• Viscosity is a measure of fuel’s resistance to flow. Expressed in Redwood or in Cst.
• Higher the viscosity higher will be the specific gravity.
• The viscosity of any oil is indirectly proportional to temperature of oil.
• Viscosity is important for handling, treatment and atomization of the fuel.
• It also is a rough indicator of its carbon and asphalt content.
• High viscosity fuels require proper preheating for good separator operation and heating before injection for good atomization, this characteristic usually can be handled without any problems.
• By preheating fuel, separation in a centrifuge is improved, but a temperature of 980C should not be exceeded because flashing of water in the separator may occur with resultant loss of the centrifuge water seal.
• Caution must be exercised when heating prior to injection to temperatures above 1350C because cracking may occur, gases may be given off, and water may vaporize forming steam pockets in the fuel line. Insufficiently heated fuel, on the other hand, can result in poor atomization and delayed burning, which may lead to higher thermal loading, scuffing problems, possible piston and piston ring failure, and to an increase in fuel consumption.
• In addition to heating prior to injection, an increase in fuel injection pressure may also be necessary to maintain design atomization spray patterns depending on fuel used.
b) Specific Gravity
✓ Specific gravity is defined as the ratio of the weight of a given volume of the product at 150C to the weight of an equal volume of water at the same temperature.
✓ It is important for oil purification. Therefore, as the specific gravity of fuel approaches 1.0, centrifuging becomes less effective.
✓ High specific gravity indicates a heavily cracked, aromatic fuel oil with poor combustion qualities, which can cause abnormal liner wear.
✓ Rate of change in specific density w.r.t water is 0.66 kg/m3 per degree Celsius.
✓ Heating the fuel prior to centrifuging assist in the separation process because the density of fuel oil changes more rapidly with temperature. A viscosity decrease also helps centrifuging.
✓ A maximum specific gravity of 0.991 (at 150C) can be handled satisfactorily. Above this value the centrifugal purifier cannot repeatedly and successfully operate, due to loss of its water seal. Specific gravity for future fuels is expected to rise to about 0.995. The major significance of this increase in specific gravity will be greater difficulty relative to water removal in settling tanks and centrifuges.
c) Carbon Residue/Asphaltenes
• Conradson Carbon Residue (CCR) is a measure of the tendency of a fuel to form carbon deposits during combustion and indicates the relative coke forming tendencies of heavy oil.
• Carbon-rich fuels are more difficult to burn and have combustion characteristics which lead to the formation of soot and carbon deposits.
• Since carbon deposits are a major source of abrasive wear, the CCR value is an important parameter for a diesel engine.
• Carbon residue is the percent of coked material remaining after a sample of fuel oil has been exposed to high temperatures.
• ASPHALTENES are those components of asphalt that are insoluble in petroleum naphtha and hot heptanes but are soluble in carbon disulfide and hot benzene.
• They can be hard and brittle and made up of large macromolecules of hydrocarbon derivatives containing carbon, hydrogen, sulfur, nitrogen, oxygen and, usually, the three heavy metals − nickel, iron and vanadium.
• A high CCR/Asphaltenes level denotes a high residue level after combustion and may lead to ignition delay as well as after-burning of carbon deposits leading to engine fouling and abrasive wear and finally thermal loading of engine.
• After burning will lead to burning of the lube oil film which leads to scuffing, cylinder wear and engine deposits.
• Fuels with high CCR values have an increasing tendency to form carbon deposits on injection nozzles, pistons, and in the ports of two-stroke engines.
• This causes reduction in the efficiency and performance of those components and increased wear.
• The maximum permissible CCR value depends on engine speed. The higher the speed, the shorter the time for combustion and the more residues deposit. Hence, acceptable CCR values should decrease as engine speed increases.
• 2-S are less affected by a high CCR than 4-S, it does contribute to increased fouling of gas ways and turbochargers, especially during low power operation or at idle. Idling should be limited to five to ten hours and be followed by running at full load to clear gas ways whenever possible.
• Continued operation at reduced output can also load up gas ways with unburned heavy fuel oils and lube oil. Here also, full load operation can help to clear gas ways.
• The combination of higher Conradson Carbon content and higher Asphaltenes content can increase the centrifuge sludge and fine filter burdens. This can require more frequent centrifuge desludging and filter element cleaning/replacement.
• Higher Conradson Carbon content also lowers the gross and net heating values (on weight basis) of a heavy fuel oil.
• Asphaltenes content is particularly important in 4-cycle engines due to their higher operating speeds, smaller bore sizes, and reduced combustion time.
• When the crude is distilled, sulphur derivatives tend to concentrate in the heavier fractions, leaving the lighter fractions with relatively low sulfur contents.
