Updated: Dec 18, 2022
In this article we're talking about knock, which is probably the number one killer of boosted engines.
We'll understand what knock and detonation is, how it differs from pre-ignition, how to avoid it, and how to deal with engine knock and pre-ignition it if it occurs.
Table of Contents:
Below are some quick definitions of the types of engine knock if you're in a hurry.
Detonation is an uncontrolled combustion event that happens following a spark plug ignition.
Pre-ignition is an uncontrolled combustion event that happens before to the occurrence of a spark.
There is also another type of engine knocking known as rod knock, which is caused by worn or damaged rod bearings - it is not related to the other types of engine knock.
Knock (also known as pinging) is the sound that may be heard if the detonation is severe enough.
Knocking refers to all types of engine knocking including pre-ignition, detonation and rod knock. However, when people talk about knock they're usually referring to the uncontrolled detonation event.
Most OEM engine knock sensors are essentially microphones that have been set to listen for a particular frequency in order to detect this knock and pre-ignition.
What Is Knock?
The most straightforward explanation for knock is that it is caused by an irregular combustion event, or combustion that should not be taking place at the time of the occurrence.
Below is a video showing what knock sounds like.
We need to go beyond that to truly understand engine knock, and we need to understand what is going on within the engine.
Assume that we have an engine cylinder that is in the compression stroke, that the piston is going upward, and that the valves are closed.
Normal combustion would take place in this manner:
The piston is moving upward, compressing the air-fuel combination.
At some point around top dead centre (TDC) the spark plug ignites.
The spark plug begins the combustion event by igniting the air fuel mixture.
The flame front spreads uniformly outward from the spark plug until most or all of the air fuel mix is burned.
Knock occurs when the combustion event is irregular; for example, pre-ignition, where combustion occurs too soon, or detonation, when the combustion process occurs chaotically and spontaneously.
What Is Detonation?
Detonation is a kind of engine knock that occurs as a result of an uncontrolled combustion event that occurs after the spark plug is fired.
It can be characterized by combustion that occurs sporadically and at the incorrect location in the cylinder, or it can be caused by combustion that occurs too rapidly and chaotically.
Detonation is accompanied by the formation of supersonic shock waves, which move at speeds greater than the speed of sound.
In fact, it is the shock waves themselves that cause resonance across the whole cylinder, and, if powerful enough, may produce the distinctive knocking sound that we can actually hear even outside the engine.
Ever thought, what's that knocking sound? (visit this article to learn might be causing those knocking sounds!)
Engine knocking is a possible cause of a clicking, pinging or knocking sound.
As opposed to explosion, it is the shockwaves that have the ability to cause harm to objects in their close surroundings.
A typical combustion event is referred to as a deflagration in certain circles.
Deflagration, in contrast to detonation, is characterised by subsonic flame fronts that travel slower than the speed of sound and have no destructive potential; instead, they simply exert pressure on the things in their immediate vicinity.
Detonation, or the self-ignition of the air/fuel combination, on the other hand, is unpredictable and chaotic, resulting in brief but very powerful and sometimes catastrophic spikes in cylinder pressure.
Detonation is a kind of engine knock that occurs later in the engine's combustion cycle than pre-ignition in terms of when it occurs.
Detonation is the term used to describe the spontaneous and chaotic self-ignition of the air/fuel combination that occurs after the spark plug ignites.
What Is Pre-Ignition?
Pre-ignition is the spontaneous self-ignition of the air/fuel mixture that occurs before it is ignited by the spark plug.
Another form of pre-ignition is called low-speed pre-ignition.
Both detonation and pre-ignition may have detrimental effects on the engine, although pre-ignition often causes more damage and does so more rapidly than detonation.
Pre-ignition causes more damage than detonation since it happens during the compression stroke, which means that the piston is actually moving toward the combustion event rather than away from it, increasing the amount of damage.
As a result, the pressure and temperatures will rise even more.
What Is Rod-Knock?
Rod knock is a kind of engine knocking that occurs when the rod bearings on the engine are worn or damaged.
The video below is what severe rod knock sounds like.
It may be caused by improper bearing clearance or installation, as well as by debris getting into the engine.
You can read this guide on bearing clearance to ensure you're using the correct clearance.
It is also possible to increase rod bearing wear by failing to service and change your car's oil on a regular basis.
Even though rod knock is not technically linked to pre-ignition or detonation, it produces a sound that is quite similar to the other kinds of engine knock.
In most cases, the sound of rod knock is continuous, as opposed to the sound of pre-ignition and detonation engine knock, which typically occurs when the engine is running at full throttle or with some amount of throttle.
Signs of Engine Knocking
Typically, the first indications of physical damage caused by knock are small, irregular patches on the surface of the piston crown, which can appear to look like erosion or even sand blasting in certain instances.
