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Why Does Knocking Occur In An Engine? [How to Get Rid Of knocking] PDF

Knocking in Engine

Have you ever wondered like “Hey, what’s that knocking noise in the engine?” probably it happens at some point while you driving. The Knocking in the engine is audible noise. A knock or a ping that you can actually hear when it occurs in your engine. The noise or sound is called as engine knocking.

Simply naming the sound, though, doesn’t tell the whole story, and you’ll soon see why.

man watching his car.

What causes the engine to start knocking? Let’s discuss what’s going on in the engine.

knocking in Engine Cylinder.

During normal combustion, the air-fuel mixture in the combustion chamber starts burning from the spark plug as soon as the sparking takes place. The mixtures burn smoothly from beginning to end, providing an even and powerful thrust to the piston.

The pressure inside the cylinder increases evenly. As soon as the sparking takes place, a wall of flame spreads out in all directions from the spark. It travels rapidly outwards through the mixture until all the charges are burning.

The movement of the flame wall through the combustion chamber during normal combustion. In this case, knocking does not occur.

How knocking occurs:

Under certain conditions, the part of the air-fuel mixture explodes before the flame wall reaches it. The temperature of the last part of the mixture increases as the flame progresses. This causes a sudden increase in pressure which imposes a sudden heavy load on the piston that is almost like a hammer blow. This creates a typical noise, known as knocking.

If knocking is severe enough, it may actually break engine parts.

It should note that knocking and detonation are synonymous terms, having the same meaning.

  • Detonation is the name giving to the sudden and violent knocking occur inside the engine cylinder. Detonation is not pre-ignition.

If the detonation is allowed to continue for a long time it may overheat the cylinder and spark plug so as to ignite the charge even before the sparking, this will cause preignition.

Thus, detonation follows the spark while pre-ignition precedes it.

1. List of Factors that Affect Knocking in Engine

  1. Compression ratio.
  2. Cool mixtures.
  3. Surface ignition and preignition.
  4. Additives.
  5. Chamber design.
  6. Operating condition.
  7. The type of the engine fuel.
  8. The position of the spark plug.
  9. The engine running.
  10. The portion of the fuel.

1.1 Compression Ratio

A higher compression ratio increases the tendency for knocking in the engine. With a higher ratio, the mixture at T.D.C has more pressure and is at a higher initial temperature.

With higher initial temperature and pressure, the temperature at which detonation occur reaches soon. Thus higher compression engines have a greater tendency to knock.

1.2 Cool Mixture

The cool mixture has less tendency to knock than hot mixtures. Moistures in the air taken in by the carburetor tend to lower the temperature and reduce a greater tendency to knock.

1.3 Surface Ignition and Preignition

Surface ignition and preignition seriously affect knocking.

The surface ignition can originate from hot spots in the combustion, chamber, such as on a hot exhaust valve or spark plug or from combustion chamber deposits.

Preignition of the fuel takes place before the spark occurs from the spark plug.

1.4 Additives

The additives mixed in the gasoline, like tetraethyl lead, reduce the tendency to knock.

1.5 Chamber Design

combustion chamber desing

The shape of the combustion chamber has a great effect on the tendency of the engine to knock. Due to the proper design of the combustion chamber, the air and fuel mix properly in the cylinder for normal combustion.

1.6 Operating Condition

Many operating conditions in an engine affect knocking. For example,

  • Higher air temperatures increase the tendency to knock,
  • Higher humidity and higher altitudes (or lower air density) reduce the tendency to knock,
  • Engine deposits advancing the spark and learning the mixture increases the tendency of the engine to knock.

Scale formation in the cooling system, blocks fuel lines (which learn out the mixture), improper ignition timing, engine deposits-all these increase knocking tendency of the engine.

1.7 Type of the Engine Fuel

The Type of engine fuel has a considerable effect on knocking. Fuel like alcohol and Benzel do not cause knocking.

But they give a higher rate of consumption because of lower calorific valves as compared to that of petrol.

1.8 The position of the Spark Plug

The position of the spark plug in the combustion chamber greatly affects knocking in the engine. The center spark plug proves quite useful.

If it is placed aside in the combustion chamber, the flame has to travel a longer distance to reach the detonation zone. The more the distance, the more are the chances of detonation.

Car running speed - giphy

1.9 Engine Running

The engine running at a higher speed for a long time highly increases the temperature of the combustion chamber which causes detonation. Excessive spark plug temperature promotes detonation.

1.10 The portion of the fuel

A rate at which the combustion of the portion of the fuel which is first to ignite just after the sparking takes place also affects detonation.

If the rate is very high it raises the temperature and pressure of the remaining portion of the fuel which is yet to burn, to very high valves thereby increasing the tendency of detonation.

2. Mechanical Factors That Affecting Knocking

The tendency of the engine to knock is affected by the shape of the combustion chamber.

The combustion of an I heap engine is bounding at the top by the cylinder head, inlet valve, exhaust valve, spark plug, and at the bottom by piston shapes-wedge and hemispherical is showing in the figure.

The shape determines turbulence, squish and quench. These three factors affect the engine to knock.

2.1 Turbulence

The proper shape of the combustion chamber imparts turbulence to the air-fuel mixture entering it.

Turbulence assures a more uniform mixing of air and fuel so that the combustion is more uniform. The turbulence also reduces the time required for the flame front to travel through the mixture.

