CHAPTER No . 5

ANTIKNOCK MATERIALS

The knock characteristics of aviation gasoline has to be determine in an engine, and to get reproducible results both the engine and the method of operation must be standardize.

In 1930 the Coordinating Fuel Research Committee (CFR) in USA develop an engine, on which the compression ratio is increased until knock occurs, but the knock is determine by a detonation meter and not by ear.

The gasoline under test is matched at the critical compression ratio with a blend of two reference fuels, iso-octane with a high antiknock value and normal heptane with a low antiknock value. The result is expressed as an octane number, which is defined as the percentage by volume of iso-octane in the blend with normal heptane that matches the gasoline in knock characteristic in the test engine, run under standard condition.

5.1 REASONS FOR KNOCKING:

Knocking in gasoline engine is due to different reasons.

the carburetor of the engine supplies the petrol air mixture to the combustion chamber which is ignited by the spark from the spark plug. If the first portion of the fuel burns in a normal manner and the last portion burn instantaneously, then a large momentary pressure imbalance is caused which sets up a pressure or shock wave in the combustion chamber resulting in knocking.
Another reason is the pre-ignition of gasoline before the regular spark occurs. It occurs at high compression ratio which increases the temperature of fuel air mixture in the combustion chamber above ignition point even before spark occurs. Hence the fuel ignites before the spark occurs. This pre-ignition of gasoline cause uneven and unsteady combustion resulting in knocking of engine.

5.2 EFFECTS OF KNOCKING:

Knocking in an engine results in wear and tear of cylinder, piston and other engine parts. Beside, it lowers the efficiency of the engine knocking reduces the life of the engine.

5.3 ANTI KNOCKING MATERIALS.

The octane number of a gasoline may be increased by the addition of fuels of higher anti knock value. To achieve this casing head gasoline, cracked and reform gasoline, benzole and alcohol may used. This process is called blending. However, relatively large amounts of the blending agent should be added for this purpose because the effect solely proportional to the amount of blending agent added and its octane number.

The anti knock properties of gasoline are usually increased by adding.

TETRAETHYL LEAD AND TETRA METHYL LEAD (TEL/TML):

The addition of TEL/TML to gasoline is called doping. TEL is a colourless liquid with sweet odour. Its specific gravity is 1.62. It boils at 200^oC with decomposition temperature. About 0.5 ml of TEL/lit is added for motor spirits and about 1ml of TEL/lit is generally added for aviation gasoline.

The addition of a very small amount of TEL to a gasoline of low antiknock value usually increases the octane number to a very considerable extent. However, some fuels are more susceptible to doping then others. Moreover the increase in octane number achieved by each successive 1ml addition of TEL lessens, and generally no appreciable increase in octane number can be obtained by using more than 7ml TEL/gallon. Hence, not more than 7ml TEL/gallon are added for Aviation Fuels.

The addition of TEL to Aviation gasoline increases the induction period by 200 – 300 % by interaction with the free radicals where by lead oxide is produce. As soon as explosive combustion occurs the lead oxide is rapidly reduced to metallic lead which is deposited on the spark plug points, the cylinder valves and the piston head. This necessitates more frequent cleaning of these parts, a part from possible damage. In order to obviate this difficulty Ethylene di bromide is also added to the dope to convert the lead formed by the combustion into the more volatile lead bromide, which is swept out with the exhaust gases.

The efficiency of TEL/TML depends on the composition of the fuel to which they are added. Increasing octane number with out lead by the refining method is an expensive business and it becomes a question of economics to what extent high octane components or lead compounds are used to obtain the required quality.

Since TEL is extremely poisonous, hence gasoline containing TEL is always coloured for identification and safety.

BENZOLE:

Benzole is a coal chemical obtained during the destructive distillation of coal and coaltar. A typical composition of benzole is: Benzene = 75%, Toluene = 15% and Xylene = 10% Since the octane number of gasoline increases in the order.

Paraffin ® Olefins ® naphthene ® Isoparaffin ® aromatics.

As benzole contains large number of aromatics, therefore, it is use as octane boosting agent.

Benzole varies some what in behaviour, according to its make up. That is to say according to the proportions of its constituents i.e. benzene, toluene and xylene, so will its effectiveness as an anti knock agent vary. Generally speaking the higher the toluene content the more effective is its antiknock value with in certain limits.

The main dis advantages to the use of high percentage of benzole in aviation gasoline is due to its principle constituents, benzene, which has a freezing point of about 5^oC, consequently only relatively small additions upto a limit of about 20% can be employed. If freezing toluene are to be avoided in the fuels lines and carburretion system of air-craft engine installations.

Benzole often depreciate rather badly in effective antiknock value when subjected to the operating conditions of some high duty engine.

METHYL TERTIARY BUTYL ETHER: (MTBE)

MTBE is the most adaptable octane enhancing oxygenate know available for blending with unleaded gasolines. However, its acceptability for use by the oil industry has not been immediate. At first, gasoline refiners were reluctant to accept MTBE; it was not a traditional part of the pattern for making gasoline.

The tougher the regulation get, the more MTBE is needed to meet them. Its features are well documented. They include its ease of use and full compatibility as a gasoline additive, its safety and its lack of toxic side affects these have prevent MTBE to be safe to people, animals and to the over all environment under the recommended use conditions.

The environmental protection agency of United States granted approval for the use of MTBE upto 7% volume an unleaded gasoline, in 1979. This was later increase to 11%. In 1988 the allowable MTBE concentration was further increased to 15% volume. The European commission has set an MTBE concentration level of 10% volume in all members states, although they has also set a maximum standard of 15% volume concentration in gasoline authorize by a single state.

5.4 ADVANTAGES OF HIGH OCTANE NUMBER:

As per an estimate, as the octane number increases from 87 to 100, the cruising speed (which is very important for air-planes) increases by 25 km/hr. There is increase in speed of climb by 80 percent and the maximum attainable height increased from 8000 meters to 11000 meters. The required take off distance decrease from 800 meters to 600 meters and pay load also increases by two tons. These all are very important for aeroplanes.