Noise control and experimental study of a highpressure oil pump enclosure

Introduction

In recent decades, with the rapid increase of vehicle ownership and the rising of living standards, people's awareness of environmental protection and requirements for quiet environment have been increasing [1]. And one of the most important requirement for car comfort is noise, vibration and harshness control (NVH),which is becoming an important part of vehicle sensory quality [2]. It can be harmful to drivers for long-term exposure to noise, distract the attention and easy to cause traffic accidents.

Previous studies have reported that engine noise is one of the most important noise sources, it consists of generator noise, combustion noise, high-pressure oil pump noise, gear noise, exhaust noise, fan noise etc [3, 4]. However of which high-pressure oil pump is used to supply the oil to the engine, and it will not only consume a part of the engine power, but also generate violent vibrations, leading to noise on the condition of high-speed operation. Therefore the noise of high-pressure oil pump cannot be overlooked. However, people mainly considered the rail pressure, displacement, type, volume efficiency, pressure pulsation and flow pressure in current designs of oil pump. Few studies have been conducted focusing on its noise problem [5]. This may be attributable to its complex structure and tedious production process [6]. In 2016, some factory has received complaints regarding high-pressure oil pump noise as high as 61, fully revealing that the noise research of high-pressure oil pump should be given more attention [7].

Lai et al. [8] designed three sound insulation boards with different structures and verified their effectiveness. The results showed that the noise of the shearing machine is reduced by 4–5 dB. Lu and Gao [9] analysed the 16∼31.5 Hz low-frequency noise characteristics before and during the control. The final test showed that the SPL in the resident areas I, II and III classified according to the severity of the interference can be reduced from 95 dB to 74, 91 dB to 66 and 83 dB to 65 dB. Cheng et al. [10] studied the energy transmission path and noise attenuation in double-wall structure based on the coupling equation, and summarised the approximation formula of the effect of gap on the low-order coupling frequency of the system. Beltman [11] set up a revised equation of vibration structure and compressible fluid by testing the noise with different frequency characteristics and modifying the DIANA element. The results show that different noise reduction methods are needed for different structures and different noise frequencies.

There are two main ways to control the engine noise: one is to optimise the design and development, the other is to use shield, including mufflers and sound shields. Noise energy is reasonably controlled by sound absorption, attenuation and reflection of sound insulation materials, and is eventually reduced to an acceptable level when transmitted to the outside world. The sound insulation cover can be made into single layer or multilayer, and can be metal or non-metal with different thickness [12].

These mathematical models are used for finite element calculation to identify and prevent engine noise. However, due to the ever-changing engine structure and the ever-changing operating conditions, the numerical calculation can only provide a theoretical basis for us [13].

Akei et al. [13] described a method of identifying surface vibration from ambient sound pressure, which was called Inverse Numerical Acoustics, and can be used to built a steady-state engine model. The noise level around the engine with sound absorbing materials and openings can be predicted by using this sound source model and boundary element method. Zheng et al. [14] used the transfer path analysis method as the boundary condition, and identified the influence of structural noise and airflow noise on subjective evaluation. The structural-borne input of hybrid FEA/SEA engine enclosure model was analysed. Augusztinovicz et al. [15] used the numerical simulation to calculation the engine with cover and without sound cover, and an experiment was carried out to compared with the calculation results. The decorative cover of the engine acts as a decorative part to play a decorative role. If a reasonable design structure and material selection are done, it will play an excellent role in noise reduction. Nowadays, nylon with fibre glass are widely used in engine cover, auxiliary materials such as polyurethane can be used to enhance sound absorption. This method is not limited to the application of automobile decorative cover [16]. The sound insulation hood is placed downstream of the noise source to prevent the difficulty of controlling the noise scattering. The area of noise reduction materials should not be too small, and should be as close as possible to noise sources. For engines, we should also pay attention to safety, heat resistance, flame retardancy and aging resistance of materials. High-pressure oil pump is one of the most important sources of vehicle noise. However little attention was given to the importance of the high-pressure oil pump as a noise source.

Materials and methods

Engine

The structure of the engine used in this study is shown in Figure 1 [17], while the red part is the high-pressure oil pump system and the high-pressure oil pump is located at the top of the engine. And the basic information of the engine is given in Table 1.

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Figure 1

The structure of engine used in the study.

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Table 1

The basic information of the engine.

Engine	Descriptions
Cylinder bore (mm)	84.5
Stroke (mm)	89
Number of cylinders	6
Cylinder pitch (mm)	90
Thick cylinder partition (mm)	5.5
Displacement (cm3)	2995
Compression ratio	10.5
Net power (kW)	245
Maximum torque (N·m)	440

Acoustic enclosure

The sound insulation ability of an acoustic enclosure depends on a variety of factors, such as the shape and size, structural rigidity, opening size, gap area, average sound absorption coefficient, sound insulation, loss factor, and so on. The commonly used sound insulation enclosure contains 5 types: open type, semi-open type, semi-open type with muffler, totally enclosed type and semi-open type with sound barrier. The structure diagrams are shown in Figure 2.