2.2 Squish

Squish means to push through a small area.

As impart the piston nears TDC, it squishes or squeezes or pushes the air-fuel mixture at the end of the compression stroke, out of the squish area, in some designs is shown in fig. As it squishes out, it promotes turbulence and thus further mixing of the air-fuel mixture.

2.3 Quench

It has already been started knocking results when the temperature of the last part the mixture goes too high and it explodes the flame front reaches it.

If some heat is extracted from this part of the mixture (or the last part of the mixture is quenched), its temperature will not reach the detonation point.

The squish area is also a point. The closeness of the cylinder head to the piston and the relative coolness of these surfaces causes the heat to be extracted from the mixture so that the tendency for detonation to occur is quenched.

2.4 Wedge Combustion Chamber

Wedge Combustion chamber

In the wedge combustion chamber, the spark plug is located to one side and hence the flame front has to travel a greater distance to reach the end of a wedge.

However, the end of the wedge has a quench area which cools the last part of the air-fuel mixture to prevent detonation. It also imparts turbulence to the mixture.

2.5 Hemispherical Combustion Chamber

Hemispherical Combustion chamber

In the hemispherical combustion chamber, the spark plug locating at the center of the dome, and hence the flame front has a relatively short distance to travel.

Also, there are no distance pockets in the last part of the mixture to detonate. The chamber has no squish or quenches area, however, there is relatively little turbulence.

2.6 Smog

Smog in car exhaust

The automobile engine and fuel system release to the atmosphere a variety of gaseous compounds, including unburned hydrocarbon and carbon monoxide. These compounds give to the formation of smog. The shape of the combustion chamber also affects the number of contaminants of the smog.

The wedge combustion chamber has a larger surface area and produces a greater percentage of contaminants per power stroke than the hemispherical combustion chamber. Incomplete combustion increases carbon deposits which ultimately cause detonation.

3. Chemical Controls of Knocking

Certain chemicals are added to gasoline to prevent the detonation of the last part of the fuel charge during combustion. These chemicals tend to increase the reaction time of the fuel, I,e., the time requires for the end gas to explode.

This increases time gives the flame front time to reach the end gas so that it enters into the normal combustion process instead of exploding.

Chemical Controls of Knocking chart -knocking in engine
The image is the form hemmings.com

Tetraethyl lead calling as ethyl or fuel is one of the most useful compounds to prevent knocking. Special scavengers are also added to the gasoline to prevent the combustion products of the lead from expositing in the combustion chambers on the plug, valves, and piston.

They tend to charge the lead compounds into forms that will vaporize and exit with the exhaust gases. Ethylene dibromide and ethylene dichloride are scavengers.

Read also: The chemical makeup of gasoline.

4. How to Get Rid of Engine Knocking

1. Increasing the Octane Level in your Fuel

the Octane Level in your Fuel chart

The antiknock value of the fuel is measured in octane number rating(ONR). The fuel iso-octane is highly resistant to knock. It is given an octane rating of 100. Another fuel normal heptane knock s very easily.

It is given a rating of zero. A mixer of 50% heptane (by volume) would have a 50 ONR. similarly, a mixture of 80% isooctane and 20% heptane would have an ONR of 80.

Iso-octane and heptane are actually the reference fuels to test the ONR of unknown fuels. The fuel to be tested is used in an engine and tolerance to knocking is noted. Now the two reference fuels are mixed in varying proportions and used to run the engine under the same condition. The mixture which gives the same knocking characteristics would identify the ONR of the fuel. There are two basic methods for testing the fuels:

  1. Laboratory method
  2. Road-test method.

With these 4 step, you can fix your engine knocking

1. Make sure you are using the proper fuel and maintain the octane level for normal cars and for some higher-end performance cars that require a higher octane level.

Fuel filling

2. Check out and make sure your engine cooling system works properly like radiators, thermostats. water pumps and cooling fans.

3. Consider cleaning your engine cylinders and combustion chamber. Using the wrong fuel can be an issue, it can leave excess contaminants in your cylinders as a result of poor combustion. Using a fuel additive like top-tier gasoline brands will provide more detergents that help keep your engine clean.

Spark plug

4. A defective spark plug OR wire can be the cause of Knocking in the engine, this is an easy job but could take about 30 minutes to 1hour. Remove the spark plug and check it. You can normally tell if there is a problem or not just by the residue left on your spark plug.

A normal plug should have only brownish-grey residue on the side electrode. You should just clean it with a wire brush and fuel injector cleaner rather than replace it.


This is how you can find and fix your engine knocking or Knocking in the engine. Engine knocking is a simple problem you can fix this by doing the above steps. If you still have any questions regarding this then leave a comment or contact us I’ll respond to you.

Hope you like our article on knocking in the engine. If yes then share this knowledge with your friends.

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About Saif M

Saif M. is a Mechanical Engineer by profession. He completed his engineering studies in 2014 and is currently working in a large firm as Mechanical Engineer. He is also an author and editor at www.theengineerspost.com

10 thoughts on “Why Does Knocking Occur In An Engine? [How to Get Rid Of knocking] PDF”

    • Yes, engine knocking here is caused by the failure of the crankshaft bearings and the loss of oil pressure or engine running without enough oil.

    • Communication is a key part of the automotive industry. Technicians and customers need to communicate effectively with each other to ensure that vehicles are properly diagnosed and repaired.


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