Why Does the Engine Knock?

[AdrielLee]

Reasons for engine knocking: 1. Excessive clearance between the piston and cylinder. This can be caused by improper assembly leading to excessive clearance or severe cylinder wear. At low temperatures, the piston clearance is larger, resulting in severe knocking noise. As the engine warms up, the piston expands, reducing the clearance and diminishing or eliminating the knocking noise. 2. Incorrect piston installation or piston deformation, disrupting the normal clearance between the piston and cylinder, leading to knocking sounds. 3. Poor lubrication of the cylinder wall: inadequate lubrication conditions, low oil pressure, or low oil viscosity prevent the formation of a proper oil film, causing direct contact between the piston and cylinder wall and resulting in knocking. 4. Bent or twisted connecting rod, causing the piston assembly to tilt excessively in the cylinder. This not only worsens sealing and lubrication but also leads to abnormal cylinder wear, resulting in knocking sounds. 5. Clogged oil passages, preventing normal pressure lubrication; overly tight connecting rod bearings or piston pins, causing the piston to strike the cylinder wall during movement. 6. Severe carbon buildup in the combustion chamber. Excessive carbon increases the compression ratio in diesel engines, raising gas pressure and temperature. Injected diesel fuel ignites prematurely upon encountering high temperature and pressure, causing detonation. During detonation, flames spread rapidly, sharply increasing temperature and pressure before the gas can expand, forming pressure waves that propagate at the speed of sound. 7. Excessive fuel injection advance angle. Early injection of diesel fuel forms a combustible mixture that ignites easily at normal operating temperatures, even before normal pressure is reached. Rapid combustion and pressure rise advance the peak combustion pressure, increasing horizontal forces on the piston and intensifying knocking against the cylinder wall. 8. Overly rapid combustion of the air-fuel mixture, causing excessive cylinder pressure and piston skirt impacts against the cylinder wall. 9. Overly thin cylinder head gasket or excessive machining of the cylinder block surface, increasing the compression ratio and causing detonation and knocking sounds.