What is Knocking in an Engine? Causes, Sound, Noise, How to Fix?

All articles are written in simple language to visualize Mechanical Engineering. Let’s explore!

What is Knocking in an Engine? Causes, Sound, Noise, How to Fix?

In this article, we will learn what is knocking in an engine, along with noise or sound, hot to fix, causes, effects, knocking during accelerating and idling, etc.

Let’s explore!

What is Knocking in an Engine?

Knocking of Engine Basics

Let’s start the basics of knocking in an engine! Some of the machines are equipped with different sets of mechanisms that either make them more efficient or cause a lag. Some of the phenomena in the engine itself make it inefficient and Knocking is one among them.

  • There are bulk loads of information available regarding the engines, but the most suppressed is the knocking mechanism.
  • It is quite prominent to get a holistic idea about this as it can help reduce the probability of knocking, thus making your engine more reliable and quite efficient.  
  • This excerpt is all about knocking off the engine and all the related information to help the readers acquire a broader perspective for the same.
  • With this material, it is quite easy to understand all the causes of Knocking.

Apart from core insights, this also holds some of the best measures to safeguard your engine from the ill effects of knocking. Here we will try to cover all the effective information regarding the Knocking of the engine. We will gradually get into the details that will help us in understanding the whole perspective of the engine knock.

Let’s start with the core introduction of the knocking process to act as a stepping stone to get all the information.

Knocking in Engine Definition

Knocking is a phenomenon in an internal combustion engine that is a sharp sound that is caused due to the premature Detonation or combustion of some compressed air-fuel mixture in the combustion chamber or cylinder.

In an optimal functioning engine, the charge combusts with the front flame, gradually progressing from the ignition towards the ignition chamber. There are instances in which an amount of charge suddenly ignites ahead of the flame front and causes uncontrolled combustion.

This also depends upon the fuel composition, which also causes high-frequency pressure waves and intense combustion. These intense pressure waves force the engine’s different components to vibrate, which produces an evident knock. This is known as knocking, which is quite audible.

  • While understanding the engine knock, it is quite important to overview some parameters, such as ignition timing.
  • It is quite important to get an idea of the ignition timing as early as this occurs. There would be more torque and power output. This also provides part-level performance of the engine.
  • In modern times, numerous innovations in technologies keep their effort nurturing towards delaying the ignition time, which ultimately helps to attain maximum power and efficiency in the operations.
  • However, there is a massive probability that if the ignition time delays across the specified limit, it may cause ineffective combustion.
  • Ultimately the efficiency of the engine suffers a lot like the energy and work produced by burning goes into vain. This is the reason there should be an effective balance between the two.

Apart from the above, another scenario where the cylinder pressure and temperatures attain a maximum height to the compression. This results in the catching of fire for the air-fuel mixture before the ignition due to the spark plug. This is known as engine knock.

After getting the idea of the Knocking, it is quite important to get a brief idea about the processes as these are quite mandatory and play a proper role in knocking the engine.

What are the Different Phases of Engine Knocking?

There are some of the processes or phases that take place while the engine knocks. These phases will provide an overview of the engine knocking. Some of them are as follows:

  • While knocking, the normal front flame travels at a speed of 20 to 30m/s
  • The residual of the burnt air-fuel mixture, radiation, and front flame causes a massive increase in the temperature, which is quite lethal for the engine components.
  • Charges auto ignites before the front flame reaches the end, an example of abnormal or incomplete combustion.
  • The velocities of different particles and components reach 400 to 900m/s during e detonation.
  • There is an abnormal increase in the pressure which can be up to 3 to 4 times.
  • While knocking, the pressure frequency in the SI engine can easily attain up to 5000 cycles/second.

With these phases, we can say that Knocking is one of the undesirable phenomena as it causes the engine to work inefficiently. It suddenly increases the large amount of heat energy which causes problems for different components of the engine.

It also puts a stagnant approach on the compression ratio, which directly affects the output and efficiency of the engine.

Is Knocking Harmful for the Engine?

Knocking is quite harmful to the engine as it affects the different components of the engine.

  • Numerous mechanisms can effectively harm the engine.
  • The engine components are quite delicate, and heavy noise, pressure waves, and continuous vibration can cause numerous problems.
  • These problems may lead to inefficiency and lower output.

However, numerous measures can be used to minimize the effects of the engine knock.

Before getting into the measures that minimize the engine knock, it is quite important to be equipped with all the information about the harmful effects of knocking.

In the ongoing paragraphs, we will get to know all the relevant information about knocking the engine.

What are the Effects of Engine Knocking?

There are numerous effects of knocking on the engine and also on its components. Here we are providing some of them which can help us to understand those effects efficiently:

  • It causes overheating of spark plug joints.
  • It also erodes the surface of the combustion chamber.
  • The whole operation of the engine gets rough as well inefficient.

These are the three most common ill effects of Knocking, which can be avoided by calibrating some of the operation and engine structural design variables, such as ignition time and compression ratio.

One of the most prominent ways to avoid knocking is to use fuel with high octane number for ignition.

