What Is Engine Knock, Detonation & Pre-Ignition (Complete Guide)
Updated: Jan 29
Engine knock, detonation, and pre-ignition are all terms that refer to abnormal ignition events that can occur inside the cylinders of an internal combustion engine. These events can cause knocking or pinging sounds, and can lead to engine damage if left unchecked.
In this guide, we will explore the causes and consequences of engine knock, detonation, and pre-ignition, and discuss ways to prevent and address these problems.
Table of Contents:
What Is Engine Knock?
Engine knock, also known as pinging or knocking, is an abnormal combustion event that occurs inside the cylinders of an internal combustion engine. It happens when the air/fuel mixture burns unevenly, too early, or too late.
There are three main types of engine knock: detonation, pre-ignition, and rod knock.
Detonation: This is an uncontrolled combustion event that occurs following a spark plug ignition.
Pre-ignition: This type of knock is caused by an uncontrolled combustion event that occurs before the spark plug ignition.
Rod knock: This is a different type of knock caused by worn or damaged rod bearings and is not related to the other types of combustion-related engine knock.
Knock is often described as a pinging or knocking sound, and it can be caused by any of these three types of engine knock.
Most original equipment manufacturer (OEM) engine knock sensors are designed to listen for a specific frequency in order to detect engine knock and pre-ignition (this does not include rod knock).
These sensors can help to prevent engine damage by allowing the engine control unit to adjust the fuel-to-air ratio or spark timing in response to the detected knock.
Understanding the different types of engine knock and how they are detected can help you identify and troubleshoot potential issues with your engine and keep it running smoothly.
Below is a video showing what knock sounds like.
To understand engine knock, it is helpful to consider the normal process of combustion in an internal combustion engine.
During the compression stroke, the piston moves upward and compresses the air-fuel mixture.
When the piston reaches the top dead center (TDC) position, the spark plug ignites the mixture.
Ignition causes the flame front to spread uniformly outward from the spark plug until most or all of the mixture is burned.
Knock occurs when this normal combustion process is disrupted, leading to irregular or unexpected combustion events.
These events can cause knocking or pinging sounds, and they can lead to engine damage if left unchecked.
Detonation
Detonation, also known as engine knock, is a type of uncontrolled combustion that occurs after the spark plug has ignited the fuel-air mixture in an internal combustion engine. It is characterized by a knocking or pinging sound and can lead to engine damage if left unchecked.
It is caused by several factors, such as high cylinder temperatures, low-quality fuel, and improper ignition timing.
Detonation can be characterized by combustion that occurs irregularly and at the wrong location in the cylinder, or it can be caused by combustion that occurs too quickly and chaotically.
Detonation produce supersonic shock waves, which move at speeds faster than the speed of sound. These shock waves can cause resonance across the entire cylinder and, if strong enough, may produce a knocking sound that can be heard outside of the engine.
It is the shockwaves themselves that can cause harm to objects in their immediate surroundings.
A typical combustion event is referred to as a deflagration in some circles.
Deflagration, in contrast to detonation, is characterised by subsonic flame fronts that travel slower than the speed of sound and have minimal destructive potential; rather, they simply exert pressure on the objects in their immediate vicinity (in this case the piston inside the engine).
Detonation, or the self-ignition of the air/fuel mixture, is unpredictable and chaotic, resulting in brief but very powerful and sometimes catastrophic spikes in cylinder pressure.
Detonation occurs later in the engine's combustion cycle, after the spark plug has ignited the fuel-air mixture. Pre-ignition, on the other hand, is a type of engine knock that occurs before the spark plug fires.
Pre-Ignition
Pre-ignition is a type of abnormal combustion that occurs in an internal combustion engine. It refers to the spontaneous self-ignition of the air-fuel mixture before it is ignited by the spark plug.
Low-speed pre-ignition (LSPI) is a specific type of pre-ignition that occurs at low engine speeds and high loads.
It is caused by the ignition of droplets of fuel that have been injected into the engine's cylinder during the compression stroke, rather than by the spark plug. LSPI can be difficult to diagnose and can cause serious damage to the engine if not addressed.
