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Research on noise control technology of automobile engine

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Noise is a by-product of industrial society, and it is considered to be the three major public hazards in the world together with air pollution and water pollution. Compared with the other two public hazards, noise has the widest influence, the most direct feeling and the most reflected by people. As a major means of transportation, cars are becoming more and more popular and growing, so the environmental pollution caused by car noise is becoming more and more serious. Because the noise generated by engine accounts for a large part of automobile noise, it is particularly important to study the mechanism of engine noise and noise control measures in automobile noise control.

1 Noise control of engine

The sound transmitted directly from the engine block and its main accessories to the space belongs to engine noise. Engine noise varies with different random types, speeds, loads and operating conditions. For example, at the same speed, the noise of diesel engine is higher than that of gasoline engine. According to the nature of noise, engine noise can be divided into combustion noise, mechanical noise and aerodynamic noise. The following mainly introduces the causes of various noises and some specific noise reduction measures.

1. 1 combustion noise

1. 1. 1 combustion noise mechanism

Combustion noise is caused by the periodically changing gas pressure in the cylinder. Mainly depends on the way and speed of combustion. In a gasoline engine, if abnormal combustion such as deflagration and surface ignition occurs, it will produce large combustion noise. The combustion noise of diesel engine is caused by the sudden increase of air pressure in the combustion chamber, which causes the vibration of various parts of the engine. Generally speaking, the noise of diesel engine is much higher than that of gasoline engine, so here we mainly take diesel engine as an example to explain how to reduce the combustion noise.

1. 1.2 combustion noise control strategy

In automobile engines, combustion noise accounts for a large proportion of the total noise, so it is of great significance to study how to reduce its combustion noise. At present, the noise reduction measures mainly include:

In the space atomization combustion system, (1) heat insulating piston is used to increase the wall temperature of combustion chamber, shorten the ignition delay period and reduce the combustion noise of direct injection diesel engine.

(2) Increasing the compression ratio and applying exhaust gas recirculation technology can also reduce the combustion noise of diesel engines. But the compression ratio mainly determines the mechanical load and thermal load level of diesel engine. By reducing the maximum cylinder pressure, the exhaust gas recirculation technology can not only suppress the production of nitrogen oxides, but also reduce the combustion noise.

(3) Pre-injection is realized by using double-spring fuel injection valve. That is, the fuel originally intended to be injected once in one cycle is injected twice. For the first time, a small part is injected first, and the pre-reaction of ignition is started before the main injection, which can reduce the accumulation of combustible mixture in the ignition delay period. This is the most effective measure to reduce the combustion noise of direct injection diesel engine. By reducing the initial opening pressure of the double-spring injector and the pre-lift of the needle valve, the formation of mixed gas can be suppressed, thus affecting the combustion noise at idle speed. By designing a two-stage lifting device, the pilot injection device is used to suppress combustion noise in a large speed range and acceleration.

(4) * * rail fuel injection system is a promising electronic control high-pressure fuel injection system for direct injection car diesel engine, which can reduce the fuel injection quantity in ignition delay period, especially for reducing combustion noise.

(5) adopt pressurization. After the diesel engine is supercharged, the charging density, temperature and pressure entering the cylinder increase, thus improving the ignition conditions of the mixture and shortening the ignition delay period. Although the maximum explosion pressure of supercharged diesel engine has increased, its pressure growth rate dp/dφ and pressure increase ratio λ have decreased, which makes the diesel engine run smoothly and reduce noise. In addition, generally speaking, the maximum rated power of turbocharged diesel engine is lower than that of non-turbocharged diesel engine in the same cylinder, which is beneficial to reduce combustion noise. After the charge air is cooled, the air temperature decreases and the charging efficiency increases, but at the same time, the effect of reducing combustion noise by charging is weakened.

(6) Selection and design of combustion chamber. For split-type combustion chamber, accurate fuel injection channel, increased channel area, control of injection direction and optimization of swirl radius at the inlet of precombustion chamber can all restrain premixed combustion and facilitated diffusion combustion, thus reducing combustion noise, fuel consumption and soot emission in a wide range from low load to high load.

For the direct injection combustion chamber, reasonable design can make it have higher turbulence kinetic energy under sufficient swirl, strengthen the diffusion of fuel and air, thus improving the combustion process and realizing low fuel consumption, low noise and low emission of diesel engine.

The structure of piston top combustion chamber has great influence on combustion noise. If the nozzle hole is smaller and deeper, the combustion noise is much smaller and the emission is obviously better. Coupled with the necking shape, the noise reduction effect will be better. Therefore, it is best to choose ω-shaped combustion chamber with necking when designing, and it should be as deep as possible within the allowable range of variation.

(7) Reduce the fuel supply advance angle. The fuel supply advance angle is small, the fuel injection time is delayed, the temperature and pressure in the cylinder are high when the fuel is injected, the fuel is atomized as soon as it is injected, and it reaches the ignition point instantly, which shortens the ignition delay period. The fuel injected first explodes and burns, and the fuel injected into the flame later will not burn immediately due to lack of oxygen. In this way, since the amount of fuel burned in the initial stage is small and the pressure rise rate is low, the combustion noise can be reduced. The combustion noise of most diesel engines decreases with the decrease of fuel supply advance angle.

