Diesel Fuel (Automobile)

8.3.

Diesel Fuel

Diesel fuel is the light oil, and is made from crude oil by the same distillation process which produces gasoline. Distillation of crude oil can continue up to about 643 K or slightly higher, before thermal cracking occurs, however diesel fuel mostly contains fractions boiling off from approximately 523 to 625 K (Fig. 8.5), as compared with about 307 to 483 K for gasoline. These higher boiling point fractions contain about 20 times more sulphur than those from which gasoline is derived. Therefore it is necessary to remove sulphur during refinement. With hydro-cracking and catalytic-cracking fractions having even higher boiling points are converted into hydrocarbons suitable for use as diesel fuels. Both hydro-cracked and catalytically cracked fuels generally have low cetane numbers (10 to 30). The oxidation resistance during storage of hydro-cracked fuel is high, whereas catalytically cracked fuels do not have, and hence they behave slightly unstable.
A typical analysis of diesel fuel would show the following properties :
Specific gravity 0.85
Cold filter plugging point 255 K
Cloud point 267.5 K
Cetane number 51
Sulphur 0.22%
Initial boiling point 453 K
Final boiling point 633 K
50% vaporisation 553 K
8.3.1.

Desirable Properties

The following properties must be controlled during blending of diesel fuels.
Volatility. Volatility should be high for better cold starting and obtaining complete combustion.
Flash Point. The flash point should be low for greater safety in handling and storage.

Fig. 8.5. The distillation curve for diesel fuels.
Cetane Numbers. This is a measure of ignitability. The higher the cetane number the more complete is the combustion and the cleaner the exhaust.
Viscosity. The viscosity should be low for good atomization.
Sulphur. Higher sulphur content gives rise to low wear and higher particulate content in the exhaust, and therefore it should be low.
Density. As the density increases the energy content of the fuel also increases.
Waxing Tendency. Wax precipitation can produce cold starting problem and subsequently stop the engine, and hence this tendency should be low.
8.3.2.

Cetane Number

The cetane number can be considered as the percentage of cetane in a mixture of cetane and heptamethyl nonane (the latter sometimes referred to as alpha-methyl naphthalene) that has the same ignition delay, in terms of degree of rotation of the crankshaft, as the fuel under test. Ignition delay is quite important because, if the delay is too long, the bulk of the charge in the cylinder catches fire almost simultaneously, causing violent combustion. On the other hand with a short delay, ignition occurs at several locations, and consequently the flame spreads progressively throughout the charge. Therefore, unless injection is properly timed for the cetane number of the fuel being used, rough running and other problems can still exist. Too high a cetane number can initiate ignition before adequate mixing has taken place causing increase in emissions. Cetane is straight-chain normal hexadecane (C16H34) and heptamethyl nonane is a multiple branched alkane (combined with seven CH3 radicals) which is defined as having a cetane number of 15. Alkanes possess better ignition quality than the aromatics.
The cetane number is defined as the %n-cetane + 0.15 times the % of heptamethyl nonane contents of the blend of reference fuel having the same ignition quality as the fuel under test. Ignition quality is obtained by varying the compression ratio to provide the same ignition delay period for the test fuel and two blends of reference fuels. One blend should have better and the other poorer ignition quality than the test fuel, but the difference between the two should be within five cetane numbers. The cetane number is determined by interpolation between the results obtained at the highest and lowest compression ratios.
Since these laboratory engine tests are not at all a convenient method of assessing the quality of a fuel, two other criteria are widely used. They are the diesel index and the cetane index. The diesel index, obtained mathematically, is computed by multiplying the aniline point of the fuel by its API gravity/100. Aniline point is the lowest temperature in degree F at which the fuel is completely miscible with an equal volume of freshly distilled aniline, which is phenylamine aminobenzene (C6H5NH2), a colourless oily liquid. API stands for American Petroleum Institutes. The degree API is measured with a hydrometer and is equal to (141.5/specific gravity at 60 degree F)-131.5. It is a measure of density for liquids lighter than water.
Cetane index is determined from API gravity and its volatility. The volatility value was originally taken at its mid-volatility, or mid boiling point, T50 (50% recovery temperature). Subsequently the formula has been modified from time-to-time and is now based on the density and volatility of four fractions of the fuel (those at the 10%, 50% and 90% distillation temperatures T10, T50 and T90 respectively). The cetane index is given by (ASTM D4737 1988),

Cetane index is usually better than diesel index as an indication of what the cetane number of a fuel would be if tested in a CFR engine in a laboratory. In general, alkanes have high, aromatics low, and naphthenes intermediate cetane and diesel indices. A value of 50 or above for either diesel or cetane index is an indication that the combustion and ignition characteristics of the fuel are good. Values below 45 are undesirable and values of 40 and less are totally unacceptable. With low values cold starting is difficult, white smoke is generated and the engine is noisy.
8.3.3.

