Fuels For Diesel Engines - BB00.40-P-0131-00A

Model All (4xWD, CAR, Heavy transporter, Light transporter, smart) 

Sheet 131.0 

The availability and quality of the diesel fuel are particularly essential preconditions for running a diesel engine. Fuels and engines must be compatible with each other in technical terms in order to ensure trouble-free operation. At any given time and place the fuel should be available at low cost and easy to access.

Mercedes-Benz diesel engines are designed for diesel fuel, which complies with respective national and international requirements (EN 590 in Europe).

Conventional diesel fuels 

Conventional diesel fuels, such as those that have been used for many years now for fast-running diesel engines, are hydrocarbon compounds, that are generated in a range between 180°C and 360°C during fractionated crude-oil distillation in the refinery plants. These hydrocarbon compounds can have very different molecular structures that naturally exhibit various properties.

Chemical structure of diesel fuel 

The are many possible variants of quadrivalent carbon C forming a compound with monovalent hydrogen H. There are linear chains and chains branched to varying highness levels as well as various ring-type systems, which can be saturated or unsaturated, and the number of multiple bonds is also different.

Alkanes are chain-like saturated hydrocarbons with the empirical formula C_n H2_n +2. These are also known as paraffins and they make up a major portion of the diesel fuel. Normal paraffins (linear) have an excellent ignition quality and favorable smoke formation characteristics, but the cold-flow property is poor. The low density also means that the volumetric calorific value is low. Compared with this iso-paraffins (branched) exhibit an unfavorable ignition quality and a better cold-flow characteristic.

Alkenes are unsaturated, chain-like hydrocarbons (linear or branched) with at least one double bond. In a double bond these are described in the empirical formula C_n H2_n . These compounds, also known as olefines, are similar in terms of characteristics to Iso paraffins, with aging however, they may lead to the formation of polymeric solid substances.

Cycloalcanes are ring shaped, saturated hydrocarbons with the empirical formula C_n H2_n . These compounds, known as cycloparaffins or even better as naphthenes, have a moderate ignition quality, but a favorable cold-flow property. Density and volumetric calorific value are average.

Aromatics, ring-shaped hydrocarbons with cyclically conjugated double bonds, have a lower ignition quality, poor smoke characteristics and a moderate cold-flow property. Density and volumetric calorific values are high.

Requirements, characteristics, parameters (DIN EN 590) 

The diesel fuel characteristics that are necessary for running a diesel engine can either be described or, with the aid of parameters, specified in more detail. Although these parameters, which as a rule are based on standardized test procedures, are indeed useful, they do not always fully satisfy the need to define important quality criteria for handling diesel fuel and for its combustion in the engine. A certain number of such specifications, for which limit values have been defined, are called on to compile standards for minimum requirements.

The additivation of diesel fuel for improving quality is always necessary, and lies within the general responsibility of the fuel manufacturer (see also Sheet 119.0 "Additives and secondary additives for fuels").

Ignition Quality 

The ignition quality represents one of the essential features of diesel fuel. In terms of significance, only a limited comparison can be drawn between this characteristic and the knock resistance of the gasoline. Looked at technically the ignition quality represents the opposite of the knock resistance.

The ignition quality is expressed as the cetane number, and it is measured as per ISO 5165 (CFR engine) or DIN 51773 (BASF test engine) in a standardized test engine and under defined testing conditions. The cetane number of a diesel fuel indicates that under these conditions, the ignition quality of this particular diesel fuel is equivalent to the ignition quality of a mixture of cetane (hexadecane) and heptamethylnonane (DIN 51773 uses 1-methylnaphthalene here). This method is well-proven for conventional diesel fuels. In contrast to this, the frequently used "cetane index" represents a value obtained by calculation which is derived from physical quantities (e.g. density, boiling characteristic). Its significance is of limited value.

