Different Grades of Diesel Fuel Used in Diesel Generators

A continuous supply of clean fuel must be provided to the fuel systems of both prime generators and standby or backup applications for optimal performance when in use. One of the most common forms of fuel used for this purpose is diesel, thus, diesel generators are in high demand. The reasons for diesel are manifold:

  1. Diesel is reliable;
  2. Diesel is commercially readily available;
  3. Diesel is generally more affordable than other fuel sources;
  4. Diesel is easily stored on-site; and
  5. Diesel allows for fast and easy starts, which is especially important for backup generators.


The diesel engine for its part is able to sustain abuse, is easier to maintain, and lasts longer than other power sources for engines, like gas or propane. Not all diesel fuel, however, is alike. Diesel fuel is available in two different grades as well as a mix (as in “cold weather mix” of #1 and #2):

  • Diesel fuel grade #1 (D1);
  • Diesel fuel grade #2 (D2); and
  • Winterized diesel (a blend of #1 and #2 diesel fuels).

These different grades are compatible with any diesel engine, which means you can use one during the winter and another during the summer without problems. That ability to switch and/or blend is another benefit of diesel because the grades and blended version offer different advantages. In fact, each grade gives up a benefit to provide another advantage. Knowing what the latter is and aligning it with your operations will be key to performance.


How Grades Differ – Viscosity & Cetane Levels

Diesel fuel grades differ based on their viscosity and cetane levels. Here is an overview of what that means.


Viscosity & Diesel Fuel Grades

Diesel fuel is measured by its viscosity, meaning its thickness. This is important because in the winter when the weather is colder, the consistency of diesel fuel becomes thicker and cloudier. When the weather is extremely cold, like during a polar vortex, the diesel’s consistency may transform to a texture more like gel and in this condition it will not flow and the engine will not run.

Diesel #1 has a lower viscosity, meaning it is less thick and flows more easily. As such, diesel #1 is more effective in the winter. Diesel #2 has a higher viscosity, so it is less efficient in lower temperatures and is better for use during warmer weather. But winters are generally becoming milder, and as they do, a mix of the two grades is often an option to reap the benefits of both grades #1 and #2.


Cetane Levels & Diesel Fuel Grades

The higher cetane level in the diesel fuel refers to its volatility, meaning how readily it ignites and how quickly it burns. Diesel #1 is more volatile than Diesel #2. At the same time, diesel #2 provides greater fuel economy, so it does not burn as fast.


#1 Diesel Fuel

As mentioned, #1 fuel has higher volatility but lower viscosity. #1 contains no paraffin wax, so it does not gel (as in the point at which diesel starts to gel) in cold weather. The high resistance to gelling in the winter is #1’s primary benefit — and given extreme weather patterns in recent years, this benefit is an important one.

While it delivers ease of flow during the winter, #1 also has its drawbacks that should be considered, like:

  • Low energy content due to higher volatility;
  • Limited lubrication due to the absence of paraffin wax, an important lubricant found in #2; and
  • Higher costs associated with it because of its low energy content requiring higher quantities to match #2’s energy content.

#1 fuel, therefore, is often a second choice to #2 and blended diesel.


#2 Diesel Fuel

#2 fuel, as also mentioned above, has lower volatility and high viscosity. #2 contains wax, which provides its desired and beneficial lubricating effect. The lubrication protects the diesel engine and all of its components from extended abuse, wear, and tear. These properties make #2 the most widely preferred diesel fuel type for diesel generators under most climates or weather conditions. The benefits of #2 are many:

  • Readily available at most fuel stations;
  • High energy content;
  • High lubrication due to the presence of wax;
  • Extended maintenance life of the diesel engine due to higher efficiency; and
  • Reduced costs as a result of all of the above.

The one drawback, however, is its low resistance to gelling in the winter due to its high viscosity and presence of paraffin wax. Wax crystals begin to form at around 10° Fahrenheit. This gelling effect in the winter means hard starts or no starts, and that has a direct impact on your operations and overall performance. Thus, #2 may not be the best option during very cold winters even if it’s a good option for the remainder of the year.


Diesel Fuel in Cold Climates

In the winter or cold climates, between the two diesel fuel grades, #1 is the best option due to its high resistance to gelling in temperatures as low as -60° Fahrenheit. That said, it is also expensive. #2 offers an economic advantage due to the price per Btu, but it starts gelling at approximately 10° Fahrenheit, depending on its overall makeup. If you operate in cold climates, here are your best options:

  • #1 due to the absence of wax;
  • #2 if additives are added to alter the size and shape of wax crystals that materialize at low temperatures; or
  • Winterized fuel, which is just another word for saying blended diesel grades #1 and #2.

The “winterized” blended option provides benefits from both grades, making it a viable and optimal option for cold climates generally and cold winters specifically. The benefits of blended diesel fuel include:

  • Good energy content;
  • Effective lubricant compounds; and
  • Reduced risk of fuel gelling at cold temperatures.

Diesel fuel in cold climates can be a challenge to balance efficiency, costs, and optimal performance. Blending addresses this challenge effectively. Smart diesel management is also crucial. Since most generator operators buy diesel in bulk when the price is low, the right blend needs to be bought at the right time of year. Depending on the type of storage tank (above ground or underground) and surrounding conditions (water and heat), diesel can last from anywhere between 6-12 months. Buying and topping up the tank at the right time takes strategic planning.


Which Grade of Fuel Is Right for Your Generator?

Under most conditions, operators do not need to know which fuel is best for their diesel generator(s) because local fuel suppliers know. They will supply you with the best option in accordance with the diesel generator, the nature of the operations, and the local weather conditions. That said, knowledge of the same makes for a more informed operator.  Keep the above in mind, and if you have further questions, always feel free to ask your fuel supplier.