Correlation between Octane Number (ON) and Cetane Number (CN)

The hydrocarbon components in petrol have a boiling range of 313 K to 473 K, whereas in iiesel fuel the range is 523 K to 623 K. Octane number is the measure of resistivity to ignition ind the cetane number is the measure of spontaneous tendency of ignition. Thus a reverse correlation holds between these numbers, and is expressed approximately as

The negative cetane or octane indicates fuel quality inferior to heptane or a – methylnaph-thalene in respective case.
8.3.4.

Wax Deposit

In cold weather, even a small wax content as little as 2%, crystallizes out and practically gells a fuel. The crystals can cause blockage of the fuel filters incorporated on the engine, and ultimately cause it to stall. In very extreme conditions, even the pipelines are blocked and a thick layer of wax may sink to the bottom of the fuel tank. Paraffins are the potential constituents, which crystallize out as wax because of their high cetane numbers.
The various measures of the tendency of a fuel to precipitate wax include cloud point (CP), which is the temperature at which the wax, coming out of solution, first becomes visible as the fluid is cooled. Then there is the pour point (PP), which is the temperature at which the quantity of wax in the fuel causes it to gel. Other tests include the Cold Filter Plugging Point (CFPP) of Distillate Fuels. This is the lowest temperature at which 20 ml of the fuel pass through a 45 urn fine wire mesh screen in less than 60 seconds. The Americans have developed the Low Temperature Fuel Test (LTFT). It differs from the European CFPP test in that it requires 200 ml of fuel to be cooled and drawn by a depression of 6 inch of mercury through a 17 p screen. The LTFT is the temperature at which 180 ml of fuel passes through the 17 um screen in less than 60 seconds. Another way of estimating operational performance of a fuel is to combine the cloud point and the difference between it and the pour point, to obtain Wax Precipitation Index (WPI). The expression is,

8.3.5.

Density

The density is important as it is related to energy content. The densities of diesel fuels obtained by the different refining processes are approximately as follows.
Straight run distilled 805 to 870 kg/m3
q
Hydro-cracked gas oil 815 to 840 kg/m
Thermally cracked gas oil 835 to 875 kg/m3 Catalytically cracked gas oil 930 to 965 kg/m3
q
Density is measured by a hydrometer with scales of specific gravity or gm/m . The sample is tested at 288 K; otherwise the appropriate correction is used. Density is different from API Gravity or degree API, since higher the number in degree API, the lighter is the fuel. The injection equipment meters the fuel on a volume basis; therefore any variation in density affects the power output, A high-density fuel products more smoke as well as more power at maximum power output.

8.3.6.

Volatility

The volatility of diesel fuel influences density, auto-ignition temperature, flash point, viscosity and cetane number. High volatility promotes vapour lock and lowers the flash point. The latter has an adverse affect on safety in handling and storage. Higher volatility also causes more easy evaporation of fuel in the combustion chamber. Consequently, low volatility components may not burn completely, thereby increasing deposits and smoke. Within the range 623 to 673 K, however, the effects of low volatility on exhaust emissions are relatively small. Surprisingly, the mid-range volatility has a remarkable influence on the generation of smoke. This may be due to the influence of this constituent of the fuel on injection and mixing, despite the fact that the cetane number can be influenced by the 50% distillate recovery temperature. In practice, the mix of volatilities is most important. High volatility components at the lower end of the curve in Fig. 11.4 improve cold starting and warm up, while low volatility components at the upper end of the curve increase deposits, smoke and wear.
8.3.7.

