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If you’ve ever ridden in or driven a semi truck, or own a modern heavy-duty pickup truck, there’s a good chance you know what a diesel engine is. For the rest of us, it can be a little confusing to suss out the differences between diesel-powered vehicles and those that burn gasoline instead.

With over one hundred years of history under its belt, the diesel engine has been a workhorse around the world for generations. Diesels are used in some of the world’s most powerful and heavily-used vehicles, and work to power many things we take for granted every day. Car Bibles’ editors have spent more than a little time driving and fiddling with diesels over the years, and are here to help you get a basic understanding of oil burners.

What Is a Diesel Engine? 

The engines are named after Rudolf Diesel and were originally intended to be replacements for steam engines. Over the years, diesels have powered everything from ocean liners to tractors, to the station wagon your parents drove. 

Diesel engines are internal combustion engines where fuel is ignited by the increased temperature of air in the cylinders. Diesel fuel itself, while derived from crude oil like gasoline, is thicker and offers better efficiency than gas.

How Does a Diesel Engine Work?

Mechanical compression causes an air temperature increase to ignite the fuel, which is why diesels are sometimes called compression-ignition engines. This is different from gas engines, in which spark plugs are used to ignite the air and fuel. That combustion and the corresponding expansion in air pressure are the forces that move the pistons in a diesel engine. 

Diesel engines can operate on either a two- or four-stroke cycle, and while gas engines can be configured in the same way, diesels only bring air into the combustion chamber on the intake stroke. 

Diesel engines generate power by burning the fuel that has been sprayed into the compressed, elevated-temperature air. Obviously, that air has to have been heated to a point that is hotter than the ignition temperature for diesel fuel, which in the case of diesel engines is typically greater than 979 degrees Fahrenheit or 526 degrees Celsius. 

When the engines are cold, or first started, heat sometimes needs to be introduced to the engine artificially, which is why you’ll sometimes see diesel engine block heaters and other devices used to raise the temperatures needed for ignition.

What’s the Difference Between Diesel and Gasoline?

Gasoline and diesel fuels and engines can almost be considered kissing cousins, as the fuels are derived from the same base materials and the hardware itself is internal combustion in the case of both engines. There are some key differences, however.

Gasoline

In gasoline engines, the fuel is mixed with air and is compressed by a piston before being ignited by a spark plug. In terms of the fuel itself, gasoline is thinner and has a much lower evaporation point than diesel. That’s why diesel has a higher boiling point than water, while gas will evaporate quickly, even at room temperature. Perhaps the biggest physical difference between the two is that diesel is considered a combustible, while gasoline is a flammable liquid. That means that diesel will put out a match, while gasoline will take that flame and run with it.

Diesel

Both gasoline and diesel fuels are derived from crude oil that has to be extracted from deep beneath the earth’s surface. After processing, diesel fuel is thicker and evaporates more slowly than gasoline. Diesel is also inherently more energy-dense than gasoline is, which plays a part in diesel engines’ typically better fuel economy than their gasoline counterparts.

Decades ago, diesel fuels could be quite “dirty,” in terms of their effects on the environment. Since then, technology and science have improved the fuels with more exact refinement techniques, and with various in-vehicle features that help mitigate emissions.

The Car Bibles Glossary for Diesel Engines

Welcome to Bible School!

Turbocharger

Many diesel engines use turbochargers to improve power, torque, and fuel efficiency. A turbocharger, or turbo, is a forced induction system that uses turbines and forces air into the combustion chamber. The bump in air flow also means a bump in fuel flow which equates to a bump in power.

DEF

Diesel exhaust fluid, or DEF, is derived from deionized water and urea. In vehicles equipped with selective catalytic reduction (SCR), exhaust gases are sent through a filter, which removes soot and other particulates. If you’ve ever seen someone “rolling coal,” you’ll know what happens without that filter. Next, the gases and DEF move into the catalytic converter, where ingredients in the diesel exhaust fluid work to convert harmful nitrogen compounds into plain old nitrogen and water.

Biodiesel

Biodiesel fuel is derived from fats, such as vegetable oils and animal fats. With some processing and blending, the oils can be used as fuel for diesel engines.

Diesel Particulate Filter

A diesel particulate filter, or DPF, is a device that works with the vehicle’s emissions system to capture harmful diesel particulates. 

The Questionnaire

Car Bibles answers all your burning questions!

Q: Can I Use Gasoline In My Diesel Engine?

A: Absolutely not. Using gas in a diesel engine or vice versa is a good way to end up needing a new engine. Only use the fuels that are recommended by your vehicle’s manufacturer. 

Q: Why Are Diesels Better at Towing?

A: Diesel engines generate more torque and return better fuel economy than their gas-fed counterparts. Gas engines are perfectly fine with towing in most cases, but their diesel brethren are better at it.

Q: Is Diesel Fuel More Expensive Than Gas?

A: As of today, yes. The national average at the end of 2020 was $2.17 for gas and $2.55 for diesel.

The Diesel Engine Tutorial

Have a diesel under your hood? It can be hard to know what to buy to keep it running smoothly. That’s why Car Bibles’ editors have grabbed a few of their favorite products to keep you going. They include Fuel Ox Winter Fuel Treatment, the Midwest Can Diesel Fuel Can, and Mechanix Work Gloves.

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