Automotive fuels are mixtures of compounds that contain mainly hydrogen and carbon, which is why they’re called hydrocarbons. Petrol and diesel fuel consist mostly of alkanes, characterised by the chemical formula CnH2n+2, where n = 1, 2, 3 and so on.  These compounds are distilled mostly from crude oil, and the process liberates a number of substances, but we will only look at those that eventually become petrol and diesel fuel.

The average barrel of crude contains:  
15-30% virgin petrol. This mixture of compounds contain from 5 to 10 carbon atoms with chemical formulas ranging from C5H12 to C10H22,according to the above formula. The boiling points range from 40ºC to about 180ºC.
15-20% diesel fuel. This contains mainly 13 to 25 carbon atoms, with chemical formulas ranging from C13H28 to C25H52. The boiling points range from 220ºC to 350ºC. These compounds can also be used to produce petrol by a process known as cracking.

Virgin petrol usually contains other substances, such as benzene, which is an aromatic, and whose percentage has to be controlled. There may also be impurities like sulphur and other chemicals that could damage engines and have to be removed.

Octane numbers
A petrol engine is designed to burn fuel in a controlled manner. This means the flame has to ignite only after the spark has occurred, and spread across the combustion chamber as a single flame. Uncontrolled combustion causes a knocking sound that is called detonation and octane improvers have to be added to avoid this.

A fuel’s anti-knock quality can be measured on a special test engine, whose compression ratio can be changed while running. The fuel to be tested is put into the tank, and the engine is run under a steadily-increasing compression ratio until detonation sets in. The test fuel is then replaced by varying mixtures of iso-octane, a fuel that is resistant to detonation, and n-heptane, a fuel that always detonates, until a mixture is found that detonates at the same compression ratio as the test fuel. This mixture is then used to determine the octane rating of the test fuel.

For example, if a mixture of 92% iso-octane and 8% n-heptane detonates at the same compression ratio as the test fuel then the fuel is rated as 92 octane. The above test is run at 600 r/min, and the octane rating is known as the research octane number (RON).

It must be emphasized that the octane rating has nothing to do with the energy content of the fuel. It only refers to the fuel’s tendency to burn without detonation. This means that an engine designed for 92-octane will not benefit from a higher-octane fuel. In fact, an engine designed for 92-octane will only suffer from using 90-octane at large throttle openings because that is when detonation is likely to occur. Also, modern knock sensors will usually retard the timing rather than to allow an engine to detonate, because the latter can damage an engine very quickly if it is severe.

Diesel differs considerably from petrol. It is less volatile and oilier. At 0,832 kg/l it’s also denser than petrol which weighs only 0,745 kg/l. Its heating value is practically the same as petrol so that the energy available per litre is about 11 per cent more. This is one of the minor reasons why diesel engines are more fuel-efficient.

The diesel combustion process relies on the heat arising from the high compression ratio, as well as the heat from the previous combustion event. Cold-starting is taken care of by glow plugs, or fuel heaters in the case of large marine or stationary diesels.

This means there is no single heat source; there are a multitude of flames, each arising from fuel molecules finding sufficient oxygen to combust. This freedom to burn when conditions are right rather than when a spark occurs means that diesel engines can run on a variety of fuel oils.

Unfortunately, diesel fuel has other disadvantages. Its viscosity varies significantly with the fuel temperature, so that at something like -15ºC it ceases to flow. This means that in winter fuel suppliers have to add an additive to keep it in a liquid state at low temperatures.

Cetane rating
Diesel fuel is expected to behave exactly opposite to petrol. It must burst into flames without the benefit of a spark. The cetane number (CN) is measure of the time delay between the fuel entering the combustion chamber and the first measurable pressure rise due to combustion.

Higher CN values usually imply a shorter delay period. Most engines will run well with a CN of 40 to 55, but modern high speed automotive diesels prefer a number of over 50. Numbers over 55 provide no further advantage. Additives are used by the suppliers to improve CN values when they are too low.

Sulphur content
Crude oil contains sulphur in percentages that vary from 0.05 to %, depending on the source, and most of this has to be removed from the by-products before they can be sold. Sulphur is not only harmful to the catalytic converters that are needed to purify exhaust gasses, but during the combustion process the sulphur gets oxidised to sulphur dioxide and sulphur trioxide. These substances dissolve in the water that is also formed as part of the combustion process and form acids that attack metal parts.
South African diesel is available in two grades containing either 500 or 50 ppm (parts per million). Most engines can run on either grade, and if you’re forced to run on the lower grade about all you can do is to change the oil more often. Sump oil formulated for diesel contains an antacid additive to take care of the acids. However, some of the latest automotive diesels on the market should only run on the 50 ppm grade because sulphur damages their diesel particulate filters.

If you’ve allowed the pump attendant to put petrol into the tank the best advice is to NOT turn on the ignition because many modern engines have electric pumps that go into action as soon as the ignition is switched on. Obviously, this also means that one should never start the engine.

Modern diesel fuel injection equipment operates at very fine clearances and pressures between 500 and 2 000 bar. (Your tyres take about 2 bar; one bar = 100  kPa). The slightly oily diesel fuel acts as a lubricant for the pump internals as well as the injector nozzles, but petrol acts like a solvent. It will wash any oiliness away and cause the metal-to-metal contact.

The resulting metal particles will be carried by the fuel to other parts of the engine where they will cause further damage. Some of the additives in the fuel may also affect seals and other non-metal parts. Most modern diesel cars are fitted with common-rail diesel engines and these are particularly vulnerable because of the number of expensive hi-tech components.

If you get petrol in the tank and the fuel pump has NOT been activated then all you have to do is clean all the contaminated fuel out of the tank, but if the pump has sent fuel though then your actions would depend on how long it has been pumping. A few seconds may not make much difference, but every component that could have been affected should be inspected.