131.0   Fuels for diesel engines

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Overview  

Particularly essential preconditions for running a diesel engine are the availability and quality of the diesel fuel. 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 have been used for many years now worldwide for high-speed diesel engines are hydrocarbon compounds that occur in the range between 180 °C and 360 °C during the fractionating crude oil distillation process in the refineries. These hydrocarbons can have extremely different molecular structures, which naturally exhibit different characteristics.

Chemical structure of diesel fuel

The quadrivalent carbon C and the monovalent hydrogen H have numerous bonding capabilities. There are linear and various branched chains as well as assorted ring-type systems, which can be saturated or unsaturated, and the number of multiple bonds is also different.

Alcanes are chain-shaped saturated hydrocarbons with the total formula CnH2n+2, which, also named paraffins, are very important for diesel fuel. Regular paraffins (linear chains, non-branched) have an excellent ignition quality and favorable smoke behavior, the low-temperature flowability however is poor, and because of the low density the volumetric calorific value is low. Compared with this iso-paraffins (branched chains) exhibit an unfavorable ignition quality and a better low-temperature behavior.

Alkenes are chain-shaped (linear-chain or branched) unsaturated hydrocarbons with a double bond; they have the total formula CnH2n. These products - also known as olefins - are similar to the isoparaffines, in smoke terms they are also less favorable.

Cycloalkanes are ring-shaped, saturated hydrocarbons with the total formula CnH2n.These products known as cycloparaffins, or better still naphthenes, exhibit a moderate ignition quality, but have a more favorable low-temperature characteristic, and they have a smoke characteristic similar to that of the olefins. Density and volumetric calorific value are average.

Aromatics, ring-shaped hydrocarbons with double bonds, have a lower ignition quality, poor smoke characteristic and a moderate low-temperature behavior. 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.

In our view diesel fuel additives are absolutely essential for improving quality. This lies within the remit of the supplier as it bears the overall responsibility for its product (see here also the section on Additives).

Ignition Quality

The ignition quality represents one of the essential features of diesel fuel. With regard to its significance for the knock resistance of the benzines however only a limited comparison can be drawn. Looked at technically the ignition quality represents the opposite of the knock resistance.

The ignition quality is expressed as the cetane number and in fact measured in accordance with ISO 5165 or DIN 51773 in a standardized test engine and under set test conditions. The cetane number of a diesel fuel means that under these conditions the ignition quality of this diesel fuel corresponds to the ignition quality of a mixture n-cetane and heptamethylnonane (here DIN 51773 uses 1-methyl naphthaline), the cetane number being calculated from the volumetric percentage of n-cetane for this mixture. This method has proved its worth with regard to conventional diesel fuel. 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 spontaneous ignition, represents a measure of ignition quality. An excellent ignition quality, i.e. a high cetane number, signifies a low ignition delay. This is primarily important when starting, in particular with a cold start. 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 lie higher than this.

Boiling characteristics

The diesel fuel's boiling characteristics lies between approx. 180 °C and 360 °C, whereby there are considerable 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, a major 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 if it is even higher then this will result in smoke formation. Extending the boiling characteristic downwards is not as critical, but also touches upon certain limitations.

The DIN standard recognizes three limit values only, namely:
  • up to 250 °C maximum 65 Vol. % vaporized
  • up to 350 °C minimum 85 Vol. % vaporized
  • 95 % by volume Point at maximum 360 °C
Suitable commercial diesel fuels are however subject to much more stringent specifications.

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 paragraph with regard to this). This can be explained by the fact that the elimination of sulfur from the fuel also removed natural lubrication improvers. The fuel lubricity however can be sufficiently improved by supplementing it with suitable additives. Experience in Scandinavia has shown that otherwise long-term damage to injection pumps is a factor that cannot be ruled out.

Low-temperature behavior

The hydrocarbon compounds generally looked on favorably for operation in diesel engines have a big disadvantage; they are not resistant to cold, i.e. even at a few degrees below zero, they form paraffins in the form of crystals. These paraffin crystals, that "coalesce", plug up the fuel filter, fuel lines and injection system and as a result they render 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. From a measuring technique point of view, attempts have been undertaken to solve the problem with the so-called cloud point, the pour point and the cold filter plugging point (CFPP).

