Cloud And Pour Point

 Cold weather performance of diesel fuel is a key consideration for users. Actual specifications for cold flow properties are based on expected temperature extremes and different test methods are used in different parts of the world. In the United States cloud point is used as an indicator of the cold flow properties of the fuel. Cloud point is the temperature at which wax begins to precipitate out of the fuel. The longer paraffin molecules in the fuel precipitate as a wax when the temperature falls below the cloud point. This wax clogs unheated fuel lines
and filters. The more paraffinic a fuel the higher will be its cloud point. In some parts of the world, pour point is used as an indication of the lowest temperature at which a fuel can be pumped. Pour points are generally 4–5◦C lower than the cloud point.

Distillation

The distillation range of diesel fuel has a significant influence on engine performance. This is especially so in medium and high speed engines. If the fuel is too volatile the engine loses power and efficiency because of vapor lock in the fuel system or poor droplet penetration into the cylinder. On the other hand if the volatility of the fuel is too low, the engine will lose power and efficiency as a result of poor atomization of the fuel. Both the front end and the back end of the distillation are important. If the 10 %vol point is too high, the engine will have difficulty starting. A low 50% point reduces particulate emissions and odor. Because heavier molecules are more difficult to burn, both soot and the soluble organic fraction (SOF) of the particulate emissions are increased if the 90% point is too high, the emission of unburned hydrocarbons will also increase.