When a diesel generator set is operating, it typically generates noise of 95~110dB(A). The operating noise can cause serious impact on the surrounding environment, so it must be controlled. A low-noise generating set mainly consists of a standard generating set, an acoustic enclosure, inlet and exhaust air noise reduction devices and an exhaust noise reduction device. By installing sound insulation and sound-absorbing layers in the acoustic enclosure, applying noise attenuation treatment to the inlet and exhaust air passages, and using a combination of dissipative and reactive silencers for the exhaust, high- and low-frequency noise can be reduced respectively.

1. Control of exhaust noise

(1) Main causes of exhaust noise

Exhaust noise is the primary component of engine noise. It is generally 10~15dB(A) higher than the engine itself. Exhaust noise is the most energetic and compositionally complex part of engine noise. Its fundamental frequency is the engine firing frequency, and the exhaust noise spectrum typically shows the fundamental frequency and its higher harmonics.

The main components of exhaust noise are as follows:

1) Periodic exhaust noise, in which the frequency of low-frequency pulsation noise caused by exhaust flow is generally 63~125Hz, with noise levels as high as 105~125dB(A).

2) Resonance noise of the air column inside the exhaust pipe.

3) Resonance noise of the cylinder.

4) Jet noise generated when high-speed airflow passes through the annular gap of the exhaust valve and through tortuous piping.

5) Vortex noise, together with regenerated noise in the exhaust system under excitation from pressure waves inside the pipe, forms a continuous high-frequency noise spectrum with frequencies above 1000Hz. As airflow velocity increases, the frequency rises significantly.

(2) Methods for controlling exhaust noise

Silencers are the basic method for controlling exhaust noise. Proper selection of a silencer (or silencer combination) can reduce exhaust noise by more than 30~40dB(A). According to silencing principles, silencers can be divided into two major categories: dissipative silencers and reactive silencers:

1) Dissipative silencer

A dissipative silencer uses porous sound-absorbing materials arranged in a certain manner inside the duct. When airflow passes through the dissipative silencer, the sound waves cause the air and fine fibers in the pores of the sound-absorbing material to vibrate. Due to friction and viscous resistance, sound energy is converted into heat and absorbed, thereby achieving noise reduction.

2) Reactive silencer

A reactive silencer uses an appropriate combination of ducts of different shapes and resonant chambers. Through reflection and interference caused by acoustic impedance mismatch resulting from changes in pipe cross-section and shape, it achieves noise attenuation. Its silencing effect is related to the pipe shape, size and structure. In general, it is highly selective and suitable for attenuating narrow-band noise and low- to mid-frequency noise.

Usually, a combination of a corrugated vibration isolator, a dissipative silencer and a reactive silencer can effectively block the transmission of exhaust vibration and exhaust noise.

2. Control of mechanical noise and combustion noise

(1) Causes of mechanical noise and combustion noise

1) Mechanical noise

Mechanical noise is mainly generated by vibration and mutual impact caused by periodic changes in gas pressure and inertial forces acting on the engine's moving parts during operation. It mainly includes noise from the piston-crank-connecting rod mechanism (mainly high-frequency noise); noise from the valve train mechanism (mainly low- and mid-frequency noise); transmission gear noise (whose noise spectrum is continuous and broad); and mechanical vibration and noise caused by unbalanced inertial forces.

2) Combustion noise

Combustion noise refers to structural vibration and noise generated during the combustion process. The sound pressure level of combustion noise inside the cylinder, especially in the low-frequency range, is very high. However, most engine components have high stiffness, and their natural frequencies are mostly in the mid- to high-frequency range. Due to the mismatch in frequency response to sound wave transmission, the very high peak cylinder pressure levels in the low-frequency range cannot be transmitted easily, whereas the cylinder pressure levels in the mid- and high-frequency range are relatively easier to transmit.

(2) Methods for controlling mechanical noise and combustion noise

1) Vibration isolation treatment

High-efficiency vibration-damping rubber pads are generally used for generator set vibration isolation. After vibration isolation treatment, surface vibration of the generator set is effectively interrupted.

2) Noise reduction treatment

Noise reduction treatment is carried out along the noise transmission path to reduce radiation from the sound source to the outside. For generator rooms with strict noise index requirements, high-efficiency sound-absorbing materials should also be applied indoors so that noise can be effectively attenuated and the generator room's overall noise reduction performance can be improved.

3. Control of cooling fan and exhaust air duct noise

Fan noise consists of rotational noise and vortex noise. Rotational noise is caused by periodic disturbances generated when rotating fan blades cut through the airflow. Vortex noise occurs when airflow separates over the cross-section of the rotating blades and, due to gas viscosity, slips off or breaks into a series of vortex flows, thereby radiating unstable flow noise. The exhaust air duct is directly connected to the outside, with high air velocity, so airflow noise, fan noise and mechanical noise are all radiated outward through this duct.

The main means of controlling fan and exhaust air duct noise is to design an effective exhaust air sound-absorbing passage. The sound-absorbing passage may consist of an air guide channel and an exhaust air noise reduction box, or of an air guide channel plus one or several sets of sound-absorbing baffles. The operating principle of the exhaust air noise reduction box is similar to that of a dissipative silencer. Sound absorption performance can be improved by replacing the absorbing material (changing its sound absorption coefficient) and by adjusting parameters such as the thickness of the absorbing material and the length and width of the exhaust air duct. When designing the exhaust air sound-absorbing passage, special attention must be paid to ensuring that the effective area of the exhaust outlet meets the heat dissipation requirements of the generator set, to avoid increased outlet resistance, which could otherwise increase exhaust noise and cause high coolant temperature shutdown.

4. Control of intake noise

Generator sets operate inside enclosed generator rooms. Broadly speaking, the intake system includes the generator set's fresh air inlet passage and the engine intake system. Like the exhaust air passage, the fresh air inlet passage is directly connected to the outside, and the air velocity is high. Airflow noise and generator operating noise are radiated outward through the intake passage. Noise from the engine intake system is caused by pressure pulsations generated by the periodic opening and closing of the intake valves, and its frequency generally falls within the low-frequency range below 500Hz.

For turbocharged engines, because the turbocharger speed is very high, their intake noise is significantly higher than that of naturally aspirated engines. Turbocharger compressor noise consists of rotational noise generated by periodic blade impacts on the air and vortex noise formed by high-speed airflow. It is a continuous high-frequency noise, with its main energy generally distributed in the 500Hz~10kHz range.

The air filter fitted to a diesel generator set inherently provides a certain silencing effect. Given that intake noise is relatively low, the engine intake system is generally not specially treated. For the generator set's intake passage, however, overall control should be implemented from aspects such as duct design and the selection of sound insulation materials. The basic approach is:

1) The net inlet area should comply with design specifications to ensure sufficient fresh air intake for both the engine intake system and the generator set cooling system.

2) The intake passage should be acoustically treated, generally by using a combination of intake louvers, air guide channels and silencing baffles. Where space permits, a combination of intake louvers and a noise reduction box can also be used.

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