• This characteristic of fuel oil is responsible for “low temperature” corrosion which attacks cylinder liners and piston rings, leading to an increase in cylinder liner wear. In cold corrosion the oxides in sulphur combine with condensing water vapor in the combustion chamber to form highly corrosive sulphuric acid. Some of this water is present in the fuel already, while another source of moisture may be in the intake and scavenging air.
• Careful control of the cooling water temperature in the inlet air coolers and/or installation of a water mist separator after the air coolers should remedy the problem of condensing moisture in the intake air.
• When operating an engine on a fuel with high sulphur content, care must be taken to avoid reaching the acid dew point temperature within the cylinder.
• One way of controlling this is to adjust the cooling water temperature at the cylinder wall.
• Sulphur is oil soluble, it cannot be removed from the fuel by centrifuging. It can be neutralized by the use of proper alkaline additives in the cylinder and/or engine lubricating oils.
• It should be noted that although the sulfur content of a fuel can be neutralized by the use of cylinder lubricating oils of proper alkalinity (TBN − Total Base Number), over-treatment for sulfur (low sulfur fuel oil) can be just as harmful as under-treatment. Over-treatment for sulfur leaves an excess of alkaline additive material free to form hard, abrasive deposits during combustion, with resultant increased abrasive wear of cylinder liners and piston rings. Therefore, when burning low sulfur fuel oil, the lube oil TBN should be lowered.
• In trunk-type engines the cylinder lube oil is scraped into the crankcase by oil control rings on the pistons. This oil has been contaminated by the sulphur in the fuel. Upon entering the crankcase, the sulfur is free to combine with moisture which may collect there. The TBN of the lubricating oil is eventually lowered to a point where it is rendered ineffective in controlling the sulfur content of the fuel.
• For 2-S engines, an increase in the sulfur content leads to slower and less complete combustion with resultant formation of more corrosive acids, more unburned carbon, and an increase in wear rates.
✓ The ash contained in heavy fuel oil includes the (inorganic) metallic content, other non-combustibles and solid contamination.
✓ The ash content after combustion of a fuel oil takes into account solid foreign material (sand, rust, catalyst particles) and dispersed and dissolved inorganic materials, such as vanadium, nickel, iron, sodium, potassium or calcium.
✓ Ash deposits can cause localized overheating of metal surfaces to which they adhere and lead to the corrosion of the exhaust valves.
✓ Excessive ash may also result in abrasive wear of cylinder liners, piston rings, valve seats and injection pumps, and deposits which can clog fuel nozzles and injectors.
✓ In HFO Ash can be removed by centrifuging.
✓ They can form hard deposits on piston crowns, cylinder heads around exhaust valves, valve faces and valve seats and in turbocharger gas sides.
✓ Effects of ash can decrease by reduction of valve seat temperatures by better cooling.
• Vanadium is a metallic element that chemically combines with sodium to produce very aggressive low melting point compounds responsible for accelerated deposit formation and high temperature corrosion of engine components.
• Vanadium is responsible for forming slag on exhaust valves and seats on 4-S engines, and piston crowns on both 2-S &4-S engines, causing localized hot spots leading eventually to burning away of exhaust valves, seats and piston crowns.
• When combined with sodium, this occurs at lower temperatures and reduces exhaust valve life.
• Vanadium is oil soluble. It can be neutralized during combustion by the use of chemical inhibitors (such as magnesium or silicon).
• Cooling exhaust valves and/or exhaust valve seats will extend valve and seat life.
• Raising fuel/air ratios also prolongs component life. Other measures which can be used to extend component life are the use of heat resistant material, rotating exhaust valves, and the provisions of sufficient cooling for the high temperature parts.
f) Cetane number
✓ Ignition quality is indicated by cetane number.
✓ Lower the Cetane Number of a fuel, poorer the fuel quality & greater the ignition delay.
✓ Higher the Cetane Number more will be the aromaticity which can increase the ignition delay, and can result in hard knocking or noisy engine running, which is undesirable over long periods of time. The result could be poor fuel economy, loss of power and, possibly, even engine damage.
✓ Diesel engines operating at speeds of less than 400 rpm are much less sensitive to fuel ignition quality.
✓ Physical factors which influence ignition and burning time are the speed with which fuel droplets are atomized, vaporized and thermally cracked to form a combustible mixture.
✓ Ignition and burning time can be improved by decreasing fuel droplet size and/or increasing swirl. Experience also has indicated that raising inlet air temperature can reduce the cetane sensitivity of higher speed diesel engines.
✓ Cetane number is normally quoted for distillate fuels only. A number of methods exist for approximating the cetane number for residual fuels.
g) Flash Point- The flash point of a fuel is the temperature at which fuel vapors can be ignited when exposed to a flame. All petroleum products will burn. However, in order for this to occur, the ratio of fuel vapor to air must be within certain limits.
h) Pour point- For pumping and handling purposes, it is often necessary to know the minimum temperature at which a particular fuel oil loses its fluid characteristics.