What is engine knocking in cars?

[BarrettAnn]

There are several main reasons that cause engine knocking: 1. Overly advanced ignition timing. To ensure the piston receives power immediately upon entering the power stroke after the compression top dead center, ignition usually occurs before the piston reaches the top dead center (because it takes time from ignition to complete combustion). Excessively advanced ignition causes most of the fuel-air mixture to burn while the piston is still in the compression stroke, leading the unburned mixture to auto-ignite under extreme pressure, resulting in knocking. 2. Excessive carbon buildup in the engine. Excessive carbon deposits in the combustion chamber not only increase the compression ratio (generating high pressure) but also create hot spots on the carbon deposits, causing engine knocking. 3. Incorrect air-fuel ratio. A too lean fuel-air mixture raises combustion temperatures, which increases engine temperature and makes knocking more likely. 4. Low fuel octane rating. The octane rating is an indicator of a fuel's resistance to knocking. The higher the octane rating, the stronger the anti-knock properties. High-compression engines have higher combustion chamber pressures, making them more prone to knocking if low-octane fuel is used.