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How Does a Diesel Engine Work?

Apr 2, 2021 ·

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Have you ever looked at a truck or large piece of machinery and wondered where its power comes from? The answer is simple: diesel engines.

From passenger cars and trucks to railroad locomotives, farming equipment, and even cargo ships, diesel engines power some of the vehicles our world relies on most. Diesel engines are often favored for their efficiency and durability, especially when compared with gasoline engines.

Keep reading to learn all about these incredible engines — what they are, how they work and what diesel technicians need to know to be successful.

What Is a Diesel Engine?

Diesel engines, like gas engines, are internal combustion engines that convert chemical energy into mechanical energy. This process moves pistons up and down inside cylinders, which then leads to the motion that turns the wheels of a vehicle.

Through a series of small combustions (or explosions), diesel engines convert fuel to energy. They are used in many different types of equipment and vehicles, including:

  • Passenger cars and trucks
  • Medium duty trucks
  • Cruise ships
  • Cargo ships
  • Power generators
  • Locomotives
  • Farming equipment
  • Construction equipment
  • Various industrial applications

There are several reasons diesel engines are chosen over gas engines for certain applications. These include:

  • Thermal efficiency: Diesel engines are about 20% more thermally efficient than gas engines. Thermal efficiency refers to the expected work from the fuel put into the engine.
  • Power and torque: An increase in thermal efficiency means diesel engines provide more power and torque. This makes diesel engines ideal for heavy loads.
  • Lower operating costs: Diesel engines are known for their increased fuel efficiency, which can result in lower operating costs.
  • Reliability: Compared with gas engines, diesel engines run with fewer revolutions per minute (RPM). This results in less wear and tear on the engine.
  • Life expectancy: Many people choose diesel engines for their long life cycles. The way diesel engines are built supports this — their castings and cylinder walls are thicker and their oiling system has a higher volume. Most diesel engines have cylinder liners that can be replaced, helping to increase the life of the engine.

Diesel Engine Components

Diesel engines are robust. They are made up of several different parts, and each part must work correctly to power the engine. Below is a breakdown of some of the most important components of these engines.

Fuel System

The fuel system consists of the fuel tank, water separator, feed pump (low-pressure), filter, high-pressure pump, injector nozzle and cylinder. Essentially, the fuel tank stores the fuel, then a low-pressure pump pulls the fuel from the tank through a filter/water separator, then pushes the fuel through another filter. From there, fuel pressure is stepped up via a high-pressure pump, whether it is an injection pump or a unit injector.

Lubrication System

The lubrication system plays an important role in the engine. It reduces wear and tear of rubbing surfaces by placing a film of oil between parts, reduces the power required to overcome friction, and removes heat from the pistons and other parts in the engine. It also separates the cylinders and piston rings.

Air Intake System

In this system, air flows into the cylinder bore through the air cleaner, which prevents dust from entering. The turbocharger compresses air from the air filter, and the intake manifold takes the air from the turbocharger to the inlet port. Finally, the intake valve allows air to pass into the cylinder bore, and a camshaft controls when the valve opens and closes.

Exhaust System

In this system, exhaust gas is passed through the diesel particulate filter, which traps solids in the exhaust stream. These solids, or particulates, are carbon and ash. The filter will need to undergo a cleaning process called regeneration periodically to convert the carbon into carbon dioxide through exposure to high temperatures.

The gas then passes through the selective catalyst reduction system, which removes the oxides of nitrogen with the help of diesel exhaust fluid. There is also an exhaust gas recirculation valve, cooler and mixer in this system. All of this equipment is there to reduce harmful emissions.

Cooling System

The cooling system helps maintain proper engine temperature, which allows all of the parts to work correctly. It keeps oil and engine components at the right temperature, which in turn helps protect the cylinder head, cylinders, valves and pistons. There are two different types of cooling used in internal combustion engines: air cooling and water cooling. While gas engines can use either method, diesel engines use water cooling exclusively.

