Car Parts and Accessories

Additives raise the octane number

Petrol has to contain a combination of properties to be suitable for use in a car engine. It has to be volatile (that is, it must vaporise easily) to give trouble-free starting in cold weather and quick warm- up with little use of the choke. But it must not be so volatile that it vaporises too readily, causing a condition known as vapour lock, or is uneconomical to use.

Petrol must also be resistant to detonation (evidenced by a noise known as knocking or pinking). It should be clean, without a tendency to leave deposits.

Knocking occurs when a car is using a grade of petrol that is too low in octane number for the compression ratio of the engine. Continuous knocking will overheat the piston and eventually make a hole in it.

If the octane number of the petrol is not high enough, the mixture detonates instead of burning smoothly across the combustion space. The detonation can often be heard at low speeds under wide throttle openings as a metallic knock.

Almost any fault that increases temperature within the engine above the normal working temperature will result in an increased octane number requirement.

Mixture strength

The chemically correct ratio for complete combustion is about 15 parts of air to one part of petrol by mass. In practice, however, this is too lean for most current engines and a balance between good performance and optimum economy will usually be found with an air/fuel ratio of about 14:1 on most cars. With this ratio, the exhaust gas CO level will be in the vicinity of 0,8 to 1,5 per cent. For maximum power, or for cold engine starts in cold weather, the air/fuel ratio may be enriched to 12:1.

A mixture that is either too rich, because the proportion of petrol is greater than it should be, or is too weak, may fail to ignite. If the engine stalls through over- richness (from excessive use of the choke, for example) the cylinders should be cleared by depressing the accelerator and operating the starter until the engine fires.

This expels excessive fuel through the exhaust system. Jabbing the accelerator will increase the difficulty by making the mixture still richer.

In an area extending from the Reef, and covering most of the northern and western Transvaal, eastwards to the Natal border and south to De Aar, all premium petrol contains a nominal 10 per cent alcohol. Alcohol will absorb small quantities of water which may find its way into the fuel system. The presence of excess water may cause phase separation which could result in costly damage to fuel components in some cars.

Condensation of moisture occurs inside all fuel tanks, especially in those exposed to low temperatures at night after the car has been in warm sunshine during the day.

As the car cools, air is drawn into the petrol tank. Condensation occurs and water droplets form in the fuel tank. Ideally, in areas where condensation levels are high, the fuel tank level should be kept as high as possible to reduce the space for air to be drawn in.

Remember that it is illegal to store large amounts of petrol in your garage. Small amounts, for use in lawnmowers, for instance, must be kept in an approved container. When storing a vehicle for a long time drain the petrol from the pump and carburettor to reduce possible formation of gum deposits.

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