Prediction of Noise in a Diesel Engine Using Sound Intensity Mappings.

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Prediction of Noise in a Diesel Engine Using

Sound Intensity Mappings.

K.V.Subbarao1,

1 Mechanical Engineering Department, Praveenya Institute of Marine Engineering and Maritime Studies, Modavalasa, Vizianagaram Dist., A.P(email:kvskanuri@gmail.com)

K.Ramji2, M.M.M.Sarcar2

2 Mechanical Engineering Department, A.U. College of Engineering, Visakhapatnam, A.P.

Abstract: Noise Source identification for any prime mover or any movable device is a key concern in today’s Industrial applications. The present paper aims at investigation of noise, sound intensity of water cooled Diesel Engine are carried out using “two micro phone” method. Sound intensity mappings are carried out at various locations of the engine surfaces. There by obtaining respective contour maps from the measurement results. From the contour maps, noise sources in engine are located. Major noise sources are identified which would help in the control the noise in an engine.

Key Words: Noise, Sound Intensity, Intensity Mappings, Diesel Engine, Noise Source, Two Microphone.

1. Introduction:

In the modern day scenario noise pollution for the environment is one of the major issues. Due to this noise reduction has become one of the prime objectives of all latest engine design and developments. This has resulted in a lot of study and research in noise source identification. The noise source identification is of utmost importance for noise control. Noise is an unwanted sound, it consists of many different frequencies and intensities. At present, there are some methods to test sound, such as sound pressure and sound intensity etc. Since the sound pressure is a scalar quantity, the results of sound pressure measurements are affected by the environment greatly.

Engine components noise is identified and sound intensity is mapped on the vibrating surfaces of the engine. The study is carried out to understand the engine noise characteristics of sound power radiation of the engine.

The “two microphone sound intensity technique” has been used to locate major noise sources in the engine. The advantage of sound intensity measurements for noise source location is that these measurements can be made in the near field and thus a high signal to noise ratio is obtained. Sound intensity maps of all the engine surfaces are obtained.

2. Literature Review:

All sound intensity measurement systems commercially available in the present day scenario are based on the ‘two-microphone’ (or ‘p-u’) principle, which makes use of two closely spaced pressure microphones and relies on a finite difference approximation to the sound

pressure gradient, from which the particle velocity is determined. The B&K 2683 standard

instrument for the measurement of sound intensity deals exclusively with this measurement principle.

The limitation of the measurement principle is the restricted dynamic range at low frequencies: because the electrical noise of the microphones at low frequencies is amplified by the time integration needed in determining the particle velocity from the pressure gradient, one cannot in practice measure sound intensity levels below, say, 50 dB re 1 pW/m2 below 100 Hz [1].

Other sources of error include the effect of airflow. Airflow gives rise to correlated noise signals that contaminate the signals from the two microphones. The result is a ‘false’ additive sound intensity at low frequencies [2].

3. Theory on Sound intensity:

Sound intensity is a well-known application that measures acoustic energy flow and provides information about sound amplitude and direction in the acoustic field. Sound intensity at a point in a sound field in a specific direction r can be expressed as

Ir = r(t) dt (1)

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The sound pressure at the midpoint of the two microphones can be obtained by taking the arithmetic average of two sound pressure signals.

p(t) = (2)

The particle velocity at that midpoint can be obtained by Euler’s equation for a zero-mean velocity medium:

Vr(t) = - (3)

By using Fourier transforms, sound intensity can also be expressed as

Ir (ω) = (4)

Where G12(ω) the one-sided cross- power spectrum density function between microphone channels1and2, ρ is the air density, ω is the angular frequency, and Δr is the separation distance between the two microphones.

4. Experimentation:

In the present study single cylinder four stroke water cooled direct injection diesel engine is used. It is installed in the engine test cell and is coupled with the eddy current dynamometer on a bed frame and the bed is mounted on the concrete floor. The exhaust line is ducted outside the room to isolate the aerodynamic noise so as to determine the engine surface radiation only in the experiment.

For three different loads, measurements are taken for investigating the effect of speed and load on sound intensity profile and sound power radiation of the test engine at the following load combinations at constant speed of 1500 rpm such as No load, 60% load & 90% load.

5. Experimental Setup:

Diesel engine with rated power 5.2 KW and rated speed of 1500 rpm is used for the present experiment (Ref. Fig 1). Test engine selection is based on the fact that this type of small single cylinder DI diesel of about 0.661 liter capacity are widely used in stationary power sources in urban areas, powering agro-appliances in rural areas .

Two microphone instrument: The sound level meter B&K 2260 is used in the experiment. The intensity probe used in the experiment is B&K 2683 two- microphone intensity probe. An 6 mm spacer is used. It is required to develop the control surfaces, which enclose the sound source i.e. test engine in this case and make appropriate grids for intensity measurement.

Fig.1 - Experimental setup Fig.2: Two microphone instrument and Sound level meter.

6. Modeling/Mapping Aspects:

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This map gives us a basic understanding of, from where the noise originates in the engine. Hot spot (Source identification of noise) identification can be found from intensity mappings.

7. Advantages of sound intensity:

The advantages of sound intensity mappings are:

 Because sound intensity is a vector quantity, the acoustic field can be represented as amplitude and direction.

 Possible determination of sound power from sound intensity measurements.  Measurements in-situ (relatively tolerant to background and uncorrelated noise).  Portable technique (can be used with a two channel sound level meter).

