What is a 4-stroke engine?

Saswata Baksi Internal Combustion Engine 1 Comment

A four-stroke engine is an Internal combustion engine, where four successive strokes (i.e. Suction-Compression-Power-Exhaust) completes in two revolutions of the crankshaft. Therefore, the engine is called Four-stroke engine.

In recent days the majority of automobile runs on a four-stroke cycle.
Basic some terms used in this article:

Top dead center (TDC):

It is the extreme position of the piston at the head end of the cylinder.

At TDC the crank angle is 0*

Bottom dead center (BDC):

The extreme position of the piston at the crank end side.

At BDC, the crank angle is a 180*

Cylinder bore:

It is the internal diameter of the cylinder.


It is the distance through which piston moves in the cylinder during 1 stroke.

NOTE: * = Degree

Four- stroke engine

Four-Stroke engine’s parts:

A Four-stroke engine consist of following parts:

  1. Cylinder: It is the heart of the engine. The piston reciprocates in the cylinder.
  2. Cylinder head: It is the top cover of the cylinder, towards TDC, is called cylinder head.
  3. Piston: It is the reciprocating member of the engine. It reciprocates in the cylinder.
  4. Piston rings: Two or three piston rings are provided on the piston. It seals the gap between the cylinder liner and piston.
  5. Crank: It is a rotating member. It makes circular motion inside the crankcase.
  6. Crankcase: It is the housing of crank and other engine parts. It is also used as the sump of lubricating oil.
  7. Connecting rod: It is used to convert the reciprocating motion of the piston to rotary motion of the crankshaft.
  8. Crankshaft: It is the rotating member, which connects the crank.
  9. Cooling fins or Water jackets: It is used for cooling purpose.
  10. Cam and Camshaft: It is provided to operate the opening and closing of the Inlet and Exhaust valve and also operate the fuel injection pump in the Diesel engine.
  11. Inlet valve: This valve controlled the admission of charge or air inside the engine cylinder.
  12. Exhaust valve: This valve controls the removal of burnt gas after combustion.
  13. Intake manifold: This is a passage which carries the fresh charge or air.
  14. Exhaust manifold: This a passage which the exhaust gas goes out of the engine cylinder.
  15. Spark plug: It is used in a Petrol engine or SI engine to ignite the fuel.
  16. Fuel injector: It is used in a Diesel engine or CI engine to sprayed the fuel inside the engine cylinder.
  17. Carburetor: It is used in a Petrol engine to mix the air-fuel properly.
  18. Flywheel: It is mounted on the crankshaft and is made of cast iron. It stores the energy in the form of inertia.

Classification of 4-stroke engine:

The four-stroke engine generally classified in 2 types:

  1. Four-stroke cycle petrol engine
  2. Four-stroke cycle diesel engine

Four-stroke Petrol engine:

This types of an engine based on the Otto cycle. The various strokes in the operation of a four-stroke cycle petrol engine are described below.


A four- stroke petrol engine consists of:

  1. A cylinder
  2. Cylinder head attach with spark plug
  3. Piston attach with piston ring
  4. Connecting rod
  5. Crank
  6. Crankshaft
  7. Valves ETC.

In four-stroke engines, Valves are used instead of Ports.

There are two valves:

  1. Suction valve
  2. Exhaust valve

And these valves are operated by means of Camshaft. It is rotated at half the speed of a crankshaft.

The working principle of the Four-stroke petrol engine:

The travel of the piston from one dead center to another is called piston stroke and a four-stroke cycle consists of four strokes:

  1. Suction Stroke
  2. Compression Stroke
  3. Power or Expansion Stroke
  4. Exhaust Stroke

Let me introduce these:

  • Suction stroke:

In this stroke the suction valve opens, exhaust valves remain close condition. The piston moves from the top dead center to the bottom dead center and the charge (Air+Fuel) enters to the engine cylinder due to the partial vacuum developed inside the cylinder.

When the piston reaches the BDC, the inlet valve closes and the flywheel makes a half revolution.

