136.0   Diesel engine (Sulfur in diesel fuel)

Select Sheet 

The occurrence of sulfur in fossil diesel fuel comes from the raw material, crude oil. Depending on the origin of the crude oil, the quantity of sulfur contained will differ and it has to be separated from the fuel components during the refining process through a desulfurization procedure.

The reduction in the fuel's sulfur content is a global process taking place for environmental protection reasons.

Normally, fuel standards specify a limit for the maximum permissible sulfur content. A minimum content is not necessary ,however, for technical operation and therefore not required.

Mercedes-Benz diesel engines are designed for diesel fuel that complies with respective national requirements (EN 590 in Europe) in the approved markets.

Sulfur content in the EU

During the course of the past few years the sulfur content in diesel fuel has been constantly reduced. Nowadays, the limit value lies at a maximum of 10 mg/kg, the diesel fuel in Europe is therefore deemed to be sulfur-free. In technical terms, there are no objections to operation with a diesel fuel that has a lower sulfur content. This is called for to a greater extent by environmental reasons. Sulfur-free diesel fuels have been specified in the EU since 1st January 2009.

Sulfur content in other countries

In many countries even today values around 0.3 % by weight are usual, in some countries as always there are sulfur values around 1.0 % by weight and above. Sheets 136.1 and 136.2 show the (incomplete) worldwide list of sulfur content in diesel fuel, divided into classes.

Effects of sulfur on basic exhaust treatment technology

By this we refer to either no exhaust-gas after treatment at all or an emission control by means of an oxidation catalytic converter.

At a sulfur content of > 0.3 % by weight in the diesel fuel, the service life of the engine is impaired. Sulfur dioxide is given off during the combustion process which together with water then forms sulfurous acid, this in turn can lead to corrosive wear, in particular when running a cold engine. One possible remedy here is the use of engine oils with a strong alkaline effect and a reduction of the oil change interval.

Sulfur contents above 0.5 % by weight apply as tougher operating conditions. With appropriate operating conditions and the use of an oxidation catalytic converter, a significant increase of the particulate masses relative to the fuel's sulfur content is detected. This originates from the catalytic oxidation of the developed sulfur dioxide to form sulfur trioxide and the associated emission of sulfate or sulfuric acids, adsorbed to the soot particles.

Effects of sulfur on advanced exhaust treatment technology

These are understood to include, for example, NOx storage catalytic converters based on earth alkali/rare metal or the "CRT system for particulate reduction.

For trouble-free operation these systems require sulfur-free diesel fuel (< 10 mg/kg). In both cases the sulfur trioxide formed from the fuel sulfur interferes with the catalytic conversion of pollutants, in addition the NOx storage catalytic converter being irreversibly blocked by the formation of earth alkali sulfates.

Supporting measures

For diesel fuels with extremely low sulfur content, special focus should be placed on the lubricity (see section on "Lubricity" Sheet 131.0). The desulfurization process impairs the lubrication properties of the diesel fuel significantly, and they have to be adjusted again through corresponding additivation.