Car Parts and Accessories

Octane numbers

A petrol’s octane number is determined by matching the petrol against a mixture of two petroleum fluids in a laboratory test engine. One, iso-octane, has a high resistance to knocking. The other, normal-heptane, has a low resistance.

A petrol is said to have a 90 octane number if it has the same anti-knock ability in the laboratory test engine as 90 parts of octane mixed with 10 parts of heptane.

The octane rating of petrol is boosted by the amount of tetra ethyl lead (TEL) it contains. In many countries the TEL content has been lowered to reduce the emission of lead, but to maintain octane values the petrol then has to undergo further costly refining.

Many other factors, like volatility, affect the way fuel is consumed.

Petrol is a complex mixture of hydrocarbons. Its characteristics change during storage and it is wise to use busy filling stations with a rapid turnover. The engine octane requirement will also vary with use, mainly due to the build-up of carbon in the combustion chambers.

There is no advantage in using a petrol with a higher octane than required, and there is the disadvantage of extra cost. In some areas of South Africa an alcohol byproduct of the fuel-from-coal process is added to high-octane petrol, improving octane values and slightly reducing the lead level.

How petrol is refined and processed to

Petrol is one of the many products derived from petroleum—more commonly known as crude oil. The crude oil is distilled in a refinery in a tall metal container known as a fractionating tower.

The oil is heated in a furnace to a temperature that will ensure that all the products to be extracted are vaporised. As the vapour rises in the column, it condenses into liquid at different levels.

Petrol obtained from the fractionating tower is low in octane number. It has to be processed to obtain a suitable octane number and treated to remove, or at least counteract, corrosive or gum-forming elements. Then it is blended into various grades and treated with additives to increase its resistance to detonation, and to control carburettor icing.

Petrol, especially some of the higher grades, is also produced as a by-product of other chemical processes, but the source of most petrols is crude oil.

Unleaded petrol

The compound known as tetra ethyl lead was added to petrol in an effort to find a solution to the problem of the pre- ignition. Pre-ignition is the result of spontaneous ignition in the combustion chamber and can cause severe mechanical damage. However tetra ethyl lead is not destroyed during combustion and is discharged into the atmosphere as a complex compound of lead. Research has shown that lead emissions from exhausts may be a health hazard and many countries have adopted emission-control measures that rely on unleaded petrol and the use of catalytic convertors.

Cars requiring unleaded fuel should operate on unleaded fuel. In South Africa, where lead-free petrol is not available, imported cars may be run on leaded petrol, but damage to the exhaust catalysts will result.

Vapour lock and icing

On some cars, the fuel supply to the carburettor can be restricted in warm weather by vapour lock. Vapour bubbles form in the fuel system between the tank and the carburettor, and prevent the pump delivering petrol.

Sometimes engine heat can cause petrol in the carburettor bowl to boil when the engine has stopped. This causes an excessively rich mixture in the induction pipe or manifold and makes starting difficult.

To avoid these problems, petrol companies alter the volatility to suit seasonal temperature changes and car manufacturers may fit a vapour lock trap, or use a T- piece in the fuel line to allow a restricted amount of warm fuel to flow back to the fuel tank.

Carburettor icing can occur in very cold, damp weather when moisture in the air supply freezes in the carburettor.

Fuel consumption

Driving methods have a considerable effect on fuel consumption; variations can occur even in similar runs made on consecutive days, because of differences in speed and traffic conditions.

A car travelling on an expressway might use 8 litres of fuel for every 100 kilometres covered at 80 km/h but it will use 10 litres at 110 km/h and in heavy traffic might easily use 13 litres / 100 kilometres.

The first kilometre, on a cold engine, uses much more fuel than a kilometre run when the engine is warm, so it is important to get the engine warm quickly. Usually, this is best done by driving the car away immediately after the engine has started and not allowing it to idle, and by being careful to use the choke as little as possible.

Measuring fuel consumption accurately is not easy, but a fair idea can be obtained by filling the tank until the bowser’s nozzle reaches its first automatic cut-off. After a long journey, refill at the same bowser, to the same first cut-off. Divide the number of litres on the second fill by the hundreds of kilometres travelled for an accurate litres / 100 km figure. For example, if the car has used 31 litres for 298 kilometres, divide 31 by 2,98 for an answer of 10,4 litres / 100km.

Some prefer to express fuel consumption in km /l. Just divide the distance covered by the fuel used. i.e. 9,6 km /l.

Possibly related posts: (automatically generated)
Octanes and Emission Control

• Emission-equipped Carburetor, mechanic Car Auto Part Repairing Guide
• Petrol injection, Auto Fuel System and Auto Part Repair Guide
• Car Fuel System/ Auto Air Cleaners
• Why Car Engine Cranks normally, but fails to start
• How the Auto Carburettor Works
• All thumbs up guide to car Glossary
• Inlet Manifold and Automobile Carburettors
• Fuel system, how to fix Auto Car Petrol supply
• Fixed Jet Carburettor Auto Parts , accelerate Car Engine for different Vehicle Speeds
• Car Won't Start, why? How to get your Auto Car checked and Repaired by yourself? step (31- 40)