Conventional diesel fuels are distillates with a boiling range of about 149°C to 371°C, obtained by the distillation of crude oil. The components of diesel fuels are straight run fractions containing paraffinic and naphthenic hydrocarbons, naphtha and cracked gas oils. The atmospheric gas oils tend to have good ignition quality (cetane number) but many contain some high melting point hydrocarbons (waxes) that can result in high cloud and high pour points.
These fractions are blended to produce different seasonal grades of diesel fuels required to meet a wide range of diesel engine uses.
Diesel fuel produces power in an engine when it is atomized and mixed with air in the combustion chamber. Pressure caused by the piston rising in the cylinder causes a rapid temperature increase. When fuel is injected, the fuel/air mixture ignites and the energy of the diesel fuel is released forcing the piston downwards and turning the crankshaft as indicated in Diagram A-7.
*Courtesy of Chevron Corporation
DIESEL FUEL QUALITY AND PROPERTIES
- Cetane Number
Cetane number is a measure of the ignition delay of a diesel fuel. The shorter the interval between the time the fuel is injected and the time it begins to burn, the higher is its cetane number. It is a measure of the ease with which the fuel can be ignited and is most significant in low temperature starting, warm up, idling and smooth, even combustion. Cetane number requirements depend on engine design and size, nature of speed and loadvariations, and starting and atmosphere conditions.
Some hydrocarbons ignite more readily than others and are desirable because of this short ignition delay. The preferred hydrocarbons in order of their decreasing cetane number are normal paraffins, olefins, naphthenes, iso-paraffins and aromatics.
This is the reverse order of their anti-knock quality. Cetane number is measured in a single cylinder test engine with a variable compression ratio. The reference fuels used are mixtures of cetane, which has a very short ignition delay, and alphamethyl naphthalene which has a long ignition delay. The percentage of cetane in the reference fuel is defined as the cetane number of the test fuel.
The importance of cetane number is very evident as low cetane number usually causes an ignition delay in the engine. This delay causes starting difficulties and engine knock. Ignition delay also causes poor fuel economy, a loss of power and sometimes engine damage. A low cetane number fuel can also cause white smoke and odor at start-up on colder days. White exhaust smoke is made up of fuel vapors and aldehydes created by incomplete engine combustion. Ignition delay during cold weather is often the cause. There is not enough heat in the combustion chamber to ignite the fuel, therefore, the fuel does not burn completely.
Diesel engines whose rated speeds are below 500 R.P.M. are classed as slow speed engines; from 500 to 1200 rpm as medium engines; and over 1200 rpm as high speed. Typical diesel fuels have cetane numbers in the low to mid 40's. These are generally satisfactory for high speed engines while low and medium speed engines may use fuels with lower cetane number.
Some refiners have used additives such as hexyl nitrate or amyl nitrate to increase cetane numbers.
The distillation characteristic of the fuel describes its volatility. A properly designed fuel has the optimum proportion of low boiling components for easy cold starting and fast warm-up and heavier components which provide power and fuel economy when the engine reaches operating temperature. Either too high or too low volatility may promote smoking, carbon deposits and oil dilution due to the effect on fuel injection and vaporization in the combustion chamber. The 10, 50 and 90% points are the principal volatility controls. The Canadian General Standard 2-3.6M82 permits a 90% point of 360°C for type B diesel fuel and maximum 90% point of 290ºC for Type A diesel fuel.
The fuel volatility requirements also depend on engine design and size, nature of speed and load variations, and starting and atmospheric conditions. For engine in services involving rapidly fluctuating loads and speeds, as in bus and truck operations, the more volatile fuels may provide better performance, particularly with respect to smoke and odour. However, better fuel economy is generally obtained from the heavier types of fuel because of their higher energy content.
Viscosity is a measure of a liquid's resistance to flow. High viscosity means the fuel is thick and does not flow easily. Fuel with the wrong viscosity (either too high or too low) can cause engine or fuel system damage.
