How does a diesel engine work?


Mar 08th, 2019

How does a diesel engine work?

Today, diesel engines are the pillar on which a lot of machines move: buses, trucks, construction equipment, and more.

In the early days, diesel engines were mostly seen as too dirty, not quick enough, and unpleasantly loud. Fast forward to the modern era, and you will find that diesel engines have undergone many improvements that make diesel fuel a lot more efficient and burn cleaner while being a lot more versatile. When talking about heat engines, diesel engines are up there with the very best, which has led to them being the preferred choice for a variety of operations, such as heavy-duty machinery, generation of power and vehicles. 

Brief Background of Diesel Engines

Basically, what powers the diesel motor is a compression ignition engine, which was invented over 100 years ago by a man called Rudolf Diesel. A lot of people who know about the diesel engine can only remember their appearance around the 1970's, which is when diesel engines began to be used in consumer cars. Back then, virtually every car manufacturer did their best to provide a passenger car that was powered by a diesel engine with the advent of the gas crunch. However, diesel motors go way back further than the 1970's. Rudolf Diesel (pictured here) is credited with inventing the diesel engine, and it is not just a recent discovery. Diesel was able to secure a patent for his diesel engine in 1892. 

That's a brief background of the invention of the diesel engine, but what really is a diesel engine?


There are various kinds of internal combustion engines, and a diesel engine is no exception. "Combustion" refers to burning. "Internal" refers to the inside of a thing. Therefore, when we say a diesel engine has an internal combustion engine, we imply that diesel engines are designed in a way that fuel gets burned right inside its cylinders, which are the engine's principal part. It is here that the production of power is carried out.

This differs greatly from external combustion engines, which are mostly found in steam locomotives. For steam engines, a huge fire gets started at one part of its boiler, which heats up the water in order to produce steam. Next, through special long tubes, the steam makes its way through twin cylinders located at the other side of its boiler. This leads to a back and forth movement of the piston, causing its wheels to move. This is external combustion, and the reason is that the fire occurs not inside its cylinder but outside.

On the other hand, in diesel engines, the fire gets burned right inside its cylinders. Energy is mostly conserved with internal combustion engines since there is no need for the heat to make its way from its point of production into the cylinder. The process occurs at the exact same spot. This is the reason that internal combustion motors have a higher efficiency than the external combustion engines.

Rudolf Diesel built the diesel engine in order to get diesel fuel converted right into mechanical energy. This internal combustion engine makes use of diesel fuel. In diesel engines, fuel burning takes place as a result of the high pressure, as well as the temperature that is built in the combustion unit.

Now let's consider the main components of a diesel engine and the functions that these components perform.


At this point we will explore the parts that constitute a diesel engine and the vital functions that they perform. We will be discussing only those components which have direct involvement in the fuel cycle, especially, for a four-stroke diesel engineSince the other components do not really have direct involvement in this cycle such as the alternator, they will not be highlighted here. 

The key parts that make up the diesel engine are as follows:

Cylinder Block Assembly

In both the two-stroke diesel engine, as well as the four-stroke diesel engine, the cylinder block is the principal component. It plays a vital role in the functioning of other compartments in the engine which support how the machine operates. The material used to make the cylinder block is cast iron, and it possesses a level of precision that is high.

The components that make up the block cylinder include:

  • The Cylinder

A press method is used to place the cylinder in the block engine in order to keep it tightly in place. It is produced using iron, as well as aluminium iron, and it is placed to help in the upward and downward movement of the piston. 

  • Water Jacket

This water sheath is placed in the engine block and is designed to help cool the engine. The water jacket has vents for the intake, as well as the outlet of water, which helps to circulate it and cool the temperature.

  • Oil Feed Lines

This is the hole for oil, which is located on the engine’s cylinder block. Its role is to ensure that an oil line goes from the head of the engine’s cylinder right through to the crankcase. This is to help in the circulation of oil to ensure that every part of the diesel engine is supplied with oil.

Cylinder Head Assembly

This component of the diesel engine is placed on its top. Just like with the cylinder block, the cylinder head assembly is produced using cast material. In recent times, cylinder heads produced with aluminium are preferred because they are lighter, as well as stronger. In this component, there are several parts, such as the camshaft and the combustion chamber. 

  • Valve and spring

The valve and spring play a vital role by serving as a door to help in the opening and closure of the intake, as well as the exhaust channels located in the combustion unit. The work of the so is to ensure that the value stays closed.

  • Camshaft

The function of the camshaft is to keep the valve pressed, and when this is achieved, the port that is responsible for the intake or the exhaust is opened.

