How does a Diesel Engine work? | How does a Diesel Cycle work?

Internal combustion (IC) engines are most common in our daily life used vehicles. These are the most efficient and high-performance engines. The IC engines have multiple types, and a diesel engine is one of them. A diesel engine is also known as a compression ignition (CI) engine.

A diesel engine is a compression engine in which the mixture process of the fuel and air is done in the engine carburetor. In this engine, the compression process occurs due to the high compression of air. In this article, we are going to discuss the different aspects of the diesel engine deeply.

What is a Diesel Engine?

An engine in which diesel fuel is ignited due to high compression of air in the combustion chamber is known as a diesel engine. A diesel engine is also known as a compression ignition engine because, in this engine, ignition occurs due to high compression of air. In this engine, there is no use of a spark plug for ignition. In 1893, Rudolph Diesel invented the first diesel engine.


four stroke diesel engine

A diesel engine has more efficiency than other internal combustion engines (i.e., petrol engines). This is because it has the highest inherent lean burn and expansion ratio, which makes the heat to be dissipated by the excess air.

These engines have two types which are 4-stroke and 2-stroke diesel engines. These types of diesel engines were utilized in place of a stationary steam engine to improve the performance.

In 1910, these engines were depleted in ships and submarines. After some time, they have been used in applications like electricity-making power plants, agricultural equipment, heavy apparatus, trucks, and locomotives.

In the 1970s, diesel engines were most commonly used in larger off-road and on-road vehicles.

A low-speed CI engine (used in applications where the overall weight of ships and other engines is relatively light) can achieve effective efficiency of up to 55%.

History of Diesel Engine

Rudolf Diesel invented 1st diesel engine in 1878. He was a student at Polytechnikum in Munich. The diesel engine is named base on the name of Rudolf Diesel.

After working many years, Diesel published his ideas about diesel engine in 1893 in the essay “Theory and Construction of a Rational Heat Motor.”

Rudolf Diesel engine
 Diesel engine built by Langen & Wolf under license, 1898


Rudolf Diesel, when he invented the diesel engine, a compressor lighter had been used as an effective and efficient way to burn the engine.

Diesel used oils such as vegetable oils to invent his first engine as he had no formula for diesel infrastructure at that time. The very high compression ratio was used to generate the high pressure and high temperature required for automatic combustion. It was the main feature of the compression ignition engine.

A method of injecting fuel directly into the combustion chamber was also required. Over time, the infrastructure of oil pollution became fuel such as gasoline (to support gasoline engines), petroleum and fuel oil (boiler room), and diesel oil.

Diesel Cycle

The diesel cycle completes a power stroke in two or four strokes of the piston. An explanation of the Diesel engine cycle working is given below with the help of T-S and P-V diagram:

diesel cycle PV diagram

 1) Suction Process (0-1)

  • For the suction of air, the piston moves from TDC to BDC (downward stroke). As it moves downward, the fresh air starts entering from the atmosphere into the compression cylinder or combustion chamber.
  • During this process, the exhaust valve remains closes, and the suction valve opens.

2) Isentropic Compression (1-2)

  • After suction, the suction valve closes, and the piston moves up (from BDC to TDC).
  • During the piston’s upward motion, it compresses the air inside the cylinder.
  • During the compression process, the temperature of the air increases from T1 to T2, the volume reduces from V1 to V2, and pressure rises from P1 to P2.
  • However, during this whole process, there is no change of enthalpy (S1 = S2).
  • This process is known as Isentropic because there is no change of enthalpy.
  • In the isentropic compression, the air is compressed up to such high temperature and pressure that the air-fuel mixture ignites itself, and it doesn’t need any extra external heat source or spark plug.

3) Heat Addition at Constant Pressure (2-3)

  • When highly compressed air reaches at point 2 (as shown in the PV and TS diagram), a fuel injector injects diesel fuel into the cylinder, which mixes with the compressed air.
  • As the diesel fuel touches the compressed air, the air-fuel mixture ignites due to the high compression of air. This ignition process adds heat into the compressed air-fuel mixture.
  • During this process, the piston becomes constant, and pressure also remains constant (P2=P3). However, enthalpy increases from S2 to S3, temperature increases from T2 to T3, and also volume increases from V2 to V3.

