What Is Diesel?

Stocks of diesel and other distillate fuel oils—such as jet fuel and heating oil—reached their lowest levels since 2008 in the United States as of October 21, according to a weekly report from the Energy Information Administration (EIA), sparking concerns from some quarters over potential shortages.

Figures from the report also show that distillate stocks have not been lower at this date since the EIA began collecting weekly data in 1982.

But what exactly is diesel and what is it used for?

Diesel is a fuel oil used in several applications to power compression ignition engines, a form of internal combustion engine invented by German engineer Rudolf Diesel in 1892. In these engines, highly compressed air is used to ignite the fuel without a spark.

Diesel is used in many freight and delivery trucks, some cars, as well as trains, buses, boats and vehicles designed for agricultural, military, construction or other industrial purposes.

Another common use for diesel is in generators that are often used to provide electricity in remote locations or as backup power supplies.

Most diesel fuel is produced by refining crude oil but some alternatives are available, such as biodiesel, which is made from biomass materials, originating from plants or animals.

"Crude oil contains thousands of individual chemicals that are separated into groups based on boiling temperature by distillation," Michael E. Sigman, a professor in the Department of Chemistry at the University of Central Florida and director of the National Center for Forensic Science, told Newsweek.

"One of the groups of molecules is known as light gas oils. Light gas oils are blended together to formulate different diesel fuels."

The United States produces most of the diesel that it consumes, the vast majority which tends to be in a form known as ultra-low-sulfur diesel or ULSD.

Prior to 2006, most diesel fuel in the U.S. contained high quantities of sulfur. But due to the health risks of sulfur in the air, the Environmental Protection Agency passed regulations that required manufacturers to lower the sulfur content of their fuels. ULSD now accounts for almost all the petroleum-based diesel used in the United States and Europe.

According to the EIA, the United States produced around 1.63 billion barrels (68.35 billion gallons) of ULSD in 2021, while around 0.33 billion barrels (14.04 billion gallons) were exported. The country also imported a small proportion of the diesel it consumed in 2021, primarily from Canada.

Once the diesel is produced, it is transported using pipelines from refineries and ports to terminals that are located around the country. The diesel is also sometimes transported using specialized barges or trains. From the major terminals, trucks are used to ferry the fuel to service stations and other businesses that use large quantities.

Diesel is among the most widely used sources of fuel today. While diesel tends to be more expensive than gasoline, more energy can be extracted from the former compared with the latter. As a result, diesel engines are more efficient and cost effective. They also tend to be more reliable.

Because diesel engines are more efficient, a car using this type of fuel will produce less carbon dioxide than a gasoline vehicle when covering the same distance. But diesel engines still contribute significantly to overall emissions, and they can also produce harmful pollutants, such as nitric oxide and fine particulate matter.

While distillate stocks are currently at low levels, this does not necessarily mean that the country is going to run out diesel, as some commentators, such as Fox News host Tucker Carlson, have suggested.

The EIA reported that the country had 25.4 days of supply distillate fuel left in storage as of October 14, but the administration told Newsweek that this does not take into account the amount of distillate fuel being produced at U.S. refineries, or the amount of distillate fuel imported into the country.

"I just don't see it happening," Denton Cinquegrana, chief oil analyst at the agency Oil Price Information Service, told Newsweek, referring to a situation where the country hypothetically run out of diesel.

Ed Hirs, a professor of energy economics at the University of Houston, explained the situation to CBS MoneyWatch.

"Inventories of diesel and gasoline are down below five-year averages, and if the entire world were to stop, we would have 25 days worth of diesel. But the world doesn't stop. We're not counting on it stopping," Hirs said.

"Your grocery store may have an inventory of three days of milk. That's because they only have three days' worth at any given point. But the cow keeps milking, the farmer keeps sending milk, the dairy keeps delivering."