Energy content of diesel fuel is measured in BTUs per gallon. The higher the energy content, the more power you can get out of your fuel. No. 2 diesel typically contains 130,000 BTU/gal.
Ryan Central has looked at the cost per BTU of its diesel fuel. "You look at all of your costs performance-wise," says Fell. "A lot of times, by using a higher BTU fuel content, you are going to come out ahead. Ultimately, you will have a lower cost per BTU."
One way to gauge how much BTU a fuel contains is by the API gravity measurement. "API gravity is a measure of fuel density, and affects the heating value (BTUs) of a fuel," says Bob Theisen, manager, technical services, CHS.
"It can also be used to predict the relative viscosity of a diesel fuel," Harvey adds. "As a rule of thumb, the lower the API gravity, the higher the energy density, viscosity and mass density of the fuel." For example, a diesel fuel with an API gravity of 30 will generally be higher in energy content, more viscous and heavier on a weight per gallon basis compared to a fuel with a 40 API gravity.
Blending diesel for winter conditions can impact the energy density. "Higher density fuels generally have higher energy content, meaning better fuel economy. But generally, these fuels are not viable for cold climates," says Gault. No. 1 fuel or fuel blends of No. 1 and No. 2 tend to be less dense, and therefore lower in energy content. "But they are required to prevent fuel flow problems in cold conditions."
Cetane number is an indication of how easy the fuel is ignited in your engine. The higher the cetane number, the shorter the ignition delay between the start of injection and combustion of the fuel. "The higher the cetane number the better, if all other aspects of the fuel are equal," says Gault.
Currently, No. 2 diesel fuel has cetane values that range from 40 to 55. "ASTM D-975 specifies a minimum cetane number of 40 for any diesel fuel," says Harvey. "Most owner manuals also require a minimum cetane number of 40." For winter fuel, you should look for a cetane level closer to 50.
"As the cetane number increases, you can expect better cold temperature starting, improved combustion and emissions, in addition to a quieter running engine," says Theisen. "We know that premium-type diesel fuels tend to have higher cetane numbers."
If the cetane number is too low, the engine will be more difficult to start, particularly in cold weather, and it may run rough. Oil sludge may accumulate more quickly, along with engine deposits. Yet, there is no benefit to running a cetane level higher than engine manufacturer specifications.
There is a tendency to misinterpret cetane level as an indication of the fuel's energy content. But a level that is higher than required will not enhance engine efficiency.
"Just because a fuel has a high cetane number does not mean it will provide optimum performance," says Harvey. "A high cetane number can - but not necessarily does - suggest a lower BTU/gal. if the No. 2 diesel fuel has been blended with kerosene to achieve the higher number."
Gault adds, "Generally the higher cetane number fuels are less dense, and therefore have lower fuel economy, so there are tradeoffs. EMA recommends a minimum cetane number of 47 for the best combination of performance and fuel economy."
Cold weather performance
Cloud point and pour point directly affect cold weather performance. "During the summer months, cloud and pour points do not carry the importance they will for winter months," says Harvey. "For winter operability, fuels exhibiting low cloud and pour points can generally provide a wider range of operability temperatures, especially at low ambient temperatures."
The cloud point is the temperature at which crystals of paraffin wax first appear. (This creates a cloudiness in the fuel.) These crystals collect and plug the fuel filter. The pour point refers to the temperature at which the diesel fuel will no longer pour; it becomes too thick.
A common practice has been to blend kerosene or No. 1 diesel fuel with No. 2 diesel to make the cloud and pour points acceptable for cold weather application. But not everyone likes this approach. "There are still a number of people who blend with a No. 1 in the wintertime," says Fell. "It is so expensive, and you can get the same benefits from using a No. 2 with an additive."