What Causes Engine Knock?
4 Answers
Reasons causing engine knock: 1. Overly advanced ignition before the piston reaches top dead center; 2. Excessive engine temperature; 3. Excessive carbon buildup in the combustion chamber; 4. Overly lean fuel-air mixture in the combustion chamber; 5. Fuel with excessively low octane rating. The solution is to use higher-octane fuel while paying attention to regular maintenance, frequently observing whether the engine exhibits abnormal phenomena, and addressing any issues promptly to prevent engine damage caused by knocking. When the engine inhales a mixture of fuel vapor and air, if the compression stroke fails to reach the designed ignition position while various uncontrolled factors cause the gas mixture to self-ignite, the tremendous impact force generated by this combustion acts opposite to the piston's movement direction, causing engine vibration. This phenomenon is known as engine knock.
In all my years of repairing cars, knocking issues have almost always been tied to fuel quality and carbon deposits. Just the other day, a Camry started making a 'knocking' sound when climbing hills after filling up with 92-octane gas from a small station—a classic case of knocking. The gasoline's octane rating was too low, causing premature ignition under compression. A modified Civic had it even worse: carbon buildup on the piston crowns pushed the compression ratio above 11:1, and at 4000 RPM, it sounded like firecrackers going off. My advice? Use fuel additives regularly to clean carbon deposits, opt for 95-octane fuel before long trips—especially for modified cars—and watch out for overheating engines, which can also trigger knocking. In summer, check coolant levels frequently.
From a mechanical principle perspective, engine knock occurs when the fuel-air mixture detonates spontaneously before the spark plug ignites it. Common causes include overly aggressive ignition timing advance or flawed combustion chamber design. For instance, an older Ford Focus experienced knock due to aging ignition coils - the ECU aggressively advanced ignition timing to compensate, but this backfired. Turbocharged vehicles require extra caution. A recent repair case involved a Lynk & Co 03+ where intake temperatures soared to 60°C during turbo engagement, causing premature combustion. For vehicles with ECU remapping, installing an intercooler is advisable. Avoid aggressive driving, especially when climbing steep grades in hot weather.
Knocking is like a ticking time bomb. If the gasoline octane rating is insufficient, the temperature at the end of compression exceeds 800°C, causing spontaneous combustion; if the spark plug heat range is incorrectly selected, the electrode temperature becomes too high, turning it into a lighter; even more terrifying is that direct injection engines are prone to fuel wall wetting, where gasoline sprayed onto the cylinder wall suddenly explodes upon encountering high temperatures. Last time, after dealing with knocking in a Magotan, switching to 98-octane fuel immediately resolved the issue. It's advised not to blindly remap the ECU to increase pressure—the factory ignition settings are the safest. If you hear metallic knocking sounds during acceleration, immediately check the combustion chamber condition, or the piston rings might get shattered.
