Knocking

KNOCKING

Definition

Knocking is a kind of explosion due to rapid pressure rise occurring in an IC engine.

1. Causes of knocking in S.J (Spark Ignition) Engine [Petrol engines]

In a petrol engine, a mixture of gasoline vapour and air at 1:17 ratio is used as fuel.

This mixture is compressed and ignited by an electric spark. The products of oxidation reaction (combustion) increases the pressure and pushes the piston down the cylinder.

If the combustion proceeds in a regular way, there is no problem in knocking. But in some cases, the rate of combustion (oxidation) will not be uniform due to unwanted chemical constituents of gasoline. The rate of ignition of the fuel gradually increases and the final portion of the fuel-air mixture gets ignited instantaneously producing an explosive sound known as “Knocking”. Knocking property of the fuel reduces the efficiency of engine. So a good gasoline should resist knocking

Chemical structure and knocking

The knocking tendency of fuel hydrocarbons mainly depends on their chemical structures. The knocking tendency decreases in the following order.

Straight chain paraffins > Branched chain paraffins > Cycloparaffins > Olefins > Aromatics.

Thus olefins of the same carbon-chain length possess better anti-knock properties than the corresponding paraffins.

2. Improvement of antiknock characteristics

Prevention

The octane number of fuel can be prevented by

1. blending petrol of high octane number with petrol of low octane number, so that the octane number of the latter can be improved,

2. the addition of anti-knock agents like Tetra-Ethyl Lead (TEL),

3. now a days aromatic phosphates are used as antiknock agent because it avoids lead pollution.

OCTANE NUMBER (or) OCTANE RATING

Octane number is introduced to express the knocking characteristics of petrol. It has been found that n-heptane knocks very badly and hence, its anti-knock value has been given zero. On the other hand, iso-octane gives very little knocking and so, its anti-knock value has been given 100.

Definition

Thus, octane number is defined as the percentage of iso-octane present in a mixture of iso-octane and n-heptane.'