressor Cabin noise

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[3] Yasuda T., Wuc C., Nakagawa N., Nagamura K., 2013, Applied Acoustics 49–57, “Studies on an automobile muffler with the acoustic characteristic of low-pass filter and Helmholtz resonator”.

[4] Mavuri, S, Watkins, S, Wang, X, St Hill, S and Weymouth, D., 2008, SAE World Congress, Detroit, United States, “An investigation of vehicle HVAC cabin noise”.

[5] Leite R.P., Paul S., Gerges S.N.Y., 2009, Applied Acoustics 636–645, “A sound quality-based investigation of the HVAC system noise of an automobile model”.

[6] Molisani L.R., Burdisso R.A., Tsihlas D., 2003, International Journal of Solids and Structures 5125–5138, “A coupled tire structure/acoustic cavity model”.

[7] Minton’s Automotive, Air Conditioning. Available online at: <http://www.mintonsautomotive.com/Air-Conditioning.html> sept. 2010.

[8] Santos, E., “Dimensionamento e avaliação do ciclo de refrigeração de sistema de climatização automotivo”, 2005, 109p., Trabalho final de curso (Mestrado Profissionalizante em Engenharia Automotiva)- Escola Politécnica da Universidade de São Paulo, São Paulo, 2005.

[9] Tian, C., Xu, H., Zhang, L. and Li, X., 2009, Applied Thermal Engineering 2824-2831, “Experimental investigation on the characteristics of variable displacement swash plate compressor”.

[10] Tian, C., Li, X. and Yang X., 2005, Applied Energy 82 1-22, “Numerical analysis of evaporator frosting in automotive air-conditioning system with a variable-displacement compressor”.

Contact Information

Rodrigo Borges Santos

MSME, Universidade Estadual Paulista – UNESP, Ilha Solteira/SP, Brazil E-mail: rbsantos10@hotmail.com João Paulo Sabioni do Nascimento

BSME, Universidade Federal Fluminense - UFF E-mail: jpaulo.sabioni@gmail.com César Abrahão Pereira Melo

BSME, Universidade Federal de Uberlândia - UFU E-mail: cesarabrahao@gmail.com Alexandre Carlos Rodrigues Ramos

BSME, Universidade de São Paulo – EESC/USP E-mail: alexandre.ramos@usp.br

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ISSN 0148-7191 ©Copyright 2014SAE International.

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The authors solely responsible for the content of the paper.