The ignition delay, in other words the time span between the injection point and the autoignition of the diesel fuel, represents a parameter for measuring ignition quality. An excellent ignition quality, i.e. a high cetane number, signifies a low ignition delay. This is primarily important when starting the engine, in particular when it is cold. The engine noise, smooth running, is also dependent on the ignition quality. DIN EN 590 specifies a minimum cetane number of 51. Good commercial fuels however have a figure higher than this.

Boiling characteristics 

The boiling-point curve of the diesel fuel lies between approx. 180°C and 360°C, whereby there are major differences around the world. A diesel engine however, is not as dependent on the boiling characteristic for its performance as is the case for a gasoline engine. If however, too large a portion of the diesel fuel does not vaporize until above approx. 350°C or if the final boiling point is too high, in the region of 380°C, or even higher, then this will result in heavy smoke formation. Extending the boiling characteristic downwards is not as critical, but also touches upon certain limitations.

The European Diesel Fuel Standard has three limit values, namely

• Up to 250°C with maximum 65 Vol.% vaporized
• Up to 350°C with minimum 85 Vol.% vaporized
• The 95 Vol.-% point at maximum 360°C

Sulfur content 

The sulfur content in diesel fuel is essentially dependent upon the origin of the crude oil, the refinery's desulfurization capabilities and is governed by standards and/or regulations.

It represents one of the most significant application-engineering parameters for diesel fuel and for this reason it is dealt with in its own Sheet 136.0 "Sulfur in diesel fuels". In general the sulfur content should be as low as possible.

The reduction in sulfur content discussed here, which over the past few years has not only taken place in the European but also in the North American economic area, has raised the problem of the lubricity of diesel fuel (see section on lubricity). This is caused by the fact that during the desulfurization process fuel components with excellent lubricating properties are also reduced. The fuel lubricity however can be sufficiently improved by supplementing it with suitable additives.

Low-temperature behavior 

At low outside temperatures, the fluidity of the diesel fuel can be negatively influenced due to precipitation of paraffin crystals.

The hydrocarbon compounds, in general terms deemed to be favorable for operation in diesel engines, have a major disadvantage; they are not resistant to cold, i.e. even at a few degrees below zero, they form crystalline structures. These paraffin crystals, that "coalesce", clog up the fuel filter, fuel lines and the diesel injection system thereby rendering operation impossible. Indeed although it is frequently possible to start the engine, it soon comes to a standstill. This does not damage the engine, and once the fuel has warmed up the fuel regains its flowability.

To prevent such problems, winter diesel fuels with improved cold viscosity characteristics are available on the market during the winter months. The excellent cold-flow properties are a result of the high-quality basic fuel and the professional additivation of flow improvers by the manufacturer. Generally, these diesel fuels are suitable for the normally prevalent outdoor temperatures.

We recommend to use only winter diesel fuels that are adapted to the local climatic conditions. 

On journeys in colder regions, the vehicle should be refueled with the winter-proof diesel fuel available all-year in certain markets either when arriving at the location in question or already before setting off. Information about all-year winter-proof diesel fuel is available on request from the filling station operator.

In Mercedes-Benz vehicles, measures have been taken that lead to quick heating of the fuel. With unsuitable fuel grades in combination with very low temperatures, however, filter clogging may occur.

When it comes to measuring technique, the cold-flow property of diesel fuel is recorded using the so-called cloud point, pour point and the cold filter plugging point (CFPP). A definite statement as to the maximum temperature at which a vehicle can still operate reliably cannot however be made on the basis of these parameters.

Nowadays, the "filterability limit value", as per the "Cold Filter Plugging Point" (EN 116), is generally used to assess the cold-flow performance. In this method, abbreviated to CFPP, the lowest temperature is determined at which a given fuel quantity flows or is sucked through a specified filter within a certain period of time. As per the German standard, the following values apply for the CFPP:

The cloud point is higher than the CFPP and it defines the temperature at which crystal precipitation starts. In principle therefore, restricted driveability is possible in cold environments above the CFPP - particularly in case of extended exposure to cold temperatures.

