The differences between diesel, fuel oil, and bunker fuel are hydrocarbons. Specifically, the difference is in the size and length of the hydrocarbons in each fuel. Hydrocarbons comprise the overwhelming majority of the components in fossil fuels — and in biofuels as well for that matter. Everything else in fossil fuels and biofuels is a contaminant. As the name implies, hydrocarbons consist of molecules with only two types of atoms: hydrogen and carbon.
Hydrocarbons are the reason fossil fuels and biofuels have value. Hydrocarbons are the reason diesel, gasoline, fuel oil, natural gas, biodiesel, etc. ignite/combust/burn. And, it is because there are different hydrocarbon categories — and classes within those categories — that there are different fuels.
Diesel and fuel oil — including bunker fuel — are heavy fossil fuels. Gasoline is a medium weight of fossil fuel. The lightweight fossil fuels gas-state fuels like methane (natural gas) and propane.

 

 

Classes of Hydrocarbons Within the Saturated and Unsaturated Categories

Saturated versus unsaturated hydrocarbons is one difference between light and heavy fossil fuels. But, there are also differences between fossil fuels because of the difference between hydrocarbons in those two categories. There is a total of four classes of hydrocarbons: paraffins, naphthenes, aromatics, and olefins.
Paraffins and naphthenes are the two classes of saturated hydrocarbons. Aromatics and olefins are the two classes of unsaturated hydrocarbons.

Paraffines A.K.A., Alkanes
Paraffins are also known as alkanes. Alkanes are single-chain hydrocarbons. The backbone of an alkane consists of carbon atoms. Alkanes are one of the two saturated hydrocarbon classes. The other class of saturated hydrocarbons is cycloalkanes, a.k.a., “naphthenes.”
The difference between paraffins and naphthenes is that naphthenes have branches. And sometimes naphthenes loop. Naphthene carbon atoms often have more than just the two carbon atom bonds along the spine of the molecule. The carbon atoms in a naphthene can have two, three, four or five bonds with other carbon atoms. And, the carbon atom on one end of the molecule chain’s spine bonds with the carbon atom on the other. A notable trait of naphthenes is that they are the most energy dense hydrocarbon class in either category of hydrocarbons.

Hydrocarbons in Heavy Fossil Fuels
Diesel and fuel oil have, essentially, the same hydrocarbon makeup. Both are composed primarily of saturated hydrocarbons. “Petroleum-derived diesel is composed of about 75% saturated hydrocarbons, and 25% aromatic hydrocarbons. The average chemical formula for common diesel fuel is C12H24, ranging approximately from C10H20 to C15H28.”

Categories of Hydrocarbons
The two categories of hydrocarbons are saturated and unsaturated. Saturated hydrocarbons are complete. They cannot take on additional hydrogen or carbon atoms. Unsaturated hydrocarbons are incomplete. They have room for the addition of hydrogen and carbon atoms. Because saturated hydrocarbons are complete, they are stable. Unsaturated hydrocarbons are incomplete and, therefore, unsaturated hydrocarbons are unstable and volatile.
Lightweight fossil fuels — gas-state fuels — have high counts of unsaturated hydrocarbon. Heavy, stable fossil fuels like diesel, bunker fuel, and fuel oil have far fewer unsaturated hydrocarbons. Gasoline is a medium weight fuel that falls somewhere in the middle.

 

Hydrocarbon Molecule and Molecule Chain Sizes
Lightweight fossil fuels, as one might expect, consist of smaller molecules and molecule chains than heavy fossil fuels. And, again, unsaturated hydrocarbons comprise a larger portion of the hydrocarbons in light fuels. Heavy fossil fuels have a greater percentage of saturated, large and long hydrocarbon molecule chains.
The larger and longer the hydrocarbon molecule chains in a fuel, the greater the density of the fuel. However, the longer and larger the hydrocarbon molecule chains in a fossil fuel, the more difficult achieving combustion efficiency is. So, while there is more energy in heavy, dense fossil fuels, the more energy is wasted. The problem with diesel, fuel oil, and bunker fuel is that a high percentage of their hydrocarbons go unburned.
Achieving the same combustion efficiency typical of lightweight fuels when combusting heavy fuels requires higher heat and more advanced technologies. The combustion efficiencies of diesel, fuel oil and bunker fuel are one of the biggest differences. And, combustion efficiency is a property of the hydrocarbons.
The biggest difference between diesel and fuel oil are the hydrocarbon sizes within each, not the hydrocarbon classes.

