Diesel fuels are distilled from crude oil. They consist of a large number of different hydro-carbon compounds including n-paraffins, i-paraffins, olefins, naphthenes and aromatic compounds. They all have boiling points in the range 160...380 °C (middle distillates).
Diesel fuel ignites on average at approxi-mately 350 °C, which is very early in com-parison with gasoline (500 °C) (lower limit 250 °C).
In order to cover the growing demand for diesel fuels, the refineries also add “conver-sion products”, i.e. thermal and catalytic-cracking products. They are obtained by cracking large heavy-oil molecules.
Quality and grading criteria
The basic fuel grade is improved by the use of a series of additives, some of which have a decisive effect (see Table 2 at the end of this section).
16 grading criteria are specified by the stan-dard EN 590 for motor vehicles which now applies throughout Europe. In many other countries around the world, the fuel stan-dards are less stringent or in some cases nonexistent. The US standard for diesel fuels ASTM D975, for example, specifies fewer criteria and applies less strict limits to these criteria. The requirements for marine and fixed-installation engines are also much less
demanding. Some of the most important grading criteria specified by EN 590 are listed in Table 1 below. It also shows the European motor manufacturers’ require-ments for diesel-fuel grade which are also subscribed to by Bosch. Such criteria help to keep vehicle emissions within present and future limits.
High-quality diesel fuels are characterized by the following features:
High cetane number
Relatively low upper boiling limit
Narrow density and viscosity spread
Low aromatic compounds (particularly polyaromatic compounds) content
Low sulfur content (≤10 ppm)
In addition, the following characteristics are particularly important for the service life and consistent function of fuel-injection systems:
Good lubricant qualities
Absence of free water
Low dirt content
The most important criteria are explained individually below.
Cetane number
The cetane number indicates the ease with which a diesel fuel ignites and is therefore of decisive importance. The higher the cetane number, the more easily combustible the fuel is.
28 Basic principles of the diesel engine Diesel fuels
Table 1
1) Diesel fuel with a sulfur content of 10 ppm will be available throughout Germany from 1/1/2003 and throughout the EU from 1/1/2005.
Criterion European motor vehicle
manufacturers EN 590
Cetane number ≥51 ≥58
Density 820...845 kg/m3 820...840 kg/m3
Aromatic compounds content – ≤20 % by vol.
Polyaromatic compounds content ≤11 % by vol. ≤1 % by vol.
Boiling point (95 %) ≤360 °C ≤340 °C
Upper boiling limit – ≤350 °C
Sulfur content1)(by mass) ≤350 ppm 5...10 ppm for compliance with Euro IV and V emission limits
Lubricity (HFRR) ≤460 µm ≤400 µm
Selected EN 590 grading criteria compared with the requirements of the European motor manufacturers
1
The cetane number is tested using a standard-ized single-cylinder testing engine. The igni-tion lag is set for the fuel under test by means of a variable compression ratio. The engine is then run on a reference fuel made up of a mixture of cetane and α-methylnaphthalene (Figure 1) using the same compression ratio.
The proportion of cetane in the mixture is al-tered until the same ignition lag is obtained.
The proportion of cetane then gives the cetane number (for example, a mixture of 52%
cetane and 48% α-methylnaphthalene has a cetane number of 52).
Paraffin fuel components have a high cetane number while aromatic compounds (chiefly cracking products) have a low cetane num-ber; i-paraffins, olefins and naphthenes have a medium cetane number.
Ignition accelerators can be added to the fuel to improve its cetane number. All types of emission, particularly NOx, diminish as the cetane number increases, as does the combustion noise.
Density
The energy content of diesel fuel per unit of volume increases with density. Fuels are sold by volume and delivered to the combustion chamber by fuel-injection systems on the same basis. If an engine is designed for use with a “medium-density” fuel, then if it is run on higher-density fuel (based on fuel grade), engine performance and soot emis-sion increase; they diminish if a lower-den-sity fuel is used. Temperature-dependent variations in fuel density are compensated for by the EDC system.
The requirement of diesel fuel is therefore
“narrow grade-based density spread”. A den-sity sensor could also provide a solution to the problem. There is a greater density spread found in fuels around the world than permitted by EN 590.
Viscosity
If the viscosity of a fuel is too low, it will lead to leakage losses in the fuel-injection system at low engine speeds in particular and there-fore also to power deficiencies and hot-start problems. If the viscosity is too high, it will impair pump function and result in poor fuel atomization. Therefore, EN 590 speci-fies narrow tolerance limits for diesel-fuel viscosity.
Boiling range
The boiling range is the temperature range within which the fuel boils.
A low initial boiling point makes a fuel suitable for use in cold weather but also means a lower cetane number and poor lu-bricant properties. A high upper boiling limit gives long-chained paraffins poor cold-start-ing properties but a higher cetane number.
Polyaromatic compounds with three or more rings also have a high boiling point but a low cetane number. As the polyaro-matic-compound content of diesel fuel increases, more soot is produced as a by-product of combustion.
Basic principles of the diesel engine Diesel fuels 29
Fig. 1 C Carbon H Hydrogen –– Chemical bond
H H highly combustible (CZ 100)
α-methylnaphthaline (C11 H10) non-combustible (CZ 0)
Reference fuels for testing cetane number
1
æ
SMK1877EWith a view to avoiding poor cold-starting properties (paraffins) and high soot emis-sions (polyaromatic compounds), therefore, the upper limit of the boiling range should not be too high. The ACEA requirement for this property is therefore 350 °C. But al-though such a requirement is valuable in terms of combustion efficiency, it is offset by a lower level of crude-oil exploitation.
