Home » Automobile engg » What Is Knocking In Engine? [Their Causes & Remedies] PDF

What Is Knocking In Engine? [Their Causes & Remedies] PDF

There’s no doubt that cars can make weird noises, and everyone agrees on that. It might sound like a clunk, a roar, or even a squeal. The knocking sound on your engine is your engine’s way of telling you it’s unhappy!

If you ignore the knock, you might experience decreased fuel efficiency—who can afford that?—as well as possible engine damage, which can be expensive to fix. Continue reading to find out what causes knocking in the engine and why it’s not something to take lightly.

Knocking In Engine

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

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

Knocking in Engine

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 occurs. 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, 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, almost like a hammer blow. This creates a typical noise known as knocking.

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

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

  • Detonation is the name given to the sudden and violent knocking that occurs inside the engine cylinder. Detonation is not pre-ignition.

If the detonation continues for a long time, it may overheat the cylinder and spark plug to ignite the charge even before the sparking. This will cause preignition.

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

You might like this: What is a Single Plate Clutch?

What Could Cause Engine Knocking?

There are three potential causes of engine knocking, as well as solutions! These are explained below:

#1 Faulty Spark Plug

The engine’s spark plugs produce the electric spark that ignites the cylinder’s fuel and air mixture. In other words, spark plugs are necessary for starting your engine. Spark plugs diminish with time, just like other components of your car.

About every 30,000 miles, according to the majority of auto manufacturers, new spark plugs should be installed; however, the condition and type of the spark plug will determine how long they last.

Your spark plugs may need to be replaced if they aren’t manufacturer-recommended or if they have seen better days.

Causes of Faulty Spark Plug

If not replaced, faulty spark plugs can reduce engine power and decrease fuel economy.

Remedies of Faulty Spark Plug

Spark plug replacement is typically quick and inexpensive because they are readily available and reasonably priced. According to the manufacturer’s recommendations, skilled technicians could replace your old spark plugs with new ones.

Consult your trusted mechanic if you’re unsure how long you’ve been operating with the same spark plugs. Your car’s power and fuel efficiency can be enhanced by regular tune-ups, as well as a small problem from turning into a costly one.

#2 Low-octane Fuel

The octane rating of gasoline differs from one pump to another, which is why there are so many choices at the pump. The greater the octane rating of fuel, the more compression it can withstand before igniting. Using regular fuel in an engine designed to run on high-octane fuel could result in excessive engine noise.

Causes of Low-octane Fuel

In general, high-octane fuel is more expensive than regular fuel. Regular fuel is less expensive, so many drivers choose to fill up their tanks with it to save money. Actually, that’s not the best way to make a choice.

Using incorrect fuel over an extended period may harm your engine and reduce fuel efficiency. Cheaper petrol won’t save you any money at all if you’re getting lower miles per gallon and may have to pay for engine repairs in the future.

Remedies of Low-octane Fuel

Initially, you have to check your owner’s manual. What kind of fuel do you suggest, and do you use it? Boost your octane level if necessary at your subsequent fill-up, or use an octane booster to improve performance. After a few fill-ups, if this doesn’t seem to be making a difference, there may be another cause for your issue.

#3 Carbon Deposits

To help prevent carbon deposits from clogging up your cylinders, carbon cleaning detergents must be included in every fuel sold. Sadly, some deposits continue to form. As a result, compression increases because there is less space for the fuel and air. When fuel compression changes, bad knocking sounds can occur.

Causes of Carbon Deposits

Excessive carbon buildup can cause issues with combustion and harm the engine’s cylinders. Due to this, it results in a decrease in performance can also lead to lower gas mileage or overheating.

Remedies of Carbon Deposits

The combustion chambers, or the cylinders, should be cleaned. For your safety, the manufacturer suggests that you should inspect your engine cylinder for carbon buildup every 100 hours of operation. Allow qualified professionals to perform a thorough tune-up and remove the carbon deposits to improve the performance of your engine.

Read Also: Understand the Working of Leaf Spring Suspension

There are other possible reasons to occur engine knocking, which are listed below.

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 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 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, and engine deposits-all these increase the 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 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.

Read Also: A Complete List of Car Body Parts & Its Function [Explained]

2. Mechanical Factors That Affecting Knocking

The engine’s tendency 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, and spark plug, and at the bottom by piston shapes-wedge and hemispherical as shown 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 the piston nears TDC, it squishes, squeezes, or pushes the air-fuel mixture at the end of the compression stroke, out of the squish area, in some designs, as 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 started knocking results when the temperature of the last part of 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; 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 that 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 plugs are located 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 in the smog.

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

Read Also: Understand Different Types of Transmission System In Vehicle

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

Chemical Controls of Knocking chart -knocking in engine
The image is from 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, knocks 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 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.

4 Steps to Fix Your Engine Knocking

1. Make sure you use 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 - Knocking in Engine

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 - Knocking in Engine

4. A defective spark plug OR wire can cause Knocking in the engine. This job is easy but could take about 30 minutes to 1 hour. Remove the spark plug and check it. You can normally tell if there is a problem just by the residue left on your spark plug.

A normal plug should have only a brownish-grey residue on the side electrode. You should 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.

Subscribe to our newsletter for daily updates:

Download PDF of this article:

Read also:

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 “What Is Knocking In Engine? [Their Causes & Remedies] 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.


Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.