High viscosity fuel will increase gear train, cam and follower wear on the fuel pump assembly because of the higher injection pressure. Fuel atomizes less efficiently and the engine will be more difficult to start.
Low viscosity fuel may not provide adequate lubrication to plungers, barrels and injectors, and its use should be evaluated carefully.
The viscosity of the fuel affects atomization and fuel delivery rate. The viscosity of diesel fuel is normally specified at 40°C. Fuels with viscosities over 5.5 centistokes at 40°C are limited to use in slow speed engines, and may require pre-heating for injection. The CGSB has a viscosity range of 1.30-3.60 for Type A fuel while the range is 1.70-4.10 for Type B fuel.
For some engines, it is advantageous to specify a minimum viscosity because of power loss due to injection pump and injector leakage. Maximum viscosity, on the other hand, is limited by considerations involved in engine design and size, fuel temperature and the characteristics of the injection system.
- Fuel Lubricity
Some processes used to desulphurize diesel fuel, if severe enough, can also reduce the natural lubricating qualities of the diesel fuel. Since engines require the diesel fuel to act as a lubricant for their injection systems, diesel fuel must have sufficient lubricity to give adequate protection against excessive injection system wear. Additives are available that can improve diesel fuel lubricity, however these additives can have unwanted side effects when used at excessive concentrations or in combination with other additives.
The Canadian General Standards Board recognizes the very high lubricity standard outlined by the High Frequency Reciprocating Rig Test (ASTM D6079) and all Canadian automotive diesel fuel must pass this standard with a wear scar diameter of less than or equal to 460 µm (micron) at 60◦C. This assures that consumers will have adequate lubricity under almost all normal operating conditions. *Please note that refiners add lubricity additive to maintain the required industry standard specification.
- Cold Weather Characteristics
Due to the extreme weather conditions that we encounter in Western Canada, diesel fuels are blended seasonally and adjusted to provide ultimate power and performance and minimize cold weather problems which are inherent to diesel fuel. As diesel fuel is cooled it will reach the "cloud point". This is the temperature at which paraffin wax falls out of solution and starts to form wax crystals in the fuel. As the fuel is cooled further, it will eventually reach its' "pour point". This is the temperature at which fuel will no longer flow or the point at which fuel gels or turns solid.
Another key property of diesel fuel is the "Cold Filter Plug Point" (CFPP). This is the temperature where fuel can no longer flow freely through a fuel filter, approximately halfway between the cloud point and the pour point.
How critical these factors become in winter operation depends on the design of the fuel system with regard to fuel line bore, freedom from bends, size and location of filters and degree of warm fuel recirculation as well as the amount and kind of wax crystals.
Additives are being used successfully to improve fuel fluidity at low temperatures. These additives, known as wax crystal modifiers, can result in satisfactory fuel flow on average of 9°C to -27°C.
Wax crystal modifiers do not change the cloud point of the fuel, but it polarizes the wax molecules and keeps the wax from forming larger crystals, therefore lowering the pour point and CFPP. As illustrated in Diagram A-9, after treating diesel fuel with a high quality diesel fuel conditioner, the wax crystals that have dropped out of solution will repel each other and improve the cold flow properties of the treated fuel.
- Flash Point
Flash point is determined by heating the fuel in a small enclosed chamber until the vapours ignite when a small flame is passed over the surface of the liquid. The temperature of the fuel at this point is the flash point. The flash point of a diesel fuel has no relation to its performance in an engine nor to its auto ignition qualities. It does provide a useful check on suspected contaminants such as gasoline, since as little as 0.5% of gasoline present can lower the flash point of the fuel very markedly.
Shipping, storage and handling regulations are predicated on minimum flash point of 40◦C. It is a very important aspect connected to legal requirements (such as the Transportation of Dangerous Goods (TDG)) regulations and safety precautions involved in fuel handling and storage and is required to meet insurance and fire regulations.
