Knock or Auto Ignition a Destructive Combustion

Knock or auto ignition in petrol engines is a destructive combustion phenomenon. We look at how motorcycle manufactures tread close to the edge of damage.

The first commercially successful internal-combustion engine was patented in 1858 by Belgian √Čtienne Lenoir. Since then, there's been an unending quest to increase combustion efficiency so that more power can be produced while using less fuel. Motorcycle manufacturers employ advanced technologies to gain even fractional improvements in efficiency.

However, the quest for lower fuel consumption and emissions amid the demand for increased power and torque does come with the risk of reaching the knock limit; push the boundaries too far and the engine will destroy itself in minutes.

In a four-stroke petrol engine, as modern motorcycles use, the intake stroke provides each cylinder with oxygen in the form of fresh air, normally at atmospheric pressure. Petrol is added to the air stream in a modern engine at the ideal ratio of approximately 1 part fuel to 14 parts air, by electronic fuel injection. During the compression stroke, the air-fuel mixture is compressed and ignited by the sparkplug just before the piston reaches its highest position, or top-dead center. A controlled burning process then releases the fuel's chemical energy, with the resultant combustion pressure pushing down on the piston during the power stroke.

What is Knock or auto ignition

The combustion process in a petrol engine is sometimes incorrectly referred to as an explosion instead of a controlled burning process. Normally, a flame front will start at the sparkplug and spread gradually throughout the entire combustion chamber and consume the air-fuel mixture. During a knock event, the air-fuel mixture ignites spontaneously if certain thresholds are breached and then combustion closely resembles an uncontrolled explosion. Normally, knock will be triggered by the pressure wave that results from the normal spark-initiated combustion, but it occurs in a different location within the combustion chamber. Carbon deposits can also lead to hot spots in the combustion chamber that trigger knock.

In older motorcycles, knock could be identified by a pinging sound from the engine. These extreme pressure waves in the combustion chamber have the potential to destroy the metal walls (piston and cylinder) of the combustion chamber in minutes.

The Knock Limit

In theory, the combustion process is more efficient at higher compression ratios. Efficiency would have been further enhanced if it were possible for the energy release to instantly take place during the top-dead-center position of the piston in the cylinder during combustion. However, the problem is that the air-fuel mixture has a tendency to self-ignite (or auto-ignite) if the temperature and pressure rises above a threshold (called the knock limit) during compression. The following criteria can result in the air-fuel mixture auto-igniting:
  • The compression ratio is too high;
  • The petrol-octane rating is too low;
  • The ignition timing is too far advanced;
  • Or the fuel mixture is too lean.

Octane versus Performance

Many people incorrectly associate the octane number of petrol with energy content. Rather, the octane number is a measure of the fuel's resistance to knock, with higher numbers denoting more resistance.
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