When this occurs, the piston crown may become somewhat rough, and if it continues for an extended period of time, it can raise cylinder pressure and temperature to the point where the piston ring ends will come into contact with each other.
Although both detonation and pre-ignition may cause pistons to melt, pre-ignition is the more frequent cause of piston melting and damage since the piston is moving against the flame front and is moving towards the higher temperatures produced by pre-ignition when it happens.
So the piston is subjected to these extreme circumstances for a longer length of time during pre-ignition than it is during detonation.
Furthermore, when it comes to bending connecting rods, pre-ignition is once again the most frequent cause of the problem.
Rod knock is often manifested by a continuous ticking, pinging, or knocking sound that may be heard.
The only other method to determine whether or not there is rod knock is to disassemble the engine and inspect the rod bearings for wear and damage.
A worn rod bearing would be characterised by scratches, grooves, and, in some cases, damage.
Why Does Detonation & Pre-Ignition Occur?
The simplest answer is that temperatures inside the cylinder become too high.
This is a problem in a petrol engine because a petrol engine compresses both air and fuel, and if cylinder temperatures become too high, this can lead to a spontaneous self-ignition of the air/fuel mixture.
This is also the reason why knock isn't a problem with diesel engines.
Diesel engines compress just air and add fuel only when the air is hot enough to ignite the fuel, which is the reason diesel engines do not need spark plugs.
Because turbochargers and superchargers effectively compress the air, adding forced induction to an engine increases the likelihood of engine knocking.
Through compression of air, they can fit more air into the same amount of space, allowing you to use more fuel and produce a stronger combustion.
This increased pressure and temperature can increase the chance of detonation and pre-ignition.
How to Prevent Engine Knock
So the simplest method to avoid knock is to keep the air entering the engine from becoming too hot.
An engine with a high chance of experiencing knock is less likely to knock when ambient temperatures are sub-zero, but it's not practical to let the weather decide when you drive your car.
Fortunately, there are other ways to avoid knock.
Use Higher Octane Fuel
One of the simplest ways to avoid knock is to simply use a higher octane fuel.
Using a higher octane fuel can prevent knock by reducing the fuels ability to burn, allowing it to withstand higher pressures and heat before igniting.
Choose a Proper Compression Ratio
Another effective method of preventing knock is to choose the proper compression ratio for your engine.
When we compress fluids like air, we bring the molecules closer together. When the molecules are closer together, they brush against each other more often, raising friction and hence the temperature of the air.
The ratio of your biggest cylinder volume when your piston is at bottom dead centre to your smallest cylinder volume when your piston is at top dead centre is your compression ratio. your ratio of static compression.
The higher this value, the larger your compression ratio, the more your piston compresses the air fuel combination, and the higher the compression ratio, the greater the possibility of knock.
Fit an Intercooler
As mentioned earlier, forced induction (turbochargers and superchargers) increase the risk of engine knocking due to increased intake air temperature.
An intercooler, whether its a water-to-air or air-to-air intercooler, uses heat exchange to remove heat from the compressed intake air.
This lowers the temperature of the intake air and significantly minimises the possibilities of knock.
There are various types of intercooler, such as tube and fin or bar and plate intercoolers.
Though, almost any type of intercooler would work providing it's big enough.
Water & Methanol Injection
Reducing the compression ratio or fitting an intercooler are passive ways of reducing the chances of knock, but there are also active ways of avoiding knock.
Probably the best and easiest way to reduce engine knock is water and methanol injection directly into the intake air stream.
Methanol is a high octane fuel that is very resistant to knock and, when combined with water, absorbs heat from the intake air, allowing you to run more boost pressure than before.
As an added bonus, water and methanol injection actually prevents carbon buildup, reducing the chances of cylinder and piston hot spots.
Does this imply that the safest strategy for avoiding knock is to run low boost, low compression, high octane fuel and a large intercooler? Probably.
Consequently, the pressure acting on the unburned air-fuel combination outside of the combustion frame front is being reduced as a result of this procedure.
It also means that we're decreasing the temperature of the unburned air fuel combination, which lowers the likelihood of it spontaneously igniting in the process.
Lowering the combustion pressure will, of course, result in less power, but this is a minor price to pay for preserving your engine's lifespan.
When the air-fuel mixture is richer, the cylinder temperatures are lower, which helps to prevent knocking.
This is also why air-cooled engines must run a richer air/fuel ratio than water-cooled engines.
Because air-cooled engines do not have coolant flowing through them, they must depend on a richer air-fuel combination to keep cylinder temperatures under control.
In spite of the fact that early ECUs could detect and react to knock, they were sluggish, and their weighted responses might spell doom for the engine over the course of a lifetime.
So, reducing compression ratio and boost, fitting a bigger intercooler and running higher octane fuel is the best way to reduce engine knock, detonation and pre-ignition.