Processes Affecting the Engine Knock

While getting deep down under towards understanding the engine knock, it is quite important to get a holistic idea about certain processes in the engine.

These processes are directly or indirectly associated with Knocking. With these, we can easily understand the whole knocking process. Two processes make all the difference in the Knocking of the engine.


The auto-ignition is the combustion cycle in which the mixture of air and fuel can react at a sudden flame which ignites the combustion.

  • The self-ignition that occurs in the absence of flame is known as auto-ignition.
  • However, the auto-ignition is not that effective as this leads to abnormal combustion, which is also known as Detonation or knocking.
  • It causes a bad effect on the engine performance and its life as it releases a massive amount of heat energy.

In addition, Autoignition is amongst his prominent cause for the engine knock. The Knocking also acts as a limiting factor for the compression ratio, which is the critical measure for the operation of any engine.


This is the ignition of a mixture of charge when the same comes in direct contact with hot surfaces.

While in the auto-ignition, it is quite evident that the spark plug gets overheated, and the exhaust valve also becomes hot due to the excessive temperature rise.

  • The whole process causes the ore-ignition of charge.
  • In accordance with ignition, this is the mechanism that initiates by some of the overheated parts, for example, exhaust valve head, metal corners, sparking plug electrodes, cylinder head, gasket rim, and many more.
  • The main reason for pre-ignition is the continuous detonation pressure shockwaves, which helps release the stagnant gases that are being used as a protection for combustion chamber walls.

This is responsible for the continuous increase in the temperature and flow of heat alongside the cylinder walls.

The above two mechanisms are quite important as it provides adequate information about the Knocking of the engine.

Without understanding these phenomena, it is quite impossible to comply with all the measures that can minimize the effect of knocking. The auto-ignition and pre-ignition are the prior steps that can generate the possibility of knocking.

Harmful Effects & Causes of Engine Knocking Sound or Noise

There are numerous ill-effects of knocking which can cause serious problems to the engine. The engine acts as a heart for any automobile, which helps in providing par level effectiveness.

Knocking is capable of ruining the engine in specific ways. Following are some of the harmful effects with their probable reasons:

  • Carbon deposits
  • Mechanical damage
  • Increase in heat transfer
  • Pre-ignition
  • Decrease in efficiency and power output
  • Noise level and roughness

Carbon Deposits

Carbon deposits

This is amongst his most prominent and lethal ill effects of knocking that causes numerous problems for the engine.

Detonation while ignition causes carbon deposits. The carbon deposits cause numerous problems that can affect the performance and driveability of the automobile.

How to Fix Engine Knocking Due to Carbon Deposits?

One of the most effective solutions for removing his carbon deposits is adding a can of fuel system cleaner to the fuel tank.

  • This can keep the carbon deposits at bay which occurs due to Detonation.
  • It is recommended to use a product that can loosen up the oil deposits from inside for massive deposits.
  • However, after the cleanup, it is quite essential to change the oil once and for all.

This is necessary as some of the oil cleaners can affect harmfully on the crankcase.

Mechanical Damage

Mechanical damage

This is one of the major defects that can occur due to the Knocking of the engine.

  • While knocking, excessive pressure waves with high magnitude generated can quickly increase the wear of parts in the ignition chamber.
  • There can be severe erosion of cylinder head, piston crown, and pitting of outlet and inlet valves can happen.
  • This is among the most lethal kind of wreckage to the heart of any machine, i.e., engine.

How to Fix Engine Knocking Due to Mechanical damage?

  • The mechanical damage of the engine’s components can be diminished with the help of taking proper care of noise level and uneven sounds of the components.
  • These sounds are audible and can be minimized with the help of using high-octane fuel.

Noise Level and Roughness

Due to knocking, there is a massive possibility of an increase in the noise level and roughness.

  • The noise level is due to the pressure waves and high pulsating noise due to Knocking or Detonation.
  • These pressure waves continuously vibrate the whole system back and forth across the cylinder, which causes ill effects to the engine functionality.
  • The roughness of the engine is being caused due to the availability of vibratory motion.
  • It also causes vibrations in the crankshaft section.
  • It is quite easy to detect any varying noise level in small engines, but it is pretty impossible for big engines.
  • Corrective measures are easy for small engines, which can easily be attained.

In some cases, if Knocking persists for a long time, it can also deteriorate the piston.

Effective measures:

The probable solution for minimizing the noise level and excessive vibration due to Knocking is a proper check.

This can be catered with the help of continuous maintenance with the help of appropriate tooling and equipment.

Sometimes the Detonation or Knocking produces a massive sound which can be harmful to the delicate parts of the engine. In the long run, it can reduce the efficiency and the power output of the engine.

Increase in Heat Transfer

One of the concerned phenomenons due to Knocking is the increase in the rate of heat.

