“What is in diesel?” is a question with two answers, one very short and the other very long. The short answer is hydrocarbons. Hydrocarbons, as the name implies, are molecules made of hydrogen and carbon atom bonds. Hydrocarbons are the components of every fossil fuel — and biofuel — that make them valuable. Hydrocarbons are the molecules in fossil fuels and biofuels that ignite, combust, burn, explode, etc. 

On a molecular level, fossil fuels are simply hydrocarbons and contaminants. Biofuels are hydrocarbons at an elemental level as well. So, diesel is simply a combination of different hydrocarbons. It is the mix of hydrocarbons — the ratio of the different hydrocarbons — that make diesel a unique fossil fuel.

More important than asking what is in diesel is asking what hydrocarbons are in diesel, and in what combination.

Hydrocarbon Classes Determine Fossil Fuel and Biofuel Types

Just as there are different types of fossil fuels and biofuels, there are different categories, classes, and types of hydrocarbons. The categories, classes, and types of hydrocarbons determine fuel type — diesel, gasoline, natural gas, kerosene, etc. — as well as the different types of biofuels. 

There are two categories of fossil fuel hydrocarbons: saturated and unsaturated. And, there are four types of hydrocarbon classes, two in the saturated category and two in the unsaturated category. Chevron.com’s technical paper Diesel Fuels Technical Review outlines four classes, “There are four major classes of hydrocarbons: paraffins, naphthenes, olefins, and aromatics. Each class is a family of individual hydrocarbon molecules that share a common structural feature but differ in size (number of carbon atoms) or geometry. The classes also differ in the ratio of hydrogen to carbon atoms and in the way the carbon atoms are bonded to each other.”

Olefins — also known as alkenes — and aromatics are unsaturated hydrocarbons. Alkanes are the only saturated hydrocarbons that occur naturally in crude oil.

Characteristics of Hydrocarbon Classes 

Large hydrocarbon molecules and long and complex hydrocarbon molecule chains equate to greater fuel density. Fuel density is the amount of energy in each volume of fuel, the amount of energy in a gallon or liter of diesel, for example. Light fuels like gasoline and gas-state fuels — natural gas (methane), propane, etc. — consist of small hydrocarbon molecules and short molecule chains. As a result, light fuels have low energy densities. 

Large hydrocarbons and long hydrocarbon molecule chains comprise most hydrocarbons in heavy fuels like diesel, diesel fuel, and bunker fuel (residual oil). That is because large hydrocarbon molecules and molecule chains have high energy density. In addition to the size and length of hydrocarbon molecules, carbon-to-hydrogen ratios play a role in density. The number of carbon atoms in relation to hydrogen atoms determines a fuel’s weight and density. The greater the number of carbon atoms — or the fewer the hydrogen atoms — in a molecule’s carbon-to-hydrogen ratio, the more energy dense the fuel. 

Hydrocarbons with high hydrogen-to-carbon ratios are lighter than hydrocarbons with higher carbon-to-hydrogen ratios. The reason being, hydrogen is the lightest element on Earth. It follows reason, then, that gas-state fossil fuels like propane and natural gas (methane) — which have high hydrogen-to-carbon counts — are extremely light. On the opposite end of the spectrum, thick and heavy fossil fuels like diesel and fuel oil are high in carbon content.  

The greater the carbon-to-hydrogen ratio, the greater the density of a fuel.

Hydrocarbons and Fuel Density

Energy density affects everything from gas mileage and emissions to engine life. Gas mileage, emissions — pollution — and the wear and tear on an engine are products of hydrocarbon molecule size and hydrocarbon molecule chain length. 

The higher the carbon-to-hydrogen ratio, the more energy in a fuel and the cleaner the fuel burns. For example, the energy density of a gallon of gasoline is considerably less than the energy density of a gallon of diesel. “Diesel and gasoline have roughly the same energy per unit mass (lower heating value, about 41 MJ/kg). The density of diesel is about 833 kg / m3 compared to 740 kg/m3 for gasoline. This gives diesel about 13% higher energy density per volume,” according to StackExchange.com. 

As a result — at least in part — diesel engines are between 25 and 35 percent more fuel efficient than their gasoline engine counterparts and last two to three times as long. 

