Noise pollution is a major environmental issue that affects not only people's mental well-being and work efficiency but also leads to serious health problems such as stress, fatigue, early hearing loss, high blood pressure, and even neurological disorders. It can significantly impact quality of life and long-term health.
Vehicle engine noise is one of the primary sources of this problem. Engine noise consists of several types, including combustion noise, mechanical noise, intake and exhaust noise, and noise from cooling fans and other components. Combustion noise occurs when the fuel-air mixture burns, causing rapid pressure increases inside the cylinder, which then vibrates engine parts. In diesel engines, an incorrect cetane number or improper injection timing can lead to rough running and increased noise. For gasoline engines, overheating, poor fuel quality, or excessive ignition advance can result in knocking and high-frequency explosions.
Engine surface noise is generated through the vibration of the outer surfaces and rigid connections of the engine. This type of noise is classified into combustion noise and mechanical noise. Combustion noise is related to the cyclic pressure changes in the cylinder, while mechanical noise arises from the interaction between moving and fixed parts during operation. At lower speeds, combustion noise is more dominant, but at higher speeds, mechanical noise becomes more significant.
To reduce combustion noise, improving the combustion process is essential. This includes optimizing spark plug and valve placement, using appropriate combustion chamber designs, and ensuring proper cooling. For gasoline engines, higher octane fuels improve ignition quality and reduce knocking. In diesel engines, choosing the right cetane number or adding ignition improvers can help prevent noise caused by combustion.
Mechanical noise comes from various parts, such as piston knocks, valve impacts, and timing gear sounds. Reducing piston knock involves minimizing the gap between the piston and cylinder wall or offsetting the piston pin. Valve clearance must be carefully adjusted to avoid excessive noise and wear. Adding buffer sections on camshafts or using hydraulic tappets can also help. High-quality valve springs with sufficient pre-tightening force are important to prevent vibration and noise.
Timing gear noise is particularly common in diesel engines. Proper alignment during assembly and using materials like plastic or cloth bakelite can reduce noise. Chain-driven systems are more effective than gear-driven ones in lowering transmission noise.
Intake and exhaust noise can be controlled by improving the valve train or exhaust system. Air filters and mufflers are widely used to reduce these noises without affecting engine performance.
For high-speed fan noise, using fluid couplings, variable-speed torsion angle fans, or water-temperature-sensing electric clutch fans can help. Replacing fan material with nylon also contributes to noise reduction.
Vibration noise is another major concern. Common solutions include vibration reduction treatment at the source, vibration isolation by inserting isolators in the path of vibration, and damping techniques such as free-layer and constrained-layer damping. These methods are especially useful for complex structures subjected to broadband excitation.
Noise control strategies typically involve three approaches: controlling the sound source, managing the noise transmission path, and implementing personal protective measures. Source control is the most fundamental, while path control, such as sound absorption and insulation, is widely applied.
Noise from machines spreads in two ways: airborne and structure-borne. Airborne noise travels through the air, while structure-borne noise causes vibrations in adjacent solid components, leading to further noise emission. To manage structure-borne noise, vibration isolation is key—replacing rigid connections with elastic ones helps reduce the spread of vibration energy.
In the engine compartment, noise reduction methods include sound absorption using micro-perforated plates and foam, vibration damping by isolating the source from the structure, and enclosing the noise within soundproof enclosures. Damping converts mechanical energy into heat, effectively reducing noise.
Materials used must meet sound insulation, sound absorption, vibration reduction, and thermal insulation requirements. Installing vibration damping pads and avoiding uneven surfaces in the engine compartment can help minimize noise. Sealing all body seams prevents diffraction waves from spreading.
Reducing vehicle engine noise is a comprehensive technical challenge involving engine design, materials, mass distribution, manufacturing processes, and assembly sealing. Manufacturers employ active measures such as vibration absorption, sound insulation, improved car structure, and muffler design. Rear suspension for passenger cars and enhanced muffler structures are also effective in reducing noise.
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