Viscosity

Increasing viscosity reduces the injector spray cone angle and fuel distribution and penetration, while increasing the droplet size. It therefore affects optimum injection timing. An upper limit has to be specified to ensure adequate fuel flow for cold starting. The viscosity of the average fuel lies about mid-way between the upper and lower limits. Too high a viscosity can caus^ excessive heat generation in the injection equipment, owing to viscous shear in the clearances between the pump plungers and their cylinders. If viscosity is too low, the leakage through these clearances, specifically at low speeds, can be excessive.
The unit of kinematic viscosity is the Stoke and is expressed in cm /s, while that of absolute viscosity is the poise. For convenience the figures are expressed in centistrokes (cSt) and cetipoises (cP), where cP = cSt x density of fluid. The SI units are m /s. The viscosity is determined by measuring the time taken for a certain volume of fuel at a prescribed temperature to flow under gravity through a capillary tube of a prescribed diameter. BS 2869 prescribes for a maximum value of viscosity of diesel fuel as 5 cSt and a minimum as 2.5 cSt at 40°C.
8.3.8.

Additives

Even after the optimization of the engines with respect to emissions and fuel economy, increasingly stringent regulations are expected to be imposed demanding maintenance of fuel quality. Because of the several variable factors, blending base stocks to produce high quality fuel is a complex operation. However this can be facilitated by introducing certain additives in required quantities that is dependent upon the blend to which they are applied. Additives must provide added benefit and also added cost of fuels. Therefore, unless they are truly cost-effective, there is no incentive for the oil companies to use them.
The crude oils from different parts of the world have different characteristics. Also, the blends demanded vary from country-to-country. Climate, too, is an important consideration, especially because of the tendency to wax formation. Additionally the fuel characteristics needed vary with engine design.
Several additives were introduced with diesel fuel as early entrants in 1988, one of which has raised its cetane number from the 48 to between 54 to 56. The others additives are corrosion inhibitor, anti-foam, cold-flow, and re-odorant additives. The benefits obtained include lower noise, 3% better fuel economy, 8.4% less black and white smoke, a general improvement in overall engine performance and durability, and a reduction in down time. Among all the
additives available now a days, the most important are the cetane improvers and those that decrease the tendency to wax precipitation in cold weather. Others used include anti-oxidants, combustion improvers, cold flow improvers, corrosion inhibitors, detergents, re-odorants and anti-foamants, stabilizers, dehazers, metal deactivators, biocides, anti-icers demulsifiers. Antistatic additives are also used, but mainly to benefit the blenders by facilitating storage handling and distribution.
8.3.9.

Cetane and Combustion Improvers

Cetane number is a measure of the ignitability of the fuel. A low cetane valve may cause starting problem in cold weather and increase the tendency for the generation of white smoke. It also enhances ignition delay (interval between injection and ignition). Consequently, since fuel is injected into the combustion chamber for a longer time prior for ignition, the final rate of pressure rise is more rapid, and therefore the engine becomes noisy. Additionally, because there is less time available for the fuel to burn before the exhaust valve opens, the hydrocarbon emissions are increased.
If the cetane number is higher than for which the injection system is timed, power is lost due to proportionally higher pressure rise when the piston is at or near TDC. Also, the fuel may ignite before proper mixing with the air causing high smoke and hydrocarbon emission. Therefore fuels having high cetane numbers perform best when the injection is retarded. But, with too much retard, enough time for complete combustion is not available causing again high smoke and HC emission. As high cetane number is difficult to obtain, the regulations in most countries specify only low limits.
Additives used as cetane improvers are mainly alkyl nitrates and others include ether nitrates, nitroso compounds and some peroxides. Iso-octyl nitrate, which is the most commonly used, improves the cetane number between 2 and 5 numbers, depending on the base fuel and the quantities of additive used.
Combustion improvers are mainly organic compounds of metals such as barium, calcium, manganese or iron and are catalytic in action. Barium compounds could be toxic ; therefore manganese and copper compounds are mainly used. Although they produce metal-based particulates, they also lower the auto-ignition temperature of the carbon-based deposits in both the cylinders and particle traps, causing them to be more easily ignited.
8.3.10.