Both detonation and pre-ignition are forms of abnormal combustion that can have detrimental effects on an internal combustion engine. However, pre-ignition often causes more damage and does so more quickly than detonation.
Pre-ignition occurs during the compression stroke, when the piston is moving toward the combustion event rather than away from it. As a result, the pressure and temperature in the cylinder increase even more, leading to more severe damage to the engine.
Detonation occurs after the spark plug has ignited the fuel-air mixture and the piston is moving away from the combustion event.
While detonation can still cause damage to the engine, it is typically not as severe as the damage caused by pre-ignition.
Rod Knock
Rod knock is a type of engine knocking that is caused by worn or damaged rod bearings in the engine. Rod bearings are small metal parts that allow the connecting rods, which link the pistons to the crankshaft, to move smoothly within the engine.
If the rod bearings become worn or damaged, they can cause a knocking or tapping sound as they move within the engine. This is oftentimes referred to as bottom end knock.
Severe rod knock is characterized by a loud knocking or tapping sound that can be heard when the engine is running. The sound may become more pronounced as the engine speed increases.
If left unchecked, rod knock can lead to serious engine damage and should be addressed as soon as possible.
The video below is what severe rod knock sounds like.
Rod knock has a variety of causes, as shown below.
It can be caused by improper bearing clearance or installation.
Debris getting into the engine can cause rod knock.
Failing to service and change your car's oil on a regular basis can also contribute to increased rod bearing wear.
Rod bearings wearing out over time.
While rod knock is not technically linked to pre-ignition or detonation, it produces a sound that is similar to the other types of engine knock.
The sound of rod knock is continuous, whereas the sound of pre-ignition and detonation engine knock typically occurs when the engine is running at full throttle or with some amount of throttle.
In most cases, the sound of rod knock is more continuous than the intermittent knocking sounds of pre-ignition and detonation.
Is Engine Knock Serious?
Yes, engine knock can be a serious issue. If left unaddressed, engine knock can cause serious damage to your vehicle's engine. The knocking or pinging sound is caused by the chaotic and uncontrolled combustion within the combustion chambers leading to harsh shockwaves.
This can lead to the pistons, connecting rods, valves and other engine components becoming damaged or breaking, which can cause engine failure.
In addition, engine knock can cause the engine to run less efficiently, which can lead to a reduction in performance and an increase in fuel consumption. It is important to have engine knock checked out by a mechanic as soon as possible to avoid further damage to your vehicle.
How Long Can a Car Go With an Engine Knock?
An engine knock is a serious issue that can cause significant damage to your vehicle if not addressed promptly. The length of time that a car can go with an engine knock depends on the severity of the knock and the conditions under which the vehicle is being driven.
You should avoid driving and starting the engine if you notice engine pinging. Continuing to drive a vehicle with an engine knock can lead to engine failure and costly repairs.
Signs of Engine Knocking
Engine knock can cause physical damage to an internal combustion engine, including damage to the pistons, cylinder head, connecting rods, valves, etc.
One of the first indications of this damage may be small, irregular patches on the surface of the piston crown or cylinder head. These patches may appear to look like erosion or sand blasting in some cases.
Engine knock, including pre-ignition and detonation, can cause physical damage to the pistons in an internal combustion engine.
When this occurs, the piston crown may become rough and, if left unchecked, the cylinder pressure and temperature can increase to the point where the piston ring ends come into contact with each other.
Pre-ignition is more likely to cause piston melting and damage than detonation because the piston is moving against the flame front and is subjected to higher temperatures for a longer period of time during pre-ignition.
Pre-ignition is also more likely to cause bending of the connecting rods than detonation.
Rod knock is a type of engine knocking that is caused by worn or damaged rod bearings. It is often manifested by a continuous ticking, pinging, or knocking sound that may be heard when the engine is running.
The only way to definitively determine whether there is rod knock is to disassemble the engine and inspect the rod bearings for wear and damage.
Worn rod bearings may show signs of scratches, grooves, and in some cases, visible damage.
