Characteristics of Diesel Fuel

 Diesel is obtained by graduated distillation of crude oil. The boiling point range of diesel fuel can vary from 180⁰C to 370℃ depending on the hydrocarbons. Diesel ignites at approximately 350℃ which is much lower than that of gasoline which ignites at approximately 500℃.

Quality and Grading Criteria of Diesel:

In Europe, the standard for diesel fuels is EN 590. The U.S. standard fir diesel is ASTM D975. High quality diesel fuel is characterized by the following features:

• High Cetane Number
• Relatively low boiling point
• Narrow density and viscosity spread
• Low aromatic compounds content
• Low sulphur content
• Good Lubricity
• Absence of free water
• Limited pollution with particulate matters

Cetane Number:

The Cetane Number (CN) indicates the quality of ignition of diesel fuel. The higher the CN, the greater is the ignition capacity of the fuel. As we know, diesel engines do not employ spark plugs to ignite the fuel, the fuel must ignite spontaneously and without any ignition lag.

CN 100 is assigned to n-hexadecane (cetane), whereas CN 0 is assigned to methyl naphthalene. As the number suggests, n-hexadecane ignites very easily, and methyl naphthalene ignites very slowly. Diesel fuel comprising of 55% cetane and 45% 𝝰-methyl naphthalene has a cetane number of 55.

CN in excess of 50 is desired for smooth running of the engine and low emissions. High quality diesel fuels consists of a higher proportion of paraffins with high CN ratings. Presence of aromatic compounds in the fuel can reduce the ignition quality.

Cetane Index:

Cetane index is another parameter which controls the ignition quality of a diesel fuel. It is calculated on the basis of fuel density and various points on the boiling curve. This does not take into account the impact of cetane improvers on ignition quality. Fuels whose cetane number has been increased by cetane improvers respond differently during combustion process than fuels with the same natural cetane number.

Nitric acid esters or alcohols are commonly used as cetane improvers. They shorten ignition lag and also reduce noise and emissions during combustion.

Boiling Range:

The boiling range of a diesel fuel depends on its composition. Boiling range is the temperature range in which the fuel vaporizes. A low initial boiling point is suitable for cold weather, but it also means lower CN and it results in poor lubrication leading to wear and tear. for this reason, the initial boiling point should not be too low.

A diesel fuel with higher final boiling point can result in increased soot production and nozzle coking (chemical deposition deposits of not easily volatized fuel on the nozzle cone and deposits of combustion residue). For this reason, the final boiling point should not be that high. The ideal boiling point would be 350℃.

Cold Flow Properties (Filtration limit):

Precipitation of Paraffin at low temperatures can result in fuel filter blockage, ultimately leading to interruption in fuel flow. The cold flow properties are assessed by means of filtration limit (Cold Filter Plugging Point (CFPP)).

To assist the flow of fuel in winter, polymer substances can be added as flow improvers. Although it cannot prevent the precipitation of paraffin crystals from the fuel, it can significantly reduce their growth so that fuel can still pass through the filter.

Flash Point:

Flash point is the temperature at which the fuel emits sufficient quantity of vapor to allow a spark to ignite the air-vapor mixture above the fuel surface. The flash point of diesel is over 55℃. Flash point determines the hazards to be taken unto consideration while transporting fuel. It should be noted that even a gasoline amount of less than 3% if mixed with diesel, it is sufficient to lower the flash point to room temperature.

Density:

The use of fuels with wide range of densities results in variations in air-fuel mixture ratios due to fluctuations in calorific value. The energy content of diesel increases if its density increases. Higher density type fuels can result in better engine performance, but also higher soot emissions.

Viscosity:

Viscosity is a measure of a liquid's resistance to flow due to internal friction. Higher the viscosity, higher will be the resistance to flow. There may be leakage of fuel in the injection pump if the viscosity is too low. Biodiesel, which has a much higher viscosity, causes a higher peak injection pressure at high temperatures in non pressure regulated injection systems (unit injection system). High viscosity also affects the spray pattern from nozzles due to the formation of larger droplets.

Lubricity:

The hydrogenation process reduces the sulphur content of the fuel, but is also removes the ionic components of the fuel which helps in lubrication. As a result, the hydrogenated fuel started to wear injection pumps. Lubricity enhancers are added to the fuel to enhance lubrication.

Lubricity is measured in a High Frequency Reciprocating Rig. A fixed clamped steel ball is ground on a plate by fuel at high frequency. The amount of wear on the ball is measured as Wear Scar Diameter (WSD) and is measured in µm. For a diesel fuel complying with European standards, its WSD must be ≤ 460 µm.

Fatty acids, fatty acid-esters or glycerin can be added to enhance the lubricity of the fuel. Biodeisel is also a fatty acid-ester, so if diesel fuel already contains a proportion of biodiesel, then no other lubricity enhancer is required.

Sulphur Content:

Diesel fuels naturally contain sulphur and the amount of sulphur depends on the quality of crude oil and components added during refining process. In particular, crack components usually have high sulphur content. As already stated above, sulphur is removed by hydrogenation process. 

Removal of sulphur helps in reducing sulphur dioxide emissions along with soot emissions. Moreover, presence of sulphur can poison the active catalyst surface present in catalytic converters. Since 2009, sulphur free fuel (sulphur content of less than 10 mg/kg) is allowed in the market.

Water in Diesel Fuel:

Even a small amount of water can damage the fuel injection pumps in a short period of time. Diesel from refineries often do not carry any amount of water, but water can enter fuel tank due to condensation of the air. For this reason, water separators are a mandatory equipment in any vehicle fuel supply system.