7.1 - Limitations:

 Frequency limitations due to pressure approximation gradient (determined by the microphone spacing): up to10 k Hz with the smallest spacing.

 Relatively time consuming.

 Accurate equipment required (accurately phase-matched microphones, and data acquisition system).

 Sources need to be stationary.

8. Results and Discussions on the intensity plots:

Sound Intensities are obtained for the selected engine on five sides, referred as front, rear, left, right and top.

(a) Front Surface (b) Back Surface

(c) Left Surface (d) Right Surface

1 2 3

1 2 3

95‐100

90‐95

85‐90

80‐85

1 2 3

1 2 3

100‐105

95‐100

90‐95

85‐90

1 2 3

1 2 3

94‐

92‐

90‐

88‐

1 2 3

1 2 3

50‐10

0‐50

‐50‐0

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Fig - 3: 2 – D Contour Maps for Engine Running at 1500 r.p.m on 90% Load

 On the front side of the engine the sound intensities are measured at three different locations (9 grid points) namely a) Valve cover b) cylinder block c) crank case

 On the rear surface of the engine sound intensity levels are measured at three different locations such as a) valve cover b) fuel injector c) fly wheel.

 On the left surface of the engine, the sound intensity levels are measured at the following locations. a) Inlet and Exhaust passages. b) Cylinder block c) crank case.

 On the right side surface of the engine sound intensities is measured at three locations namely a) fuel injector b) cylinder block c) crank case.

 On the top surface of the engine the sound intensity measurements are taken on the following locations a) valve cover b) ports c) fuel injector

The Sound intensity mappings along with sound intensity levels are obtained for further analysis. The data is taken at different working conditions of the engine namely a)No load b)60% load and c)90% load respectively, while keeping the speed of the engine constant at 1500 r.p.m.

Amongst the three above mentioned investigations observational details for 90% load are given as:  The maximum noise level on the front face at the valve casing is 89 dB, at the crank case it is 96 dB

and at the cylinder block is 93 dB. Hence it is seen that, of three sources the highest noise level is observed at the crank case. This high noise level on the front face in the crank case region may be attributed to the presence of the gear transmission system.

 Investigating the noise intensities on the rear side of the engine the following observations are made. The maximum noise at the valve case is100 dB, at the cylinder block it is 96 dB and at the crank case region it is 102 dB. On the rear surface of the engine also the maximum noise is observed at the crank case region, same as the front surface. The high level of noise is attributed in the vicinity of the flywheel due to the aerodynamic action.

 Investigating the left surface of the engine the maximum intensity of noise at the inlet and exhaust ports is 95 dB, at the cylinder block is 94 dB and at the crank case also around 94 dB. It is observed that the cylinder head with the exhaust and inlet passages has almost the same noise level as the crank case. This may be due to the fact that the exhaust ports produce a higher noise.

 The right side surface of the engine exhibited the following sound intensities. The intensity level at the injector is 94 dB, at the cylinder block is 96 dB and at the crank case is also 96 db. The maximum noise level at the cylinder block is attributed to the presence of the fuel pump.

 On the top control surface of the engine the following observations are made. Maximum intensity of noise at the ports is 96 dB, at the valve cover it is 97 dB, at the injector it is 94 dB. The highest intensity of sound at the valve cover is due to the rocking action of the rocker arms assembly which is located on the central part of the cylinder head.

2D and 3D sound intensity (contour) mappings are plotted for 90% load for various surfaces i.e., front, rear, left, right& top surfaces and are shown in the figures3&4 respectively

(e) Top Surface 1 2 3

1 2 3

95‐100

90‐95

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Front

Surface Back Surface

Left Surface Right Surface

Top Surface

Fig - 4 : 3 D Contour Mappings for Engine Running At 1500 rpm On 90% Load

9. Conclusions:

The conclusions made from the present work are

 Maximum intensity of sound is observed at crank case region in the rear side as 102 dB.  On the left surface of the engine the intensity variation is observed around 94 dB.  On the right surface of the engine the intensity variation is observed as 96 dB.  On the top surface of the engine the intensity variation is observed to be 96 dB.

 With the decrease of load, it is observed that the sound intensity decreases at rated speed of 1500 rpm and the same can be observed in the 3 D contour mappings also.

10. References:

[1] F. Jacobsen, J. Sound Vib. 166, 195-207(1993).

[2] F. Jacobsen, ‘Intensity measurements in the presence of moderate airflow’ in Proc. Inter-Noise 94 , Yokohama, Japan, 1994, pp. 1737-1742.

[3] ZHANG Bao-Cheng, ZHAO Peng-Fei and LI Bao-ji: Identification of Noise Source on a Diesel Engine using Sound Intensity Measurements, Advanced Research Vols. 268-270 (2011) pp 205-209.

[4] Zhang Jun Hong and Han Bing: Analysis of engine front noise using sound intensity techniques, Mechanical Systems and Signal Processing 19 (2005) 213-221.

Figure

Fig - 3: 2 – D Contour Maps for Engine Running at 1500 r.p.m on 90% Load

Fig -

3: 2 – D Contour Maps for Engine Running at 1500 r.p.m on 90% Load p.4
Fig - 4 : 3 D Contour Mappings for Engine Running At 1500 rpm On 90% Load

Fig -

4 : 3 D Contour Mappings for Engine Running At 1500 rpm On 90% Load p.5

References

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