Vehicle engine noise control

1. Main hazard of noise

Noise pollution not only has a negative impact on people's self-perception and work ability, but also can lead to serious health disorders, fatigue, early deafness, high blood pressure, and neurological diseases.

2. The formation and countermeasures of vehicle engine noise

Engine noise mainly includes combustion noise, mechanical noise, intake and exhaust noise, noise from cooling fans and other components. The combustion noise is noise generated when the combustible mixed gas is burned and the gas pressure in the cylinder rises sharply to impact the components of the engine and vibrate. The cetane number in diesel is not suitable or the injection time is too early, which will cause the engine to work rough and the noise will increase sharply. The gasoline engine causes high-frequency explosion sounds and knocks on the cylinder due to overheating, poor gasoline quality, and excessive ignition advance angle.

The noise generated by the combustion process and structural vibration inside the engine is radiated to the atmosphere through the vibration of the outer surface of the engine and the rigid connection structure with the outer surface of the engine, and is therefore referred to as engine surface noise. According to the mechanism of engine surface noise, it can be divided into combustion noise and mechanical noise. The combustion noise is mainly caused by the cyclically varying pressure in the cylinder, which is closely related to the combustion mode and combustion speed of the engine. The mechanical noise is the period of action between the moving parts and between the moving parts and the fixed parts during engine operation. It is caused by the force of sexual change, which is related to the magnitude of the excitation force and the dynamic characteristics of the engine structure. In general, at low speeds, combustion noise dominates, and at high speeds, mechanical noise dominates.

To reduce combustion noise, it is necessary to improve combustion conditions and improve combustion quality to achieve a smooth pressure waveform. The most efficient combustion can be achieved by properly arranging the spark plugs and valves and using the appropriate combustion chamber type and cooling. In terms of fuel, the higher the octane number of gasoline, the better the ignition quality and anti-knocking performance; for diesel engines, the choice of suitable cetane number of diesel oil, if not, can be added to the ignition accelerator to improve Ignition quality, which can effectively prevent noise caused by fuel combustion.

Mechanical noise includes piston knocking sound, valve mechanism impact sound, timing gear running sound, and the like. To reduce the piston knocking sound, a method of reducing the gap between the piston and the cylinder wall and offsetting the center of the piston pin from the center of the crankshaft may be employed. The magnitude of the impact noise of the valve mechanism is determined by the valve clearance. If the valve clearance is too small, the sealing will not be good. If it is too large, it will cause a great impact when the valve is opened and closed, which will generate strong noise and aggravate the wear. During use, the valve clearance should be adjusted frequently to make it within the specified clearance. Generally, a buffer section is added before and after the basic shape (working section) of the camshaft to reduce the impact, or a hydraulic tappet is used. Insufficient rigidity of the push rod and the rocker arm in the valve transmission mechanism may cause severe vibration and noise when the valve moves, and even cause the valve to lose control; the pulsation of the valve spring may also generate noise, so the valve spring requires high quality and elastic force. And give a certain pre-tightening force when installing.

The noise when the timing gear is running is a great noise source in the diesel engine. The timing gear should be aligned during assembly. The timing gear made of different materials (such as plastic, cloth bakelite, etc.) can be greatly reduced. Noise, chain transmission ratio gear transmission can significantly reduce transmission noise.

The way to control the intake and exhaust noise is the same. In the case of ensuring that the engine output power is not reduced, the intake and exhaust noise can be reduced by improving the engine valve train or the exhaust mechanism. The paper air filter and exhaust muffler widely used at present can significantly reduce the intake and exhaust noise.

In order to reduce the noise of high-speed engine fans, a fan with a fluid coupling or variable-speed torsion angle can be used, or a water-temperature-sensing electric clutch fan can be used. After changing the fan material to nylon, it also has a certain effect on reducing fan noise.

3. Vibration noise

The main solutions to vibration noise are: 1 vibration reduction treatment. Mainly used at the vibration source to reduce the amplitude of the vibration wave; 2 vibration isolation treatment. A vibration isolator is installed in the path of vibration wave transmission to achieve the purpose of reducing the vibration source; 3 damping treatment. Modern structures are mostly complex multi-degree-of-freedom systems, mostly broadband excitation, and increased damping can effectively suppress vibration. Common methods of processing are damping free layer processing and constrained layer damping processing.

4. Noise control approach and identification of noise sources

The path of noise control is: sound source control, noise transmission path control and personal protective measures. Sound source control is the most fundamental way to control mechanical noise, and the control noise propagation path is the most commonly used noise reduction method. The common measures are sound absorption, sound insulation, noise reduction, vibration isolation and so on.

The noise generated by the machine spreads out from two ways, one is directly transmitted through the air medium, causing noise pollution. This airborne noise is called air sound; the other is to directly excite adjacent solid components through the sound source. Vibration, which propagates in adjacent members in the form of elastic waves and radiates large noises into the air at weak points. This solid-propagating noise is called solid sound. The main technical measure to control solid sound is vibration isolation, which is to change the rigid connection between the vibration source and the foundation or other objects to elastic connection, thereby weakening or hindering the propagation of vibration energy and achieving the purpose of noise reduction.

5. Vehicle engine noise control measures

Specific methods for noise reduction in the engine compartment include: sound absorption - absorption of sound energy by microplates and foam; vibration damping - isolation of vibration source from structure by damper; closure - using sound enclosures or other soundproofing devices The noise is closed, making it difficult to spread; damping—converts mechanical kinetic energy into heat and dissipates it.

The materials selected must meet the requirements of sound insulation, sound absorption, vibration reduction and heat insulation. The method of sound insulation and vibration reduction of the engine is as follows: the mechanical energy is converted into heat energy by installing a vibration damping pad on the engine and the water radiator fixing bracket; the interior of the engine compartment should be as far as possible to avoid unevenness, and a porous material and a damping layer can be used. The method should be closed to all the skeleton seams on the body to avoid diffraction waves.

Solving the noise of the vehicle engine is a technical issue involving all aspects of the machine, including engine structure, material, mass distribution, process level, assembly sealing and so on. Active measures taken by manufacturers to prevent noise on the structure of vehicle engines include: vibration absorption, sound insulation technology, improvement of automobile structure, sound insulation board and muffler within a certain distance from the sound source, and rear suspension engine for passenger cars. , improve the structure of the muffler and so on.