  • Rocker arm

This oscillating lever is responsible for ensuring that radial movement is conveyed into a linear movement in order to get the poppet valve opened from the engine’s cam lobe. A lobe that rotates raises and lowers one end, and the other end serves as the stem for the valve. 

  • Combustion chamber

This small space is designed mostly for combustion activities. What emanates from here is a fire blast that helps push down the piston. You will mostly find the combustion chamber in diesel engines that use indirect injection. 

Piston & Connecting Rod

Pistons are built to help in the adjustment of cylinder volume. It is very important to regulate cylinder volume because it helps facilitate the operation of the engine. The downward movement of the piston will lead to an enlargement of the cylinder volume while an upward movement will cause the cylinder volume to retract. The function of the connecting rod is to ensure that the piston makes consistent upward and downward movements. 

The  parts that make up the piston are as follows:

  • Ring compression

The rings ensure that air leakages do not occur with every compression stroke. Basically, the rings ensure that the gap between the walls of the piston, as well as the main liner, is closed.

  • Oil ring

With the downward movement of the piston, the oil ring is responsible for wiping oil that comes in contact with the cylinder walls. It also performs the vital functions of creating a separation between the combustion and oil chambers, which helps ensure that there are no pressure leaks in the combustion chamber. 

  • Pin piston

This is responsible for creating a connection between the piston, as well as the connecting rod. It performs the function of a hinge and it is tubular in nature. 


The material that is used to produce the crankshaft is cast-iron. The function of the crankshaft is to ensure that the upward and downward movement of the piston is done in a rotary manner. You can compare the principle by which the crankshaft works to what happens when you ride a bicycle. The crankshaft is built in such a way that it can we stand pressure which emanates from the piston. A customized iron alloy, which is very strong, is used to make this component. 

Some of the parts that are found on the crankshaft:

  • Crank pin

This pin connects to the connecting rod's large end.

  • Crank journal

This pin performs the function of helping the crankshaft spin. You will basically find it resting on the bearings, and it mostly is attached to the lower part of the engine’s connecting rod. 

  • Weight balance

The crank pin is located opposite to the weight balance. The function of this component is to perform counterweight duties right inside the machine. 

Oil Pan

The oil pan holds the engine oil. It is made with very strong and durable materials, such as thin iron. 

Timing Chain Assembly

In the system that contains the valve mechanism, is the timing chain. It provides an angular connection between the crankshaft, as well as the camshaft rotation. This component is located in the front of the diesel engine. The connection between the sprocket gears of the crankshaft and camshaft is made possible by the timing chain. 

Fly Wheel

The function of this component is to give balance to the speed of the diesel engine. The material used to make this component is solid iron, which serves as the torque storage. The flywheel basically acts as an energy reservoir. Energy is stored as kinetic energy, which is essential in helping the diesel engine to run, even if the production of power does not take place. 

Fuel System Assembly

The first system goes all the way from the fuel tank to the engine injector. Fuel systems are designed to supply fuel during strokes right inside the combustion chamber. When it comes to diesel engines, fuel systems are categorized into two distinct types which are the conventional fuel systems and the common rail fuel systems. Common rail fuel systems are known to be much more efficient, as well as, economical.

All of the components that are discussed above play a vital role in ensuring that the diesel engine operates smoothly as required. If issues occur to any of the parts that have been discussed above, then, it will lead to a disruption in the working process of your diesel engine.


Before we go into details about how diesel engines work, you need to keep in mind that what basically differentiates diesel and gasoline engines is that for the diesel engines, the combustion chambers get supplied with fuel via the nozzles for fuel injection as soon as there is a very high air pressure which is appropriately hot to get the fuel ignited. Here is what really takes place when you start your vehicle that is powered by a diesel engine –

  1. First, the key is turned in the vehicle's ignition

Then, you exercise some patience to allow the engine to develop the needed heat that will get it to start in its cylinders. Once you turn the key, you initiate a process which involves the injection of fuel into the engine’s cylinders. This is done in the presence of very high pressure which causes the air to build up the heat by itself in the engine's cylinders. These days, the amount of time that is needed for this process to occur is significantly reduced.

In combustion chambers that have already been preheated, diesel fuel starts quicker, and this is the reason that some manufacturers get to install glow plugs which use energy from the battery to get the air heated up beforehand in the cylinders at the time when the engine is first ignited. Innovation in the techniques for fuel management, as well as, the injection pressure is now able to build up considerable heat to get the feel ignited without the need for glow plugs. 

