How Diesel Engines Work

By: Marshall Brain & Kristen Hall-Geisler  | 

Diesel Fuel

diesel fuel
Diesel fuel has both pros and cons when it comes to its eco-friendliness. nexus 7/Shutterstock

Petroleum fuel starts off as crude oil that's naturally found in the earth. When crude oil is processed at refineries, it can be separated into several different kinds of fuels, including gasoline, jet fuel, kerosene and, of course, diesel.

If you have ever compared diesel fuel and gasoline, you know that they are different. They certainly smell different. Diesel fuel is heavier and oilier. It evaporates much more slowly than gasoline — its boiling point is actually higher than the boiling point of water.


Diesel fuel evaporates more slowly because it is heavier. It contains more carbon atoms in longer chains than gasoline does. It takes less refining to create diesel fuel, which is why it used to be cheaper than gasoline. According to the U.S. Energy Information Administration, diesel accounts for about 24 percent of the petroleum products used in the United States; gasoline accounts for 56 percent.

Diesel fuel has a higher energy density than gasoline. On average, 1 gallon (3.8 liters) of diesel fuel contains approximately 139,000 BTUs, while 1 gallon of gasoline contains 124,000. This, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines.

Diesel fuel is used to power a wide variety of vehicles and operations. It of course fuels the diesel trucks you see lumbering down the highway, but it also helps move boats, school buses, city buses, trains, cranes, farming equipment and various emergency response vehicles and power generators.

In terms of the environment, diesel has some pros and cons. The pros — diesel is more efficient, so it requires less refinement, and it emits lower amounts of carbon dioxide, which leads to climate change. The cons — high amounts of nitrogen compounds are released from burning diesel fuel, which lead to acid rain, smog and poor health conditions. Next we'll look at some recent improvements made in these areas.