Engine Detonation: What is it, and how to prevent it?

Engine knocking (pre-ignition)
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Engine detonation, commonly known as "engine knocking," is a negative phenomenon that occurs during combustion in petrol engines and is acoustically manifested by a knocking sound.

In short, engine detonation is an incorrect fuel mixture combustion at the wrong moment. In this article, we will show how engine knocking manifests and how to prevent it.

Spis treści

What Is Engine Detonation?

When the fuel mixture enters the cylinder, it is heated through compression. At the right moment, when the temperature is almost high enough to cause self-ignition, the fuel mixture is ignited by the spark from the spark plug.

If the mixture self-ignites, its burnout is inhomogeneous and of poor quality. In this case, shock waves begin to spread in the cylinder space, reflected from the internal walls of the combustion chamber, which is acoustically manifested by the already mentioned knocking sound.

This phenomenon is thus commonly called engine knocking because of the knocking sound the engine is making.

What Does Engine Detonation Cause?

If you ignore this problem for long enough, all sorts of problems will begin to manifest, such as the burning of pistons, piston rings, valves, or gaskets under the cylinder head. In addition, the lifespan of the catalytic converter is rapidly reduced.

How To Prevent Engine Detonation?

3D model of a working V8 engine. Pistons and other mechanical parts are in motion.
  • By using fuel with a higher octane number - the higher it is, the higher the temperature and thus also the pressure required for self-ignition of the fuel, which prevents self-ignition

  • By reducing the load on the engine - for example, shifting to a lower gear (in case of a manual transmission) or releasing the accelerator pedal

  • By reducing the timing advance - modern engines have engine detonation sensors and adjust the ignition timing automatically

How Does Compression Ratio Relate To The Engine Detonation?

Another important parameter is the compression ratio of the engine. It is the ratio of the volume of the cylinder between the piston in the bottom and the piston in the top dead center, respectively, the ratio of the entire working volume of the cylinder to the volume of the compression space.

In other words, it is the ratio between the volume of the fuel mixture sucked into the cylinder and the volume of the compressed mixture. The greater the compression ratio, the greater the compression of the fuel mixture before ignition.

To prevent engine detonation, the compression ratio for spark-ignition engines is no greater than 10:1. However, engines equipped with an engine detonation sensor, an engine control unit, and other devices can achieve a compression ratio of up to 14:1.

In turbocharged gasoline engines, the compression ratio is around 8.5:1 because part of the compression of the working substance is done in the turbocharger.

Diesel engines have a compression ratio of 20:1 or even higher because they work on the principle that the injected fuel ignites from the heat of compression. This is why the compression ratio for diesel engines must be higher than gasoline engines. The load from the pressures in the engine cylinder limits the compression ratio of the diesel engine.

Why Is There No Risk Of Engine Detonation In Diesel Engines?

There is no risk of engine detonation within diesel engines because the fuel is injected into the cylinder only when the air is fully compressed. Thus, the ignition occurs when optimal; therefore, self-ignition due to increasing pressure cannot occur.

Watch a short video demonstration of how engine detonation takes place: