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We often associate diesel with bigger vehicles. Diesel trucks are the workhorses of our roads, and the reason for this is quite clear: a diesel engine can be one of the most reliable and powerful machines you could hope to meet, and they tend to have a torque advantage over their gasoline counterparts that makes them well suited to hard work. 

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However, they may also be heavier, larger and hungrier; diesel-guzzlers, if you will. As nations around the world continue to take steps to become more environmentally friendly, the auto universe follows suit, and this has led to a wide range of innovations. EVs and hybrids are some of the most notable, but there’s another interesting area that diesel-powered vehicles are expanding into: renewable diesel and biodiesel.

Both forms of diesel are created to have less of a harmful impact than standard diesel, which is refined from petroleum. Conceptually, both biodiesel and renewable diesel are similar, but the two terms are far from interchangeable. Renewable diesel is a hydrocarbon closer to petroleum diesel, according to the EIA, and it can be mixed with or used in place of regular diesel. Biodiesel can also be combined like this, but it is not a hydrocarbon, and there are limitations to its use in comparison.

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Petroleum diesel is not renewable, having its roots in crude oil just as gasoline does. Renewable diesel is instead produced through a process known as hydrogenation, where vegetable oil or animal fat is treated and blended with hydrogen atoms, then distilled into its composite parts, including the ever-volatile naptha.

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The result is a diesel that is chemically equivalent to petroleum diesel. This accounts for the ease of blending them or substituting one for the other entirely, though not without considerable cost. According to a 2023 University of Illinois study, the technology for making renewable diesel is substantially higher because the same crude oil refining technology is used in its production. Often, there isn’t so much a logistical cost to using renewable diesel as a financial one.

The Foreign Agricultural Service also raises concerns that the market for producing renewable diesel just isn’t there without federal and state policies that would mandate minimum qualities and offer tax credits. As a result, it’s not as simple as replacing traditional diesel outright. Unlike biodiesel, however, renewable diesel is chemically very close to doing so, as the comparable renewable diesel meets ASTM International’s requirements for ASTM D975.

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One of the most important differences between biodiesel and renewable diesel is the production process itself. Biodiesel is created through a process called transesterification, which typically sees methanol and animal fat or waste vegetable oils catalyzed with a strong alkali, the primary result of which is an unrefined form of biodiesel. The same process also produces an unrefined form of glycerin, a multi-purpose substance that is used in moisturizers, as a sweetener in food, and more. 

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To purify it, the crude biodiesel is washed either with water or dry-washed chemically. The latter process is the more eco-friendly of the two, as it removes concerns over the great quantities of water the process requires. There’s no understating how important this may be. In a 2011 issue of the science journal Applied Energy, scientists noted that each liter of biodiesel produces roughly ten times as much wastewater. Techniques such as ceramic membrane separation -– whereby residue is filtered through pores of a ceramic filter -– have been introduced to help make the process more efficient while using less water. 

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Experimentation to develop and improve other forms of diesel have been ongoing since before the creation of the diesel engine itself, such as J. Patrick and E. Duffy engaging in the transesterification of oils in 1953. As some aspects of the auto world continue to lean towards the more sustainable, the benefits of these options are becoming clearer to see, too. A U.S. Department of Energy report shows that renewable diesel reduces carbon intensity by nearly 65%. 

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There are other important benefits to these greener forms of diesel, too. The potential risks of spillage, which can happen even in the best-case scenario, are lessened, and the reduced volatility with regard to heat is vital to consider, too. Biodiesel will only ignite in the air at temperatures of over 130°C. As such, it can be a far safer fuel source. Biodiesel, though, is a fatty acid methyl ester rather than a hydrocarbon, which means that it’s used in limited blends of up to around 20% biodiesel to 80% petroleum diesel, meaning it is typically used to blend and not typically used as a transportation fuel. Some drivers might wonder what happened to biodiesel, but like renewable diesel, it has considerable challenges to overcome.

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