Do you know where the noise in the operation of the diesel generator set comes from?

Many customers have a headache about the operating noise of diesel generator sets. If you want to deal with this problem, the key is to find the source of the noise. So, let’s follow Haixing Power to see the common noise sources and categories.

The main noise sources of diesel generator sets are the exhaust noise of diesel engines, intake noise, cooling fan noise, combustion noise, mechanical noise and electromagnetic noise of generators, and vibration during unit operation.

1) Mechanical noise

The mechanical noise of a diesel engine is a noise stimulated by impact and vibration caused by the action of gas pressure and the inertia of the machine parts. Mechanical noise mainly includes piston strike noise, gear mechanism noise, air distribution mechanism noise, bearing noise, high-pressure oil pump noise, body vibration and noise caused by unbalanced inertia forces, etc.

The mechanical noise of diesel engines increases rapidly with the increase of rotational speed. When operating at low speed, mechanical noise is not important compared to other noises; but when operating at high speed, mechanical noise is often the main source of noise.

2) Intake noise

Intake noise is one of the main aerodynamic noise sources of diesel engines. It is formed by the pressure fluctuations in the intake pipe caused by the periodic opening and closing of the intake valve. When the intake valve is opened, a pressure pulse is generated in the intake pipe. As the piston continues to move, the pressure wave is quickly damped; when the intake valve is closed, a pressure pulse that lasts for the corresponding time is also generated, which is also damped and disappears quickly. During the operation of the diesel engine, the two pressure pulses appear alternately, which creates periodic intake noise.

For non-supercharged diesel engines and gasoline engines, the intake noise is lower than the exhaust noise, close to the level of mechanical noise generated by moving parts of the internal combustion engine.

For large diesel generators with superchargers, superchargers produce strong high-frequency noise. The airflow pulsation fundamental frequency noise generated by the supercharger during inhalation and its respective harmonic noise and the strong eddy current noise of the air in the intake pipe can be up to about 130dB.

3) Exhaust noise

When the diesel engine is working, the high-temperature and high-pressure exhaust gas in the cylinder is periodically injected into the exhaust pipe as the exhaust valve is disconnected and closed. The exhaust pipe port discharges the high-temperature and high-speed pulsating air flow, thereby generating periodic exhaust noise. Exhaust noise is the main source of noise for diesel engines. Its intensity is related to factors such as power and speed of diesel engines, and varies with the change of the speed and load of the diesel engine.

The spectrum of diesel engine exhaust noise shows obvious medium and low frequencies, but the high frequencies have also reached the corresponding level. The intermediate frequency noise is caused by the higher harmonics of the fundamental frequency. High-frequency noise mainly refers to the turbulent sound generated during exhaust, the combustion and explosion sound in the cylinder, and the noise added to impact, vibration of the machine part, and the self-vibration of the pipe wall.

The exhaust noise spectrum often contains the following frequency components: exhaust noise with the number of exhaust gases per second as the base frequency, gas column resonance noise in the pipeline, cylinder Helmholtz resonance noise, exhaust injection and impact noise, turbulence noise at the inner wall of the exhaust system pipeline, and Carmen vortex noise on the back of the gas valve stem, etc.

A. Basic frequency exhaust noise.

Basic frequency noise is caused by the fact that when the exhaust valve of each cylinder of the diesel engine is opened, the combustion gas in the cylinder suddenly ejects at a high speed, and the airflow hits the gas near the air valve in the exhaust channel, causing it to generate a drastic pressure change and form a pressure wave, thereby stimulating noise. This noise is a typical low-frequency noise. The fundamental frequency is related to the number of exhaust gases per second, that is, it is the same as the cylinder burst frequency.

In the exhaust noise spectrum, peaks usually occur near the fundamental frequency f0 or its second and third harmonics 2f0 and 3f0. When the frequency is high, the exhaust noise sound pressure level with the exhaust frequency as the fundamental frequency is not large. According to the frequency domain analysis of the exhaust noise signal, it can be seen that exhaust noise is mainly composed of harmonics in the low frequency band, and is composed of a series of discrete harmonics.

Generally speaking, the main component of diesel engine exhaust noise is periodic noise with the diesel engine ignition frequency as the base frequency, but there is clutter in the middle. This is because the combustion conditions of each cylinder and the wear of each exhaust valve are not the same, so the exhaust noise periods of each cylinder are slightly different.

B. Air column resonance noise

The air column in the exhaust system pipeline generates air column resonance noise due to resonance due to periodic exhaust noise. The acoustic components of the air column in the exhaust pipe include resonant sound caused by various parts inside the exhaust pipe, tail pipe, and muffler.

In the diesel engine exhaust system, a gas column with a closed valve that is connected to the atmosphere through the exhaust passage and exhaust pipe to the atmosphere forms a closed air column at one end (valve end) and an open one end (atmospheric end). Usually, noise peaks appear at its natural frequency and its higher harmonics.