- Sulphur Content
Sulphur in diesel fuel can cause combustion chamber deposits, exhaust system corrosion, and wear on pistons, rings and cylinders, particularly at low water-jacket temperatures. Sulphur tolerance by an engine is dependant on the type of engine, the type of service and lube oil used. Sulphur is totally non-compatible with exhaust treatment technology in on-road 2007 (and later) model year engines.
Ultra Low Sulphur Diesel (ULSD) is an initiative of the Canadian government to significantly reduce the amount of sulphur in on-road clear, and off-road dyed diesel. It was an aggressive iniative to reduce fuel sulphur to 15 parts per million (PPM) by mass.
If you own a 2006 or earlier model diesel fuel powered on-road vehicle you may use either Ultra Low Sulphur Diesel (ULSD) or Low Sulphur Diesel (LSD). If you own a 2007 or newer diesel fuel powered on-road vehicle you must refuel with Ultra Low Sulphur Diesel (ULSD) only.
Presently, all UFA clear and dyed diesel meets the ULSD specification of 15 PPM or less sulphur content.
Your present diesel engines will perform as well with Ultra Low Sulphur Diesel as they did with Low Sulphur Diesel and will benefit by burning cleaner, producing cleaner exhaust and, in turn, helping the environment. ULSD is also guaranteed to meet the very high lubricity standard described in the Fuel Lubricity Section (#4).
Diesel engine crankcase oils are formulated to combat various levels of fuel sulphur content and it is important to use the engine builders' recommended crankcase oil quality and oil change intervals. These often relate to the sulphur level of the fuel as well as to other service conditions.
- Carbon Reside
Carbon residue gives a measure of the carbon depositing tendencies of a diesel fuel after evaporation and pyrolysis under prescribed conditions. While not directly correlating with engines deposits, this property is considered a guide.
Ash forming material maybe present in diesel fuel in two forms: (1) abrasive solids and (2) soluable metallic soaps. Abrasive Solids contribute to injector, fuel pump, piston and ring wear, and also engine deposits. Soluable metallic soaps have little effect on wear but may contribute to engine deposits.
Diesel fuel acidity if not controlled, can cause poor fuel stability, cause corrosion of mild steel, and it could cause deposit formation in some types of fuel injection equipment.
- Copper Strip Corrosion
If a diesel fuels corrosive tendencies are not controlled the possibility of corrosion to copper, brass or bronze parts in the fuel system may occur.
- Electrical Conductivity
The ability of a fuel to dissipate electric charge that has been generated during pumping and filtering operations is controller by its conductivity. If a fuel’s conductivity is sufficiently high, the static electric charge dissipates fast enough to prevent its accumulation and dangerously high electrical potentials are avoided.
- Thermal Stability
Heat transfer is a design function of diesel fuels in many modern diesel engines. Only a portion of the fuel that is circulated and pressurized by the fuel injection system is actually combusted. The remainder of the fuel is recycled back to the fuel tank. The bulk fuel temperature can be well above ambient levels. Inadequate high-temperature stability of a diesel fuel can result in the formation of insoluable degradation products that can then cause filter plugging.
In colder temperatures, the recycling of heated fuel back to a colder fuel tank, can cause condensation problems, that could lead to increase in free moisture in the fuel. This problem could possibly lead to filter plugging and necessitate a higher frequency of fuel separator filter maintenance and/or drains.
A) DIESEL FUEL CLASSIFICATION
The Canadian standard for diesel fuels is CAN/CGSB-3. 517-2007. This specification recognizes the low to high temperatures (Types A and B) and wide area distribution patterns in the Canadian markets (also shown in Chart A-1).
UFA diesel fuels are formulated to give satisfactory performance at the temperatures indicated by the 2.5% low end design temperature data for the period and location of intend use This is the same measure applied for Canadian Government standards. It means that the cloud point or, in the case of a flow improved fuel, the low temperature performance of the fuel will meet the lowest temperature pumping and filtering requirements 97.5% of the time on average.
