How Diesel Engines Work

By: Marshall Brain  | 

The 4.5-liter V-8 Duramax improves efficiency by 25 percent when compared with gasoline engines, while reducing pollutants and emissions.
The 4.5-liter V-8 Duramax improves efficiency by 25 percent when compared with gasoline engines, while reducing pollutants and emissions.
2008 HowStuffWorks

­O­ne of the most popular HowStuffWorks articles is How Car Engines Work, which explains the basic principles behind internal combustion, discusses the four-stroke cycle and talks about all of the subsystems that help your car's engine to do its job. For a long time after we published that article, one of the most common questions asked (and one of the most frequent suggestions made in the suggestion box) was, "What is the difference between a gasoline and a diesel engine?"

Diesel's story actually begins with the invention of the gasoline engine. Nikolaus August Otto had invented and patented the gasoline engine by 1876. This invention used the four-stroke combustion principle, also known as the "Otto Cycle," and it's the basic premise for most car engines today. In its early stage, the gasoline engine wasn't very efficient, and other major methods of transportation such as the steam engine fared poorly as well. Only about 10 percent of the fuel used in these types of engines actually moved a vehicle. The rest of the fuel simply produced useless heat.


­I­n 1878, Rudolf Diesel was attending the Polytechnic High School of Germany (the equivalent of an engineering college) when he learned about the low efficiency of gasoline and steam engines. This disturbing information inspired him to create an engine with a higher efficiency, and he devoted much of his time to developing a "Combustion Power Engine." By 1892 Diesel had obtained a patent for what we now call the diesel engine.

­If diesel engines are so efficient, why don't we use them more often? You might see the words "diesel engine" and think of big, hefty cargo trucks spewing out black, sooty smoke and creating a loud clattering noise. This negative image of diesel trucks and engines has made diesel less attractive to casual drivers in the United States — although diesel is great for hauling large shipments over long distances, it hasn't been the best choice for everyday commuters. This is starting to change, however, as people are improving the diesel engine to make it cleaner and less noisy.

If you haven't already done so, you'll probably want to read How Car Engines Work first, to get a feel for the basics of internal combustion. But hurry back — in this article, we unlock the secrets of the diesel engine and learn about some new advancements.

Diesel Engines vs. Gasoline Engines

­In theory, diesel engines and gasoline engines are quite similar. They are both internal combus­tion engines designed to convert the chemical energy available in fuel into mechanical energy. This mechanical energy moves pistons up and down inside cylinders. The pistons are connected to a crankshaft, and the up-and-down motion of the pistons, known as linear motion, creates the rotary motion needed to turn the wheels of a car forward.

Both diesel engines and gasoline engines convert fuel into energy through a series of small explosions or combustions. The major difference between diesel and gasoline is the way these explosions happen. In a gasoline engine, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs. In a diesel engine, however, the air is compressed first, and then the fuel is injected. Because air heats up when it's compressed, the fuel ignites.


The following animation shows the diesel cycle in action. You can compare it to the animation of the gasoline engine to see the differences.

diesel engine animation
Diesel engine
© 2019

The diesel engine uses a four-stroke combustion cycle just like a gasoline engine. The four strokes are:

  1. Intake stroke — The intake valve opens up, letting in air and moving the piston down. ­
  2. Compression stroke — The piston moves back up and compresses the air.
  3. Combustion stroke — As the piston reaches the top, fuel is injected at just the right moment and ignited, forcing the piston back down.
  4. Exhaust stroke — The piston moves back to the top, pushing out the exhaust created from the combustion out of the exhaust valve.

Remember that the diesel engine has no spark plug, that it intakes air and compresses it, and that it then injects the fuel directly into the combustion chamber (direct injection). It is the heat of the compressed air that lights the fuel in a diesel engine. In the next section, we'll examine the diesel injection process.

Diesel Fuel Injection

Photo courtesy DaimlerChrysler Atego six-cylinder diesel engine
Photo courtesy DaimlerChrysler Atego six-cylinder diesel engine

One big difference between a diesel engine and a gas engine is in the injection process. Most car engines use port injection or a carburetor. A port injection system injects fuel just prior to the intake stroke (outside the cylinder). A carburetor mixes air and fuel long before the air enters the cylinder. In a car engine, therefore, all of the fuel is loaded into the cylinder during the intake stroke and then compressed. The compression of the fuel/air mixture limits the compression ratio of the engine -- if it compresses the air too much, the fuel/air mixture spontaneously ignites and causes knocking. Because it causes excessive heat, knocking can damage the engine.

Diesel engines use direct fuel injection -- the diesel fuel is injected directly into the cylinder.


