Combustion Process. - Od162070010

Principles of Gasoline And Diesel Fuel Systems
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b. Combustion Process. The normal combustion process in the combustion

chamber goes through three stages when producing power.

They are as

follows:

(1) Formation of Nucleus of the Flame. As soon as a spark jumps the

gap of the spark plug electrode, a small ball of blue flame develops in the

gap. This ball is the first stage, or nucleus, of the flame. It enlarges

with relative slowness and, during its growth, there is no measurable

pressure created by heat.

(2) Hatching Out.

As the nucleus enlarges, it develops into the

hatching out stage. The nucleus is torn apart so that it sends fingers of

flame into the mixture in the combustion chamber. This causes only enough

heat to give a slight rise in the temperature and pressure in the entire

air-fuel mixture. Consequently, a lag still exists in the attempt to raise

pressure in the entire cylinder.

(3) Propagation. It is during the third, or propagation stage that

effective burning occurs. The flame now burns in a front that sweeps across

the combustion chamber, burning rapidly and causing great heat with an

accompanying rise in pressure.

This pressure causes the piston to move

downward.

Burning during normal combustion is progressive.

It increases

gradually during the first two stages, but during the third stage, the flame

is extremely strong as it sweeps through the combustion chamber.

c. Detonation.

If detonation takes place, it will happen during the

third stage of combustion.

The first two stages are normal, but in the

propagation stage, the flame sweeps from the area around the spark plug

toward the walls of the combustion chamber. Parts of the chamber that the

flame has passed contain inert gases, but the section not yet touched by the

flame contains highly compressed, heated combustible gases.

As the flame

races through the combustion chamber, the unburned gases ahead of it are

further compressed and are heated to higher temperatures.

Under certain

conditions, the extreme heating of the unburned part of the mixture may

cause it to ignite spontaneously and explode.

This rapid, uncontrolled burning in the final stage of combustion is called

detonation. It is caused by the rapidly burning flame front compressing the