Unsaturated Hydrocarbons
The two classes of unsaturated hydrocarbons are aromatics and olefins. Aromatics occur naturally in crude oil. Olefins are a byproduct of crude oil refinement and do not occur naturally in crude oil. Unstable and volatile, aromatics and olefins produce the most toxic emissions. Additionally, aromatics and olefins also produce emissions with the greenhouse gases with the greatest global warming potential.
Another difference between light and heavy fossil fuels is the size of the hydrocarbons they contain. The molecules and molecule chains in light fossil fuels are short and small. Those in heavy fossil fuels are long and large.

 

Diesel: Hydrocarbon Types, Sulfur Content, and Cetane Rating
Unlike the hydrocarbons in gasoline vs diesel, the hydrocarbons in diesel and fuel oil are very similar. In fact, they are almost the same in several cases. The hydrocarbon makes up of diesel fuels, “are approximately similar to fuel oils used for heating (fuel oils no. 1, no. 2, and no. 4),” according to the U.S. Health and Human Services Department. Diesel and fuel oils consist of mixtures of aliphatic and aromatic hydrocarbons. “The aliphatic alkanes (paraffins) and cycloalkanes (naphthenes) are hydrogen saturated and comprise approximately 80-90% of the fuel oils. Aromatics (e.g., benzene) and olefins (e.g., styrene and indene) compose 10-20% and l%, respectively, of the fuel oils.”
The hydrocarbon makeup of diesel and fuel oils is very similar. But still, there are different types of diesel. The differences in diesel grades are dependent on two things. The number of contaminants — sulfur specifically — is one difference between fuel grades. The cetane level of different grades is the second.

Regular Diesel Vs Low-Sulfur
Sulfur is the contaminant in diesel that generates the most concern for those worried about the environmental and health effects of diesel emissions. Sulfur is not toxic nor a major pollutant in its natural state. But, when sulfur oxidizes to create sulfur oxides, the molecules become dangerous for both the environment and for the health of people, flora, and fauna.
Sulfur oxides are one of the two contributors from diesel fuel emission responsible for producing to acid rain. The U.S. Environmental Protection Agency explains, “Acid rain results when sulfur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents. The SO2 and NOX react with water, oxygen and other chemicals to form sulfuric and nitric acids. These then mix with water and other materials before falling to the ground. While a small portion of the SO2 and NOX that cause acid rain is from natural sources such as volcanoes, most of it comes from the burning of fossil fuels.”
It is because of acid rain that emissions governing bodies from around the world have, together, mandate the use of low-sulfur diesel in most commercial and passenger vehicles.
With respect to sulfur content, there is a significant difference between regular sulfur and low-sulfur. The U.S. Department of Energy explains, “ULSD is a cleaner-burning diesel fuel that contains 97% less sulfur than low-sulfur diesel (LSD). ULSD was developed to allow the use of improved pollution control devices that reduce diesel emissions more effectively but can be damaged by sulfur.”

