How Does a Diesel Engine Work?

Have you ever looked at a truck or large piece of machinery and wondered where its power comes from?

The answer is simple: its diesel engine.

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 about these incredible engines—what they are, how they work and what diesel technicians need to know to advance in their career!

Key points:
- Diesel engines are internal combustion engines that convert chemical energy into mechanical energy, powering a wide range of vehicles and machinery including passenger cars, trucks, ships, locomotives and construction equipment. They are chosen for their efficiency, power and durability.

- Diesel engines operate on four strokes: intake, compression, power and exhaust, using compression ignition rather than spark ignition, which means they ignite fuel through the heat of compressed air rather than a spark plug.

- Key components of a diesel engine include the fuel system, lubrication system, air intake system, exhaust system, cooling system and electrical system, each playing a crucial role in the engine's operation and efficiency.

- Diesel engines offer several advantages over gasoline engines, such as higher thermal efficiency, more power and torque, lower operating costs, increased reliability and longer life expectancy due to their design and operation principles.

- Diesel technicians need comprehensive knowledge of diesel engines and their components to diagnose, repair and maintain them effectively. With the continuous evolution of diesel technology, ongoing education and training are essential for success in the field.

What Is a Diesel Engine?

Diesel engines, like gas engines, are internal diesel 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.

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Diesel Engine Components

There are several systems in a diesel engine and each must work correctly to power it. Below is a breakdown of some of the most important components of a diesel engine.

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

Ever wondered, “How does a turbo work in a diesel engine?” 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.

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How Does a Diesel Engine Start?

To get a diesel engine up and running, the process begins with the starter motor cranking the engine, which in turn rotates the crankshaft. This movement causes the pistons to move up and down within the cylinders. As the piston ascends, it compresses the air inside the cylinder, significantly increasing its temperature. At the peak of this compression stroke, diesel fuel is injected into the hot, compressed air.

What ignites fuel in diesel engines? The high temperature causes the diesel to ignite spontaneously, creating a small explosion that forces the piston back down. This cycle repeats rapidly, powering the engine. Diesel engines rely on this compression ignition process, which is different from gasoline engines that use spark plugs for ignition.

How Does a Diesel Engine Turn Fuel Into Power?

Diesel engines operate using the same four strokes as gas engines:

• 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. Rather, it is waiting behind the intake valve under pressure. As soon as the valve opens, the air rushes in.
• 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.
• 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.
• 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.

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.

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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 works together is what allows technicians to diagnose, repair and maintain these engines.

Because of their many benefits, diesel engines are widely used in vehicles across the country. 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 keep their knowledge and skills relevant.

Being able to service and repair diesel engines offers 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 apply 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).¹

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.

Read: How Do Diesel Hydraulic Brakes Work?

FAQs

1. How does fuel ignite in a diesel engine?

Fuel ignites in a diesel engine due to the high temperature created by air compression in the cylinder rather than by a spark like they would in gasoline engines. This process, known as compression ignition, causes the fuel to combust and drive the piston.

2. What are the diesel engine basics?

A diesel engine operates through compression ignition, where air is compressed in the combustion chamber to a high temperature and diesel fuel is then injected. The fuel combusts due to this high temperature, creating a force that pushes the piston and powers the engine.

3. What are the common uses of diesel engines?

Diesel engines are commonly used in heavy-duty vehicles such as trucks, buses and construction equipment due to their high torque output and fuel efficiency. They are also used in marine applications, generators and some passenger vehicles.

4. What are the main components of a diesel engine?

The key components of a diesel engine include the cylinder, where air is compressed and combustion takes place; the piston, which is driven by the combustion process; and the crankshaft, which converts the up-and-down movement of the piston into rotational motion needed for vehicle propulsion.

5. What are the diesel mechanic basics?

A diesel mechanic inspects, maintains and repairs diesel engines. They need to have a solid understanding of diesel engine components, functions and troubleshooting techniques, and may work on heavy vehicles, marine engines or stationary diesel power plants.

6. How does a diesel engine work?

A diesel engine works by drawing air into a cylinder and compressing the air to significantly increase its temperature before injecting diesel fuel into the cylinder. This fuel immediately combusts in the high-temperature air, creating a force that drives the piston and turns the crankshaft.

7. What is the role of compression in diesel combustion?

Compression is crucial in diesel combustion because it increases the temperature of the air in the cylinder to a point at which the injected diesel fuel spontaneously ignites.

8. What are the advantages of diesel engines over gasoline engines?

Diesel engines offer higher fuel efficiency and greater torque than gasoline engines, making them ideal for heavy-duty and long-distance applications. They are also generally more durable and require less maintenance since they don’t have electrical ignition parts.

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Ready To Get Diesel Engine Training?

UTI’s Diesel Technology program is designed to be completed in just 45 weeks and is available at campus locations nationwide!⁷ UTI's diesel technician classes begin every three to six weeks, so you can get started and be taught diesel engine servicing skills ASAP.

To learn more about our program, request more information and an Admissions Representative will get in touch. Apply now and start learning diesel mechanic basics!