P93691.vp

Diesel Engines and the Environment - Noise
Contents:
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
What is Noise?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Intensity of Sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sound Level Measurement Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Influence of Sound Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Primary Sources of Engine Noise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Two-Stroke Engine Noise Emissions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Exhaust gas noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Airborne noise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Structure-borne noise excitations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Noise Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Bridge wing − Exhaust gas noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Engine room − Airborne noise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Accommodation − Structure-borne noise excitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Conclusion
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Diesel Engines and the Environment - Noise
Introduction
Intensity of Sound
as undesirable and, therefore, as noise.
The physical intensity of sound, I − which e.g. air. When the air starts to pulsate, the variations in air pressure will spread ous nitric oxides from exhaust gases.
necessary, but harmless, evil. Today,excessive noise is considered a form ofpollution which, in the long run, may Intensity of sound
cause permanently reduced hearing.
As a consequence, authorities nowdemand that noise levels are kept ρc, the corresponding mean effective sound pressure (p) and the pulsation velocity (u) may be stated as follows: a standard for noise limits was theFederal Republic of Germany which, noise levels permitted on its ships.
Today, there are numerous national = Velocity of sound in medium (air) (m/s) The greater demand for noise limitationin the maritime area has, of course, greater demands are now made on theengine designer to provide more de- tailed and precise information regardingthe various types of noise emission Reference for sound levels
After a brief definition of what noiseactually is, this paper will attempt to clarify ‘noise’ as applied to MAN B&W’s Given a sound intensity I = 10−12 W/m2 and using the above formulas, we can state the corresponding reference mean effective sound pressure (p ) and mean effective pulsation velocity (u ) as follows: emissions, noise level limitation, andthe current situation in relation to noise.
= 20 x 10-12 = 2 x 10−5 Pa (Pascal = N/m2) What is Noise?
A popular definition of noise is ‘anundesirable sound’. To what extent a is, of course, a personal evaluation.
However, if the sound level is so highas to be damaging to health, it will Sound Level Measurement
has doubled in intensity, a linear division of troduced as a unit for measuring sound.
which is measured, and when nothingelse is given, it will be re 2 x 10−5 Pa.
tensity of 10−12 W/m22 corresponds to a the level at which the ear begins to feel The above-mentioned intensity andpressure reference values represent At the so-called ‘far-field’, i.e. where no distance will reduce the intensity of the Sound level - far-field law
Sound level at distance R compared to distance R : Reference sound intensity . . . . . . . . . . . II = 10−12 W/m2 Sound intensity level (dB) at R . . . . . . .L = 10 x log Area of a sphere:. . . . . . . . . . . . . . . . . . . . . . . . . . . A = 4 πR2Sound intensity at R . . . . . . . . . . . . . . . . . . . . l = l x (R /R )2 Sound level at R . . . . . . . . . . . . . . . . . . . . . . . . . . L = L − 20 x log (R /R ) Spherical
propagation
In general, the sound level will be reduced by 6 dB for each doubling of the distance Bridge wing
Fig. 1: Spherically propagated sound waves - far-field law. Distance R from noise source (point source assumed) The Influence of
Sound Frequency
The sensitivity of the human ear is closely As a result of the ear’s varying sensitivity special noise curves, and ISO’s ‘Noise Rating’ curve sheet is very often used, are made in the so-called octave bands.
quency is twice as high as the lower.
characteristics of the ear with the sound dB Attenuation
Centre frequencies of octave bands
Fig. 2: ISO´s Noise Rating curves Octave band pressure levels, re 2 x10 5 Pa sound reduction of 6 dB, see Fig. 1.
used by MAN B&W Diesel in this paperand corresponds to the mentioned ISOreference figures I and p .
m/s in this norm then corresponds to34.0 dB.
Centre frequencies of octave bands
when the vibrational energy transferredfrom the engine feet to the ship hull ismeasured.
Fig. 3: Filtering (weighting) curves for sound level meters If the sound pressure levels of the vari- respectively, the average ‘Noise Rating’ trical filtering curves, the so-called A, B, highest NR-figure will give the resulting In particular, the A filter is often used quency range(s) should be attenuated.
sating for the ear’s subjective percep- A sound level obtained by linear mea-surement, i.e. without any correctionfor the sensitivity of the ear, is desig-nated dB(Lin).
Primary Sources of
Engine Noise
Exhaust gas
pulsations
ments and frequency analyses, it canbe ascertained that noise emissionsfrom the two-stroke engine primarilyoriginate from: Airborne
The best way of reducing engine-relatednoise is, naturally, to reduce the vibra-tional energy at the source or, if this is neither feasible nor adequate, to atten-uate the noise as close to its source as Vibration in
engine feet
The different noise sources of the die-sel engine, of which the primary ones 1. Exhaust gas noise
2. Airborne noise
3. Structure-borne noise
sion to the environment. The types ofengine-related noise emission will bediscussed in the next section.
Fig. 4: Typical sources of engine noise Two-Stroke Engine Noise
dB Attenuation
Emissions
On the basis of theoretical calculationsand actual measurements, we employcomputer models − please refer to ourpaper: ‘MAN B&W Computerised En- gine Application System’ − to provideour customers with data regarding thesound levels of the following engine-related noise emissions, which are typi-cal of our two-stroke engines. See also Centre frequencies of octave bands
Fig. 5b: Typical noise attenuation for a 25 dB(A) absorption silencer 1. Exhaust gas noise
the funnel to the bridge wing. The curvesheet shows that the noise level in the accumulating the intensities of the octave band sound levels, including the A-weighted attenuation, as shown in Fig. 3.
