The Triton

Engine Room

Diesel Digest: Diesel basics boil down to chemistry


Diesel Digest: by Capt. Jeff Werner

The petroleum that is pumped out of the ground via an oil well is called crude oil. This crude can vary in color and density depending on the ratio it contains of different hydrocarbon molecules, the amount of sulfur and the traces of metals such as iron, nickel and copper.

Plainly stated, hydrocarbons are molecules composed of hydrogen and carbon atoms. Depending on their configuration, these two different atoms can link together to form molecules that resemble a linear chain or a circular ring. The linear molecules can be straight or branching and the ring molecules can have five or six-sides with appendages of varying complexity. Simpler hydrocarbons have shorter chains and fewer rings, and more complex hydrocarbons have longer chains and more rings. Petroleum can be likened to a stew whose main ingredients are simple and complex hydrocarbon molecules, with a seasoning of sulfur, oxygen, nitrogen and metals, and sometimes a dash of bacteria.

The four classes of hydrocarbon molecules that make up crude oil are paraffins, naphthenes, aromatics and asphaltenes. Diesel is composed of an elaborate mixture of these same groups of hydrocarbons with the exception of asphaltenes, since those tar-like substances would clog fuel filters.

It is the job of the oil refinery to distill the crude oil into the molecular components that make up diesel fuel, and blend them together in the proper proportions for the most efficient combustion by a diesel engine. Distilling is a process that separates two or more components in a liquid based on a physical property, such as the boiling point. At modern oil refineries, fractional distillation remains the major process used to produce diesel. It separates crude oil into its component parts, or fractions, by boiling the crude. Petroleum is heated by high pressure steam and as the temperature of the oil rises, the different hydrocarbon molecules are boiled off and collected for blending. For diesel fuel, the range of boiling points for all of its component hydrocarbons ranges between 275 F and 750 F.

The proper blend of aromatics, naphthenes and paraffins is important because that is what determines the diesel fuel’s capabilities. For example, paraffins have excellent combustibility, but a low energy release. Since paraffins are waxy, they have very poor cold weather flow properties. Aromatics have very good cold weather flow properties and energy release, but very low combustibility. The characteristics of naphthenes fall somewhere between those of paraffins and aromatics.

Once diesel fuel leaves the refinery, its shelf life depends on a variety of chemical processes that affect the fuel. As fuel ages it oxidizes and become unstable.  Oxidation takes place when the oxygen present in the small amount of air that is dissolved in fuel attacks chemical compounds in that fuel. According to research by Chevron, an oil industry leader, “Unstable diesel fuels can form soluble gums or insoluble organic particulates. Both gums and particulates may contribute to injector deposits, and particulates can clog fuel filters.” In addition, if trace amounts of copper and iron become dissolved in diesel fuel, they will accelerate the fuel instability. Without proper treatment, fuel will become unstable within six months of leaving the refinery.
In addition, organic acids found in the fuel blend lead to the formation of insoluble compounds that cause incomplete combustion and poor engine performance. Unfortunately, asphaltenes are also found in most diesel fuel in small concentrations because they were not completely removed during the refining process. Asphaltenes are not soluble in fuel and they have a tendency to agglomerate. Agglomeration means they form clusters, and while these clusters are only the size of a grain of table salt, they are large enough to be a key component to premature fuel filter blockage.

Preventing or reversing the process of fuel breakdown can only be addressed with a proper fuel preventive maintenance program. This program solves the problem through a combination of testing, filtration, conditioning and restoration of the fuel to keep it in optimal condition for peak performance.

Capt. Jeff Werner is a 25-year veteran of the yachting industry as a captain on private and charter yachts, both sail and power, and a certified instructor for the RYA, MCA, USCG and US Sailing. He also owns Diesel Doctor ( Comments are welcome below.

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