4) Isentropic Expansion (3-4)

  • In this process, the mixture expands into the cylinder.
  • Due to the expansion, the heat of the ignited air-fuel mixture works on the piston and forces it to move down, which rotates the crankshaft. This rotation of the crankshaft further moves the car.
  • During this whole process, the pressure of the mixture falls from P3 to P4, volume increases from V3 to V4, and temperature also reduces from T3 to T4. However, entropy doesn’t change S3=S4.

5) Constant Volume Heat Rejection (4-1)

  • After the expansion process, the piston further moves downward to remove the waste heat from the cylinder.
  • In this process, entropy falls from S4 to S1, temperature up to T1, and the pressure falls further to P1. However, volume remains unchanged (i.e.V4 = V1).
  • After the removal of all waste heat, the piston again sucks air, and the whole process repeats.

Working Principle of Diesel Engine

The working of the diesel engine is different than the petrol or SI engine. A diesel engine works on the basic principle of the diesel cycle. A diesel engine cycle consists of four processes those are:

  1. Suction
  2. Compression
  3. Expansion and
  4. Exhaust Process

diesel engine working

1) Suction Stroke

  • The piston moves downward inside the combustion chamber in the early stage and produces a vacuum inside the cylinder.
  • Due to the creation of a vacuum, a pressure difference generates between the outside and inside the cylinder.
  • Because of the pressure difference, the inlet valve opens, exhaust valve closes and air moves from the atmosphere into the combustion chamber.

2) Compression Stroke

  • After the suction stroke, the inlet valve and outlet valve are closed, and the piston starts to move up (from BDC to TDC) for air compression. In this air compression process, the pressure and temperature of the air increase but volume decrease.
  • At the end of the compression stroke, the piston starts to move at a constant rate for some time, and fuel is injected into the combustion chamber, which mixes with compressed air.
  • Due to high compression of air, the fuel-air mixture ignites, and the internal heat of the mixture increases. During this heat addition process, the pressure of the air-fuel mixture remains constant (as shown in the above PV diagram of the Diesel engine cycle).

3) Power Stroke

  • Due to the ignition of the air-fuel mixture, the heat is released by the air-fuel mixture.
  • The released heat performs wok on the piston and pushes it downward.
  • As the piston moves downward, the combusted mixture expands into the combustion chamber. This down movement of the piston rotates the crankshaft and moves the vehicle.

4) Exhaust Stroke

  • After power stroke, the piston reaches at BDC, opens the exhaust valve, and pushes the exhaust gases out of the chamber.
  • After exhaust stroke, the piston moves again upward and repeats the whole cycle.

Read also: Petrol Engine Working

Components of Diesel Engine

A compression ignition (CI) engine or diesel engine has below given major components:

  1. Fuel system
  2. Cooling system
  3. Fuel filters
  4. Fuel system
  5. After-cooler
  6. Fuel injector
  7. Crankcase
  8. Turbocharger
  9. Camshaft
  10. Crankshaft
1) Fuel System

The fuel system has a separator, injector, fuel pump, fuel rails, fuel pressure regulator, fuel injection pump, and many other units. This system also has fuel filters used to filter the fuel and make it free from dust and other dirt.

2) Fuel Separator

This part of the diesel engine uses to stops the poor-quality fuel from engine failure.

3) Fuel Filters

The filters play a vital role in the safety of the engine. They filter the fuel and remove dust and other contamination from the fuel. In this way, it also extends the engine life.

4) Fuel Injector

The injector plays an important role in the combustion process of the fuel. It works in such a way that it injects fuel into the combustion chamber as the compressed air enters into the chamber and mixes the fuel with compressed air.

5) Turbocharger

A turbocharger allows the engine to suck more air into the combustion chamber and increases the horsepower of the engine.

6) After-cooler

 It uses to decrease the intake air temperature.

7) Lubrication System

The lubrication system has the following major purposes:

  1. Eliminates foreign material from the engines.
  2. Make a connection between the piston ring and cylinder.
  3. Decreases the wear and stops seizure of rubbing surface.
  4. Eliminates heat from the pistons and other components.
  5. Decreases the power required to disable frictional resistance.

In the oil lubricating system, the various parts of the diesel engine lubricate under a high-pressure supply. This oil for lubricating the engine parts is stored in an oil sump. An oil pump uses to pump the oil and transfers it into a filter.

After passing through the filter, the oil falls into the main gallery. The main gallery oil uses to lubricate the main bearings.

After bearing lubrication, some amount of the oil goes back into the sump, some oil is used to lubricate to the cylinder wall, and the left-over oil goes into the crankpin. The oil lubricates the piston ring by flowing from the crankpin to the piston pin through connecting rod’s hole.

