by Chris Woodford. Last updated: October 20, 2021.
Have you ever gazed in amazement as a giant truck crawls slowly up a hill? Probably not! Such things happen everyday. But stop and think for a
moment about what's happening—how a huge, heavy load is being
systematically lifted against the overwhelming power of gravity using
nothing more than a few cups of dirty liquid (fuel, in other words)—and you might agree
that what you're seeing is quite remarkable.
Diesel engines are the power behind our biggest machines—trucks,
trains, ships, and submarines. On the face of it, they're
similar to ordinary gasoline (petrol) engines but they generate more power,
more efficiently by working in a subtly different way. Let's take a
Photo: Diesel engines (like the one in this railroad locomotive) are ideal for pulling heavy trains. This is a superbly preserved (and highly polished!) British Rail Class 55 ("Deltic"), number 55022, called Royal Scots Grey dating from 1960. Here's a picture of the
Napier Deltic diesel engine that powers it.
What is a diesel engine?
Like a gasoline engine, a diesel engine is a type of internal combustion engine. Combustion is another word for burning, and internal
means inside, so an internal combustion engine is simply one where
the fuel is burned inside the main part of the engine (the cylinders)
where power is produced.
That's very different from an external
combustion engine such as those used by old-fashioned steam
locomotives. In a steam engine, there's a big fire at one end of a
boiler that heats water to make steam. The steam flows down long
tubes to a cylinder at the opposite end of the boiler where it pushes
a piston back and forth to move the wheels. This is external
combustion because the fire is outside the cylinder (indeed,
typically 6-7 meters or 20-30ft away). In a gasoline or diesel engine, the fuel
burns inside the cylinders themselves. Internal combustion wastes
much less energy because the heat doesn't have to flow from where
it's produced into the cylinder: everything happens in the same
place. That's why internal combustion engines are more efficient
than external combustion engines (they produce more energy from the
same volume of fuel).
Photo: A typical diesel engine (from a fire truck) made by Detroit Diesel Corporation (DDC). Photo by Juan Antoine King courtesy of US Navy and
How is a diesel engine different from a gasoline engine?
Gasoline engines and diesel engines both work by internal combustion, but in
slightly different ways. In a gasoline engine, fuel and air is
injected into small metal cylinders. A piston compresses (squeezes)
the mixture, making it explosive, and a small electric spark from a
sparking plug sets fire to it. That makes the mixture explode,
generating power that pushes the piston down the cylinder and
(through the crankshaft and gears) turns the wheels. You can read
more about this and watch a simple animation of how it works in our
article on car engines.
Diesel engines are similar, but simpler. First, air is allowed into the
cylinder and the piston compresses it—but much more than in a
gasoline engine. In a gasoline engine, the fuel-air mixture is
compressed to about a tenth of its original volume. But in a diesel
engine, the air is compressed by anything from 14 to 25 times. 
If you've ever pumped up a bicycle tire, you'll have felt the pump
getting hotter in your hands the longer you used it. That's because
compressing a gas generates heat. Imagine, then, how much heat is
generated by forcing air into 14–25 times less space than it normally
takes up. So much heat, as it happens, that the air gets really
hot—usually at least 500°C (1000°F) and sometimes very much
hotter. Once the air is compressed, a mist of fuel is sprayed into
the cylinder typically (in a modern engine) by an electronic
fuel-injection system, which works a bit like a sophisticated aerosol
can. (The amount of fuel injected varies, depending on how much power
the driver wants the engine to produce.) The air is so hot that the
fuel instantly ignites and explodes without any need for a spark
plug. This controlled explosion makes the piston push back out of the
cylinder, producing the power that drives the vehicle or machine in
which the engine is mounted. When the piston goes back into the
cylinder, the exhaust gases are pushed out through an exhaust valve
and, the process repeats itself—hundreds or thousands of times a
How does a diesel engine turn fuel into power?
Animation: How a four-stroke diesel engine works.
Like a gasoline engine, a diesel engine usually operates by repeating a cycle of four stages or strokes,
during which the piston moves up and down twice (the crankshaft rotates twice in other words) during
- Intake: Air (light blue) is drawn into the cylinder through
the open green air inlet valve on the right as the piston moves down.
- Compression: The inlet valve closes, the piston moves up, and compresses the air mixture, heating it up. Fuel (dark blue) is injected into the hot gas through the central fuel injection valve and spontaneously ignites. Unlike with a gas engine, no sparking plug is needed
to make this happen.
- Power: As the air-fuel mixture ignites and burns, it pushes the piston down, driving the crankshaft
(red wheel at bottom) that sends power to the wheels.
- Exhaust: The green outlet valve on the left opens to let out the exhaust gases, pushed out by the
In a two-stroke diesel, the complete cycle happens as the piston moves up and
down just once. Confusingly, there are really three stages in a two-stroke cycle:
- Exhaust and intake: Fresh air is blown into the side of the cylinder, pushing the old exhaust out through
valves at the top.
- Compression: The inlet and exhaust valves close. The piston moves up, compresses the air, and heats it up. When
the piston reaches the top of the cylinder, fuel is injected and spontaneously ignites.