For all Mercedes-Benz passenger cars and commercial vehicles, the addition of kerosene  or aviation turbine fuel  to improve the cold resistance is no longer  permissible due to possible negative effects on the diesel injection system due to insufficient lubricity. The use of gasoline  is prohibited due to fuel lubricity deterioration and safety reasons (reduced flash point).

Flow improver additives (secondary additives) have been on the market for some time now. Due to subsequent addition, they generally do not provide the required effect. The refueling of diesel fuel that has a sufficient cold-flow quality is therefore essential to ensure reliable winter operation of the vehicle.

Polar diesel with extreme cold-flow quality is available for Arctic climate zones. Compared with diesel fuel specified for moderate climates, this not only differs in terms of its cold-flow properties, but also mainly through the density, viscosity and the boiling characteristics.

Density 

In Europe, a density value that lies between 820 and 845 kg/m³ at 15°C has been defined for diesel fuel. Up to the end of 1999 an upper limit value of 860 kg/m³ was permissible. Around the world, some markets still permit similarly high values, some even without any density specification.

For fuel manufacturers and suppliers, it is important to have as wide a range of permissible densities as possible.

A range that is specified as narrow as possible is desirable, bearing in mind that the fuel is purchased by volume and the injection pump is metering in volumetric terms, but the thermal value usually increases as density increases. It is not possible to achieve the necessary performance with the given injection-pump settings and an ultralight fuel (low density) nor to comply with the specified emission-control requirements with a very heavy fuel (high density).

Viscosity 

The viscosity of the diesel fuel is responsible for the level of flow and wear prevention in the diesel injection system, and it influences the aerosol formation in the combustion chamber. As per the EN standard, when measured at 40°C, it can lie between 2.0 and 4.5 mm² /s. Generally, this large tolerance range is not fully utilized, which is advantageous.

additives 

The use of diesel fuels with high additivity levels is a necessary measure, and in the long term also a cost-effective one, for ensuring the service life and cleanliness of the engines and fuel systems, maintaining favorable exhaust emission values as well as achieving a good performance overall.

In terms of the supply of such fuels, the individual customer must rely on the filling stations that he/she visits selling such fuels with additives; the opinion of large companies passed-on to us has shown that this is the case nationally, and is usually the case in respect of independent filling stations not tied to major suppliers. Fleet owners should therefore ensure that products with additives are delivered when negotiating on a bi-lateral level.

We would expressly like to point out that according to our assessment the slight percentage increase in fuel costs is more than compensated for by the savings in terms of maintenance and servicing and the lower susceptibility for repair work. Typical complaints that can be prevented from occurring through the use of fuels with high additivity levels are, e.g. coking of injection nozzles, wear and corrosive damages throughout the entire fuel system. Apart from this constantly improving exhaust-emission values help to take the burden off the environment.

The fuel additive gains greater significance when the problems associated with lubricity in sulfur-free fuels is entered into the equation (see section on "Lubricity"). Looked at in this way optimizing the additive process is no longer an option, but a necessity.

The additivity process should be undertaken by the supplier as the party responsible for fuel quality. The use of secondary additives (see Sheet 119.0) is always at the risk of the vehicle operator, since their use may negatively impact warranties issued both by the manufacturer of the vehicle and the fuel supplier.

Storage and transportation 

The following instructions are of particular relevance to those of our customers who own their own filling station.

Diesel fuel is a valuable energy carrier. If it is to be used in the vehicle - in accordance with the customer's wishes - without any problems then certain basic technical rules must be observed.

Fuel tanks may never be operated alternately. Alternate refueling with diesel and spark-ignition engine fuel must thus be avoided. (At least) two clearly separated fuel tanks should be set up if both fuel types are required. If this instruction is not followed, then alternating contamination effects are inevitable.

In particular, customers who do not purchase diesel fuel often, should completely use up their stocks of summer-grade and transitional fuel before receiving a delivery of winter-grade quality.