8) Cooling System

A cooling system has many purposes in an engine. The most purposes of a cooling system are:

  • It maintains an ideal temperature for the best efficiency of the engine in all situations.
  • This system maintains the oil lubricating properties.
  • It avoids excess heat and prevents the engine parts such as valves, pistons, cylinder head, and cylinder.

A cooling system performs two kinds of cooling:

  • Water cooling
  • Air cooling

The engine cylinder surrounds via a water jacket. This jacket has water that absorbs heat from the cylinder.

The water-cooling method has three types:

  • Forced circulation method
  • Thermosiphon method
  • Non-direct or direct method

Diesel Engine Efficiency

A diesel engine has high efficiency because of its high compression ratio. The lack of a throttle valve means very little air exchange loss, which consumes less fuel, particularly at medium and low loads. Due to these reasons, diesel engines are very economical.

According to Rudolf Diesel, the actual productivity of a diesel engine should be from 43.2% to 50.4% or more.

The actual efficiency of a diesel engine in the latest passenger cars can reach 43%, while engines in heavy diesel buses and trucks reach maximum efficiency of up to 45%. But, the driving cycle has less average efficiency than the maximum efficiency.

The maximum efficiency of a diesel engine is about 55%, which can achieve with a large 2-stroke marine diesel engine.

Diesel Cycle Engine Theoretical Efficiency

The compression stroke and power stroke of the diesel engine are reversible adiabatic. Therefore, the efficiency of the Diesel cycle can be measured from the constant volume and constant pressure processes. 

Diesel cycle TS diagram, diesel cycle efficiency

The below-given formula can calculate the efficiency of the Diesel cycle:

diesel cycle efficiency

As we discussed that,

Work done = Heat added to the system – Heat rejected by the system

The efficiency is:

diesel engine efficiency

The compression ratio (r) is given below:

compression ratio

Cut-off ratio is given below:

Cut-off ratio

The expansion ratio is given below;

Expansion ratio

After calculating all parameters, now let’s calculate the heating addition process at constant pressure (1-2),

constant pressure heating process of diesel engine

So, in the case of adiabatic compression (4-1),

adiabatic compression process

In the case of the adiabatic expansion (2-3),

adiabatic expansion


By putting the value of T1 in equations (v) and (iv), 

diesel engine efficiency formula, after substituting the value of T1

Now putting the values of T3, T2, and T1 in the above equation (iii),

final efficiency

Types of Diesel Engines

The diesel engine has two major types that are:

  1. 4-stroke diesel engine
  2. 2-stroke diesel engine

1) 2-Stroke Diesel Engine

The 2-stroke diesel engine is a type of compression ignition engine that completes a power cycle in only two strokes of the piston.  It ignites the fuel due to the high compression of fuel.  

Advantages and disadvantages of Two-stroke Diesel Engine: –

They have a low weight.The two-stroke diesel engine has unstable idling.
They have low cost.These engines produce high pollution.
These engines can work in any position.They have high noise.
These IC engines are easy to start.They have high vibrations.
They have a simple mechanism.These have scavenging issues.
They require low maintenance costs.Low volumetric and thermal efficiency.

Read more: Two-stroke Engine working

2) 4-Stroke Diesel Engine

It completes a power cycle after two revolutions of the crankshaft or four strokes of the piston. You can find these engines in heavy vehicles like buses, coaches, tractors, cars, etc.

four-stroke engine diesel engine

Advantages and disadvantages of 4-stroke Diesel Engine: –

It has high compression ratio.This diesel engine has high cost.
It has high fuel efficiency compared to a two-stroke engine.They have a complex design.
They generate less pollution.They are less powerful than a two-stroke engine.
They have high durability.These engines have more parts.
They have high fuel efficiency.They have a high weight.

Read more: Four-stroke Engine working

Difference between Diesel Engine and Petrol Engine

Diesel EnginePetrol Engine
A diesel engine operates on the diesel cycle.A petrol engine operates on the Otto cycle.
It is more efficient.It is less efficient.
They are depleted in heavy-duty automobiles like buses, tractors, and cars, etc.They are most commonly depleted in small vehicles like vans and bikes etc. 
These are very expensive.They have a low cost.
A diesel engine uses diesel which is very expensive.A petrol engine uses petrol which is less expensive.
It has a high compression ratio.It has a relatively low compression ratio.
These have high maintenance and initial costs.These engines have low maintenance and initial costs.
Diesel fuel is more difficult to ignite.The petrol fuel is easy to ignite.
It produces high noise while working.It produces less noise.
A diesel engine has low fuel consumption.A petrol engine has higher fuel consumption.

