Noise Source of Volvo Generator Engine
by Starlight Generator dieselgeneratortechVolvo Generator Engine noise
Engine noise is the noise emitted
from the engine structure. The pie chart below shows how large a relative part
of total engine noise emissions given engine components have. Engine noise is
created by forces that arise during the combustion process and the mechanical
shocks between pistons and liners, valves and gear wheels etc. When a new
engine is designed, a great deal of effort is put into minimizing engine noise.
The most important means for achieving this is optimizing the injection system,
controlling engine block stiffness and special treatment of the gear wheels in
the engine timing system. Sound dampening materials can be placed on large
noise emitters such as the oil sump and valve cover. Engine noise in an
installation is usually reduced by screening off the engine. Such screening can
be made even more effective by lining it with absorbent material. Usually, the
level of screening is governed by engine cooling capacity. Finally, engine
noise is to a large extent dependent on engine speed. For a given power an
engine with low rpm is preferable to one with high rpm.
Exhaust Noise
Without a muffler, exhaust noise is the absolute largest source of noise on a diesel engine. High pressure pulses from combustion causes the exhaust flow to oscillate. On a six-cylinder four-cycle engine, exhaust noise basic frequency is equivalent to three times engine rpm. This means that the muffler in most applications must have a high silencing capability across a wide frequency range. The two most usual diesel engine muffler types used are:
- Absorbing mufflers
Absorbing mufflers have a chamber
that is covered with sound absorbing material. Acoustic energy is transformed
into heat in the absorbent material. This type of muffler has its best
dampening ability in the medium to high frequency range.
- Expansion mufflers
The inside of the muffler is
constructed so that sound is reflected, which reduces the acoustic force
transmitted through the muffler. By allowing the pipe to discharge into a
chamber and then leading exhaust gasses through several chambers, good sound
dampening is achieved across a broad frequency range.
The price paid for good sound
dampening in both muffler types is a higher back pressure in the exhaust
system. The development of mufflers is often a compromise between acceptable
back pressure and necessary noise reduction. Volvo Penta provides both muffler
types. Their use ensures large noise reduction and low back pressure.
Inlet Noise
As is the case with the exhaust
system, pressure variations in the inlet system causes the airflow to
oscillate. These pressure variations are of course much lower than those in the
exhaust system. On modern turbocharged engines the compressor functions as an
effective sound muffler. The pressure waves (noise) are dispersed by the
rotating compressor blades. This means that inlet noise does not have any
practical significance on Volvo Penta engines.
Auxiliary Noise
The cooling fan is one of an
engine's largest noise sources. According to measurements the sound power level
increases by 2 to 3 dBA when a fixed fan is used. Because fan noise is relative
to engine rpm, it is necessary for the radiator to have a large capacity. An
efficient radiator allows fan speed, and thus also noise, to be reduced. On
applications where maximum cooling capacity is not a constant requirement,
thermostatically or viscously coupled fans may be used. Viscously coupled fans
are available as accessories for most Volvo Penta engines for mobile
applications. Other equipment such as generators and air compressors have less
significance compared to the fan when it comes to noise.
Structure Borne Noise
In many applications engine
vibrations that affect the engine bed can cause sound emissions from connected
structures. In many installations this can be a bigger problem than noise from
the engine itself. By using resilient engine mounts the forces influencing the
engine bed are reduced and therefore also frame noise. In the case of
insulation for heavy vibrations, the engine bed must be at least 10 times as
stiff as the insulator.
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Created on Jul 27th 2018 04:27. Viewed 301 times.