Why Detonation & Pre-Ignition Occurs
Detonation and pre-ignition are types of abnormal combustion that can occur in an internal combustion engine. They are caused by high temperatures inside the cylinder, which can lead to the spontaneous self-ignition of the air-fuel mixture.
This is more likely to occur in a petrol engine, which compresses both air and fuel. In a diesel engine, on the other hand, fuel is only added when the air is hot enough to ignite it, so the risk of engine knock is much lower.
Diesel engines do not need spark plugs because they rely on the heat of the compressed air to ignite the fuel, rather than using a spark.
Turbochargers and superchargers are devices that are used to increase the power of an engine by compressing the air and forcing it into the engine. This increased pressure and temperature can increase the risk of engine knock, such as detonation and pre-ignition.
By compressing the air, turbochargers and superchargers allow more fuel to be burned, which can produce a stronger combustion and increase the power of the engine. However, this increased pressure and temperature can also increase the risk of engine knock.
How to Prevent Engine Knock
One of the most effective ways to prevent engine knock is to keep the air entering the engine from getting too hot. The following are also ways to reduce the likelihood of engine knocking.
Using higher octane fuel
Choosing the proper compression ratio for your engine
Fitting an intercooler
Using water and methanol injection
These measures can help to reduce the likelihood of knock, even in warmer weather.
It's worth noting that an engine with a higher risk of experiencing knock may be less prone to it when ambient temperatures are very low, but it's not practical to rely on the weather to determine when you should drive your car.
By taking proactive steps to prevent knock, you can help ensure that your engine runs smoothly and efficiently.
Use Higher Octane Fuel
One way to prevent engine pinging is to use a higher octane fuel. Higher octane fuel is less prone to ignition under high pressures and heat, which can help to reduce the likelihood of knock.
An octane rating (or octane number) is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without detonating.
When the fuel in your engine is able to withstand higher temperatures and pressures before igniting, it can help to prevent knock and ensure that your engine runs smoothly and efficiently.
Keep in mind that while using higher octane fuel can be an effective way to prevent knock, it may not be the only solution. Other factors, such as the compression ratio of your engine and the ambient temperature, can also affect the likelihood of knock.
By addressing these factors and using a higher octane fuel, you can help to prevent engine knock and keep your vehicle running smoothly.
Choose a Proper Compression Ratio
Choosing the proper compression ratio for your engine is another effective way to prevent engine knock. Changing pistons and connecting rods can alter the compression ratio, some piston and rod combinations have a lower or higher compression ratio depending on what you choose,
The compression ratio is the ratio of the volume of the cylinder when the piston is at the bottom of its stroke (bottom dead center) to the volume of the cylinder when the piston is at the top of its stroke (top dead center).
It is a measure of how much the air-fuel mixture is compressed in the cylinder before ignition.
Higher compression ratios can increase the likelihood of engine knock because they result in higher temperatures and pressures in the cylinder.
Lower compression ratios reduce the likelihood of engine knock since they don't compress the air-fuel mixture, resulting in lower temperatures and pressures in the cylinder.
When the air-fuel mixture is compressed more, the molecules are brought closer together and friction increases, which raises the temperature of the mixture. If the temperature gets too high, the mixture may ignite prematurely, causing knock.
By choosing the proper compression ratio for your engine, you can help to prevent knock and ensure that your vehicle runs smoothly and efficiently.
It's important to note that the compression ratio is just one factor that can affect the likelihood of knock, and other measures, such as using a higher octane fuel or fitting an intercooler, may also be necessary to prevent knock.
Fit an Intercooler
Using an intercooler is another effective way to prevent engine knock, especially in engines that use forced induction (such as turbochargers or superchargers).
Forced induction increases the risk of knock by increasing the temperature of the intake air, which can cause the air-fuel mixture to ignite prematurely.
An intercooler helps to reduce this risk by using heat exchange to remove heat from the compressed intake air, lowering its temperature and reducing the likelihood of knock.
There are two main types of intercoolers: water-to-air and air-to-air. Both types use a system of tubes and fins to transfer heat from the compressed intake air to a cooler medium, which helps to lower the temperature of the intake air and minimize the risk of knock.
