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What Is Engine Knock, Detonation & Pre-Ignition (Complete Guide)

Updated: 1 day ago

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:


inside view of an engine


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.


  1. During the compression stroke, the piston moves upward and compresses the air-fuel mixture.

  2. When the piston reaches the top dead center (TDC) position, the spark plug ignites the mixture.

  3. 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.


Diagram showing the process of detonation type engine knocking
Diagram showing the process of detonation type engine knocking

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.


A diagram showing deflagration combustion (1) vs detonation type knock (2)
A diagram showing deflagration combustion (1) vs detonation type knock (2)

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.