What is engine knocking? Does the engine knocking damage the engine?


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In a four-stroke engine, the entire cycle or two revolutions of the crankshaft is completed in four-stroke (2 upward and 2 two downward movements of the piston). The four strokes are,

  • Suction stroke
  • Compression stroke
  • Power stroke
  • Exhaust stroke

In a petrol engine, the air-fuel mixture enters the engine cylinder because of the vacuum or pressure depression created by the downward movement of the piston. This downward movement of the piston is called the suction stroke. At this time the inlet valve is open and the exhaust valve remains closed. So the air-fuel mixture enters the cylinder through the inlet port.

After suction, the piston moves upwards, at this time both the valves are closed. The movement of the piston compresses the air-fuel mixture, this is called compression stroke.

At the end of compression, the air-fuel mixture is ignited with the help of a spark plug. So the piston moves downward, this stroke is called the power stroke. This is the stroke that creates energy to run the vehicle. At this time both the valves are in the closed position.

After the power stroke, the piston moves upward. At this time, the exhaust valve opened and the exhaust gas comes out of the engine cylinder.

In a diesel engine, atmospheric air enters the engine cylinder at a suction stroke. The air is compressed in the compression stroke. At the end of the compression stroke, the fuel is injected by the injector. The piston moves downward due to the ignition of the fuel, and the power stroke is complete. 

After the power stroke, the piston moves upward and removes exhaust gases from the engine cylinder. The valve operations are the same as of a petrol engine.

The engine knocking occurs due to the ignition of fuel automatically, because of high temperature and pressure. This process is known as detonation.

In a petrol engine, the knocking occurs at the last of the compression stroke.


In compression stroke, the piston moves upward, which compresses the air-fuel mixture. At the end of compression, fuel burns with the help of a spark plug. The ignition starts near the spark plug and gradually spreads around, This is the normal process.

But in some cases, due to high pressure and temperature, the ignition occurs different areas of the combustion chamber. When these are combined together, there creates a big blast. This makes vibration and knocking sound, this is actually known as engine knocking.

In a diesel engine, knocking generally occurs when there is an ignition delay happens. Due to the ignition delay, there accumulates more fuel near the injector. So there creates more pressure and temperature, which leads to a big blast rather than normal ignition.

Engine knocking reasons


The low octane rating of fuel: Simply, the fuel can resist ignition till there create a specific temperature and pressure. If using a fuel that has a low octane rating, there is a chance of ignition before the normal ignition time. Or after the normal ignition, the fuel burns automatically without the help of a spark, if there create a temperature and pressure that are required to ignite that octane-rated fuel.

Compression ratio: If there happens any change in compression ratio, this will lead to engine knocking or detonation. If the compression ratio increases, the pressure increases which also increases the temperature.

Carbon deposits: Cabon deposits in the combustion chamber and in the piston head are causing to increase in compression ratio. In this way, detonation happens ( Engine knocking ).

Lean air-fuel mixture: Lean air-fuel mixture may cause engine knocking. In this way the number of air increases which will allow raisin the pressure.

Ignition timing: If any change happens in ignition timing, which will cause engine knicking. To avoid this, some engines use temperature sensors. Fuel injection vehicles use knock sensors to sense engine knocking. If any changes happen, automatically adjust the ignition timing.

Engine overheat: If for any reason engine overheat happens, there is a chance of engine knocking.

Weather conditions: Hot weather may cause engine detonation. In these conditions, the intake air will be hot which increases the pressure and temperature

Does the engine knocking damage the engine?

  • Engine knocking leads to damage to the piston, and Engine knocking leads to damage to the piston, which is breaking the piston ring land.
  • Engine knocking leads to breaking the valves.
  • It will melt the spark plug center electrode.
  • Engine knocking increases the temperature, it may be caused to an expansion of the piston. That Will leads to piston ceasing.

It is important to service the vehicle if engine knocking happens. It happens due to several engine problems, so take special care, maybe otherwise it leads to pre-ignition or engine ceasing.
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