DIESEL KNOCK

One of the characteristics of combustion in a diesel engine is diesel knock.
Knocking in a gasoline engine and knocking in diesel engine are the same in that they occur due to a sudden pressure increase during the combustion process. However, there are a number of fundamental differences betwen the two in terms of the timing, cause, and state of knocking.

Diesel knock occurs when the combustible gas mixture produced during the ignition lag period burns explosively and the pressure rises suddenly.

Knocking in gasoline engine occurs when self ignition occurs too easilly, but diesel knock occurs when self ignition does not occurs easilly enough. Therefore, the causes of two types of knocking are completely opposite from one another.

In gasoline engine , there is large difference between normal combustion and knocking combustion. In a diesel engine, knocking combustion is hard to distinguish during operation. Therefore, knocking is distinguished from normal combustion according to whether or not a sudden pressure increase generates a hitting noise or brings about shock to engine parts.

Because of the nature of its cause, diesel knock can be prevented by shortening the ignition lag period. Fuel injection nozzles are generally designed to lower fuel injection during the ignition lag period.

COMBUSTION IN DIESEL ENGINE

The combustion process in a diesel engine is explained in detail as follows.
Fuel particles injected rom a nozle into the cylinder in the form of high pressure mist are heated by the high temperature and high pressure air. They ignite and burn when they begin to evaporate and are mixed with hot air.

The ilustration shows this process in terms of the pressure in reference to the crank angle. The combustion process can be divided into the following 4 periods.

1. Ignition Lag period.
During the period from A to B, fuel is injected from A in mist form, heated by compressed air in the cylinder and approaches the ignition temperature. Although this period is short, and the pressure does not increase suddenly, because the length o this period heavily influences combustion, it should be as short as possible. The length of this period is influenced by the ignitability o the fuel, the compression pressure and temperature of the air, and the injection state of the fuel.

2. Flame propagation period (Explosive combustion period).
This is the period from B to C in the ilustration. At point B in the ilustration the fuel prepared for combustion during the ignition lag period ignites at one or more locations in the gas mixture. This propagates very quickly to all parts, causing nearly simultaneous combustion. Fuel injected betwen B to C burns at the same time. As a result, the pressure increases suddenly. The increase in pressure is related to the quantity and the atomized state of the fuel injected during the ignition lag period. Most of the injected fuel is completely burnt by the end pf this period (C).

3. Direct combustion period.
This is the period from C to D. Fuel injection continues after point C. Fuel injection and combustion take place simultaneously because of the flames produced betwen B and C. Therefore, the pressure change between C and D can be regulated to some extent by controlling the rate of the fuel injection.

4. Post-combustion period.
Injection ends at point D in the illustration and the burnt gas expands. Any fuel that has not burnt completely burns during this period of expansion. The period after point D is called the post-combustion period.

If this period is too long, the exhaust temperature becomes too high and the thermal efficiency is lowered. Therefore, this period must be short. Combustion during this period is heavily influenced by the size and distribution of the fuel particles and their contact with the air.

Thus, combustion can be divided into four periods. The ignition lag period and the flame propagation period can be regarded as a preparatory period for the direct combustion period ; the quality of these periods gives a large influence on combustion.

Therefore, the initial injection pressure of the nozzle, the state of atomization, the compression pressure and the injection timing are important maintenance items for diesel engines.