Noise balance in internal combustion engines.

Kishore, K. ; Yousufuddin, Syed ; Subbaiah, G. Venkata 等

Introduction

High levels of noise have adverse effects on human beings like loss of hearing, dilation of pupils increase in blood pressure, contraction of muscles, headache, and stress. It is well known that noise and vibration are interrelated. Traditional design engineers used to control noise by reducing the level of vibration using vibration isolators etc. In the present studies the various components of internal combustion (IC) engine are ranked in this order of sound intensity. Further the flow of sound around the IC engine is plotted taking the advantage of vector notation. Sound intensity is a vector it can locate the source of sound. Further these measurements can be taken in any field.

Sound intensity is the time-averaged product of the pressure and particle velocity [1]. A single microphone can measure pressure. However to measure the particle velocity, which is relative to pressure gradient cannot be measured. This can be done by using two microphones.

We know I = p x [bar.u]

When the microphones are used.

Average pressure P = [P.sub.a] + [P.sub.b]/2

From classical Euler equation

u = -1/[rho] [integral] [partial derivative]p/[partial derivative]r dt

Where [rho] is the density of the median

Then

I = -[P.sub.a] + [P.sub.b]/2[rho][DELTA]r [integral] ([P.sub.a] + [P.sub.b])dt

It is proved that sound intensity in magnitude is equal to the imaginary part of the cross spectrum of FFT applied to a cross correlation function of the time domain signal recorded by the two microphones [2].

Plan of Investigation

Generally to reduce the noise levels in IC engines it is necessary to know how much noise is radiated by the machine. In what pattern it is following. We there fore need to know the sound power of individual components that make the most noise then we can consider steps to reduce it. Hence ranking of the various components like fuel pumps, turbo unit, alternator, exhaust manifolds, mufflers and radiator are ranked in the order of highest to lowest sound intensity [3]. A grid of size 1000 mm X 3000 mm is considered and a model of rectangular box enclosure around the IC engine is assumed. Readings are taken at an interval of 500mm and the flow pattern is recorded.

Instrumentation

Sound intensity can be measured by using a dual channel FFT analyzer. However in the present investigation B&K make sound intensity measuring probe with dedicated hard ware for obtaining the imaginary part of the cross spectrum directly was used. The IC engine in present investigation is of four-cylinder diesel engine coupled to an AC generator of 125 kVA capacity. The probe used comprises of two microphones fitted in a fixture manufactured by B&K.

Experimentation

Sound intensity readings are recorded at several points on the grid as shown in figure 1.

Although several readings are taken at different plans along the points marked as A's B's and C's framing a box pattern. Sample readings only at one plane at a height of 1000mm from the base of the generator are reported in table-1.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Observations and Conclusions

Turbo charger is identified as the maximum noisy component of the engine. Exhaust manifold and turbo chargers are on the same side of the engine hence the sound distribution level is always positive on one side. Due to different components like fuel pump, alternator, air filters on one side a non-uniform sound distribution is obtained. There are some points of noise cancellation only at the left side of the engine no noise cancellation points are recorded near the radiator side. Positioning the fuel pump under the turbo unit may create another noise cancellation zone on the right side of the engine. Noise levels are high in load condition when compared to no load condition by 5 dB. Sound intensity measurements are more convenient than traditional vibration analysis for noise suppression.

Acknowledgments

The authors sincerely thank the Management of Vasavi College of Engineering for giving permission to carry out the experiments in the college premises.

References

[1] Tandon, N., Nakra, B.C., Sarkar, B., and Adyanthya, V., 1997, "Noise Control of Two Wheeler scooter Engine," Applied Acoustics, 51(4), pp.369-380.

[2] Watts, G., and Hothersall, D.C., 1994, "Acoustic performance of new designs of traffic noise," Journal of Sound and Vibration, 177, pp.289-305.

[3] Said, W., Nakra, B.C., and Nassir, A., 1981, "Investigations on acoustic mufflers," Journal Acoustical Society of India, 9, pp.68-73.

K. Kishore (1), * Syed Yousufuddin (2) and G. Venkata Subbaiah (3)

(1,2,3) Mechanical Engineering Department, Vasavi College of Engineering, Ibrahimbagh, Hyderabad-500031, Andhra Pradesh, India

* E-mail: syedyousufuddin@rediffmail.com

Table 1: Sound intensity in decibel (dB).

SNo.   Grid point    dB   SI. No   Grid point   dB

1      [A.sub.1]    -90     16     [A'.sub.1]   -91
2      [A.sub.2]    -93     17     [A'.sub.2]   +92
3      [A.sub.3]     0      18     [A'.sub.3]   +92
4      [A.sub.4]    +94     19     [A'.sub.4]   +93
5      [A.sub.5]    +96     20     [A'.sub.5]   +96
6      [B.sub.1]    -87     21     [B'.sub.1]   +88
7      [B.sub.2]    -90     22     [B'.sub.2]   -90
9          B3        0      23        B'3       +91
8      [B.sub.4]    +92     24     [B'.sub.4]   +93
9      [B.sub.5]    +93     25     [B'.sub.5]   +94
10     [C.sub.1]    -84     26     [C'.sub.1]   +86
11     [C.sub.2]    -85     27     [C'.sub.2]   +87
12     [C.sub.3]     0      28     [C'.sub.3]   -89
13     [C.sub.4]    +87     29     [C'.sub.4]   +90
14     [C.sub.5]    +90     30     [C'.sub.5]   +91