Cold-weather properties
At temperatures ≤0 °C, diesel fuels may pre-cipitate paraffin crystals which can clog up the fuel filter. For this reason, oil companies add flow enhancers to diesel fuel in the win-ter to limit crystal formation so that their size still allows them to pass through the filter pores.
The previously common practice of adding gasoline or kerosene is no longer necessary and also dangerous because it lowers the flash point. In cold parts of the world, the oil industry produces winter diesel fuel with a CFPP rating (Cold Filter Plugging Point, i.e. the point at which it clogs the filter in cold weather) (e.g. at least –20 °C for Germany). For Arctic regions, the CFPP is substantially lower (as much as –44 °C).
Lubricant properties (“lubricity”) In order to reduce the sulfur content of diesel fuel, it is hydrogenated. In addition to removing sulfur, the hydrogenation process also removes the ionic fuel compo-nents that aid lubrication. After the intro-duction of low-sulfur diesel fuels, wear-related problems started to occur on distrib-utor-type fuel-injection pumps which are lubricated by the fuel. The oil industry was able to fully restore the lubricant qualities, however, by adding lubricant additives.
Since 1998 lubricity has been standardized on the basis of the HFRR method (High Frequency Reciprocating Rig) (in which a steel ball is moved rapidly to and fro) by EN 590 and ISO 12 156-1 and 12156-2.
A maximum permissible WSD (Wear Scar Diameter, i.e. caused by the steel ball)
deter-mined according to the HFRR method of 460 µm, is adequate to protect fuel-injection pumps. For brand new pumps, Bosch rec-ommends the use of a diesel fuel with a WSD ≤400 µm.
Water in diesel fuel
Diesel fuel can absorb water in solution in varying proportions depending on tem-perature, e.g. 50...200 ppm (by weight) at 25...60 °C.
EN 590 permits a maximum water con-tent of 200 mg/kg. In many countries, how-ever, analysis of diesel fuels reveals higher water concentrations. Dissolved water does not harm the fuel-injection system. Free wa-ter, however, which cannot be dissolved in the fuel, can cause damage to fuel-lubricated injection pumps within a very short space of time and even when it is present only in very small quantities.
The presence of water in the fuel tank as a result of condensation from the air cannot be prevented. A water separator and a water sensor on the fuel filter are therefore ab-solutely essential. In addition, the vehicle manufacturer must design the tank ventila-tion system and the fuel-filler neck so as to prevent additional water from entering.
Overall contamination
Overall contamination refers to the sum total of undissolved foreign particles in the fuel such as sand, rust and undissolved organic components. EN 590 permits a maximum of 24 mg/kg. However, this figure is too high. Particularly the very hard sili-cates that occur in mineral dust are harmful to precision-made high-pressure fuel-injec-tion systems. Even a fracfuel-injec-tion of the permis-sible overall contamination level of hard particles would produce erosive and abrasive wear (e.g. at the seats of solenoid valves).
Such wear causes valve leakage which lowers the injection pressure and engine perfor-mance as well as increasing exhaust particu-late emissions.
30 Basic principles of the diesel engine Diesel fuels
A particle size of 6...7 µm in the fuel is critical, especially considering the fact that 100 ml of fuel can contain millions of such particles. High-efficiency fuel filters that not only achieve very good filtration results but also have long replacement intervals can help to solve the problem.
Sulfur content
Diesel fuels contain varying amounts of sul-fur in chemically bonded form depending on the quality of the crude oil. The sulfur is extracted from the middle distillate by hy-drogenation at high pressure and tempera-ture in the presence of a catalyst. The initial by-product of this process is hydrogen sul-fide (H2S) which is subsequently converted into pure sulfur.
Since the beginning of 2000 the EN 590 maximum limit for the sulfur content of diesel fuel has been 350 ppm. From 2005 onwards the EU (European Union) will require all diesel fuels to contain less than 10 ppm of sulfur.
Emission-control systems such as NOX
catalytic converters and particulate filters function on the basis of catalytic effects and have to be run on sulfur-free fuel (≤10 ppm). Otherwise, instead of the NOX
and HC reactions, sulfur reactions would take place and the catalytic converter would
be to a greater or lesser degree “contami-nated” for the purposes of emission elimina-tion, and therefore incapable of performing its intended function.
Regardless of the function of the systems used for emission control in the future, sul-fur dioxide (SO2) and sulfate particle emis-sions can also be eliminated by the use of sulfur-free fuels.
Coking
The coking tendency of a fuel is an ex-tremely complex process. The coking factor indicates the degree to which the fuel injec-tors “coke up” (resulting in restriction of flow).
Flash point
The “flash point” indicates the storage tem-perature at which flammable vapors are produced. For diesel fuels, it is above 55 °C (Hazard Class A III).
Additives in diesel fuel
The most important additives and their effects are listed in Table 2. Their concentra-tion level in the fuel is generally < 1 %.
Basic principles of the diesel engine Diesel fuels 31
Table 2
Different additives can have similar effects.
The arrows indicate the effects of each additive independently of other components.
Additives Effect
Ignition accelerators (cetane improvers) Increase cetane number Improve
Engine starting characteristics
Exhaust white-smoke emission
Engine noise levels
Exhaust emission levels
Fuel consumption
Detergents Keep nozzles cleaner
Flow improvers Improve reliability at low temperatures
Wax anti-setting additives Improve storage properties at low temperatures Lubricity enhancers Reduce fuel-injection component wear especially with
hydrogenated low-sulfur fuels
Antifoaming additives Make refuelling easier (reduce tendency to slosh over) Anti-corrosive additives (corrosion inhibitors) Protect the fuel system
Effects of the most important diesel-fuel additives
2