The injector on a diesel engine is its most complex component and has been the subject of a great deal of experimentation -- in any particular engine, it may be located in a variety of places. The injector has to be able to withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. Getting the mist circulated in the cylinder so that it is evenly distributed is also a problem, so some diesel engines employ special induction valves, pre-combustion chambers or other devices to swirl the air in the combustion chamber or otherwise improve the ignition and combustion process.

Some diesel engines contain a glow plug. When a diesel engine is cold, the compression process may not raise the air to a high enough temperature to ignite the fuel. The glow plug is an electrically heated wire (think of the hot wires you see in a toaster) that heats the combustion chambers and raises the air temperature when the engine is cold so that the engine can start. According to Cley Brotherton, a Journeyman heavy equipment technician:

All functions in a modern engine are controlled by the ECM communicating with an elaborate set of sensors measuring everything from R.P.M. to engine coolant and oil temperatures and even engine position (i.e. T.D.C.). Glow plugs are rarely used today on larger engines. The ECM senses ambient air temperature and retards the timing of the engine in cold weather so the injector sprays the fuel at a later time. The air in the cylinder is compressed more, creating more heat, which aids in starting.

Smaller engines and engines that do not have such advanced computer control use glow plugs to solve the cold-starting problem.

Of course, mechanics aren't the only difference between diesel engines and gasoline engines. There's also the issue of the fuel itself.

Diesel Fuel

A sample of diesel fuel
A sample of diesel fuel

­­­­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. You will often hear diesel fuel referred to as "diesel oil" because it's so oily.


Diesel fuel evaporates more slowly because it is heavier. It contains more carbon atoms in longer chains than gasoline does (gasoline is typically C9H20, while diesel fuel is typically C14H30). It takes less refining to create diesel fuel, which is why it used to be cheaper than gasoline. Since 2004, however, demand for diesel has risen for several reasons, including increased industrialization and construction in China and the U.S. [source: Energy Information Administration].

Diesel fuel has a higher energy density than gasoline. On average, 1 gallon (3.8 L) of diesel fuel contains approximately 155x106 joules (147,000 BTU), while 1 gallon of gasoline contains 132x106 joules (125,000 BTU). 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. Think about how important diesel is to the economy -- without its high efficiency, both the construction industry and farming businesses would suffer immensely from investments in fuels with low power and efficiency. About 94 percent of freight -- whether it's shipped in trucks, trains or boats -- relies on diesel.

In terms of the environment, diesel has some pros and cons. The pros -- diesel emits very small amounts of carbon monoxide, hydrocarbons and carbon dioxide, emissions that lead to global warming. The cons -- high amounts of nitrogen compounds and particulate matter (soot) are released from burning diesel fuel, which lead to acid rain, smog and poor health conditions. On the next page we'll look at some recent improvements made in these areas.

Diesel Improvements and Biodiesel

Cleaner fuel and stricter emissions levels should reduce the number of early deaths due todiesel engine pollution.
Cleaner fuel and stricter emissions levels should reduce the number of early deaths due todiesel engine pollution.
Photo courtesy Mario Tama/Getty Images

­During the big oil crisis in the 1970s, European car companies started advertising diesel engines for commercial use as an alternative to gasoline. Those who tried it out were a bit disappointed -- the engines were very loud, and they would arrive home to find their cars covered from front to back in black soot -- the same soo­t responsible for smog in big cities.

Over the past 30 to 40 years, however, vast improvements have been made on engine performance and fuel cleanliness. Direct injection devices are now controlled by advanced computers that monitor fuel combustion, increasing efficiency and reducing emissions. Better-refined diesel fuels such as ultra low sulfur diesel (ULSD) will lower the amount of harmful emissions and upgrading engines to make them compatible with cleaner fuel is becoming a simpler process. Other technologies such as CRT particulate filters and catalytic converters burn soot and reduce particulate matter, carbon monoxide and hydrocarbons by as much as 90 percent. [source: Diesel Technology Forum]. Continually improving standards for cleaner fuel from the European Union will also push the auto industries to work harder at lowering emissions -- by September 2009, the EU hopes to have particulate matter emissions down from 25mg/kilometer to 5mg/kilometer [source: EUROPA].


You may have also heard of something called biodiesel. Is it the same as diesel? Biodiesel is an alternative or additive to diesel fuel that can be used in diesel engines with little to no modifications to the engines themselves. It's not made from petroleum -- instead it comes from plant oils or animal fats that have been chemically altered. (Interesting fact: Rudolf Diesel had originally considered vegetable seed oil as fuel for his invention.) Biodiesel can either be combined with regular diesel or used completely by itself. You can read more about biodiesel in How Biodiesel Works.

Originally Published: Apr 1, 2000

Lots More Information

Related Articles

More Great Links