Views: 0 Author: Site Editor Publish Time: 2016-09-13 Origin: Site
Many customers have a headache when it comes to the operating noise of diesel generator sets. To deal with this problem, the key is to identify the source of the noise. Therefore, let’s follow Haixing Power to take a look at the common sources and classification of noise.
The main noise sources of diesel generator sets are exhaust noise, intake noise, cooling fan noise, combustion noise, mechanical noise, electromagnetic noise of the generator, and vibration of the unit during operation.
1) Mechanical noise
The mechanical noise of a diesel engine is caused by the impact and vibration of relatively moving parts caused by the gas pressure and the inertia of the machine parts. Mechanical noise mainly includes piston knocking noise, gear mechanism noise, valve train noise, bearing noise, high-pressure oil pump noise, body vibration and noise caused by unbalanced inertial force, etc.
The mechanical noise of diesel engines increases rapidly with the increase of rotation speed. When running at low speed, mechanical noise is not important compared with other noises; but when running at high speed, mechanical noise is often the main noise source.
2) Intake noise
Intake noise is one of the main aerodynamic noise sources of diesel engines. It is caused by the pressure fluctuations in the intake pipe caused by the periodic opening and closing of the intake valve. When the intake valve opens, a pressure pulse is generated in the intake pipe. As the piston continues to move, this pressure wave is quickly damped; when the intake valve is closed, a pressure pulse lasting a corresponding period of time is also generated, which is also affected by Damping and disappears quickly. During the operation of the diesel engine, these two pressure pulses appear alternately, which forms periodic intake noise.
For non-supercharged diesel and gasoline engines, the intake noise is lower than the exhaust noise and is close to the level of mechanical noise generated by the moving parts of the internal combustion engine.
For large diesel generator sets with superchargers, the superchargers will produce strong high-frequency noise. The airflow pulsation fundamental frequency noise and its harmonic noise generated when the supercharger is inhaled are superimposed with the strong eddy noise of the air at the intake pipe, which can reach 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 intermittently closed. The exhaust pipe discharges high-temperature and high-speed pulsating airflow, thus producing periodic exhaust noise. Exhaust noise is the main noise source of diesel engines. Its intensity is related to factors such as the power and speed of the diesel engine, and changes with the changes in the speed and load of the diesel engine.
The spectrum of diesel engine exhaust noise shows obvious mid- to low-frequency characteristics, but high frequencies also reach a corresponding level. Intermediate frequency noise is caused by higher harmonics of the fundamental frequency. The high-frequency noise is mainly the turbulence sound generated during exhaust, the sound of combustion and explosion in the cylinder, as well as the additional noise caused by impact, vibration of machine parts, and self-vibration of the pipe wall.
The exhaust noise spectrum often includes the following frequency components: exhaust noise with the number of exhausts per second as the fundamental frequency, air column resonance noise in the pipeline, cylinder Helmholtz resonance noise, exhaust gas injection and impact noise, exhaust gas Turbulence noise on the inner wall of the system pipe and Karman vortex noise on the back of the valve stem, etc.
A. Fundamental frequency exhaust noise
Fundamental frequency noise is due to the fact that when the exhaust valve of each cylinder of the diesel engine is opened, the gas in the cylinder is suddenly ejected at high speed, and the air flow impacts the gas near the valve in the exhaust passage, causing a drastic change in pressure to form a pressure wave. Excite noise. This kind of noise is a typical low-frequency noise. The fundamental noise frequency is related to the number of exhaust gases per second, that is, it is the same as the cylinder explosion frequency.
On the exhaust noise spectrum, peaks usually appear near the fundamental frequency f0 or its second and third harmonics 2f0 and 3f0. When the frequency is higher, the exhaust noise sound pressure level based on the number of exhausts as the fundamental frequency is not large. Frequency domain analysis of the exhaust noise signal shows that the exhaust noise is mainly composed of harmonics in the low frequency band and consists of a series of discrete harmonics.
Normally, the main component of diesel engine exhaust noise is periodic noise with the ignition frequency of the diesel engine as the fundamental 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 each cylinder The period of exhaust noise is slightly different.
B. Air column resonance noise
The air column in the exhaust system pipe resonates due to the excitation of periodic exhaust noise, resulting in air column resonance noise. The components of the air column sound in the exhaust pipe include the resonance sound caused by various parts inside the exhaust pipe, tail pipe, and muffler.
In the diesel engine exhaust system, from the closed valve through the exhaust duct and exhaust pipe to the atmosphere, an air column is formed with one end closed (valve end) and one end open (atmosphere end). Usually at its natural frequency and its higher harmonics Noise peaks appear at the wave.