(8) When the fuel with high cetane number is selected, the ignition delay period is short, which affects the formation of combustible mixture during the ignition delay period, reduces the pressure rise rate and reduces the combustion noise.

1.2 mechanical noise

Mechanical noise is caused by periodic mechanical motion between moving parts and between moving parts and fixed parts, which is related to the magnitude of excitation force and the structure of moving parts. There are mainly piston knocking noise and valve mechanical noise.

1.2. 1 piston knocking sound

When the engine is running, the piston moves laterally from one side to the other under the action of lateral force near the upper and lower dead center, thus forming a strong knock on the cylinder wall and generating piston knocking noise. The main causes of knocking are the gap between the piston and the cylinder liner and the gas pressure acting on the piston.

Measures to reduce piston knocking noise are:

(1) The piston pin hole is offset, that is, the piston pin hole is appropriately offset to the main thrust surface 1 ~ 2mm.

(2) The skirt of the piston is provided with a transverse heat insulation groove, the adjusting steel sheet is embedded in the piston pin seat, and the steel cylinder is embedded in the skirt, so that the cylinder gap of the piston at 40℃ is reduced.

(3) Increasing the rigidity of the cylinder liner can not only reduce the knocking sound of the piston, but also reduce the noise caused by the friction between the piston and the cylinder wall. In order to increase the rigidity of the cylinder liner, the method of increasing the thickness of the cylinder liner or setting the reinforcing ribs can be adopted.

(4) Improving the lubrication between the piston and the cylinder wall and increasing the damping when the piston knocks on the cylinder wall can also reduce the knocking noise of the piston. For example, on a D= 180mm single cylinder testing machine, the vibration of the engine block is reduced by 6dB(A) by spraying engine oil on the cylinder wall with special lubricating oil. Obviously, this measure is limited in practice. Recently, Maru Energy Company of Japan has successfully developed a new lubricant containing ceramic particles, which forms a "ceramic membrane" on the surface of cylinder metal to prevent direct contact between metals. Therefore, while reducing friction noise, it can also improve lubrication performance, save fuel and prolong service life.

1.2.2 Transmission gear noise

The noise of transmission gear is caused by the impact and friction between teeth in the process of gear meshing. On the internal combustion engine, the gear bears alternating dynamic load, which will deform the shaft. The shaft will cause dynamic load on the bearing, and the dynamic load of the bearing will be transmitted to the engine housing and the gear room housing, which will make the housing excite noise. In addition, the torsional vibration of crankshaft will destroy the normal meshing of gears and excite noise. The noise of transmission gear is related to the design parameters, structural type, machining accuracy, gear material matching, gear chamber structure and running state.

Measures to reduce transmission gear noise are as follows:

(1) Control gear tooth profile, improve gear machining accuracy and reduce gear meshing clearance, that is, reduce the energy when gears collide with each other, thus reducing gear meshing transmission noise.

(2) Using new materials, such as high damping engineering plastic gear, replacing the original steel gear with engineering plastic gear, the noise of the whole machine is reduced by about 0.5dB(A), and the effect is obvious.

(3) Reasonably arranging the position of the gear transmission system, such as arranging the timing gear at the flywheel end, can effectively reduce the influence of torsional vibration of the crankshaft system on gear vibration.

(4) Using timing gear synchronous belt drive instead of timing gear rotation can obviously reduce noise.

1.2.3 reduce the noise of the valve train

Most internal combustion engines use cam and valve mechanism, which includes camshaft, tappet, push rod, rocker arm, valve and other parts. The valve train has many parts and poor rigidity, and it is easy to cause vibration and noise in the process of movement, including the impact between the valve and the valve seat, the transmission impact caused by the valve clearance, the friction vibration between the tappet and the cam working surface, and the noise caused by the irregular movement of the valve at high speed. The noise of valve train is related to the type of valve train, valve clearance, valve seating speed, material, cam profile, lubrication state of cam and tappet, internal combustion engine speed and other factors.

Measures to reduce the noise of valve train mainly include:

(1) Good lubrication can reduce friction and noise. It is suggested that the minimum oil film thickness between cam and tappet is 2Lm at idle speed and 3Lm at 1000r/min. The higher the cam speed, the thicker the oil film. Therefore, when the internal combustion engine runs at high speed, the friction vibration and noise of the valve train are not prominent.

(2) Reducing the valve clearance can reduce the impact between the rocker arm and the valve, but it cannot make the valve clearance too small. Using hydraulic support column can fundamentally eliminate valve clearance and reduce noise. In recent years, valve hydraulic drive system has also appeared, with lower noise.

(3) Shortening the length of the push rod is an effective measure to reduce the weight of the system and improve the stiffness. The overhead camshaft cancels the push rod, which is especially beneficial to reduce noise.

1.3 aerodynamic noise

The noise generated by gas disturbance and the interaction between gas and other objects is called aerodynamic noise, including intake noise, exhaust noise and fan noise in the engine.