  • The heat produced can quickly transfer to the combustion or ignition chamber.
  • The probable reasons for the increase in heat transfer are due to the fast completion of combustion.
  • Other than that, the prominent one is the scouring of the protective layer of the cylinder walls.
  • The inactive gas on cylinder walls reduces the heat transfer, which ultimately increases the heat accumulation.
  • This decreases the overall power output as well as the efficiency of the engine.

Effective measures:

  • Excessive temperature can be catered to with the help of proper cooling.
  • There is a huge need for using an appropriate cooling system that effectively cools down the combustion chamber or cylinder wall, which gets at optimal temperature when the air-fuel mixture ignites.


As we know, the Knocking of the engine causes the increase in the rate of heat transfer, but there is one thing that is the chain reaction of the same. This is known as pre-ignition, which occurs due to local overheating in the sparking plug, reaching optimal temperature.

  • This temperature rise is as high as it can easily ignite the charge before the spark passing.
  • When the engine detonates for a long time, it may cause pre-ignition, which is quite dangerous for the engine.

Effective measures:

This is the commonest of all the harmful effects. It can be reduced with the help of proper operational procedures for sparking, which ultimately ignites the air-fuel mixture.

Knocking can easily be minimized with the proper knowledge of these practical measures and information about harmful processes.

How to Fix or Reduce the Engine Knocking Sound?

After getting all the information regarding the ill effects of knocking on the engine, it is quite mandatory to get the ideas to help us reduce the engine’s Knocking.

  • As we all know, modern engines are equipped with numerous electronic components in which is a knock sensor.
  • The sudden automatic explosion in different parts of the cylinder causes excessive noise caught by the sensors.
  • This part of this excerpt provides some of the effective measures to prevent engines from knocking.

The detailed analysis about the same is as follows:

Delaying the Time of Ignition

The Knocking can be controlled by effective delay in the ignition timing.

  • When the knock sensor receives the starting of the Knocking inside the cylinder, it transmits the signal to ECU.
  • Following that, the ECU commands the spark plug to delay or stop the ignition time.
  • Hence, the Detonation or ignition takes place in the later stage.
  • At this time, the high temperature and pressure are avoided at any cost.
  • All these functions take place in just milliseconds which is quite a rapid time-space.
  • This happens due to the electronic control. After this, the combustion cycle continues, and the Knocking doesn’t happen again.

This is amongst the best self-correcting methods that can fully diminish Knocking and its ill effects.

Use of High Octane Fuel

The use of high octane fuel is gaining popularity for all good reasons. However, there is some confusion related to the extra spending on premium fuel.

  • The high octane fuels are pretty effective and helpful in diminishing the effects of knocking or preventing it.
  • The high octane fuels are comparatively more resistant to engine knocking.
  • It effectively makes it quite efficient as this property doesn’t enable them to withstand high pressure, temperature, or fire.
  • It is quite prominent for fuel only to get ignited from the spark plug. This property is one of the foremost ones.

The high octane fuels are knock resisting, and here if the ignition timing is advanced, then the mixture of air and fuel will not burn at an incredible pace.

Reducing the Compression Ratio

It is one of the susceptible methods as the compression ratio is directly proportional to the internal combustion engine’s performance and efficiency.

  • With the higher compression ratio, the engine can be more efficient, and one can attain better performance; however, one of the main reasons for engine knock is the high compression ratio.
  • The vulnerability of the knocking mechanism increases when there is a high compression ratio attained.
  • This is one of the foremost reasons that make ECU a critical aspect that determines the exact ignition time.
  • The electronic control is vital as it also lowers the compression ratio that makes knocking of the engine relatively low.

An optimal balance can help control the ignition time, which ultimately helps the engine perform at par level with maximum efficiency and avoids the engine’s Knocking.

Decreasing the cylinder temperature

As we know that the auto-ignition takes place when the air-fuel mixture inside the cylinder occurs due to a massive increase in the temperature.

However, it is critical to maintaining the temperature inside the cylinder so that it doesn’t reach the top position where Knocking can be evident.

  • It means that there should be a par level of cooling around the cylinder.
  • The walls of the cylinder should not be heated up due to the Detonation.
  • It is also prescribed that the intake air should not get hot.
  • It can be diminished with the air coolers, which are commonly installed in big engines.

All the above prerequisites are critical in reducing the engine knock as the auto-ignition inside the cylinder is the main factor. These points should be considered to safeguard all the components of engines.

An engine is a delicate machine that should be appropriately taken for longer life and efficient operation. This also helps in keeping the mechanical components up to the mark.

With the above preventive points, we can easily avoid Knocking the engine to a large extent.


Here we have tried to gather all the information about one of the lethal mechanisms that deteriorated the engine. This excerpt has all the possible data about the engine’s Knocking, but there are many more left.

The discussion of the knocking phenomenon helps us clear the perception about one of the most deficient phenomena and allows us to cater the engine against that. The probable defects can help understand the same, while the effective measures can help us understand it fully. We have also formed a section that consists of preventive measures which can keep the probability of knocking at bay.

With this information, we can quickly get a holistic idea about the Knocking or Detonation of the engine. We will try to gather some of the critical insights about the engine’s Knocking-in other excerpts.

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