Difference Between Knocking and Detonation

Key Difference – Knocking vs Detonation
 

Knocking and detonation are often confusing, but they are both different terms that are used to explain problems in engines. Knocking is the creation of vibrations or sharp sounds in an engine due to improper initiation of the combustion as a response to ignition by spark plug. Knocking should not be confused with pre-ignition. Detonation is either the pre-ignition or auto-ignition of a fuel in an engine’s combustion chamber. The key difference between knocking and detonation is that, knocking brings several drawbacks to the engine such as, overheating of spark plug points, erosion of the combustion chamber surface and rough, inefficient operation whereas detonation can cause abrasion, mechanical damage and overheating in engines.

CONTENTS

1. Overview and Key Difference
2. What is Knocking
3. What is Detonation
4. Similarities Between Knocking and Detonation
5. Side by Side Comparison – Knocking vs Detonation in Tabular Form
6. Summary

What is Knocking?

Knocking is the making sharp sounds due to uneven combustion of fuel in the cylinder of a vehicle engine. It happens because the air-fuel mixture inside the cylinder does not properly initiate the combustion as a response to ignition by spark plug. A spark plug is a device that can deliver an electric current from an ignition system to a combustion chamber to ignite an air-fuel mixture by an electric spark. When put in simple terms, knocking is the vibration of engine due to pressure waves produced from the uneven combustion. This produces an audible knock.

There can be several reasons that cause knocking in engines. One reason is faulty spark plugs. Spark plugs can age and break down overtime. The life of spark plug depends on the condition and type of the spark plug. Another possible reason for knocking is use of low octane fuel.

Octane rating/octane number: This is a figure indicating the anti-knock properties of a fuel, based on a comparison with a mixture of isooctane and heptane. Gasoline from refineries comes in different octane ratings. Higher the octane rating of a fuel, the greater the compression it can withstand before igniting.

Another cause for knocking is carbon deposits in cylinder. Most of the times, carbon cleaning agents are used to prevent carbon deposits that can clog the cylinder. But there can be still a little amount of deposits form. When these deposits are formed, there is less space available for air and fuel in the cylinder. Therefore, fuel compressions can occur that may lead to knocking.

Knocking brings some several drawbacks to the engine such as,

• Overheating of spark plug points
• Erosion of the combustion chamber surface
• Rough, inefficient operation

What is Detonation?

Detonation is the process of pre-ignition or auto-ignition of a fuel in an engine’s combustion chamber. This is often occurred due to use of fuel with low octane rating. This means that the fuel starts to burn before the spark plug fire and electric current. Detonation is characterized by an instantaneous, explosive ignition.

Some of the causes for detonation are low-grade engine fuel usage and overheated spark plug tips. Low-grade engine fuels cause deterioration of engine parts. An overheated spark plug tip may cause pre-ignition. Some several preventive measures for denotation are as below:

• Use of high-grade engine fuels
• Enhancing air-fuel ratio in the cylinder
• Reduce ignition timing
• Reducing the load on engine

What are the Similarities Between Knocking and Detonation?

• Both Knocking and Detonation are problems arising in vehicle engines.
• Both Knocking and Detonation can cause unfavorable operation in the engine.

What is the Difference Between Knocking and Detonation?

Summary – Knocking vs Detonation

Knocking and detonation are problems in engines that are often found in vehicles. Most of the times, the two terms are confusing and used interchangeably. But they are two different situations in engines, as discussed above in this article. The difference between knocking and detonation is that, knocking brings some several drawbacks to the engine such as, overheating of spark plug points, erosion of the combustion chamber surface and rough, inefficient operation whereas detonation can cause abrasion, mechanical damage and overheating in engines.

Reference:

1.The Editors of Encyclopædia Britannica. “Knocking.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 15 June 2015. Available here  
2.“What Causes Engine Knocking? | Firestone Complete Auto Care.” Completely Firestone, 14 Feb. 2017. Available here  
3.Gable, Christine & Scott. “What Is Detonation?” ThoughtCo, Aug. 27, 2017. Available here  
4.“Engine knocking.” Wikipedia, Wikimedia Foundation, 25 Feb. 2018. Available here

Image Courtesy:

1.’530i Touring Msport LCI 59’By E60 Forums, (CC BY-SA 3.0) via Commons Wikimedia  
2.’SOHC Hemi Chamber’By Marshmallow bunnywabbit, (CC BY-SA 3.0) via Commons Wikimedia