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MOTOR SPIRIT/ GASOLINE(M S)

VAPOUR LOCK

This is the most important limit to the maximum volatility of motor gasoline and the only limit under hot weather conditions. It is caused by premature volatilization resulting in the stoppage of fuel flow to the carburetor and is obviously accentuated, if the gasoline itself is of high volatility, as in the case with motor gasoline and power alcohol

Cars show a high variation in their susceptibility to vapour lock; this is due entirely to differences in engine design. However, for a given car there are three main factors which influence the tendency to vapour lock; fuel volatility, ambient temperature and altitude.

The vapour locking tendency of a gasoline is minimized by controlling its volatility, particularly its Reid vapour pressure.

MIXTURE DISTRIBUTION

Gasoline is largely, but not completely vaporized before it enters the engine cylinders. The uniform distribution of the unvapourised fuel is difficult to achieve with result that some cylinders receive a richer mixture than others and general efficiency suffers. Recovery of heavier ends up to 90% volume, give indication  for mixture distribution.

A suitable upper limit on the final boiling point of the gasoline helps to minimize the risk of crankcase oil dilution, particularly under cold starting conditions.

CLEANLINESS AND STABILITY

Inadequately refined or addition of large percentage of cracked stream in gasoline may lead to gummy and unstable compounds liable to form gum in storage, particularly under high temperature conditions. Gum tends to leave deposits in the carburetor and the inlet system of an engine. These deposits are harmful because they can give rise to (a) valve sticking with loss of power, (b) valve burning and (c) deposit build-up in the inlet system causing reduction in air flow and engine output.

SULPHUR AND MERCAPTANS

Gasoline normally contains very little sulphur, but this can be harmful on account of low temperature corrosion caused by condensed products of combustion. (Sulphur on combustion forms sulphur oxides, which when dissolved in water becomes sulphuric acid). However, it is pertinent to mention here that wear in the modern car engine is predominantly non-corrosive in nature. Nevertheless, it must be appreciated that the presence of sulphur reduces the the antiknock effect of T.E.L.

It is important that, motor gasoline be free of those sulphur compounds which in themselves attack metal parts of the engine or the fuel system. This characteristic is tested by the Copper Strip Corrosion Test, a severe dis-coloration or pitting of the polished strip indicates the presence of corrosive sulphur compounds in the fuel.

Mercaptans are sulphur compounds with objectionable odour. When these are present, the gasoline is subjected to a "sweetening" process to remove the objectionable odour. The Doctor's test is used to detect the presence of mercaptans, a negative result indicating their absence.

T.E.L.

Octane number of cracked stream form FCCU is higher than that for straight run stream from CDU. However, to attain required octane number, percentage of cracked stream is restricted by its poor oxidation stability. Tetraethyl lead is added to motor gasoline to improve its anti-knock value (Octane rating). During combustion TEL forms an oxide of lead which is not volatile and so should tend to deposit on the combustion chamber, valves, spark plugs, etc. with harmful effects. To counteract this. "Scavenger" compounds, such as ethylene dibromide and ethylene dichloride, are mixed with TEL so that on combustion more volatile compounds of lead are formed, resulting in most of the metal passing out with exhaust gas. The TEL together with the scavengers and a dye to indicate the poisonous nature of the compound is known as "ethyl fluid". Despite the use of decavenger compounds, it can give rise to such difficulties as: sparkplug fouling, surface ignition, exhaust valve failure and octane requirement increase.