1.3. 1 intake noise

When the engine is working, the high-speed airflow enters the cylinder through the air filter, intake pipe and valve, which will produce strong aerodynamic noise, sometimes about 5 dB(A) higher than the noise of the engine itself, and become the main noise source after the exhaust noise. Noise increases with the increase of engine speed, regardless of the change of load. Its main components include: periodic pressure pulsation noise, eddy current noise, cylinder Helmholtz vibration noise and intake pipe column vibration noise.

The control strategies of intake noise mainly include:

(1) Reasonable design and selection of air filter. The reasonable design of intake pipe and intake port of cylinder head can reduce the pressure pulsation intensity in the intake system and the eddy current intensity at the valve passage.

(2) Introduce noise elimination measures.

1.3.2 exhaust noise

Exhaust noise is mainly at the beginning of exhaust. Exhaust gas is discharged from the gap of the exhaust valve in the form of pulses, and quickly rushes into the atmosphere from the exhaust port, forming high-energy and complex-frequency noise, including the components of fundamental frequency and its higher harmonics. This kind of noise is the biggest and most important noise source in cars and engines, and its noise is often higher than that of the whole engine 10 dB (a) ~ 15 dB (a). The exhaust noise includes not only the fundamental frequency noise and its higher harmonic noise, but also the vibration noise of the exhaust manifold and the air column in the manifold, the eddy current noise on the back of the valve stem and the turbulent noise on the inner wall of the exhaust system pipeline. In addition, exhaust noise also includes exhaust gas injection and impact noise.

The control strategies of exhaust noise mainly include:

(1) Start with the design of the exhaust system, such as reasonably designing the length and shape of the exhaust pipe to avoid airflow vibration and reduce eddy current.

(2) The application of exhaust gas turbocharger can reduce exhaust noise, but the most effective method is to adopt high noise elimination technology and use an exhaust muffler with small power loss and wide noise elimination frequency range.

1.3.3 Fan noise

Fan noise is a noise source that can not be ignored in the engine, especially in the air-cooled engine, and its noise is even comparable to the intake and exhaust noise at high speed and full load. It is mainly aerodynamic noise, which consists of rotating noise and eddy current noise. Rotating noise is caused by air pressure pulsation caused by periodic impact of rotating blades on air particles. Vortex noise is caused by the vortex generated by the surrounding air when the fan rotates. Due to the viscous force, these vortices split into a series of independent small vortices. These vortices and the splitting of vortices will disturb the air and form pressure fluctuations, thus exciting noise. Eddy current noise is generally broadband noise.

The fan noise of the engine is dominated by eddy current noise at low speed and rotating noise at high speed. The higher the speed and diameter of the fan, the greater the air volume of the fan and the higher the noise. The lower the fan efficiency, the greater the power consumption and the greater the fan noise.

The control strategies of fan noise mainly include:

(1) Properly control the speed of the fan, and the noise of the fan is much greater than other noises with the increase of the speed. Under the condition that the cooling requirement is unchanged, in order to reduce the rotating speed, the fan diameter or the number of blades can be appropriately increased within the structural size range; Fully applying the theory of fluid mechanics to design a high-efficiency fan can reduce the rotating speed on the premise of ensuring the cooling air volume and wind pressure.

(2) Using a fan with uneven distribution of blades, even distribution of blades will often produce some parts with higher sound pressure level. When the blades are unevenly arranged, the prominent line spectrum components in the fan can generally be reduced, making the noise spectrum smoother.

(3) Replacing steel fan with plastic fan can reduce the noise and power consumption of fan, but the current cost is slightly higher than that of steel fan. Plastic fans have been widely used in small and medium power internal combustion engines abroad. You can also use a "flexible fan" whose installation angle can be changed. The blades of this fan are made of very thin steel plate or plastic. When the speed of the fan increases, due to the aerodynamic force, the blades are twisted and flattened (the installation angle becomes smaller), so the power consumption and noise of the fan are reduced. When the rotating speed is reduced, due to the small aerodynamic effect, the torsion of the blade becomes smaller, ensuring sufficient air flow.

(4) Fan automatic clutch is used in vehicle internal combustion engine. The test shows that when the vehicle is running, the time required for the fan to work is generally less than 10%. Therefore, the fan clutch can not only make the internal combustion engine work at a suitable temperature, reduce power consumption, but also achieve the effect of noise reduction.

(5) Reasonable design of fan and radiator system. Such as the distance between engine and fan, the distance between fan and radiator, the position and shape of fan and fan cover, the resistance of air passing through radiator, etc. , will have an impact on the full use of cooling air volume. Reasonable arrangement and design can achieve the purpose of reducing the speed of the fan.

2 conclusion

To sum up, there are many factors that affect the noise of automobile engines, and it is difficult to greatly reduce the noise by using a certain noise reduction method. To reduce the noise of automobile engine, we should start with the noise source and transmission way of engine noise, and make clear the object and goal of noise reduction. Through comprehensive consideration and various technical means, combustion noise, mechanical noise and aerodynamic noise can be effectively controlled and reduced to some extent, so as to achieve the purpose of reducing automobile engine noise.