Viscosity

• The viscosity of diesel has a strong influence on fuel atomization, a high viscosity fuel resulting in larger fuel droplets.
• An increase in viscosity reduces spray cone angle and increases spray penetration.
• Low viscosity on the other hand, results in an increase in leakage of fuel past the pumping elements and loss of fuel system calibration.
• High viscosity of fuel is necessary for lubrication and protection of the injection equipment from wear.

Most fuel specifications limit kinematic viscosity of diesel fuel in the range 2.0 to 5.0 centistokes.

Chemical Composition

• Olefins adversely affect oxidation and storage stability of diesel fuels. Poor fuel oxidation stability results in formation of gummy substances which cause plugging of fine fuel filters and sticking of injector needles.
• As the aromatic content of fuel increases, the particulate and PAH emissions increase.
• Fuels with high aromatic content have a lower natural cetane number and its consequent effect on combustion and emissions.
• Aromatics have higher flame temperatures and hence increase in aromatic content of fuel is expected to result in higher NO_x emissions.
• An increase in polycyclic aromatic content generally results in higher PM emissions. .

The diesel fuel specifications in the USA, Europe and several other countries now limit aromatic content to 10 percent maximum. Also, limits on the poly-aromatic hydrocarbons are being specified. One side effect of reduction in aromatic content is reduction in lubricity characteristics of the diesel fuels causing durability problems of injection pump and injectors.

Sulphur Content

• Sulphur on combustion produces sulphur dioxide (SO₂), of which about 1 to 3% is oxidized to sulphur trioxide (SO₃) and forms sulphates found in particulate emissions. The balance of SO₂ is exhausted as gas.
• Typically, increase of 500 ppm in sulphur content contributes to about 0.01 g/kWh increase in diesel PM emissions.
• The sulphur trioxide on combining with water forms sulphuric acid that causes wear of engine cylinder liner and piston rings.
• Sulphur increases deposit formation in the combustion chamber and the deposits become harder in presence of sulphur.
• Fuel sulphur has deleterious effect on functioning of advanced after-treatment devices such as NO_x storage- reduction (NSR) catalysts, continuously regenerating diesel particulate traps (CRT) and catalyzed diesel particulate filters (CDPF). The sulphur dioxide and trioxide poison the catalyst.   NSR catalysts require practically sulphur free (< 5 ppm) fuel.
• In CRT, conversion of   NO to NO₂ ahead of CRT is reduced by sulphur as it poisons the catalyst.  Sulphur lower than 30 ppm is necessary for functioning of CRT

In most countries during early 1990s, sulphur content of diesel fuel was in the range from 0.2 to 0.5% (2000 to 5000 ppm) by mass. After the year 2000, a number of European countries made available the diesel fuels with less than 0.005% (50 ppm) sulphur. A large number of countries around the world have diesel fuels with sulphur below 0.05% for road vehicle application. Trends in diesel fuel sulphur content in some countries are given in Table 8.10.