  1. A start light is activated

As soon as the indicator gives the signal, you push down hard on the accelerator of your vehicle, and you turn the key in the vehicle's ignition to get it started. At this point, fuel is delivered by the fuel pumps into the engine. As the fuel makes its way, it comes in contact with fuel filters that get it cleaned before letting it through to the nozzles for fuel injection. Getting the feel cleaned is an ideal maintenance technique because contaminated fuel will get the injector nozzles clogged up.

  1. Fuel is pressurized by the pump for fuel injection into the delivery tube

Also referred to as a rail, the delivery tube keeps the feel highly pressurized at about 23,500psi at the minimum while ensuring that every cylinder gets a supply of pressurized fuel at the right time. From the fuel injectors, diesel fuel is sprayed right into the cylinder's combustion chambers via the nozzles which are powered by the ECU of the engine. It is the engine control unit which regulates the pressure, determines fuel spray time, the duration of the fuel spray, etc. There are diesel fuel systems where other methods are employed for controlling the injection of fail. Today, new innovations are being explored to manufacture diesel engines which have higher power and responsiveness.

  1. In the cylinders, fuel meets with air and fire

In the previous step, we analyzed how the fuel gets to where it needs to be. While that process is ongoing, a different process takes place at the same time to transport air to the appropriate location which will result in a firepower play. An air cleaner is used in conventional diesel engines to supply air. However, today, turbochargers have taken over the game as they have the ability to pump very large air volumes right into the cylinders. If the conditions are right, turbochargers help in fuel economy while supplying even greater power. Turbochargers significantly enhance diesel vehicle power by at least 50% and can significantly lower the consumption of fuel by at least 20%. 

  1. Combustion begins

At this point, the little fuel which is highly pressurized in the engine's pre-combustion chamber spreads combustion to the fuel, as well as, the air that is already present combustion chamber. 

When the weather is cold, it may be quite a challenge to get a diesel engine started since the cylinder block has a cold metal, as well as, a cold head which draws out whatever heat is produced as compression strokes take place in the cylinder. This inhibits ignition. In order to circumvent this problem, manufacturers equip diesel engines with glow plugs. These little electric heaters are placed inside the cylinder and provide assistance in igniting the fuel when the engine needs to be started. 

Resistive grid heaters are, also, sometimes used to get the inlet air heated up to get the engine to operating temperature. Electric resistive heaters that are placed right inside the engine block (engine block heaters) and are linked to the car’s utility grid get used for prolonged periods even if the engine of the vehicle is no longer running. This helps to lower the time needed to get the engine started subsequently.

When the weather is cold, diesel fuel stands a risk of getting waxed. This implies that it can solidify and be transformed into crystalline. In the fuel, the crystals continue to expand, and this keeps the engine starved of fuel. In order to tackle this issue, electric heaters which give off a low output are placed in the engine's fuel tank, as well as, around the fuel lines.

Then again, there are many diesel engines that possess a system for returning spills, and this ensures that any diesel that is spilled by the injector pump, as well as, the injectors gets sent back to the engine’s fuel tank. Once the engine of the car is warmed up, warm fuel being sent back will help to ensure that waxing does not occur in the fuel tank. Due to the innovations in fuel technology, there are special additives that have been added to ensure that waxing does not occur anymore.

Another very important component of diesel engines is the governor which could be either mechanical or electronic. It is responsible for limiting engine speed by exercising control over how much fuel is being delivered. In diesel engines, air that comes in is never throttled. Therefore, if a diesel engine does not have a governor, over speeding is very likely. For fuel injection systems that are governed in a mechanical way, the gear train of the engine is what drives them. Fuel delivery control in mechanical systems functions by combining weights and springs. However, modern diesel engines that are controlled electronically manage fuel delivery, as well as, limit the revolutions that occur through an Electronic control unit (ECU). The ECU works by getting signals on the speed of the motor and uses actuators to regulate fuel amounts and the timing of fuel injections.

Now that we have analyzed how a diesel engine basically works let us analyze the types of diesel engines and see the principles on which they work.


To better understand how diesel engines work, we need to consider the different kinds of diesel engines that we have today. There are different ways to classify diesel engines, but one very popular way to do so is to classify them based on their working mechanism. In that case, diesel engines are classified into two types which are the two-stroke diesel engine, as well as, the four-stroke diesel engine. 

