DIESEL

Sulphur occurs naturally in crude oil and the sulphur content can range from 100 TO 33 000 parts per million (ppm). If the sulphur is not removed during the refining process it will contaminate the Diesel fuel. Depending on the crude oil used, as well as the refinery configuration, sulphur levels in Diesel fuel can be as low as 10 ppm or exceed 10,000 ppm.

Since January 2006, 2 grades of Diesel fuel have been available in South Africa : 

• Standard Grade Diesel (500 ppm sulphur (max); Standard Diesel 500 ppm)
• Low sulphur grade Diesel (50 ppm sulphur (max) Low-sulphur Diesel 50 ppm)

The presence of Sulphur in Diesel fuel can lead to the corrosion and wear of engine components and this can have a significant effect on engine life. As the sulphur level decreases, the engine life increases.

During combustion, the formation of sulphur oxides and water vapour combine, resulting in sulphur acids (sulphuric and sulphurous acids). During combustion, nitrogen from the air is also oxidised, forming nitrogen oxides (NOx) which in turn become nitric and nitrous acids. These acids are extremely corrosive to engine components, so good quality engine oil fortified with appropriate levels of neutralising agents are necessary to protect the engine components from damage.  Exhaust Gas Recirculation (EGR), is now widely used in Light Duty and Passenger vehicles and to a lesser extent also in heavy duty engines. EGR effectively returns a portion of the exhaust gas back to the engine under part-load operating conditions. Since NOx formation is extremely sensitive to temperature, by replacing some of the intake air with inert exhaust gas, the flame temperature can be reduced by a small amount and the NOx emissions can be reduced without seriously affecting the combustion efficiency. The exhaust gas is usually cooled before returning it to the engine via the intake manifold and the small amount of sulfuric acid in the exhaust may condense, and this presents a risk of internal engine corrosion. The lower the sulphur level in the Diesel fuel, the lower the risk of internal engine corrosion.

Since the 1920’s, lead has been widely used as a petrol additive.  However, leaded petrol cannot be used by cars equipped with catalytic converters designed to reduce harmful exhaust emissions, as lead very rapidly and permanently renders the catalyst completely ineffective. This, together with the realization that lead emitted from vehicle exhausts has the potential to adversely affect human health, resulted in it being phased out, first in North America and then across Europe and, increasingly, the rest of the world.