Research On Diagnosis Of Abnormal Sound Caused By Vehicle Air-conditioning System

The use of vehicle air-conditioning(AC) system is to improve interior environment and riding comfort. However, it could produce abnormal sounds under different working conditions, which is against the original purpose. These abnormal sounds possess different subjective performances and spectral characteristics. Convenient for describing, in this essay, the interior squeaking noise generated under specific engine speed is called “constant-speed abnormal sound”; the interior impacting noise when AC system is activated is called “suck-in abnormal sound”; and the interior lasting roar after AC system is activated is called “start-up abnormal sound”. The three above are the main forms of abnormal sound caused by AC system in terms of a domestic SUV, which is used as testing vehicle. Many other vehicles on the market also have these problems.In order to diagnose and improve the abnormal sounds, three aspects are taking into consideration based on “source-path-receiver” pattern. First, no matter what kind of form the abnormal sound is, the reason always has something to do with the transferring and transforming among three energy forms, which are air-borne sound, structural vibration and fluid pulse in the pipeline. In terms of AC system, the source of those three energy forms is the compressor. Meanwhile, the compressor receives effects from other front-end accessories such as engine and alternator. However, since the compressor and engine had already been strictly adjusted before the vehicle was put on the market, the source control is hardly considered available. Second, controlling and improving the receiver’s properties is considered passive control, and not able to find out the real cause. Therefore and third, the basic and effective way is to find and control the transfer path. Till now, most domestic vehicle manufacturers still don’t possess the ability to efficiently diagnose and solve this problem. Due to the lack of systematical, logical diagnosis methods, massive effort was made but the outcome was barely ideal.Therefore, this essay looks into the reason of the three forms of abnormal sound, using a domestic SUV as researching object.First, interior noise signals under different working conditions were attained to perform stationary spectrum analysis, increasing speed spectrum analysis and order analysis etc. Due to these methods, the frequency, spectral properties and engine speed when the interior abnormal sound occurred were recognized. Then the target frequency of interior abnormal sound was confirmed by interactive filtration and subjective evaluation.Second, the probability that compressor’s working noise contributes to interior noise through air-borne sound transfer path was investigated. Noise Transfer Function(NTF) from engine cabin to driving cabin was tested and analyzed, which is used as the evaluation standard of vehicle acoustic wrapping level.Third, the operational modal analysis(OMA) was carried out on the pressure-low(PL) pipeline, one of the most important components and the main transfer paths for noise, vibration and fluid pulse in AC system. Based on the OMA, the operational modal frequency and modal shape of PL pipeline under specific working condition(when interior abnormal sound occurred) were calculated, and the location with massive movement in the operational modal shape was found.Besides, in terms of several components or measuring points with correlated vibrations in the AC system and front-end pulley system, the priority ranking analysis was applied to find the most prior component or measuring point on the target frequency(frequency of interior abnormal sound), so as to avoid the influence of correlation among the signals. According to the result, a series further investigations were carried out, including experimental modal analysis(EMA) for the alternator support and finite element analysis(FEA) for compressor’s armature. Based on the researches, the reason of compressor’s squeaking noise and the constant-speed abnormal sound in the driving cabin were concluded.Furthermore, a series traditional experimental methods and the wavelet energy spectrum analysis using suitable mother wavelet and reasonable wavelet parameters were conducted. The reason of suck-in abnormal sound and start-up abnormal sound were concluded, respectively.At last, in order to efficiently and fundamentally solve the problem in future work, a set of overall, systematical and feasible testing and diagnosis methods and experiment outline for abnormal sounds caused by AC system were composed.