Four Classes of Hydrocarbons 

Again, there are only four hydrocarbon classes: paraffins, naphthenes, olefins, and aromatics. Because there are only four classes of hydrocarbons, necessarily, that means the hydrocarbons in fossil fuels fall within one of the four classes. However, there are hundreds of different hydrocarbon types in diesel as well as every other type of fossil fuel.

Diesel alone contains more than 500 different types of hydrocarbons. And, many of the hydrocarbons in diesel also exist in gasoline and other fossil fuels. While fossil fuels have hydrocarbons in common, it is the ratio of large-to-small hydrocarbon molecules that separate fossil fuels. There is a formula for the hydrocarbon mixture in every fossil fuel. 

Likewise, there is hydrocarbon formula for diesel. 

Categories and Classes of Hydrocarbons in Diesel

The four classes of hydrocarbons fall into one of two categories. Regardless of which of the four classes a hydrocarbon falls under, a hydrocarbon is either a saturated hydrocarbon or an unsaturated hydrocarbon. The ratio of saturated hydrocarbons to unsaturated hydrocarbons varies in diesel. 

But, the ratio is typically around four saturated hydrocarbons to every one unsaturated hydrocarbon. 

With respect to hydrocarbon classes, the hydrocarbons in diesel fall under one of three categories: paraffins, aromatics, and naphthenes. Olefins are very uncommon in diesel fuel because they are uncommon in crude oil. “Olefins rarely occur in crude oil; they are formed by certain refinery processes.” As the Energy Technology Network’s Advanced Motor Fuels Department explains, “Diesel fuel consists mainly of paraffins, aromatics, and naphthenes. Diesel fuel contains hydrocarbons with approximately 12–20 carbon atoms and the boiling range is between 170 and 360 °C.”

What Saturated Hydrocarbons — Alkanes — Are

Saturated hydrocarbons constitute the majority of the hydrocarbons in diesel fuel. About 75 percent of the hydrocarbons in diesel are saturated hydrocarbons. Like there are different types of hydrocarbons, there are different types of saturated hydrocarbons or alkanes. 

Different kinds of alkanes — saturated hydrocarbons — have different numbers of hydrogen atoms and carbon atoms. And, different alkanes have different hydrogen-to-carbon ratios.  There are two types of alkanes in diesel fuel: paraffins and naphthenes. “Paraffins and naphthenes are classified as saturated hydrocarbons because no more hydrogen can be added to them without breaking the carbon backbone.” 

Paraffin Hydrocarbons

Normal paraffins are single-chain molecules. There is a backbone of carbon atoms. Attached to the carbon atoms are between one and three hydrogen atoms. “Normal paraffins have carbon atoms linked to form chain-like molecules, with each carbon – except those at the ends – bonded to two others, one on either side.” But, akin to the fact that there are different fossil fuels because there are different types of hydrocarbons — saturated and unsaturated — there are different kinds of paraffins. In addition to normal paraffins, there are also isoparaffins.

Isoparaffin Hydrocarbons

Isoparaffins have the same carbon backbone as normal paraffins. However, in addition to the backbone of carbon atoms, isoparaffins have carbon branches. The implication is, a different paraffin hydrocarbon can have the same number of carbon and hydrogen atoms in a chain, but a different structure. 

“Isoparaffins have a similar carbon backbone, but they also have one or more carbons branching off from the backbone. Normal decane and 2,4-dimethyloctane have the same chemical formula, C10H22, but different chemical and physical properties. Compounds like this, with the same chemical formula but a different arrangement of atoms, are called structural isomers.”

Cycloalkane Hydrocarbons (Naphthenes)

In addition to single-chain molecules and chain molecules with branches, paraffin hydrocarbons also develop into chains in which the two ends bond to make a loop. Paraffin hydrocarbons that loop are cycloalkanes or naphthenes.  

And, each alkane type can have a large number of different types.

