Fuel Cold Flow Properties
The operational performance of diesel fuel is significantly impacted by its low-temperature properties. Below a certain temperature, various changes occur which affect its ability to flow and be effectively utilized in a combustion engine.
Cloud Point and Pour Point
Two key indicators of a fuel's cold-weather performance are its cloud point and pour point. The cloud point is the temperature at which wax crystals begin to form, causing the fuel to become cloudy. The pour point is the lowest temperature at which the fuel will still flow under prescribed conditions. These points are determined through standardized laboratory testing procedures and are dependent on the fuel's specific composition, particularly its paraffin content.
Wax Crystallization and Fuel Gelation
As the temperature decreases, paraffin waxes present in diesel fuel begin to precipitate out of solution, forming crystals. These crystals increase in size and number, eventually leading to a significant increase in the fuel's viscosity. This process is referred to as wax crystallization and ultimately results in gelation. The resulting gel-like structure hinders the fuel's ability to flow through the fuel system, leading to engine malfunction or complete failure.
Factors Affecting Cold Flow Properties
- Fuel Composition: The concentration and type of paraffin waxes significantly influence the cloud point and pour point. Fuels with higher paraffin content will gel at higher temperatures.
- Additives: Cold flow improvers (CFIs) are commonly added to diesel fuel to modify the crystal structure of the waxes, reducing their tendency to form large, interlocking structures and lowering the pour point and gel point.
- Fuel Storage and Handling: Proper fuel storage and handling practices, including avoiding contamination and ensuring adequate fuel temperature, are critical to maintain optimal cold weather performance.
Determination of Cold Flow Properties
Specific test methods, such as ASTM D2500 (cloud point) and ASTM D97 (pour point), are used to determine these critical fuel properties. These methods provide standardized procedures for laboratory determination of the temperature thresholds at which fuel performance is affected.
Consequences of Fuel Gelation
Gelation can lead to a range of problems, including clogged fuel filters, difficulty starting the engine, poor fuel atomization, reduced engine power, and complete engine failure. This is particularly problematic in cold climates where temperatures frequently fall below the fuel's gel point.