C. Injection noise, turbulent noise and eddy

current noise The fundamental frequency noise is caused by fluctuations in the exhaust air flow velocity. Even if the airflow injection speed is constant, noise will be stimulated, called spray injection noise, whose sound power is proportional to the 8th power of the airflow velocity, and its frequency is proportional to the first time of the airflow velocity.

The friction between high-speed airflow and cylinder walls and pipe walls will lead to turbulence. When the airflow passes through throttle elements such as throttle valves, valves and nozzles, it will also cause turbulence. When the airflow flows through the valve stem, it will also deflow and form a vortex. These will cause noise. These noises form the medium and high-frequency part of the exhaust noise, and its frequency distribution moves to the high-frequency band as the internal combustion engine speed increases.

D.

When the exhaust valve of the Helmholtz resonance noise internal combustion engine is opened, the cylinder forms a Helmholtz resonance cavity, the exhaust passage and the exhaust pipe, which has a resonance frequency. The frequency component consistent with the resonant frequency in the noise excitated by the exhaust gas flow is sufficiently amplified in this resonance cavity.

The characteristic of Helmholtz resonance noise is that its frequency is independent of the speed of the internal combustion engine, but its resonance frequency varies with the working volume of the cylinder. Helmholtz resonance noise is more prominent in single-cylinder engines and can also be found in two-cylinder engines and three-cylinder engines. However, in multi-cylinder engines with more than four-cylinder engines, the exhaust branch and main pipe are longer, so the noise is not prominent.

4) Fan noise

Fan noise consists of rotational noise and eddy current noise. Rotating noise, also known as blade noise, is caused by the rotating blade periodically hitting the air particles, causing the pressure pulsation of the air to produce noise. Its basic frequency f1=nZ/60 (n is the fan speed; Z is the number of blades). When the fan rotates, the surrounding gases produce vortexes, which split into a series of discrete small vortexes due to the action of viscous forces. Eddy current and vortex splitting disturbs the air, forming a compression and sparse process and producing eddy current noise, generally wideband noise. When a muffler is installed in the inlet and exhaust pipe of the internal combustion engine, the cooling fan becomes an important source of noise. Fan noise can generally reach about 100dB(A).

5)

The structural vibration caused by the combustion of the combustion noise mixer is transmitted to the surface of the internal combustion engine through external and internal transmission channels, and air sound is formed by radiation from the surface of the internal combustion engine, which is called combustion noise.

During the combustion process of the diesel engine, after the diesel oil is sprayed into the combustion chamber in an oil mist, it mixes with air in a short time and then burns on its own after it is mixed with the air in a very short time. The air pressure in the cylinder rises rapidly to 6 to 9MPa, and the temperature also rises to 2000 to 2500K. While the combustion is carried out, combustion noise is generated. Compared with gasoline engines, diesel engines have a higher compression ratio, generally 16-22, and gasoline engines are generally 6-9, so the combustion noise of diesel engines is much higher than that of gasoline engines.

6) Electromagnetic noise

Electromagnetic noise is caused by alternating electromagnetic fields causing vibrations in certain mechanical components or space volumes. The main characteristics of electromagnetic noise are related to factors such as alternating electromagnetic field characteristics, forced vibrating components and the size and shape of space. The noise emitted by motors, generators, transformers, neon ballasts, etc. is typical electromagnetic noise.

7) Vibration

diesel engine is a reciprocating machine, and vibration during operation is inevitable. In addition to radiating noise directly into the space, the vibration generated during the operation of the generator set also spreads on the foundation surface connected to the unit. During the propagation process, the vibrations will arouse vibrations of the foundation, walls, ceilings, doors and windows, pipes, etc., and radiate noise again on the surfaces of these objects, generating solid sound. Therefore, it is very necessary to perform vibration control of diesel engine units.

The noise source analysis

The noise source in the machine room comes from the diesel generator set. Referring to the above analysis of the main noise sources of the generator set, the main noise sources of the unit include: fan noise, inlet and exhaust noise, combustion noise, mechanical noise and electromagnetic noise and unit vibration sound transmission, etc.

The main ways for the computer room noise to spread outward are the sound transmission of the exhaust duct outlets, inlets and exhaust vents, doors and walls.
According to empirical analysis, when the high-power generator set is started at full load, the indoor noise can be as high as 100dB(A). Although the noise in the factory boundary and sensitive point has attenuated, the sound pressure level is still high (because the unit has not yet been in place, the above is empirical data), which has a relatively large impact on the surrounding sound environment. According to theoretical analysis and relevant experience, the noise frequency of high-speed diesel engines (>800rpm) shows a higher sound level in the range of 800 to 5000Hz, and at the same time, due to the influence of cooling fans, there are also higher sound levels in the range of 125 to 500Hz. Since the unit is located on the negative floor, the noise transmission through the wall has little impact on the sensitive points, while fan noise, exhaust noise and mechanical noise propagate outward through the air outlet as the main ways.

The above is the main source and manifestation of the noise of diesel generator sets. Only by clarifying the source of the noise can targeted measures be carried out to reduce or avoid environmental pollution caused by noise, not only create a good working environment for employees, but also ensure that production does not affect surrounding residents or residents.