- Power: As the air-fuel mixture ignites, it pushes the piston down, driving the crankshaft
that sends power to the wheels.
Two-stroke engines are smaller and lighter than four-stroke ones, and tend to be more efficient since they produce power
once during each rotation (instead of once during every two rotations, as in a four-stroke engine).
This means they need more cooling and lubrication and suffer higher wear and tear.
What makes a diesel engine more efficient?
Photo: A typical diesel engine being tested under laboratory conditions.
Photo by Pat Corkery courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Diesel engines are up to twice as efficient as gasoline engines—around 40–45 percent
efficient at best. 
In simple terms, that means you can go much further on the same amount of fuel
(or get more miles for your money). There are several reasons for
this. First, they compress more and operate at higher temperatures.
A fundamental theory of how heat engines work,
known as Carnot's rule, tells us that the efficiency of an engine depends
on the high and low temperatures between which it operates.
A Diesel engine that cycles through a bigger temperature difference
(a higher hottest temperature or a lowest colder temperature) is more efficient.
Second, the lack of a sparking-plug ignition system makes for a
simpler design that can easily compress the air much more—and
this makes the fuel burn hotter and more completely, releasing more energy.
There's another efficiency saving
too. In a gasoline engine that's not working at full power, you need
to supply more fuel (or less air) to the cylinder to keep it working;
diesel engines don't have that problem so they need less fuel when
they're working at lower power. Another important factor is that
diesel fuel carries slightly more energy per gallon than gasoline
because the molecules it's made from have more energy locking their
atoms together (in other words, diesel
has a higher energy density than gasoline). Diesel is also a better
lubricant than gasoline so a
diesel engine will naturally run with less friction.
How is diesel fuel different?
Diesel and gasoline are quite different. You'll know this much if you've
ever heard the horror stories of people who've filled up their car or
truck with the wrong sort of fuel! Essentially, diesel is a
lower-grade, less-refined product of petroleum made from heavier
hydrocarbons (molecules built from more carbon and hydrogen
atoms). Crude diesel engines that lack sophisticated fuel injection
systems can, in theory, run on almost any hydrocarbon fuel—hence the
popularity of biodiesel (a type of biofuel made from, among other
things, waste vegetable oil). The inventor of the diesel engine,
Rudolf Diesel, successfully ran his early engines on peanut oil and
thought his engine would do people a favor by freeing them from a
dependency on fuels like coal and gasoline, and centralized
sources of power. 
If only he'd known!
Photo: Have grease will travel: Joshua and Kaia Tickell, a couple of
environmentalists, use this trailer (the Green Grease Machine) to make biodiesel fuel for their van (attached to the front) using waste cooking oil thrown out by fast food restaurants.
The fuel costs an impressive $0.80 per gallon. Photo by Warren Gretz courtesy of US
Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Advantages and disadvantages of diesel engines
Diesels are the most versatile fuel-burning engines in common use today,
found in everything from trains and cranes to bulldozers and
submarines. Compared to gasoline engines, they're simpler,
more efficient, and more economical. They're also safer, because diesel fuel is less
volatile and its vapor less explosive than gasoline. Unlike gasoline engines, they're particularly good for
moving large loads at low speeds, so they're ideal for use in
freight-hauling ships, trucks, buses, and locomotives. Higher compression
means the parts of a diesel engine have to withstand far greater
stresses and strains than those in a gasoline engine. That's why
diesel engines need to be stronger and heavier and why, for a long
time, they were used only to power large vehicles and machines. While
this may seem a drawback, it means diesel engines are typically more
robust and last a lot longer than gasoline engines.
Photo: Diesel engines aren't just used in vehicles: these huge stationary diesel engines generate electricity in a power plant on
San Clemente Island. Photo by Warren Gretz courtesy of US
Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Pollution is one
of the biggest drawbacks of diesel engines: they
produce a mixture of pollutants, including nitrogen oxides, carbon monoxide,
hydrocarbons, and soot particles, which are dirty and hazardous to health.
In theory, diesels are more efficient, so they
should use less fuel, produce fewer carbon dioxide (CO2) emissions, and
contribute less to global warming.
In practice, there's some argument over whether that's really true.
Some laboratory experiments have shown average diesel emissions
are only slightly lower than those from gasoline engines,
although manufacturers insist that if similar diesel and gasoline cars are
compared, the diesels do indeed come out better.
Other recent research suggests that even new diesel cars
are highly polluting. The European Environment Agency, for example, notes that even a typical, "clean" diesel car
that meets EURO 6 emissions standards produces about 10 times as much nitrogen
oxide pollution as a comparable gasoline vehicle. 
What about CO2 emissions?
According to the British Society of Motor Manufacturers
and Traders: "Diesel cars have contributed massively to reducing CO2 emissions. Since 2002, buyers choosing diesel have saved almost 3 million tonnes of CO2 from going into the atmosphere."
Diesel engines do tend to cost more initially than gasoline engines, though their lower running costs and
longer operating life generally offsets that.
