Short Answer:

Knocking is an unwanted sound or vibration produced in internal combustion engines due to abnormal combustion of the fuel-air mixture. It occurs when the fuel in the engine cylinder ignites before the spark plug fires or when multiple flame fronts collide. Knocking causes a metallic pinging sound and reduces engine efficiency.

The main causes of knocking include high compression ratio, low-octane fuel, overheating of the engine, incorrect ignition timing, and carbon deposits in the combustion chamber. Knocking must be controlled as it can damage the piston, cylinder walls, and other engine components over time.

Detailed Explanation:

Causes of Knocking

Knocking is a common problem in spark ignition (SI) engines and sometimes in compression ignition (CI) engines. It occurs when the combustion of the fuel-air mixture does not happen smoothly as designed. In a normal combustion process, the spark plug ignites the mixture near the end of the compression stroke, and the flame spreads evenly through the cylinder. However, in knocking, part of the mixture auto-ignites before the flame front reaches it, leading to sudden pressure rise and vibration.

The following are the main causes of knocking in internal combustion engines:

High Compression Ratio:
When the compression ratio of an engine is too high, the temperature and pressure of the air-fuel mixture increase beyond the fuel’s self-ignition point. This causes premature ignition of the mixture, leading to knocking. Using a lower compression ratio or high-octane fuel can prevent this problem.
Low Octane Rating of Fuel:
The octane number of a fuel indicates its ability to resist knocking. Fuels with low octane numbers tend to ignite earlier than required. Hence, when low-octane fuel is used in high-compression engines, knocking becomes more likely. High-octane fuels are specifically designed to reduce this tendency.
Overheating of Engine:
Excessive engine temperature increases the cylinder wall and combustion chamber surface temperatures. This heat can cause the unburned mixture (end gas) to ignite automatically before the spark, producing knocking. Proper cooling and lubrication are necessary to prevent engine overheating.
Incorrect Ignition Timing:
If the spark occurs too early (advanced ignition timing), the mixture starts burning while the piston is still moving upward, creating excessive pressure. This premature combustion results in knocking. Correct ignition timing ensures smooth power delivery and avoids sudden pressure changes.
Carbon Deposits:
Carbon deposits on the piston head or cylinder walls reduce the combustion chamber volume, effectively increasing the compression ratio. These deposits can also retain heat, acting as ignition sources that cause local pre-ignition. Regular maintenance and use of clean fuel help in avoiding such deposits.
Lean Air-Fuel Mixture:
A lean mixture contains more air and less fuel than the ideal ratio. This slows down the combustion process and increases the temperature of the unburned gases, promoting knocking. Maintaining the correct air-fuel ratio helps achieve complete and smooth combustion.
High Inlet Air Temperature:
If the air entering the cylinder is already hot, the temperature of the compressed mixture will also rise. This can lead to auto-ignition of the fuel-air mixture. Using intercoolers in turbocharged engines or improving air intake systems can control inlet air temperature.
Engine Speed and Load Conditions:
Knocking is more likely to occur at high engine loads and low speeds. Under these conditions, the temperature and pressure in the cylinder increase significantly, promoting abnormal combustion. Proper control of engine load and maintaining appropriate speed can reduce this issue.
Improper Fuel Mixture Distribution:
In engines with poor mixture distribution, some cylinders may receive a richer mixture and others a leaner one. The lean mixture burns slowly, increasing the chance of knocking. Efficient carburetion or fuel injection systems ensure uniform mixture distribution and prevent knocking.
Use of Contaminated Fuel:
Impurities or additives in fuel that do not burn cleanly can also cause irregular combustion. This leads to hot spots in the combustion chamber, which promote auto-ignition and knocking. Ensuring fuel purity and using proper additives can minimize this cause.

Conclusion:

Knocking is a harmful condition in internal combustion engines that results from abnormal combustion of the fuel-air mixture. It can cause severe damage to engine components if not controlled. The main causes include high compression ratio, low fuel octane number, overheating, incorrect ignition timing, carbon deposits, and improper fuel mixture. To prevent knocking, proper engine design, correct fuel selection, regular maintenance, and proper ignition settings are essential. Controlling knocking not only extends engine life but also improves performance, efficiency, and smooth operation.