Low Vs High Cetane Diesel
The cetane rating of diesel is analogous to the octane rating of gasoline, but the opposite. Octane additives increase the compression combustion resistance of gasoline. Cetane additives reduce the compression combustion resistance of a fuel. Both cetane and octane are measures of how much pressure a fuel can withstand before autoignition. Straight-run gasoline — gasoline without octane additives — is often weak to pressure and requires greater resistance.
Straight-run diesel, on the other hand, is often too resistant. That means a diesel engine with straight-run diesel will not start in cold weather, low-temperature conditions. Increasing the octane rating and weakening diesel’s pressure resistance allows engines to fire in the cold more easily.
The cetane rating of diesel is simply a measure of the API gravity of diesel, the weight. “A low-density fuel contains fewer BTUs and consequently provides less power to a diesel engine. A typical gravity for #2 diesel fuel is in the 32-34 range compared to a high-cetane fuel which typically has a gravity rating in the 36-38 range and more closely resembles a #1 diesel fuel,” explains GrowMark Incorporated.
While the hydrocarbons in diesel and several types of fuel oils are minimal, there is a rather large difference between those found in diesel and other types of fuel oils, bunk fuel particularly.

Fuel Oils:

Types and Classes, Including Bunker Fuels
During the crude oil distillation process, light, medium, and heavy hydrocarbons separate, a.k.a., “fractionalize.” As the temperature of the oil inside a crude oil distillation column increases, hydrocarbons vaporize. The light hydrocarbons vaporize at lower temperatures that heavy hydrocarbons. Once vaporized, the hydrocarbons are drawn into storage tanks.
The vaporized distillates separate into gas, naphtha, kerosene, light diesel, and heavy diesel (distillate fuel oil).
But, there are also hydrocarbons in diesel that are so heavy, they will not vaporize. Instead, if the temperature gets too high, they will auto-ignite. The hydrocarbons that will not distill are residuals. From residuals come residual fuel oil. Since there are both distillate fuel oils and residual fuel oils, clearly, not all fuel oil is the same.

Classes of Fuel Oils
The two types of fuel oils divide into any number of classes. In the United States and North America, there are six classes in total: Number 1 through Number 6 fuel oil. The United Kingdom breaks down fuel oils into eight classes, four distillate and four residual.
There are two characteristics that separate the different classes of fuel oil. The first is the minimum flash point. The second difference between fuel oils is the minimum and maximum kinematic viscosity.

Flash Point of Different Classes of Fuel Oils
Flashpoint is the temperature at which an organic compound — fuel oil in this case — emits enough vapors to ignite in air. For example, Number 1 fuel oil has a flash point of around 109 degrees Fahrenheit. Number 6 fuel oil has a flash point around 150 degrees Fahrenheit.
Flashpoint plays a role in combustion traits of a fuel. Flashpoint is an indicator of the compression resistance of a fossil fuel. The compression of a gas generates heat. Exposed to enough heat, and fossil fuels auto-ignite. The higher the flashpoint of a fuel, the more pressure it can sustain before auto-igniting.
Another difference between fuel oils is their kinematic viscosities.

Kinematic Viscosity of Fuel Oil Classes
Kinematic viscosity is a measure of the fluidity of a fuel. CSC Scientific Company’s Amanda Ranowski explains, “Kinematic viscosity is the measure of a fluid’s inherent resistance to flow when no external force, except gravity, is acting on it.” Kinematic viscosity is a symptom of fuel density. And, fuel density is a measure of how much energy is in a fuel on a volume scale.
But, while high fuel density is generally thought of as a positive attribute, the high kinematic viscosity is often negative. Because fuels with high kinematic viscosity do not flow quickly, it is difficult to use them in combustion engines. Ideal fuels for combustion engines have high density and low kinematic viscosity.
Bunker fuel has the highest kinematic viscosity of all fuel oils and the highest flashpoint.

Essential Difference Between Diesel Fuel, Fuel Oil, and Bunker Fuel
With respect to chemical makeup, the difference between diesel, fuel oil, and bunker fuel is hydrocarbon size. Of the three, diesel contains the smallest, shortest hydrocarbon chains. Bunker fuel has the longest, largest hydrocarbon molecule chains. No. 1 through 5 fuel oil falls in the middle.
The other differences between diesel, fuel oil, and bunker fuel are their flashpoints and their kinematic viscosities. The flashpoint and kinematic viscosity of diesel is the lowest of the three heavy fuels. Bunker fuel has the highest flashpoint and kinematic viscosity of the heavy fossil fuels.