Electrical System

The electrical system is made up of the alternator, starter motor and battery. The starter motor, which is powered by the battery, rotates the flywheel, which rotates the crankshaft. This is what causes the pistons to move in the cylinders. The piston compresses the air in the cylinder to create heat, which ignites fuel injected into the cylinder.

How Does a Diesel Engine Start?

So how does a diesel engine turn fuel into power? Diesel engines operate using the same four strokes as gas engines:

  1. Intake stroke: In this phase, the piston travels from top dead center (TDC) to bottom dead center (BDC). This results in low pressure inside the cylinder. All diesel engines are turbocharged, so air isn’t drawn in but is waiting behind the intake valve under pressure. As soon as the valve opens, the air rushes in.
  2. Compression stroke: The piston travels from BDC back to TDC and prepares for combustion. The compression creates heat, which ignites atomized diesel fuel. There is no spark necessary — diesel fuel will self-ignite if it is atomized, or sprayed into the cylinder in a fine mist.
  3. Power stroke: In the power stroke, the piston goes from TDC to BDC. The combustion creates pressure to move the piston downward, and this shifts the power to the crankshaft.
  4. Exhaust stroke: In the last stroke, the piston travels from BDC to TDC. As it rises, exhaust gases are pushed from the cylinder and through the exhaust valve. They go into the emissions system and are then pushed from the vehicle.

Diesel engines do not use spark plugs to ignite the fuel mixture. The higher compression ratio in the engines produces enough heat to ignite the mixture. Additionally, diesel fuel itself allows for combustion without the use of a spark plug.

Diesel Engine Diagram

A key difference between diesel and gas engines is fuel management. Gas engines control the flow of air into the cylinder. By increasing airflow, gasoline increases by default. Diesel engines directly control fuel flow into the cylinder, and more fuel equals more engine speed.

To overcome the existing air pressure in the combustion chamber, diesel engines need to generate a significant amount of fuel pressure. The pictured diesel engine relies on a pressurizing fuel pump and a high-pressure common rail system to create, maintain and deliver fuel at precisely the correct time.

Diesel engine diagram

The high-pressure common rail sits under the valve cover. It distributes fuel to the individual cylinders via the fuel injectors. Unused fuel cools down in the fuel cooler before the return line transports it back to the fuel tank.

Common rail diagram

How This Applies to a Diesel Technician Career

It’s important for diesel technicians to know diesel engines inside and out. Understanding how each of the different components work together is what allows technicians to diagnose, repair and maintain these engines.

Because of their many benefits, diesel engines are here to stay — and as with anything in the transportation industry, they will likely continue to evolve and improve. Today’s diesel techs must be able to keep up with these changes to stay relevant.

Being able to work on diesel engines has the potential to provide a variety of career opportunities, whether it be in a dealership, repair shop, truck stop or even the railroad industry. Skilled diesel techs can take their skills to many different industries, including:

  • Trucking and transportation
  • Railroad
  • Marine
  • Agriculture
  • Power generation
  • Service writing

In some cases, diesel techs are able to learn skills on the job, but many employers prefer candidates who have completed a postsecondary training program, like the Diesel Technology program at Universal Technical Institute (UTI).

Students who train at UTI can learn everything from truck brakes and chassis to fuel systems, preventive maintenance, and transportation refrigeration. They also dive into diesel engines — learning the principles of operation of four-stroke engines, diesel combustion fundamentals and other characteristics unique to these powerful engines.

Ready to Get Trained for a Diesel Career?

UTI’s Diesel Technology program is designed to be completed in just 45 weeks7 and is available at campus locations nationwide. Classes begin every three weeks, so you can get started and prepare for your dream career sooner.1

To learn more, visit our program page and request information to get in touch with an Admissions Representative today.

UTI Campuses That Offer Diesel Mechanic Training

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1) UTI is an educational institution and cannot guarantee employment or salary.

2) For program outcome information and other disclosures, visit

7) Some programs may require longer than one year to complete.

Universal Technical Institute of Illinois, Inc. is approved by the Division of Private Business and Vocational Schools of the Illinois Board of Higher Education.


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