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Diesel Fuel and Cold Weather

Background

What is the lowest temperature at which a vehicle will start and run? This temperature is often referred to as the cold weather operability limit. The cold weather operability limit of a particular vehicle varies and is dependent upon the properties of the fuel and the design of the vehicle fuel system.

 

Fuel Properties

Diesel Fuel is a complex mixture of a wide variety of hydrocarbons. Depending on how the fuel is blended at the refinery, diesel fuel can contain up to 20% paraffin molecules, also known as “wax”. These wax molecules tend to stick together (agglomerate) as the temperature drops. The colder the fuel, the more the agglomerate until they form a crystal. These “crystals” can become so large that they will separate out of the fuel as a solid. At this point they are too large to pass through the fuel filter, so they stick to the surface of the filter. If the fuel temperature continues to fall, eventually enough wax crystals will form to completely block the filter surface, resulting in lack of fuel flow thereby stalling the engine.

 

Fuel Operability Predictors

There are three primary methods, which as currently used to predict the operability temperatures of fuel. Each method has its limitations in predicting cold weather operability, and no test currently can accurately predict operability for all vehicles.

 

The three predictor methods are:

  • Cloud Point
  • Pour Point
  • CFPP (Cold Filter Plug Point)

 

    • CLOUD POINT
      The Cloud Point of a fuel is usually 5 to 15° Fahrenheit above the operability temperature of the fuel, making Cloud Point a conservative measure of vehicle operability. Cloud Point is an important winter fuel property because it is a commonly measured specification. Knowing the fuel’s Cloud Point, combined with previous experience can be a very good place to start. Blending #1 diesel fuel will generally lower the Cloud Point by about 3°F for every 10% of #1 blended. Cloud Point is generally unaffected by common additives.Cloud Point is the temperature at which a wax crystal grows large enough to become visible. Cloud Point is measured using ASTM D-2500 test method. Generally, each diesel fuel supplier sets his or her own cloud point maximums, which may vary seasonally. For No. 1 diesel fuel, the winter Cloud Point will typically be -30°F or lower. For No. 2 diesel fuel, the winter Cloud Point will typically be +15°F or lower.

 

    • POUR POINT
      Pour Point is the temperature at which diesel fuel becomes so thick that it will no longer pour from a container. At this temperature the fuel is essentially unpumpable.Typical winter Pour Points for No. 1 diesel fuel are -30°F or lower. Typical winter Pour Points for No. 2 diesel fuel are 0°F of lower.

      Pour Point is typically well below the temperature at which the fuel will plug a fuel filter. Therefore Pour Point is a useful measure for fuel handling properties but not a good indicator of vehicle operability.

 

    • CFPP
      CFPP (Cold Filter Plug Point) is the temperature at which a fuel will plug a 45-micron screen under prescribed test conditions. CFPP is measured using European Test Method IP 309, and is currently the most commonly used cold weather operability indicator.Although widely used, CFPP has its limitations. While most vehicles will operate at outdoor temperatures down to the CFPP of the fuel, some will not. This is primarily due to verifying designs in the fuel systems.

 

NON-FUEL RELATED FACTORS FOR WINTER OPERABILITY

 

Fuel temperatures are generally higher than air temperature. Thus, the temperature at which a filter plugs generally does not occur until the outdoor air temperature drops below the critical fuel temperature. Fuel temperatures are affected by many factors including fuel storage conditions, engine types, fuel system designs, fuel heaters, and operating conditions.

    • FUEL STORAGE
      During the winter, fuel stored underground will tend to stay warmer than fuel stored above ground. In addition, the temperature of delivered fuel will have an impact on operability. As an example, fuel delivered at 50°F to a 125 gallon truck fuel tank in typical 8-mph wind conditions, at 0°F temperature will take about 12 hours to drop down to 10°F. However, fuel delivered at 30°F in similar conditions will take only 8 hours to get to the same point.

 

    • HARDWARE EFFECTS
      Although virtually all diesel engines recirculate fuel through the fuel injection system back to the fuel tank, the location of the vehicle’s fuel filter can impact cold weather operability. Some fuel systems place the fuel filter closer to the warm engine or shield the filter from the wind, while others are located on frame rails for ease of changing and draining. Locations close to the engine or shielded from the wind will tend to warm the fuel more than filters exposed to the wind and elements.Additionally, the high wind speeds will also increase the heat loss from equipment fuel tanks and lines. Windshields and insulation will help reduce heat loss. Many of the newer trucks have aerodynamic covers over the fuel tanks. These not only lower the power equipment of the vehicle, but also affect the convective heat loss from the tanks.

      Various types of heaters may be used to warm the diesel fuel and engines. Heaters can be places in a fuel tank, along fuel lines, at the fuel filters, and on the engine block to name a few locations. All of these devices tend to create a larger difference between actual fuel temperature and outdoor air temperature.

 

    • OPERATING CONDITIONS
      How a vehicle is operated also contributes to differences in fuel temperature. A vehicle operated 24 hours a day will have warmer fuel than an identical vehicle that s shut down for several hours each day. Additionally, differences in vehicle parking location can influence fuel tank temperatures. As noted earlier, a tank fueled with 50°F fuel and subject to 8-mph winds takes about 12 hours to get its fuel temperature down to 10°F. A truck that is parked in an area that is shielded from the wind will require almost 17 hours for its fuel temperature to reach 10°F

 

    • Near empty fuel tanks will cool off much more quickly than full tanks. Therefore, refueling prior to parking a vehicle will help slow fuel cooling.

 

IMPROVING WINTER FUEL PERFORMACE

There are many methods which help decrease fuel filter plugging. The most common are:

      • Blending #1 diesel fuel with #2 diesel fuel
      • Blending additives into the diesel fuel
      • Keeping the fuel warm
      • Keeping the fuel dry
      • Using a combination of the above methods

 

BLENDING #1 DIESEL FUEL

#1 diesel fuel contains a smaller concentration of paraffin molecules than #2. Reducing the percentage of paraffin molecules available for crystallization will lower the cold weather operability predictors for the fuel. For every 10% of #1 blended into #2 fuelse, the cold weather operability predictors (Cloud Point, Pour Point, and CFPP) will be reduced 3 to 4°F. However, #1 has lower BTU content than #2, thus decreasing energy available.

 

WAX MODIFYING ADDITIVES

Wax modifying additives (also referred to as flow improver additives) were developed to give fuels better low temperature filterability. When fuel cools, paraffin molecules tend to stick together to form wax crystals. Wax modifying additives are very long polymers, which tend to adsorb to the surface of paraffin molecules, hindering the growth of the wax crystals, resulting in smaller, more compact wax crystals. These smaller crystals do not separate out of the fuel and do not block fuel filters as easily.

While each method listed above will contribute to lowering fuel filtering plugging, no method will work in all circumstances.

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