What Causes Knocking Sound in Diesel Engines?
2 Answers
Introduction to the causes of knocking sound in diesel engines: 1. Overly advanced ignition timing: To ensure the piston gains power immediately upon entering the power stroke after compression top dead center, ignition usually occurs before the piston reaches top dead center (as it takes time for complete combustion from ignition). Excessively advanced ignition causes most of the fuel-air mixture to burn while the piston is still in the compression stroke, leading to spontaneous combustion of the unburned mixture under extreme pressure, resulting in knocking. 2. Excessive carbon buildup in the engine: Too much carbon deposits in the combustion chamber not only increases the compression ratio (generating high pressure) but also creates hot spots on the carbon surface, causing engine knocking. 3. Overheating engine: An engine operating in too hot an environment, leading to high intake air temperature, or poor engine coolant circulation, can cause the engine to overheat and knock. 4. Incorrect air-fuel ratio: An overly lean fuel-air mixture raises combustion temperature, which in turn increases engine temperature, making knocking more likely. 5. Low fuel octane number: The octane number indicates fuel's resistance to knocking; higher octane numbers mean better anti-knock properties. Engines with high compression ratios have higher combustion chamber pressures, making them prone to knocking if low-octane fuel is used.
I've been driving diesel vehicles for many years, and knocking sounds are quite common. They usually occur due to abnormal fuel combustion, such as incorrect fuel injection timing. If the fuel is injected too early or too late, it can easily cause knocking, which sounds like a pounding on the cylinder. It could also be due to excessive clearance between the piston and cylinder wall, where wear and aging cause looseness and collision. Poor fuel quality with high impurity content can also lead to this issue. In winter, low temperatures during startup can make the knocking more likely. Not fixing it in time can affect the engine's lifespan, and in severe cases, it may even cause cylinder burn-out. I recommend first checking the fuel system and injectors, and trying a higher-quality diesel fuel. Don’t skimp on regular maintenance—changing the oil and filters on schedule can help reduce such problems. Safety comes first; if you hear the sound, shut off the engine immediately and inspect it. Don’t risk driving further.