Even so, car buyers no longer seem convinced: there's been a substantial drop in sales since
the Volkswagen emissions scandal
in 2015, where the German car maker misrepresented emissions of its diesel cars to make them seem less
There's no question that diesel engines will continue to power heavy vehicles—trucks,
buses, ships, and railroad locomotives all depend on them—but their future in cars and lighter vehicles is becoming increasingly uncertain. The push toward electric cars has provided a powerful impetus to make gasoline engines lighter, more economical, and less polluting, and these improved gas engines undermine some of the perceived advantages of using diesels in cars. In the growing competition between affordable electric vehicles and improved
gasoline cars, diesels may find themselves squeezed out altogether. Then again, diesels themselves
are constantly evolving; in 2011, the US Department of Energy predicted that future engines could rise in efficiency from today's 40 percent to 60 percent or more. If that happens, diesel could remain
a contender in smaller vehicles for many years to come, especially if their sooty emissions
can be properly addressed.
Who invented the diesel engine?
Not surprisingly, it was German engineer Rudolf Diesel (1858–1913). Here, briefly, is the story:
- 1861: French engineer Alphonse Beau de Rochas (1815–1893) outlines the basic theory of the four-stroke engine and files a patent for the idea on February 16, 1862, but he fails to put together a working machine.
- 1876: German engineer Nikolaus Otto (1832–1891) builds the first, successful, four-stroke internal combustion engine.
- 1878: Scotsman Dugald Clerk (1854–1932) develops the two-stroke engine.
- 1880: Aged 22,
Rudolf Diesel goes to work for refrigerator engineer Carl von
Linde (1842–1934), where he learns about thermodynamics (the science
of how heat moves) and how engines work.
- 1890: Diesel figures out how to make an improved internal combustion
engine using higher pressures and temperatures, which doesn't need a sparking plug.
- 1892: Diesel begins patenting his ideas to prevent others from profiting by them.
Artwork: Rudolf Diesel's original internal combustion engine, as he drew it in his 1895 patent.
The cylinder (1) is at the top. 2) A "plunger" (as Diesel referred to it) is attached by a crank and connecting rod (3) to a flywheel (4). A gear driven by the flywheel (5) is attached to a centrifugal governor (6) that keeps the engine speed constant (shutting off the fuel supply if the engine runs too fast, then switching it back on when the engine slows down again). Artwork courtesy of US Patent and Trademark Office (with colors and numbering added by us to simplify the explanation). You can read more in
US Patent #542,846: Method of and apparatus for converting heat into work by Rudolf Diesel.
- 1893: Diesel builds a huge, stationary engine that works for one entire minute, under its own
power, on February 17, 1894.
- 1895: Diesel's engine patent is granted in the United States on July 16, 1895.
- 1898: With Diesel's help, the first commercial engine is built in a
factory in St Louis, Missouri, United States by Adolphus Busch
(1839–1913), brewer of Budweiser beer.
- 1899: Manufacture of diesel engines begins at Diesel's plant in Augsburg.
Diesel begins licensing his ideas to other firms and soon becomes
- 1903: Petit Pierre, one of the first diesel ships, begins working on the Marne-Rhine canal in France.
- 1912: MS Selandia, the first ocean-going diesel ship, makes its maiden voyage.
- 1913: Diesel dies in mysterious circumstances, apparently falling overboard from the ship Dresden while traveling from London, England to Germany. Rumors circulate that he has been murdered or committed suicide, but nothing is ever
- 1931: Clessie Cummins,
founder of the Cummins Engine Co., builds one of the first successful diesel-engine cars and demonstrates its efficiency by driving it from Indianapolis to New York City on just $1.39 of fuel.
- 1931: Caterpillar revolutionizes agriculture by introducing the Diesel Sixty,
its first diesel-powered crawler tractor, based on the popular Caterpillar Sixty.
- 1936: Mercedes introduces the
260D, one of the first mass-produced diesel-engined passenger cars, and it
remains in production until 1940. Over the next four decades, Mercedes sells nearly two million diesel-powered cars.
- 1939: General Motors unveils its EMD FT, a powerful diesel-electric locomotive, and sends the first one (Number 103) on a year-long voyage to demonstrate its worth. Proving beyond doubt the superiority of diesel, this sounds the death knell for steam locomotives.
- 1970s: A global fuel crisis prompts renewed interest in using small, efficient diesel engines in cars.
- 1987: The world-famous Queen Elizabeth 2 (QE2) ship is
refitted with nine diesel-electric engines (each the size of a double-decker bus), making it the most powerful diesel-powered merchant ship at that time.
- 2000: Peugeot introduces the world's first particle filters (PF) for diesel engines on its 607 model, claiming a 99 percent reduction in soot emissions.
- 2015: Volkswagen is plunged into a huge global scandal after systematically cheating on diesel engine emissions tests. Diesel car sales plummet for the first time in years.
- 2017: Volvo becomes the first major carmaker to shift away from gasoline and diesel engines, announcing that
all new cars will be hybrids or fully electric from 2019.