Two-stroke diesel engines

This basically refers to a diesel engine that operates in just two strokes. Its inventor was Hugo Guldner in the year 1899. The technology on which 2 stroke diesel engines work was advanced during the 1930's other to produce machines that had an enhanced power to weight ratio, as well as, output range. Diesel stream liners in the 1930's were the first mobile vehicles on which day 2-stroke diesel power was applied. As time went on, the two-stroke diesel engines began to be modified for use in the locomotive and marine industries. In fact, this is what paved the pathway towards dieselising railroads from the 1940's and 50's. 

Diesel engines basically work by using compression ignition. This refers to the process of fuel injection after having the compression chamber compress the air, thus, resulting in the self-ignition of the fuel. This process differs from gasoline engines which make use of the Otto cycle where fuel mixes with air and, then, gets into the combustion chamber to be lighted up by spark plugs. 

Internal combustion engines that work in two strokes have a simpler mechanism than the four-stroke engines. However, they have a more complex nature with regards to thermodynamics and aerodynamics. Basically, the theory on how internal combustion engines operate involves four cycles which are –

  • The Intake
  • The Compression
  • The Ignition
  • The Exhaust

In two-stroke diesel engines, this process takes place in just a single revolution and in 360 mechanical degrees. This is in contrast to four-stroke diesel engines where this process takes place in 2 full revolutions and in 720 mechanical degrees. For two-stroke diesel engines, multiple functions can take place at any point in time as the engine runs. 

How it works

Intake is initiated at the point where the piston rests at BDC (bottom dead center). At this point, the cylinder wall ports let in air through the cylinder. Artificial aspiration is very important in the operation of a two-stroke diesel engine, and they utilize a blower that is driven mechanically to get the cylinder charged with needed air. Alternatively, turbo-compressors are used too. During the early phase, remnant combustion gases are forced out from the last-occurring power stroke using the charged air. This process is called scavenging.

The piston rises, and the air charge that was let in during the intake is compressed. Fuel injection takes place at the TDC (top dead center) which leads to combustion emanating from the very highly pressurized air charge, as well as, the heat that results from compression, thus, causing the piston to be driven downward. 

With the downward movement of the piston right inside the cylinder, it gets to a certain level in which the exhaust port opens up so that combustion gases with high pressure can be expelled. In recent times, a 2 stroke diesel engine makes use of a poppet valve that is mounted on top, as well as, uniflow scavenging. As the piston continues to move downward, the ports for the intake of air will become exposed which causes the cycle to begin all over again.

Four-stroke Diesel Engines

These, also, internal combustion engines which make use of about 4 piston strokes (the intake, the compression, the ignition, and the exhaust) in a single operating cycle. For an operating cycle to take place, the crankshaft must complete two revolutions at 720 degrees. This kind of diesel engine is probably the most widely seen small engine out there. Practically, four-stroke diesel engines make use of about 5 strokes to complete an operating cycle. These 5 strokes include –

  • The Intake

In this event, air and fuel mix together and get supplied into the combustion chamber. With the movement of the piston from the top dead center to bottom dead center, the intake event takes place as an opening of the intake valve occurs. The piston moves downward towards the bottom dead center which leads to the creation of low cylinder pressure. The mixture of air and fuel goes right through the intake valve with force created by ambient atmospheric pressure. This pressurized air-fuel mixture goes right into the cylinder where it fills the low-pressure area that has been created from the motion of the piston. 

The mixture of air and fuel keeps flowing, and the cylinder keeps getting filled a little bit past the bottom dead center, and the direction of the piston starts changing. The intake valve stays open a few degrees from the rotation of the crankshaft after the bottom dead center. The intake valve gets closed to seal the deal mixture of fuel and air in the cylinder

  • The compression

This is the point where the mixture of fuel and air is compressed tightly in the cylinder. The sealing of the combustion chamber forms the charge, which refers to the amount of air and fuel mixture that is compressed and trapped, ready to be ignited. The compression of the air and fuel mixture makes it possible for the release of greater energy with the ignition of the charge. In order for the cylinder to get sealed so as to provide the needed compression, the valves for intake, as well as, exhaust needs to be closed. By compression, we refer to the process by which a charge is reduced or squeezed from large to small volumes inside the combustion chamber. The momentum that is needed for charge compression is maintained by the flywheel.

With the compression of the charge by the engine’s piston, the compressive force is increased, and this leads to the generation of heat. The air and fuel mixture which gets compressed and heated in the charge causes a rise in the charge temperature, as well as, a rise in the vaporization of the fuel. The rise in the temperature of the child takes place uniformly in every part of the combustion chamber, and this leads to the production of faster combustion after ignition.

The rise in the vaporization of fuel takes place as little fuel droplets undergo total vaporization due to the generated heat. The rise in the droplet surface area that is now exposed to the flame of the ignition makes room for the charge to be burned more completely inside the combustion chamber. 