Cold Weather Additives

Cold flow improvers, or wax anti-settling additives (WASA), were among the first additives in diesel fuels to have practical value. But, it is difficult to distribute them uniformly throughout the fuel in adequate quantities for their effective use. Therefore new additives have been developed which modifies the shape of the wax crystals for enabling them to pass the Cold Filter Plugging Point (CFPP) test. These additives include ethylene vinyl acetate, polyolefin ester, and polyamide, which also work as flow improvers. They modify the shapes of the wax crystals from flat platelets which tend to get together.
There are three types of modifier, such as pour point depressants, flow improvers and cloud point depressants. The use of particular one among these depends basically on local requirements and the type of wax to be treated. The latter is mainly dependant on the boiling range of the distillates and the origin of crude oil. Fuels with a narrow boiling range produce large wax crystals that are less sensitive to treatment by additives than smaller and more regular shape crystals formed in fuels having a wider boiling range.
Pour point depressants for diesel fuels interact with the wax crystals to reduce their size and modify their shape. Flow improvers are more effective which form small multi-axial needle crystals instead of larger platelets. Olefin-ester copolymers such as ashless copolymers of ethylene and vinyl acetate are some of the flow improvers presently in use. These additives are usually used in proportions ranging from 100 to 500 parts per million. Basically, these additives improve cold filterability, but they also lower the pour point. Cloud point is an indicator of the quality of the base fuel. Although a cloud point depressant by itself lowers the cold filter plugging point of a base fuel, using it in a fuel also containing a flow improver may have the opposite effect. Olefin-ester-copolymer cloud point depressants generally provide only small improvement of the order of about 276 K and they are costly. Therefore they are unattractive, except where cloud point is^ included as a part of a diesel specification.
8.3.11.

Dispersants and Corrosion Inhibitors

Ash-less polymers and organic amines are some of the dispersant additives, which restrict the size of the particles formed within the fuel, and additionally, remove them from the surface. They should be used continuously, otherwise they may dislodge gums causing blockage of the filters. However, there are dispersant modifiers, or detergents, such as polyamides, polyisobutane and succinimides, which keep the surfaces of the combustion chambers and injection nozzles clean but, if used in excess, can actually form gums.
Corrosion inhibitors serve not only to protect fuel system components against corrosion but also bulk storage tanks and barrels. Alkyl phosphate is perhaps among the most commonly used corrosion inhibitors. Fuel surfaces like metals that can be oxidised in contact with air, require attention. Such oxidation can cause the formation of gums, sludges and sediments. However it can be reduced or prevented by adding surface-active sulphonates and polymers or alkylated phenol. These must be added immediately after refining the fuel and while it is still warm.
8.3.12.

Detergents and Anti-corrosion Additives

The main purpose of the detergents is to remove carbonaceous and gummy deposits from the fuel injection system. Gum can cause sticking of injector needles. Lacquer and carbon deposited on the needles can restrict the flow of fuel, distort the spray and even totally block one or more of the holes in a injector. As a result misfiring, loss of power, increase in noise, fuel consumption, HC, CO, smoke and particulate emissions in general, and difficult starting due to large fuel droplets (because of the reduction in flow rate) may take place. The detergents used commonly are amines, amides and imidazones, at concentrations of about 100 to 200 ppm. Sometimes about 200 ppm of alkenyl succinimide, or about 600 ppm of hydrocarbyl amine polymeric dispersants are added with them for better dispersal of the particulates.
Anti-corrosion additives, of about 5 ppm, are employed to protect pipelines in which diesel fuel is transported, but only trace proportions remain at the delivery end. Therefore, heavier treatment is necessary if vehicle fuel system is to be protected. The additives used are generally esters or amine salts of alkenyl succinic acids, alkyl phosphoric acids, or aryl sulphuric acids.
8.3.13.

Anti-foamants and Re-odorants

Anti-foamants allow the fuel tank to be filled completely and more rapidly avoiding splashing of diesel fuel on the person filling the tank. These additives are usually silicon surfactant additives applied in quantities of between about 10 and 30 ppm.
Many diesel vehicle owners are uncomfortable to the lingering and unpleasant smell of diesel fuel. The problem is less serious with gasoline, owing to mainly of its rapid rate of
evaporation. On the other hand elimination of the smell is undesirable because it facilitates the detection of leaks. Therefore re-odorants are used to modify the fuel by partially masking it with a more acceptable odour. The rate off treatment is usually about 10 to 20 ppm.