Depending upon the method of fuel production and the vehicle configuration it was possible in practice to transfer these parameters more or less successfully. Today the "filterability limit value" is generally given in terms of the "Cold Filter Plugging Point" (EN 116). In this method, abbreviated to CFPP, the lowest temperature is calculated at which a given fuel quantity flows through a specified filter within a definite period of time. According to the standard this limit value lies below 0 °C in summer, and it lies below -20 °C in winter. During the transitional period a maximum of -10 °C is permitted. Even when normally a provision of at least 3 °C is also given, it is obvious that when the temperatures drop, difficulties cannot be ruled out.

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 injection system due to insufficient lubricity. The use of gasoline is prohibited due to fuel lubricity deterioration and safety reasons (reduced flash point).

Special additive flow improvers have been on the market for some time now. If one reads the manufacturer's specifications, then it is apparent that such additives can indeed be of use. Unfortunately, they do not have the same desired affect on every kind of fuel (see to Sheet 137.0).

Due to the fact that several suppliers provide diesel fuel with guaranteed resistance to low temperatures, we recommend using such fuel only (refer also to Sheets 137.0 and 137.1).

Density

Density is not specified in every country's standard. DIN EN 590 specifies that the density of the diesel fuel is between 820 and 845 kg/m 3 at 15 °C. This density fluctuation permissible in the European market has been significantly reduced as of 1.1.2000; up to the end of 1999, 860 kg/m 3 was permissible as the upper limit. Similar values always still apply in various countries: In principle a large span, when you think that fuel is in fact bought by volume and the injection pump measures volumetrically, but on the other hand the calorific value depends on the mass. Fuel producers

and suppliers place value on having as wide a range of permissible densities as possible. It is not possible to achieve the necessary performance with the given injection-pump settings and an ultralight fuel nor to comply with the specified emission-control levels with a very heavy fuel.

Viscosity

Viscosity, in other words the internal friction, the fuel's tenacity, is responsible for the flow processes and the wear resistance in the injection system and influences the pulverization capability in the combustion chamber. According to the DIN standard, it can lie between 2.0 and 4.5 mm 2 /s at 40 °C; generally, this large tolerance range is advantageously not fully utilized.

additives

The use of diesel fuels with high additivity levels is a necessary measure, and in the long term also cost-effective one, for the service life and cleanliness of the engines and fuel systems, maintenance of 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. Large customers are generally in a position to enter into bilateral negotiations that guarantee the supply of products containing additives; we would recommend these customers to demand that they are provided with such fuels only.

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 increased additives in the fuels 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 application of secondary additives is always at the risk of the operator of the vehicle, since their use may impair any warranty issued both by the manufacturer of the vehicle and the fuel supplier. An exception is given by specific flow improvers or microbiocides (see also Sheet 119.0).

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.

Never operate the tanks alternately, in other words do not fill them alternately with diesel and gasoline, but if demand exists for both fuel types (minimum), then two dedicated tanks should be used. 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 ground tank must not contain any water or other dirt (e.g. from contamination with microorganisms, see Sheet 138.0). This applies particularly prior to filling the tank with winter diesel fuel. If this should however occur, have the tank cleansed thoroughly. Check the bottom tanks 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 blocking of the filter.

Nonobservance of this rule can lead to premature blockage of the fuel system filter and performance problems during the winter months.

Ignition point/hazard class

The diesel fuel's ignition point, as measured by ISO 2719, must be higher than 55 °C. For combustion within the engine, this is in fact meaningless, but important so that the diesel fuel falls into hazard class A III (fluids which are not soluble in water with a flash point between 55 °C and 100 °C) (see also Sheet 112.0).

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.

The water content must not be higher than 200 mg/kg in order to prevent corrosion from occurring. 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. For this reason, the coke residue is limited to 10 % petroleum stock (as measured by Conradson).

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 any wear that had occurred in the past on the injection equipment. In the meantime, the addition of these lubrication-enhancing additives is no longer required in diesel fuels that contain fatty acid methyl ester ("biodiesel", "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 in existence both as a CEC specification (CEC F-06-A96) and an ISO testing technique (ISO 12156-1). The maximum permissible limit value for the lubricity in the HFRR test has been defined in EN 590 at 460 µm at 60 °C.

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

FAME content

The EN 590 standard, as per the 2009 issue, expressly permits the maximum addition of 7 % (vol.) of fatty acid methyl ester ("biodiesel", "FAME"). This must not exert any negative influence on the diesel fuel quality, i.e. the EN 590 requirements must still be adhered to. Special attention must be paid to ensuring that only a FAME quality is permitted for additive purposes that complies with the EN 14214 regulations for FAME. Additional information hereto is available in Sheet 135.0 ("FAME as diesel fuel").

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.