The floor tank must not contain any water or other dirt. This applies particularly prior to filling the tank with winter diesel fuel. If any contamination is present, the tank should be cleaned. Floor tanks must be checked at regular intervals!

If the fuel supply is changed from fuels without additives to fuels with additives then special care must be taken to ensure that the storage tanks are clean. The detergents present in the fuels containing additives, which serve to keep the vehicle fuel system clean, can also carry dirt particles from the storage tanks into the vehicle's fuel system and thus contribute to a faster clogging of the filter.

Failure to observe these rules can lead to premature blockage of the fuel filter and performance problems during the winter months.

Flash point/hazard class 

The flash point of the diesel fuel as measured according to ISO 2719 must be more than 55°. The flash point is the temperature at which the diesel fuel vapor could be ignited through use of an ignition source. In terms of engine combustion this is irrelevant, but it is however important for safe handling of diesel fuel. This regulation means that simplified transport conditions apply for the diesel fuel.

Even very small admixtures of gasoline will significantly lower the ignition point of diesel fuel. Although the ignition point of diesel fuel is higher than that of gasoline, the self-ignition temperature for diesel fuel is lower than that for gasoline.

Purity 

Diesel fuel must be free of any organic acids and solid matter and be clear when at ambient temperature.

To avoid corrosion, the water content must not be higher than 200 mg/kg. In order to ensure that the diesel fuel does not contain any organometallic, wear-enhancing compounds, the permissible ash content has been set at maximum 0.01 percent by weight.

Diesel fuel components which tend to promote carbonization can cause considerable engine-related problems, e.g. nozzle coking and excessive combustion-chamber deposits. This is why the coke residue is limited by the 10% distillation residue.

Lubricity 

The reduction in sulfur content for environmental reasons which has taken place during the past few years has brought with it the problem of the diesel fuel's lubricity, because hydrogenation of the middle distillate which was required to gain the reduction, also caused the removal of the natural lubrication enhancers.

The addition of lubrication-enhancing additives was absolutely essential after the lowering of the sulfur content in the diesel fuel, to avoid wear on the injection equipment that had occurred in the past. In the meantime, the addition of these lubrication-enhancing additives is no longer required in diesel fuels that contain fatty acid methyl ester ("FAME"), because FAME itself exhibits a lubrication-enhancing influence.

EN 590 regulates the lubricity through specifications in the "HFRR Test" ("High Frequency Reciprocating Rig Test"), in which a ball is put into forced oscillation under load on a plate, whereby the diesel fuel to be tested serves as lubricating medium. This test is described in the ISO test method 12156-1. The maximum permissible limit value specified for lubricity in the HFRR test is 460 urn at 60°C for Europe.

Although the method is largely accepted in the industry, point of criticisms regarding precision and meaningfulness (i.e. correlation with practice) of the test still exist.

FAME content 

The EN 590 as of version 2009 expressly permits a maximum addition of 7 Vol.-% fatty acid methyl ester ("FAME"). This may not exert any negative influence on the diesel fuel quality, i.e. the EN 590 requirements must still be adhered to. It should be especially noted that in Europe only a FAME quality that complies with the EN 14214 regulations for FAME is permitted for additive purposes. Further information on this is available in Sheet 135.0 "Fatty acid methyl ester (FAME) as diesel fuel".

Microorganisms in diesel fuel 

A high water content with formation of a separate water phase in the fuel tank can lead to microbiological growth in the fuel tank (diesel bug). Lengthy idle times promote microbiological infestation. Besides corrosive waste products from the microorganisms, biological sludge is also generated that could clog up the fuel system.

The following symptoms indicate that a vehicle or floor tank is infested with microorganisms:

• Frequent fuel filter change necessary due to loss of performance, gellike substance in filter
• Increased occurrence of internal tank corrosion

Miscellaneous 

Almost all previously mentioned characteristics or parameters are dependent on each other. This applies in particular to density, boiling characteristic, viscosity, ignition point, low-temperature behavior and ignition quality. If one of these characteristics is altered, the others inevitably change too.