Thanks to its ideal location, i.e. close tothe noise source, this gas receiver also tance from the funnel top to the bridgewing is 7 metres.
Fig. 7a curve 1 shows a 6L80MC Mk 5engine, running at nominal MCR, where on the bridge wing, a relatively voluminous absorption type will normally be adequate,as this attenuates the dominant fre- Fig. 7a curve 2 shows the average air-borne noise level calculated for a nomi-nally rated 6L80MC Mk 5 engine witha noise level of approximately NR 101and 105 dB(A) for an engine with highefficiency turbochargers, (curve 2A) andapproximately NR 98 and 103 dB(A) foran engine with conventional turbochargers(curve 2B). The difference in noise levelsoriginates from the difference in noiseemission from the turbo- chargers them-selves. In general, the higher the turbo-charger efficiency, the higher the noiseemission from the turbocharger and theengine.
Fig. 7b shows the corresponding aver-age airborne noise level calculations fora 6S26MC Mk 6 engine.
Fig. 6: Example of location of measuring points on a diesel engine in accordance with CIMAC’s recommendations of measurement based noise levels measured in the ves-sel may be 1-5 dB higher than the cal-culated sound intensity based noise levels.
total average airborne noise level, an in- close to the funnel top as possible.
lencer is a flow silencer, i.e. a pipe with noise level of the engine, using the high figure for high efficiency turbochargers.
the specified noise limit requirements.
Especially in large diesel engines, it may sometimes − to meet the noise limit re- quirements − be necessary to introduceadditional noise reduction measures, 2. Airborne noise
type of engine, the average airbornenoise level of a nominally rated engine times more, depending on their extent.
engine, and normally near a turbocharger, 3. Structure-borne noise excitation
Vibrational energy in the engine ispropagated, via the engine structure, energy is transferred to the ship’s tank the engine and the reciprocating move-ment of the pistons.
amplitude-dependent, so the velocitycan normally be employed as a unit of Centre frequencies of octave bands
The reference velocity used correspondsto the previously used intensity and Fig. 7a: ISO´s NR curves and noise levels for a 6L80MC Mk 5 engine for example, 105 dB(A) instead of110 dB(A), especially in view of theinfluence of sound reverberations and Helmholtz resonator lining in scavenge air pipe the noise emitted by other machinery.
The possibility of reducing the noise External insulation of scavenge air receiver from an existing engine is greatly lim-ited because, as previously mentioned, External insulation of scavenge air cooler the noise stems from many differentsources, and because the noise trans- Additional absorption material at the engine and/or mission paths − through which vibrational at the engine room walls (yard’s responsibility) Additional turbocharger intake silencer attenuation Additional attenuation material at the turbocharger’s inspection cover However, in principle, the transmissionof airborne noise from the engine room Low noise diffusor for turbocharger compressor, if available to other locations, e.g. accommodationquarters, normally has no influence onthe actual noise level in these locations.
Table 2: Additional noise reduction measures on diesel engines Noise Limits
builder, or indirectly by referring to na-tional or international legislation on the Organisation) recommendations. TheIMO noise (sound pressure) limits for different ship spaces are listed in Table 3.
national standards for noise levels in ships has, in general, resulted in a considerable reduction of the noise levels in newly-built ships, especially in the accommo- Bridge wing
exhaust gas noise that predominates,there are certain limitations, as the Centre frequencies of octave bands
3. Structure-borne noise - engine feet, vertical always be met by installing a suitableexhaust gas silencer.
Fig. 7b: ISO´s NR curves and noise levels for a 6S26MC Mk 6 engineMCR: 2,400 kW at 250 r/min It is often seen that ships built and reg-istered in the Far East are not equippedwith exhaust gas silencers, whereas shipsbuilt in Europe are normally equipped mentioned, the reference value 10−9 m/s The reason for omitting silencers is that, to save the cost of silencers, shipowners rules required in Europe, for ships built ance with the rules is not required.
a vertical vibration velocity level in theengine feet.
Engine room − Airborne noise
levels in the diesel engine feet can, with 5 x 10−8 m/s, the structure-borne noise levels shown should be 34.0 dB higher.
Incidentally, the vibration velocity level yards, or their consultants, normally have to those required in passenger ships.
These noise requirements can, as a Work spaces
Machinery spaces (continuously manned) ** Machinery spaces (not continuously manned) ** Conclusion
Navigation spaces
Listening posts, including navigating bridge Radio rooms (with radio equipment operating accommodation will not be a problem.
On the other hand, the airborne noiseemitted from the engine in the engine Accommodation spaces
there is a risk that the noise limits for additional noise reduction measuresare introduced.
In future, therefore, it must be expectedthat it will be very important, from a ** Ear protectors should be used when the noise level is above 85 dB(A), and no individual’s daily exposure duration should exceed four hours continuously or eight Table 3: IMO noise limits (sound pressure level) and the introduction of stricter require- has increased over the years becauseof the higher rated engines.
Accommodation − Structure-borne
seems to have constituted a seriouslimitation for the engine builders. How- The introduction of a ‘floating floor’ ever, it is a recognised fact that a noise

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