“Under standard conditions of temperature and pressure (STP), the first four members of the alkane series (methane, ethane, propane, and butane) are in gaseous form, and compounds starting from C5H12 (pentane) to n-heptadecane (C17H36) are liquids (constituting large fractions of hydrocarbons found in liquid fuels (e.g., gasoline, jet fuel, and diesel fuel), whereas n-octadecane (C18H38) or heavier compounds exist in isolation as wax-like solids at STP. These heavier paraffins are soluble in lighter paraffins or other hydrocarbons and can be found in diesel fuel and fuel oils. Paraffins from C1 to C40 usually appear in crude oil (heavier alkanes in liquid solution, not as solid particles) and represent up to 20% of crude by volume.”

The remaining components of diesel fuel are aromatics, unsaturated hydrocarbons. 

What Unsaturated Hydrocarbons — Aromatics — are in Diesel

Aromatics are unsaturated hydrocarbons. Aromatics constitute the non-alkanes in diesel fuel.  “Diesel fuels have aromatics contents in the range 15 to 37% volume.” There are three types of aromatics: mono-aromatics, di-aromatics, and tri-aromatics. While there are hundreds of specific types of aromatics in diesel, there is only a half dozen that constitutes a major portion of contemporary diesel.

The most common aromatics in diesel are:

1. Benzene
2. Toluene or methylbenzene
3. m-xylene or 1,3-dimethylbenzene
4. Ethylbenzene
5. Propyl benzene
6. Isopropylbenzene

Benefits and Drawbacks of Aromatics

Aromatics play a crucial role in two important qualities of diesel. First, the greater the number of aromatics, the greater the viscosity. So, the more aromatics in diesel, the more fluid it is. Additionally, aromatics are volatile hydrocarbons which mean aromatics help with diesel engine cold weather starts. And, the more aromatics, the higher the cetane rating of a diesel fuel. But, there are also drawbacks of aromatics, particularly with respect to the environment.   

Aromatics, when combusted, produce dirtier emissions than alkanes generate. “Aromatics may lead to carcinogenic compounds in exhaust gases, such as benzene and polyaromatic compounds. Olefins in gasoline may lead to an increase in the concentration of reactive olefins in exhaust gases, some of which are carcinogenic, toxic or may increase ozone-forming potential.”

Good Vs Bad Hydrocarbons in Diesel

Diesel fuel is not a “clean” fuel according to traditional environmentalist. The reason being, the particulate matter, and smog associated with burning diesel. However, the black smoke produced by diesel engines is hardly any different than the smoke from chimneys, volcanoes, and forest fires. While ugly, the black smoke that diesel engines of the past produced was simply unburned hydrocarbons.

Arguably the most dangerous emissions from combustion engines are the invisible gases found in emissions. Carbon monoxide, for example, is produced in extremely low quantities by diesel engines. On the other hand, gasoline engines produce significant amounts. 

There is one reason — the prevalence of one hydrocarbon over another — that separate diesel from other fossil fuels: naphthenes. “Naphthenes are a class of cyclic aliphatic hydrocarbons or simply Cycloalkanes.” Simply, naphthenes are alkanes that loop. That means naphthenes are energy-rich and extremely dense. More importantly, the emissions from naphthenes are not toxic.

While all fossil fuels have large numbers of paraffins (alkanes) and aromatics, not all fossil fuels have large quantities of naphthenes. Instead of naphthenes, the third component in light fuels like gasoline and gas-state fossil fuels are alkenes. Alkenes are toxic. So, at a fundamental level, diesel is different than gasoline and other light gas-state fuels because it has naphthenes, valuable, non-toxic emissions producing hydrocarbon. 

Diesel Hydrocarbons Compared to Those of Other Fossil Fuels

Fortunately, hydrocarbons that are “clean” are the same hydrocarbons that have high energy density. Likewise, those hydrocarbons that are toxic to people and damage the atmosphere and the environment are also those that are not energy dense. While diesel has issues with respect to emissions pollution, those issues are not hydrocarbon-related. While the sulfur in regular diesel is toxic, low sulfur diesel is considerably less polluting than gasoline. 

Diesel has a higher alkane content than gasoline and a lower aromatics content. Additionally, gasoline has a high alkenes content. Alkenes produce toxic emissions. Rather than alkenes, diesel contains naphthenes, a form of alkanes. Alkanes are the cleanest class of hydrocarbons with respect to emissions. And, naphthenes have the highest energy density of all hydrocarbons. 

With respect to hydrocarbon emissions, diesel is the cleanest fossil fuel.