How Diesel Engines Work

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 graphic shows the diesel cycle.

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

Remember that the diesel engine has no spark plug that intakes air and compresses it; instead it 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.

One big difference between a diesel engine and a gas engine is in the injection process. Most car engines use a combination of port injection, which injects fuel just before the intake stroke (outside the cylinder), and direct fuel injection. Port injection is used at lower engine speeds because it has a more stable mixture of air and fuel. Direct injection is used at higher speeds to provide more power and less chance of knocking, which is when the air is compressed too much and the fuel spontaneously ignites.

Diesel engines only 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. Direct injection technology has advanced far enough that glow plugs aren't usually necessary for touching off the spark anymore, but many cars still have them anyway. Their heat helps burn the fuel more efficiently.

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

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.

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.

Since then, vast improvements have been made on engine performance and fuel cleanliness. Direct injection devices are now controlled by computers that monitor fuel combustion, increasing efficiency and reducing emissions. The sophistication of these computers turned out to be a double-edged sword, however, when we learned in 2014 that Volkswagen had been using the technology to cheat on emissions testing. The company had to buy back diesel vehicles it had sold with the promise of being a cleaner alternative fuel and pay billions in fines, and in some cases VW executives served jail time.

Since 2010, all diesel fuel in the United States has been ultra-low sulfur diesel (ULSD), which contains very little sulfur. Why does that matter? The devices that lower pollution and emissions from diesel engines can be damaged by sulfur, so having less of that element means cleaner-burning fuel. Modern catalytic converters for diesel engines are 90 percent efficient, so they don't stink or put out clouds of black smoke anymore.

But these changes might not be enough. Diesel still emits a lot of particulates that can harm humans and their environment, especially in dense urban areas. That's why several cities, states, and even countries have decided to ban diesel vehicles within the next decade or so.

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 in some engines used completely by itself.