  • Compression Stroke:

In this stroke, the piston moves from bottom dead center (BDC) to the top dead center (TDC), and both the suction and exhaust valves are in closed condition. The mixture, which was sucked into the cylinder during the suction stroke, is compressed into the combustion chamber. The heat produced during the compression stroke and vaporize the mixture. As the piston approaches the top dead center, the compression stroke completes and then the spark plug gives the spark.

  • Power Stroke or Expansion stroke:

Again both the inlet and exhaust valves are closed in this stroke. The compressed charge ignited by the high intensity of spark and expands adiabatically, therefore, it pushes the piston downward i.e. towards BDC then the stroke is completed.

  • Exhaust Stroke:

During the exhaust stroke, the inlet valve is in a closed condition and the exhaust valve is opened. The piston rises up from BDC to TDC. The stroke is completed when the piston reaches the Top Dead Center. And then again stokes are recycled.

In the exhaust stroke in one situation both the valves are in opened condition for some seconds so that fresh charge entered to engine cylinder and push the burnt gases out of the cylinder. This process is called Scavenging.

This is how a Four-stroke Petrol engine works.

Four-stroke Petrol engine diagram:

4-Stroke Petrol engine diagram


Valve-timing diagram for four-stroke Petrol engine:


  • Inlet valve opens when piston at TDC
  • Inlet valve closes when piston reaches BDC
  • Spark produce, when piston reaches TDC
  • Exhaust valve open when piston at BDC
  • Exhaust valve close when piston at TDC

Actual valve-timing diagram:

  • Inlet valve opens about 10*-20* before TDC
  • Inlet valve close 30*-40* after BDC to take advantage of rapidly moving gas
  • Spark occurs 20*-35* before TDC
  • Exhaust valve opens about 30*-50* before BDC
  • Exhaust valve close about 10*-15* after TDC


Applications of Four-stroke Petrol engine:

It has various types of usages like:

  1. In automobiles
  2. Motorcycles
  3. Cars
  4. Buses
  5. Trucks
  6. Airplanes
  7. Small pumping sets
  8. Mobile electric generator

Four-stroke Diesel engine:


A four-stroke Diesel engine consists of:

  1. A fuel injector
  2. Fuel pump
  3. Cylinder
  4. Cylinder head
  5. Inlet and exhaust valves
  6. Piston
  7. Piston ring
  8. Connecting rod
  9. Camshaft
  10. Crankshaft
  11. Cam ETC.

Working principle of Four-stroke Diesel engine:

The cycles of Four-stroke diesel engine is the same as the Petrol engine.

  1. Suction Stroke
  2. Compression Stroke
  3. Power or Expansion Stroke
  4. Exhaust Stroke
  • Suction Stroke:

In a suction stroke, the inlet valve is in the opened condition and the exhaust valve remains closed. The atmospheric air is drawn into the cylinder as the piston moves from TDC to the BDC due to the partial vacuum air is sucked inside the cylinder.

  • Compression Stroke:

In this stroke Piston moves from BDC to TDC, as a result, the air inside the cylinder is compressed and heated.

In this stroke both the valves are in closed condition.

The air temperature is around 800*C

At the end of the compression stroke, fuel injector inject the fuel at high pressure into the compressed hot air, thus the ignition takes place inside the cylinder.

  • Power or Expansion Stroke:

During this stroke, both the valves are in closed condition. The piston at the TDC is pushed downward by the expansion of burned gas.

Therefore a Power stroke is produced.

  • Exhaust stroke:

In this stroke, the exhaust valve is in opened and the inlet valve is in the closed condition. The piston travels from BDC to TDC and pushed the burned gas out of the cylinder.

And here is also the scavenging process happened to removed the burnt gas completely.