There are also various types of intercooler designs, such as tube and fin or bar and plate, but almost any type of intercooler will work as long as it is large enough to effectively cool the intake air.
By using an intercooler, you can help to prevent engine knock and ensure that your engine runs smoothly and efficiently.
It's worth noting that an intercooler is just one of several measures that can be taken to prevent knock, and other factors, such as the compression ratio and the type of fuel used, should also be considered.
Water & Methanol Injection
Water and methanol injection is an active way to prevent engine knock by directly injecting these substances into the intake air stream of an engine.
Methanol is a high octane fuel that is very resistant to knock, and when combined with water, it absorbs heat from the intake air, allowing you to run more boost pressure without increasing the risk of knock.
In addition to reducing the risk of knocking, water and methanol injection can also help to prevent carbon buildup in your engine, which can lead to hot spots in the cylinders and pistons. This can help to improve the overall performance and efficiency of your engine.
While water and methanol injection is a effective way to prevent knock, it is not the only solution.
Other measures, such as reducing the compression ratio or using an intercooler, can also be effective in reducing the risk of knock. By taking a comprehensive approach to preventing knock, you can help to ensure that your engine runs smoothly and efficiently.
Aftermarket ECU and Knock Sensor
One way to prevent engine knock is to use an aftermarket engine control unit (ECU) or knock sensor. An ECU is a computer that controls various aspects of an engine's operation, including the fuel-to-air ratio, ignition timing, boost pressure, etc.
Most modern engines already utilise knock sensors so this approach may not be needed. However, some aftermarket ECUs have better performing knock sensor strategies, making their use preferable over OEM knock sensors and ECUs.
An aftermarket ECU allows you to fine-tune these settings to optimise the performance of your engine and reduce the risk of knock.
A knock sensor is a device that detects vibrations and sound frequencies caused by knock and sends a signal to the ECU to adjust the engine's parameters in response. By using a knock sensor, you can help to prevent knock in real-time.
While aftermarket ECUs and knock sensors can be effective in preventing knock, it's important to keep in mind that they are just one part of a comprehensive approach to preventing knock.
Preventing Knock
To prevent engine knock, it may be advisable to adopt a strategy that includes running low boost, low compression, high octane fuel, and a large intercooler.
Detonation can be prevented by any or all of the following techniques.
Retarding ignition timing (often used by ECUs when knocking is detected by a knock sensor).
The use of a fuel with high octane rating, which increases the combustion temperature of the fuel and reduces the proclivity to detonate.
Enriching the air-fuel ratio which reduces the combustion temperature and increases the margin to detonation (this can sometimes be used by the ECU as method of reducing or preventing knock if it is detected).
Reducing peak cylinder pressure (reducing boost pressure or reducing the compression ratio).
Decreasing the manifold pressure by reducing the throttle opening or boost pressure (can often be used by ECUs to reduce or prevent engine knock).
Reducing the load on the engine.
This approach can help to reduce the pressure and temperature of the unburned air-fuel mixture in the cylinder, which lowers the risk of spontaneous ignition and knock.
Lowering the combustion pressure and temperature can also help to extend the lifespan of your engine by reducing the risk of damage caused by knock, detonation, and pre-ignition.
While this strategy may result in lower power output, it is a small price to pay for the long-term health of your engine.
It's worth noting that air-cooled engines, which do not have coolant flowing through them, may require a richer air-fuel ratio to keep cylinder temperatures under control.
This is because the lack of a cooling system means that the engine must rely on the cooling effect of the air-fuel mixture to prevent overheating.
Early engine control units (ECUs) were able to detect and react to knock, but their responses were often slow and may not have been enough to prevent damage over the long term.
Modern ECUs often use a knock sensor to detect engine knocking allowing the ECU to alter ignition and fuel injection timing to reduce knock. These can be fitted by the manufacturer or can be added with aftermarket ECUs.
By taking proactive measures to reduce the risk of knock, such as reducing the compression ratio and boost, fitting a larger intercooler, and using higher octane fuel, you can help to ensure that your engine runs smoothly and efficiently.