Types of Diesel Fuel: What You Need To Know

Diesel fuel is a crucial energy source for heavy-duty trucks, construction equipment, generators and industrial machinery. But did you know that not all diesel fuel is the same?

If you're passionate about working with diesel engines and want to dive deeper into topics like fuel systems, emissions and maintenance, consider enrolling in the diesel technician training program at Universal Technical Institute (UTI). UTI offers hands-on training that prepares graduates to work with diesel technologies in real-world settings.^1,18

In this guide, we’ll break down the different grades and types of diesel fuel, how they work and when to use them. Whether you’re an aspiring diesel technician or a diesel vehicle owner, knowing the right fuel to use can help extend engine life and optimize performance.

What is Diesel Fuel?

Diesel fuel is a liquid fuel derived from crude oil or alternative sources, such as biodiesel, which is produced from renewable materials like vegetable oils or animal fats. Specifically formulated for use in diesel engines, diesel is denser and less volatile than gasoline, and it ignites through compression rather than spark ignition.

One of diesel's key advantages is its higher energy content per gallon, which contributes to greater fuel efficiency and higher torque output, making it ideal for heavy-duty applications. This power and efficiency make diesel a preferred fuel in commercial vehicles, agricultural equipment, construction machinery and industrial engines.

Diesel fuel is available in different grades and types—including seasonal blends and ultra-low sulfur diesel (ULSD)—each affecting performance, cold-weather operation and emissions compliance. Choosing the right type of diesel fuel is essential for engine longevity, optimal performance and regulatory compliance.

Understanding Diesel Fuel Grades

Diesel fuel is categorized by grades that determine its performance characteristics, viscosity and cold-weather reliability.

Diesel #1

Diesel #1 is a lighter, lower-viscosity fuel that performs well in extremely cold temperatures. It has a lower cloud point, meaning wax crystals begin to form at much colder temperatures than in Diesel #2, making it ideal for winter use or arctic conditions.

However, Diesel #1 provides less energy per gallon compared to Diesel #2. This is because it contains less carbon, which directly affects its energy density. As a result, engines may see a slight drop in fuel economy or power output when running exclusively on Diesel #1, though its superior cold-weather flow properties often outweigh that trade-off in frigid climates.

Diesel #2

Diesel #2 is the most used diesel fuel for vehicles, heavy equipment and industrial applications. It has a higher energy content and delivers better fuel economy than Diesel #1 due to its higher carbon content and greater lubricity. However, Diesel #2 is less effective in cold temperatures, as it tends to thicken and gel more quickly when exposed to freezing conditions.

To overcome this, many operators use a blend of Diesel #1 and Diesel #2 during the winter months. This approach helps maintain the cold-weather flow properties of Diesel #1 while still preserving much of the energy efficiency and lubricating benefits of Diesel #2, offering a balanced solution for performance and reliability in colder climates.

Diesel #4

Diesel #4 is a heavier, high-viscosity fuel typically used in large industrial applications, such as marine vessels, power plants and heavy stationary engines. Due to its thickness, Diesel #4 requires preheating before use to lower its viscosity and allow the fuel to flow properly through the fuel system.

This type of diesel also produces higher emissions compared to lighter fuels. That’s largely due to its higher sulfur content and heavier hydrocarbon structure, which result in more soot, particulate matter and sulfur dioxide when burned. Because of this, it is not suitable for modern diesel engines and is generally limited to equipment designed specifically for heavy fuel oil use.

Read more: Diesel Terminology Explained

What Are the Different Types of Diesel Fuel?

Beyond grades, are there different types of diesel fuel? And how many types of diesel fuel are there? There are various types, each with unique properties and applications.

Petroleum diesel

Petroleum diesel, also known as petrodiesel, is the traditional diesel fuel refined from crude oil through a distillation process. It’s the most widely used type of diesel fuel and is commonly available in Diesel #1 and Diesel #2 grades, each suited for different operating conditions.

While petroleum diesel is efficient and energy-dense, it emits higher levels of pollutants, including particulate matter, nitrogen oxides (NOx) and sulfur compounds. These emissions have led to stricter environmental regulations and increased interest in alternative diesel fuels such as biodiesel and renewable diesel, which offer cleaner combustion profiles.

Biodiesel

Biodiesel is a renewable fuel made from vegetable oils, animal fats or recycled cooking grease. It is chemically processed through transesterification, a method that converts raw oils into fatty acid methyl esters (FAME), which can be used in diesel engines.

Biodiesel is biodegradable and generally produces fewer emissions than petroleum diesel. It can help reduce carbon monoxide (CO), particulate matter and unburned hydrocarbons, although it may result in slightly higher nitrogen oxide emissions under certain conditions.

To ensure compatibility with standard diesel engines, biodiesel is often blended with petroleum diesel in ratios like B5 (5% biodiesel) or B20 (20% biodiesel), offering environmental benefits without requiring engine modifications.

Synthetic diesel

Synthetic diesel is artificially produced from natural gas, biomass or coal using processes like Fischer-Tropsch synthesis. It burns cleaner than petroleum diesel, producing lower emissions and better lubricity for engines. Synthetic diesel is becoming more popular due to its high efficiency and reduced environmental impact.

Renewable diesel

Renewable diesel is similar to biodiesel in that it’s derived from biomass sources such as vegetable oils, animal fats and other renewable feedstocks. However, unlike biodiesel, renewable diesel is produced through a process called hydrotreatment, which removes oxygen and impurities, resulting in a fuel that is chemically identical to petroleum diesel. This means it does not contain fatty acid methyl esters like biodiesel does.

Because of its chemical similarity to traditional diesel, renewable diesel meets the same ASTM D975 fuel specifications and can be used without blending in any diesel engine or infrastructure—no modifications are required. It provides comparable energy efficiency and performance but with the added benefit of lower lifecycle greenhouse gas emissions and reduced particulates.

For these reasons, renewable diesel is increasingly being adopted by fleets, transit agencies and government operations as a cleaner alternative to petroleum diesel.

Dyed diesel

Dyed diesel, also known as off-road diesel, is a tax-exempt fuel intended for use in non-road vehicles and equipment such as tractors, construction machinery, generators and other stationary engines. It is dyed red to clearly distinguish it from taxable on-road diesel. The red dye indicates that the fuel is exempt from federal and state road taxes because it is not meant for vehicles operating on public highways.

Using dyed diesel in on-road vehicles is illegal and can result in significant fines and penalties. Authorities use various enforcement methods, such as fuel tank inspections and dip tests, to detect illegal use. These tests can identify the presence of dye in the fuel system even if the tank has been flushed, making it difficult to avoid detection.

Learn More About the Different Diesel Types at UTI

Understanding diesel fuel types and grades is crucial for anyone working in the diesel industry. Choosing the right diesel ensures optimal performance, fuel efficiency and compliance with environmental regulations.

UTI’s Diesel Technology program trains students in fuel system maintenance, emissions standards and alternative diesel technologies. Our hands-on curriculum prepares graduates to pursue careers in the diesel industry, working on trucks, heavy equipment and industrial engines.

Interested in a career as a diesel technician? Request more info and gain the skills needed for a career in diesel technology! If you’re ready to get started, you can fill out an application today!