What is Detonation in an Engine?

Detonation in an engine means the rise of sudden pressure in an internal combustion engine, which affects the engine components. In this article, I will discuss the meaning of Detonation in an Engine, the factors affecting detonation, etc.

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Detonation in an Engine

At present the amount of power that can be developed in the cylinder of a petrol engine is fixed by the liability of a fuel to detonate, i.e., just before the flame has completed its course across the combustion chamber and the remaining unburnt charge fires throughout its mass spontaneously without external assistance.

The extreme pressure causes damage to the cylinder wall and the piston wall, also the excess pressure causes damage in the combustion chamber, which is called "ping". It is the ping that manifests detonation. Thus, a sudden rise of pressure during combustion ac­ companied by a metallic hammer-like sound is called detonation.

The area in which knocking or the detonation occurs is farthest removed from the sparking plug, and is named the •detonation zone" and even with severe detonation, this zone is rarely more than one-quarter the clearance volume.

Process of Detonation in an Engine

Detonation is called the engine knocking. The process/phenomenon of detonation or knocking may be explained by referring to Fig., which shows the cross-section of the combustion chamber with flame advancing from the spark plug location A.

>The pre-combustion flame front compresses BB'D(the end charge) farthest from the spark plug, thus raising its temperature. The temperature of the end charge also increases due to heat transfer from the hot advancing flame front.

>Also, some pre-flame oxidation may take place in the end charge leading to a further increase in its temperature.

Read more - What is the difference between petrol engine and diesel engine 

>If the end charge BB'D reaches its auto-ignition temperature and remains for some time to complete the pre-flame reactions, the order will auto-ignite leading to knocking combustion. During the pre-flame reaction period, the flame front could move from BB' to CC', and the knock occurs due to auto-ignition of the charge ahead of CC'.Here we have combustion unaccompanied by flame, producing a very high rate of pressure rise.

> The diagram of the pressure time of detonating combustion in S.I. engines is drawn and labeled below :

> The intensity of detonation' will depend mainly upon the amount of energy contained in the 'end-mixture' and the rate of chemical reaction which releases it in the form of heat and a high-intensity pressure wave.

Thus, the earlier in the combustion process the detonation commences, the more unburnt end-mixture will be available to intensify the detonation. As little as 5 percent of the total mixture charge when spontaneously ignited will be sufficient to produce a very violent knock.

Theories of Detonation

There are two general theories of detonation- 

1. The Auto-Ignition Theory 
2. The Detonation Theory.

The Auto-Ignition Theory

Auto-ignition does not require any flame for combustion in the engine. The auto-ignition theory of knock assumes that the flame velocity is expected before the onset of auto-ignition and that gas vibrations are created by a number of end-gas ele­ments auto-igniting almost simultaneously.

The Detonation Theory

In the auto-ignition theory, it is assumed that the flame velocity is normal before the onset of auto-ignition whereas in detonation theory a true detonating wave formed by preflame reactions has been proposed as the·mechanism for explosive auto-ignition. Such a vibration or shock would travel through the chamber at about twice the sonic velocity and would compress the gases to pressures and temperatures where the reaction should be practical.

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In fact, knocking or detonation is a complex phenomenon and no single explanation may be sufficient to explain it fully.

Effects of Detonation

The effects of detonation are - 

> Noise and roughness.

> Mechanical damage.

>  Carbon deposits.

> Increase in heat transfer.

> Decrease in power output and efficiency.

> Pre-ignition.

Control of Detonation

The knocking of the engine can be controlled or even stopped by the following methods :

> Increasing engine r.p.m.

> Retarding spark.

> Reducing pressure in the inlet manifold by throttling.

> Making the ratio too lean or too rich, preferably the latter.

> The use of high-octane fuel can eliminate detonation. High-octane fuels have dopes (such as tetra-ethyl lead, benzol, xylene, etc.), to petrol.

Factors Affecting Detonation/Knocking

The detonation of the engine can happen in many processes. It has many factors that affect the knock or the detonation. Particular factors are listed below-

> Fuel choice. A low self-ignition temperature promotes knock.

> Induction pressure. An increase in pressure decreases the self-ignition temperature and the induction period. Knock will tend to occur at full throttle.

> Engine speed. Low engine speeds will give low turbulence and low flame velocities (combustion period is constant in angle) and knock may occur at low speed.

> Ignition timing. Advanced ignition timing increases peak pressures and promotes knock.

> Mixture strength. Optimum mixture strength gives high pressures and promotes knock.

Conclusion

The engine detonation is an annoying problem for the driver and the passengers sitting inside the car. The engine detonation can cause several engine damage and the lubrication system can cause problems. Regular checkups and time-to-time servicing can save the engine from detonation.