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Figure 2

Several structural types of sound insulation.

Taking sound absorption material as inner material will greatly improve the insulation of the enclosure [18]. On account of this, a double-layer enclosure was developed using PA66 (Polyamide-66) with 4 mm thickness as the outer structure, and polyurethane(PU) foam with 8 mm thickness as the inner material, which belongs to semi-open enclosure. PA66 exhibits good strength and impact resistance, while PU is lightweight, waterproof and moisture-proof, convenient to process, and has excellent absorption capacity. Models of high-pressure oil pump, acoustic enclosure, and the assemble model are shown in Figure 3.

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Figure 3

The models of high-pressure oil pump (a), acoustic enclosure (b), and assemble model (c).

Insertion loss IL is generally used to express the noise reduction effect of the acoustic enclosure. Since the sound insulation cover designed in this paper is semi-open, the sound isolation performance can be calculated in the following form [18]: Where S₁ and S₀ are the areas of closed surface and non-closed surface, m²; TL is the sound insulation of the enclosure; α is the sound absorption coefficients.

Acoustic performance test of the enclosure

Transfer Function Method was utilised to test the sound absorption performance of the inner PU foam and insulation of the double-layer enclosure. The test equipment is SW422 impedance tube and four-channel acoustic analyser coming from Beijing Prestige Sound Electrical Technology Co., LTD. The diameter of the impedance used here is 100 mm, and the test frequency bend is 63∼1600 Hz. The impedance tube test system and test sample are shown in Figure 4. The specific test principle is detailed in the international standard 15010534–2 and GB/T 18696.2 2002 [19].

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Figure 4

The impedance tube test system (a) and test samples (b).

Test of the engine noise spectrum

In order to obtain the noise spectrum of the engine, 5 microphones are distributed around the engine, and are respectively located onthe top, the bottom, the front, the right, and the left of the engine. Under the 0% and 100%loads, the SPL of 5 points under one metre noise were tested, and the five-point average is obtained finally. Schematic diagram(a) and test chart(b) of microphone field distributionof engine acoustic test are shown in Figure 5.

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Figure 5

Schematic diagram (a) and test chart (b) of microphone field distribution of engine acoustic test.

Instrumentation and signal processing

The instrument used for sound signal measurement consists of a sampling device, a pretreatment device, an input-output device and a processing device. The instrument specifications are shown in Table 2. The physical signal of the sound collected by the microphone is converted into an electrical signal, and the analog voltage signal is filtered by the sound level meter, and is output to the computer. The computer processes the signal through SoolEdg Pro software and stores it in 16-bit quantisation precision and.wav lossless file format. The sampling bottle rate should not be too high or too low. On the one hand the 48 kHz sampling frequency can cover the ear monitoringaudio frequency range (20Hz–20kHz), on the other hand this method satisfies the Nyquist–Shannon sampling theorem.

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Table 2

Specifications of the used instruments.

Aisle	Microphone B&K		Preamplifier B&K
	Model	Serial number	Model	Serial number
1	4190	2486389	2669L	2502215
2	4190	2486388	2669L	2502214
3	4190	2486387	2669L	2502213
4	4190	2486386	2669L	2502212
5	4190	2486385	2669L	2502211

Notes: Piston sounder: B&K4231; Test standard: GB 1859–2000; Full anechoic chamber. Structure: Space: 9 m × 8 m × 7 m, flat sound absorbing material. Environmental control system produced by IMETCH. Ensure that the ambient temperature in the full anechoic chamber is 25 ± 3°C.

Conclusions

In this study, the noise reduction effect of a double layer acoustic enclosure of the high-pressure oil pump was experimentally measured. The data obtained from the experimental study for the SPL of the engine with and without the acoustic enclosure are presented. The results revealed that the SPL of the engine evidently increased after the high-pressure oil pump was installed, especially at frequencies below 4000 Hz. And the average SPL increased at approximately 8 dB, revealing that the high-pressure oil pump is a very important noise source for the engine. A double layer enclosure was developed using 4 mm thickness PA66 as an outer structure and 8 mm thickness polyurethane foam as the inner material, which is owning to the fact that PA66 has good impact resistance and the polyurethane foam showed an excellent absorption effect. Finally, one metre SPL was tested under 0% and 100% load conditions. The results indicated that the SPL is significantly reduced throughout the spectrum when the acoustic enclosure is installed, the SPL reduced by almost 2 dB within the entire test frequency, in addition to the SPL in a small band near 2000Hz changed little, showing a remarkable noise reduction effect of the high-pressure oil pump enclosure.