  • The Ignition

This event takes place with the ignition and oxidization of the charge via a chemical reaction causing heat energy to be released. By ignition (or combustion), we refer to the very quick oxidizing chemical reaction which involves the chemical combination of fuel and oxygen to release energy as heat. 

Only a short time is required for problems combustion to get flame spread all through the entire combustion chamber. Sparks from spark plugs and results in combustion at about 20 degrees of the rotation of the crankshaft before top dead center. A progressive flame front, which refers to the wall that acts as a separation between the charge and the byproducts of combustion consumers the atmospheric oxygen, as well as, the fuel vapor. As the flame front continues to make its way throughout the combustion chamber, the whole charge gets burned.

  • The power

This refers to a stroke of operation in the engine where the head of the piston is forced apart from the head of the cylinder by hot expanding gases. Torque is applied to the crankshaft when the force and movements of the piston get transferred via the connecting rod. The rotation of the crankshaft is initiated by the application of torque. What determines how much torque is produced is the piston pressure, piston size, and engine throw. Both valves remain closed when the power stroke occurs.

  • The Exhaust

The stroke takes place with the expulsion of spent gases out of the combustion chamber which is released right into the atmosphere. This happens to be the last stroke, and it takes place with the opening of the exhaust valve while the intake valve stays closed. It is the movement of the piston which gets the exhaust gases evacuated into the atmosphere.

When the piston gets to bottom dead center during this stroke, a complete combustion cycle has occurred while the cylinder gets filled up by exhaust gases. At this point, an opening of the exhaust valve takes place and the flywheel, as well as, other parts that move cause the piston to be pushed back to top dead center. This process causes the exhaust gases to be forced out of the exhaust valve. When the exhaust stroke takes place, you will find that the piston stays at top dead center and this signals the completion of one operating cycle. 

Now that we have understood how two stroke and four stroke diesel engines operate, another important aspect to consider is the fuel injection process in diesel engines. 


In diesel engines, fuel injection systems are located at its heart. The system pressurizes and injects fuel which is forced into compressed air inside the combustion chamber. Here are the components of the fuel injection system in a diesel engine -

  • The fuel injection pump which has the function of getting fuel highly pressurized
  •  The high-pressure pipe that performs the function of supplying the injection nozzle with fuel
  • Injection nozzle that gets fuel injected right into the cylinder
  • The feed pump which performs the function of sucking fuel out of the fuel tank
  • The fuel filter which filters the fuel

Key functions of a fuel injection system

The fuel injection system in a diesel engine performs 4 key functions as follows –

  • Feeding fuel

The body of the injection pump has several pump elements built into it. With the elevation of the plunger by the cam, the high-pressure compression of the fuel takes place which is, then, forwarded to the injector.

  • Injection timing adjustments

When we talk about the delay in ignition, we refer to that time period which exists between fuel injection, ignition, and combustion, and the time when the highest combustion pressure is attained. Basically, this time period can be said to be almost constant regardless of the speed of the engine, and a timer helps in the adjustment or alteration of the injection timing. This helps in the achievement of optimum combustion. 

  • Fuel quantity adjustments

Air intake constantly occurs in diesel engines regardless of the speed as well as, the rotating load. As the quantity of injections is altered with engine speed while the timing of injection remains constant, it will create a change in the output, as well as, the consumption of fuel. Basically, the output of the diesel engine is nearly proportional to the quantity of fuel injected, and the accelerator pad helps to adjust this. 

  • Fuel Atomizing

With injection pump pressurization of fuel and the injection nozzle atomization, fuel is thoroughly mixed with air which helps to enhance ignition. This produces total combustion. 

How fuel injection systems work

Fuel injection pumps are very complex. They are all calibrated in a way that helps them to always supply appropriate amounts of fuel inside the combustion chamber through fuel injectors. The result is that power output, and fuel efficiency are maintained. Most of the time, injection pumps are indirectly driven by gears, by chains, or even a timing belt from the crankshaft.

Fuel injection pumps are times precisely to supply appropriate diesel volumes to the cylinder chamber just before the arrival of the cylinder at TDC position of the compression stroke. What this accomplishes is to ensure the availability of fuel at just the right point where it is needed. Here, heat which is produced from high-pressure piston compression ignites the fuel which supplies the power that the diesel engine needs. Fuel injection pumps are designed to function up to thousands of times in a single minute.

Did you enjoy this educational piece on how a diesel motor works? Check out How a car works from A - Z.