In Canada, because the diesel grades are adjusted to meet local winter conditions, blending with stove oil or kerosene by the customer is seldom practised. It requires about 30% of No. 1 Diesel (stove) in No. 2-D fuels to achieve a 3C lowering of the cloud point and, if blended cold, wax crystals in the cold 2-D fuels are not dissolved. Better results can be achieved by using a wax crystal modifier additive.
Engine and Tractor Specific Diesel Fuel Recommendation can be found in the Tractor and Farm Equipment Lubrication Guide.
CANADIAN GOVERNMENT CHART A-1 AND CAN/CGSB-3.517-2007 TYPICAL
*1 All UFA Ultra Low Sulphur Diesel is guaranteed to have 0.0015% by weight or less total sulphur content.
*2 ULSD testing has indicated that most diesel fuels at 15 PPM or less sulphur by mass, will have cetane No’s in the high end of the range.
DIESEL FUEL PERFORMANCE ISSUES
- Diesel Engine Smoke
Diesel engine smoke is caused by incomplete combustion.
White smoke is caused by tiny droplets of unburned fuel resulting from engine misfiring and low temperature. This smoke should disappear as the engine warms up.
Black smoke could be caused by a faulty injector, insufficient air and overloading and/or over fueling the engine.
Blue gray smoke is the result of burning lubricating oil and is an indication the engine is in poor mechanical condition.
- Diesel Fuel Contamination
One of the more common contaminants in fuel is water. Water gets into diesel fuel storage and vehicle tanks in several ways including condensation during transportation, by leakage through faulty pipes or vents and by careless handling. Water can cause injector nozzle and pump corrosion, bacteria and fungi growth and fuel filter plugging with materials resulting from corrosion or microbial growth. Both vehicle and storage tanks should be checked frequently for water and drained or pumped as necessary. In extreme cases, biocides may be required to control bacterial growth.
In cold Canadian winters, ice formation in fuel containing water creates severe fuel line and filter plugging problems. Regularly removing the water is the most effective means of preventing this problem; however, small quantities of a water dispersing diesel fuel treatment may be used to help prevent line and filter freeze-ups. Most new diesel engines require almost daily draining of water separators to avoid excessive or critical moisture build-up.
Dirt is another common contaminant of fuel and may cause poor performance and extreme wear in fuel pumps. Fuel tank caps, dispensing nozzles and hoses should be kept clean to eliminate potential sources of contamination.
- Diesel Fuel Colour
There is no relationship between natural diesel fuel colour and such desirable diesel fuel qualities as BTU/gallon, viscosity, cloud point, cetane number or distillation range. Diesel fuel colour varies with the crude source, refinery methods and the use of dyes. However, if the fuel colour darkens during storage, this could indicate oxidation and/or other sources which can cause operating problems.
- Blending Lubricating Oil into Diesel Fuel
It was a common practice to blend lubricating oil into diesel fuel to provide added lubricity for fuel pumps and injectors. This practice may adversely affect fuel quality features and could lead to fuel system and piston deposits, increased exhaust emission, and fuel filter plugging. This practice may also result in the diesel fuel being out of compliance with federal regulations or other specifications.
Today’s seasonal blends of ultra low sulphur diesel fuel are monitored at the refinery level to ensure adequate lubricity.
Water and sediment contamination is by far the most probable cause of fuel system failure
DIESEL FUEL HANDLING
One of the most critical parts of the diesel engine is the fuel injection system. Many of the components of this system are highly finished and operate with clearances as small as 0.0025 mm. Since any foreign material that finds its way into the system can damage these parts and seriously impair engine performance, clean fuel is essential to the proper operation of the engine.
UFA ensures that reasonable care has been taken to provide cleanliness in production, storage and shipment of diesel fuel. However, the customer's storage facilities often contain small quantities of fine rust particles and condensed water from humid air. These contaminants become well dispersed in the diesel fuel during the filling of storage tanks.