C. Injection noise, turbulence noise and eddy noise
Fundamental frequency noise is caused by fluctuations in exhaust gas flow velocity. Even if the airflow injection speed is constant, noise will be excited, called injection noise. Its sound power is proportional to the eighth power of the airflow speed, and its frequency is proportional to the first power of the airflow speed.
The friction between the high-speed air flow and the cylinder wall and tube wall will cause turbulence. The air flow will also form turbulence when it passes through throttling components such as the throttle, valve and nozzle. When the air flow passes through the valve stem, it will break off and form a vortex. These will These noises constitute the mid- to 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. Helmholtz resonance noise
When the exhaust valve of an internal combustion engine is opened, the cylinder, exhaust passage, and exhaust pipe form a Helmholtz resonance cavity, which has a resonance frequency. The frequency component consistent with the resonance frequency in the noise excited by the exhaust gas flow is fully amplified in this resonance cavity.
The characteristic of Helmholtz resonance noise is that its frequency has nothing to do with the engine speed, but its resonance frequency changes with the cylinder working volume. Helmholtz resonance noise is more prominent in single-cylinder engines, and can also be found in two-cylinder and three-cylinder engines. However, in multi-cylinder engines with four or more cylinders, due to the mutual interference between the cylinders, the exhaust gas The branch pipes and main pipes are longer, so the noise is not prominent.
4) Fan noise
Fan noise consists of rotational noise and eddy current noise. Rotating noise is also called blade noise. It is caused by the rotating blades periodically hitting the air particles, causing the pressure pulsation of the air to produce noise. Its fundamental frequency is f1=nZ/60 (n is the fan speed; Z is the number of blades). When the fan rotates, the surrounding gas generates a vortex, which is split into a series of discrete small vortices due to the viscous force. Eddy currents and vortex splitting cause disturbance in the air, forming compression and rarefaction processes to produce eddy current noise, which is generally broadband noise. When a muffler is installed on the intake and exhaust pipes of an internal combustion engine, the cooling fan becomes an important source of noise. Fan noise can generally reach about 100dB(A).
5) Combustion noise
The structural vibration caused by the in-cylinder gas produced by the mixer combustion is transmitted to the surface of the internal combustion engine through external and internal transmission channels, and is radiated by the surface of the internal combustion engine to form airborne sound, which is called combustion noise.
During the combustion process of the diesel engine, after the diesel oil is sprayed into the combustion chamber in the form of oil mist, under the action of high temperature and high pressure, it mixes with air in a short time and then ignites and burns on its own. The air pressure in the cylinder rises rapidly to 6~9MPa, and the temperature also increases. When the temperature rises to 2000~2500K, combustion noise will be generated while combustion is doing work. Compared with gasoline engines, diesel engines have a higher compression ratio, generally 16 to 22, while gasoline engines generally have a compression ratio of 6 to 9, so the combustion noise of diesel engines is much higher than that of gasoline engines.
6) Electromagnetic noise
Electromagnetic noise is generated by the alternating changes in the electromagnetic field that cause certain mechanical parts or space volumes to vibrate. 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 and neon light ballasts is typical electromagnetic noise.
7) Vibration
The diesel engine is a reciprocating machine, and vibration during operation is inevitable. In addition to directly radiating noise into space, the vibrations generated by the generator set during operation also propagate on the foundation surface connected to the generator set. During the propagation process, they will arouse vibrations in the foundation, walls, ceilings, doors, windows, pipes, etc. In these The surface of the object radiates noise again, producing solid-state sound. Therefore, it is very necessary to control the vibration of diesel engine units.
Noise source analysis
The noise source of 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 are: fan noise, intake and exhaust noise, combustion noise, mechanical noise and electromagnetic noise and the unit Vibration sound transmission, etc.
The main channels through which the noise in the computer room spreads outward include the flue exit, air inlet and exhaust vents, and sound transmission through doors and walls.
According to empirical analysis, when a high-power generator set is started at full load, the indoor noise can reach more than 100dB(A). Although the noise at the factory boundary and sensitive points has been attenuated, the sound pressure level is still high (because the unit is not yet in place, the above is Empirical data), which has a relatively large impact on the surrounding acoustic environment. According to theoretical analysis and relevant experience, the noise frequency of high-speed diesel engines (>800rpm) presents higher sound levels in the range of 800 to 5000Hz. At the same time, due to the influence of the cooling fan, there are also higher sound levels in the range of 125 to 500Hz. Since the unit is located on the negative floor, the outward transmission of noise through the wall has little impact on sensitive points, while fan noise, exhaust noise and mechanical noise are the main channels for outward transmission through the air vents.
The above are the main sources and manifestations of noise from diesel generator sets. Only by clarifying the source of noise can we take targeted measures to reduce or avoid environmental pollution caused by noise, which not only creates a good working environment for employees, but also ensures Production will not affect surrounding households or residents.