Advantages and Disadvantages of Diesel Engine

Advantages and disadvantages of the diesel engines are given below:

Adavantages of Diesel Engine

  • The main benefits of using a compression ignition engine is the direct fuel injects inside the combustion chamber. It doesn’t require spark plug for combustion. Also, gas is not needed to manipulate the power supply of the compression ignition engine.
  • A diesel engine has high efficiency than a SI engine.
  • It has high speed. 
  • It uses a smoke trap to improve the torque. If only a reduction of SW and protection is in operation, the pumping and pumping capacity can be reduced, and the capacity of the inlet channel can be increased. HC and CO can also be reduced.
  • A diesel engine or compression ignition engine also consumes less fuel compared to SI engine.
  • This type of engine consumes low amount of fuel.
  • These engines require low maintenance as compared to petrol engines.
  • CI engine doesn’t need a spark plug to ignite the fuel.

Disadvantages of Diesel Engines

  • These engines require a high compression ratio to generate the conditions required for automatic ignition
  • A diesel engine has a high chance of crashing as compared to the petrol engine.
  • If this engine will not control properly, it can permanently damage its components under high fire conditions.
  • High compression ratio affects on its performance.
  • These engines have a high maintenance cost.
  • The automobiles that have diesel engines are more expensive.
  • Diesel fuel has a high price as compared to petrol fuel.
  • These are expensive as compared to Spark ignition engines.

Applications of Diesel Engine

  1. The compression ignition engine uses in heavy industrial machineries.
  2. IC engines use to power different compressors, pumps, and large motors.
  3. These internal combustion engines use in hydroelectric powerplants.
  4. They use with turbines to generate electricity.
  5. They use to power the ships.
  6. High-speed engines use to power cars, buses, yachts, trucks, tractors, and different cars.
  7. They also use to power the railway train.

FAQ Section

Is diesel engine better or petrol engine?

A diesel engine is better than a petrol engine because it emits less CO2 compared to a petrol engine. The diesel engines are more efficient than petrol engines, but they have a high cost. 

The diesel engines have a high capability to move heavy weights because they produce more torque than petrol engines. Therefore, diesel engines are most commonly used in heavy-load vehicles. 

What are the problems with diesel engines?

  1. The diesel engines are difficult to start.
  2. They have a high cost.
  3. Heavyweight
  4. The diesel engine is used diesel fuel which is very high thicker than petrol fuel. Due to this, diesel fuel has a high chance to get contamination.

Diesel engine works on which cycle?

A diesel engine works on the diesel cycle.

What are the types of Diesel engine?

A diesel engine has the following two major types:

  1. 4-stroke engine
  2. 2-stroke engine

What are the parts of a diesel engine?

The diesel engine has the following parts:

  1. After-cooler
  2. Fuel system
  3. Crankshaft
  4. Cooling system
  5. Camshaft
  6. Fuel filters
  7. Fuel injector
  8. Crankcase
  9. Fuel system
  10. Piston

Do diesel engines have spark plugs?

A diesel engine doesn’t have a spark plug. The reason is that, in this engine, ignition occurs due to the high compression of air. 

Who invented the diesel engine?

Rudolf Diesel discovered the diesel engine in the 1890s.

What happens if you put gas in a diesel engine?

Diesel fuel is designed in such a way that it doesn’t require any external heat source (such as a spark plug) for ignition, while gasoline fuel can’t ignite without a spark plug. Therefore, if you put gas into your diesel engine, your engine will not ignite it, and the engine will not start. 


The diesel engines are most widely using all over the world. The reason behind their popularity is that these are more efficient than petrol engines. These engines are using to power different heavy vehicles as well as to power industrial heavy machinery.

The CI engine consumes less fuel as compared to a petrol engine. So, the diesel engine has high power and speed than the SI engine. But these engines have a high cost as compare to petrol engines.

Read More
  1. Different types of Engines
  2. Types of Internal Combustion Engines (ICE)
  3. Working and types of Petrol Engine
  4. Types of Rotary Engines
  5. Working of Carnot Engine
  6. Working and types of Stirling Engines
  7. Types of Jet Engines

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