The best way to protect a diesel engine from such contaminants is simply to allow them to settle out in the storage tank.
The time required for this settling will vary with the size of tank used. The following table indicates how fast a very small particle (5 microns) will settle in diesel and other fuels. It is evident from these settling times that diesel fuel should be allowed to stand for a day or, at least, overnight before it is removed from storage vessels.
A properly designed storage tank is essential if clean fuel is to be delivered to the engine. The impurities that settle out must remain separated from the fuel.
To facilitate this, the following practices should be observed (as demonstrated in Diagram A-10):
- The tank should be sloped slightly and a drain placed at the low end. Through this drain, settled impurities can be removed easily.
- The fuel suction outlet should be located well above any possible level of settled impurities so that only clean fuel is pumped.
- A filter should be provided in the suction line to remove any impurities that have not had time to settle. UFA offers various types of filters designed to keep sediment and moisture from entering our customers equipment.
- The storage tank itself and any interior tank coating should be insoluble in the fuel and non-reactive with the fuel.
- The tank should be as large as possible consistent with the user's normal fuel requirements, since use of a number of small containers increases the chance of contamination.
- Proprietary additives should not be blended into the diesel fuel without first consulting the fuel supplier.
This sketch of a typical fuel storage tank illustrates the type of installation that is
designed to minimize the carry-over of fuel contamination to the engine.Diagram A-10
Clean fuel in the storage tank does not ensure clean fuel in the engine. Care must be taken to minimize contamination when the engine fuel tanks are filled. Also, engine tanks should be filled at the end of each day to reduce the amount of water that will condense in the tanks overnight. The engine tank should be provided with a drain and the engine tank discharge line should have a filter. Both the drain and the filter should be serviced regularly to eliminate water and dirt that has accumulated and thereby ensure that the filter will not become "plugged" in service.
Although we always try to ensure that fuels are never mixed when transferring product, inevitably it does occasionally happen. Whenever a situation such as this arises, we are usually faced with what should be done with the contaminated product. The best alternative is to not use the mixed product, which could mean disposal or returning product to a refinery for re-processing.
- Gasoline in Diesel Fuel
Very small quantities of gasoline in diesel fuel will sharply reduce the flash point of diesel fuel. Low flash point diesel fuel or heating fuel is a serious safety concern in the handling and transporting of the fuel.
Gasoline in heating fuel could cause violent, explosive burning in the heater. Extreme precaution should be taken to ensure that heating fuel is never contaminated.
Gasoline in diesel fuel can cause fuel pump or diesel injector damage.
Product mixes are rarely a complete mixing of one product with another. This results in higher concentrations of gasoline in some parts of the tank than in others. This creates areas of much lower flash points and greater hazards than a completely mixed situation.
The following table shows the reduction of flash point when gasoline becomes completely mixed into diesel fuel or heating fuel in small proportions:
2 litres of gasoline in 1000 litres of diesel fuel will reduce flash point by 3C)
- Diesel Fuel in Gasoline
Contamination of gasoline by diesel fuel or heating fuel will not cause low flash point problems, but can result in other problems that may be serious and costly.
The main difficulty will be the reduction of octane. Diesel fuel, typically has an octane of approximately 40 and will reduce the octane of a mixture in direct proportion to the level of contamination.
The following table indicates the effect certain percentages of diesel fuel in gasoline can have on octane numbers:
In today's octane sensitive fuel injection systems, it is of extreme importance to avoid product mixes and lowering of octane.
Octane that has been lowered only slightly, under some conditions, can be sufficient to cause discernible engine knock or "pinging". Higher levels of diesel contamination can result in blown pistons, cracked heads, or other engine damage due to the reduction in octane. If diesel fuel is present in appreciably large amounts, dirty combustion and poor operation will result in further engine damage.
As can be seen, maximum effort must be made to avoid possible product mixing..