Four-stroke Diesel engine diagram:



Valve-timing diagram for four-stroke Diesel engine:

Actual valve-timing diagram:

  • Inlet valve open at 10*-30* before TDC
  • Inlet valve closes at 20*-50* after BDC
  • Exhaust valve opens approximately 40* before BDC
  • Exhaust valve close 15* after TDC
  • Fuel injection starts 5*-15* before TDC
  • Fuel injection stops 15*-25* after TDC


Applications of Four-stroke Diesel engine:

Four-stroke Diesel engine is one of the most popular prime overs. It has various types of applications like:

  1. Small pupming sets
  2. In automobile industry
  3. Air compressor
  4. Drilling jigs
  5. Boats and Ships
  6. Power saw
  7. Bulldozers
  8. Tanks
  9. Diesel electric locomotives ETC.
4-Stroke Petrol engine animation (SOURCE: WIKIPEDIA)

2-Stroke engine vs 4-Stroke engine

Difference between 2-stroke engine and 4-stroke engine

Petrol Engine (SI) vs Diesel Engine (CI)

Petrol engine vs diesel engine

Advantages and Disadvantages of Four-Stroke engine:

Advantages of 4-Stroke engine:

  1. More torque
  2. More fuel economy than 2-stroke engine
  3. Less consumption of lubrication oil
  4. Less pollution because of proper burning of fuel
  5. More durability
  6. Well managed cooling system, so that longer engine life
  7. Less noisy
  8. Scavenging happens really well, therefore Thermal efficiency and HP of the engine is high

Lets talk about some disadvantages:

  1. There are many moving parts and hence there is more friction, therefore the engine has less mechanical efficiency.
  2. The design is complicated
  3. The weight of the engine is more because of heavier flywheel construction.
  4. Due to more parts the maintenance required quite often.

Fuel supply system in Four-stroke engine:

Petrol engine fuel supply system consists of:

  • Fuel feed system
  1. Gravity system
  2. Vacuum system
  3. Pressure system
  4. Pump system
  5. Fuel injection system

At first, a electrical petrol pump or mechanical petrol pump sucked the fuel from the fuel tank via fuel filter as shown in figure then the fuel through fuel lines goes to the carburetor where air+fuel mixed with proper ratio and enters into the engine cylinder for combustion.

To know more, checkout Fuel supply system in petrol engines

layout of fuel supply system in petrol engine

Layout of Fuel supply system in Petrol engine

Diesel engines fuel supply system consists of:

  1. A Diesel tank
  2. A feed pump
  3. A filter
  4. An injection pump
  5. An injector
  6. Connecting lines

There are two types of fuel injection system in Diesel engine:

  1.  Air injection system or Indirect injection
  2. Solid injection system or Direct injection

And there are two types of solid injection system:

  1. Common rail fuel injection system
  2. Individual pump fuel injection system.

Lubrication system in Four-stroke engine:

The supply of lubricating oil between two moving parts of vehicle is called lubrication.

Why we need lubrication system?

The primary objectives of lubrication are:

  1. To reduce friction between moving parts to its minimum so that power loss is less
  2. To reduce wearing out of moving parts
  3. To provide cooling effect
  4. To provide cleaning action

4-stroke engine lubrication system consists of:

  1. Oil pump
  2. Oil cooler
  3. Oil strainer
  4. Oil filter
  5. Oil pressure gauge
  6. Crankcase dilution

Parts to be lubricated:

The following parts in the engine require lubrication.

  1. Crankshaft bearings
  2. Crankpins
  3. Big end of connecting rod bearing
  4. Small end of connecting rod bearing
  5. Bushes of gudgeon pin or Wrist pin
  6. Inner wall of the cylinder
  7. Valve operating mechanism
  8. Timing gears
  9. Camshaft bearing

Lubrication system in engines:

  1. Petroil system
  2. Splash system
  3. Pressure system
  4. Dry-sump system
  5. Combined splash and pressure-feed system

Cooling system in Four-Stroke engine:

All the heat produced by combustion of fuel is not totally converted into power at crankshaft.

Only about 20 percent of the heat is used as power at the crank shaft. And 35 percent of the heat is transferred to the cylinder wall which cause pre-ignition of the charge and also lubricant burned away due to the heat, therefore the engine maybe seized.

Keeping above factor in mind we need to install a proper cooling system.

In a Four-stroke engine their are two types cooling method.

  1. Cooling by air
  2. Cooling by water





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About the Author

Saswata Baksi


Saswata completed his diploma in Mechanical Engineering from I.C.V.P and now studying B-Tech in School of Mechanical Engineering at KIIT University, one of the best university in India. Apart from that, he loves blogging, Saswata is also the founder of jhargramhub.in and partsofspeech.net

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