Detonation in Petrol Engines-Problems and Remedies

Contents (Click to skip to the relevant part):

1. Introduction: What is Detonation?
2. Ideal, detonation free working of a petrol engine
3. Mechanism of detonation
4. Causes of detonation
5. Steps to prevent detonation

Introduction: What is Detonation?

While a petrol engine is running, it may sometimes vibrate unevenly and produce noise which is uncomfortable and irritating. Such a phenomenon is called detonation. Apart from being uncomfortable, it is also detrimental to the life of the engine.

Before understanding how detonation works and how to eliminate it, we must have a basic understanding of how a petrol engine works.

Ideal, detonation free working of a petrol engine

In a petrol engine, the petrol-air mixture is compressed inside the engine cylinder to a high temperature and pressure. Then it is sparked to catch fire and expand. The expansion is pushes the piston and eventually rotates the crankshaft and causes the vehicle to move.

In an ideal case, when the air-petrol mixture is sparked, only a small part of it in the vicinity of the spark plug should catch fire. The flame front should propagate slowly and eventually engulf all of the charge. This would ensure in a smooth expansion and decrease of pressure inside the engine cylinder. Under such a condition, there would be no unwanted vibration or noise and the operation would be free from detonation.

Mechanism of Detonation

But, in a practical petrol engine, when the charge is compressed to high temperature and pressure, it may get self-ignited at localised points due to the high pressure and temperature. And all these localised flames would propagate radially outward and two or more such flame fronts may collide or impinge against each other. This would cause a haphazard expansion of the charge and would result in what we call as detonation.

Anything that tends to increase the pressure and temperature of the petrol-air mixture would make detonation more probable to happen.

Causes of Detonation

1. High intake pressure
2. High intake temperature of air-petrol mixture.
3. High compression ratio
4. Supercharging of the engine
5. Low engine speed
6. Using a richer petrol-air mixture

Steps to prevent Detonation

1. Adjust the carburetor setting to use a leaner petrol-air mixture.
2. Ensure proper cooling of the engine
3. Use a good quality fuel having higher Octane number
4. Reduce the compression ratio
5. Retard (delay) the instant of firing the spark plug.

Click Here to know the differences between a petrol and a diesel engine.