Reciprocating Engine: Overview, Types, Parts & Working

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Reciprocating Engine: Overview, Types, Parts & Working

Editing Date：2025-05-12

Reciprocating Engine: Overview, Types, Parts & Working

Introduction
A Reciprocating Engine is a classification of the internal combustion engines in which the piston moves up and down. The top portion of the piston where valves are located is called Top dead center, while the opposite extreme down portion of the piston is called Bottom dead center. The air or air-fuel mixture is drawn into the cylinder through the Intake valve and the combustion products are expelled from the cylinder through the Exhaust valve. An Engine is a powerhouse of the vast majority of automobiles such as trucks, aircraft, ships, electric power, generator and so on.
The piston reciprocates in the cylinder between the Top Dead Center (TDC) and Bottom Dear Center (BDC). The largest distance in one direction traveled by the piston from TDC TO BDC is called a Stroke of the engine. The diameter of the piston is called BORE. The minimum volume formed in the cylinder when the piston is at TDC is called Clearance volume. The volume displaced by the piston as its move between TDC to BDC is called Displacement volume. The ratio of the maximum volume formed in the cylinder to the minimum (Clearance) volume is called the Compression Ratio. Mathematically, compression ratio= (Vmax or BDC ) / (Vmin or TDC )

Types of Reciprocating Engine:

The reciprocating engine has been categorized into the following types :

According to the thermodynamic cycle:
•	OTTO or Constant Volume Cycle – In this the energy released during the combustion of a fuel occurs at constant volume.
•	DIESEL or Constant Pressure Cycle – In this the energy released during the combustion of a fuel occurs at constant pressure.
•	DUAL or Limited Pressure Cycle – In this the energy released during the combustion of a fuel happens partly at constant volume and pressure.

Number of Stroke Per Cycle:
•	4 Stroke engine – In 4 stroke the cycle of engine complete 4 strokes of piston
•	2 Stroke engine – In 2 stroke the cycle of engine completes 2 strokes of the piston

Ignition System:
•	Spark Ignition (SI) Engine – A homogenous mixture of air and fuel vapor is supplied to the engine and the combustion is initiated by spark plug
•	Compression-Ignition (CI) Engine – Air sucked inside the cylinder is compressed to a higher pressure and temperature. This temperature exceeds the self-ignition temperature of the fuel.

The engine cycle of both SI and CI engines can be completed in 2 strokes or 4 strokes of the piston.

Fuel Used:
•	Petrol – The engine works on Petrol fuel such as Motorcycle
•	Oil or Diesel – engine works on Diesel
•	Gas – CNG, LPG, Producer Gas
•	Multi-Fuel Engine – Use gasoline or diesel oil for starting the engine and kerosene as there primary fuel

Cooling System:
•	Water Cooling – Engine is cooled by circulating water
•	Air Cooling – Engine is cooled by blowing atmospheric air over the hot surface

Multicylinder Engine:
•	The power output of an engine is directly proportional to its speed.

Parts of a Reciprocating Engine:

A reciprocating Engine consist of the following parts:

•	Piston
•	Piston Cylinder
•	Connecting Rod
•	Valves (Inlet and Outlet) or Port (Inlet and Outlet)
•	Spark Plug or Fuel Injector
•	Piston Ring
•	Cooling jackets

Piston:
	A piston is circular in shape and it is the moving component of a reciprocating engine. The piston is connected to the connecting rod and transfer power from the engine to the crankshaft. In a 2-stoke engine, piston also work as a valve by closing and opening inlet and outlet port.

Piston Cylinder:
	It is also circular in shape. It is a cylinder in which the piston is move. It is made by the same material as of the piston.

Connecting Rod:

The main function of the connecting rod is to convert the reciprocating motion of the piston to the rotary motion. One end of the connecting rod is connected to a piston with the help of a gudgeon pin or piston pin and another end is connected to the crankshaft.

piston-cylinder arrangement

Valves or Port :
	Valves are used in Four-Stroke Engine, and Ports are Used in Two-Stroke Engine. When the engine is in the suction condition then charges enter through the inlet valve and after burning of charge the burned gas goes out of the cylinder with the help of the outlet valve.

Spark Plug or Fuel Injector:
	Spark plug is used for igniting the charge in a petrol engine and the fuel injector is used in diesel engines where high-pressure fuel is sprayed by the injector. However, fuel injector also controls the timing and quantity of fuel sprayed inside the cylinder. Here I mentioned the brief differences between Spark Plug vs Fuel Injector, you may check this guide too.

Piston Ring:

Piston rings are used for the following:

1	To save the piston and cylinder from wear out due to direct contact.
2	To clean the surface of the piston, and also it scratches down extra lubrication oil from the cylinder surface to the sump.
3	It provides a tight seal so that charges inside the cylinder can not get out of the cylinder.

Cooling jackets:
	It fits inside or outside the cylinder to save the engine cylinder from excessive heating. We may use water or coolant inside the cooling jacket which is continuously circulating.

How does a Reciprocating engine work?

The reciprocating engine has four types of strokes.

1	Suction Stroke
2	Compression Stroke
3	Power Stroke
4	Exhaust Stroke

In suction stroke, fuel goes to the engine cylinder via inlet valve or port. When the piston goes from TDC to BDC then there is pressure difference between atmosphere. Due to this vacuum is created inside the cylinder. Because of the Vacuum created the fuel is pumped in through the inlet valve. At this stage the outlet valve is in closed condition.

In compression stroke, the charge (Fuel or air+fuel) is compressed. Due to this its pressure and temperature gets higher and higher.

At this stage, a high thrust is produced by the burned gas. Due to this the piston is shifted to the BDC. The piston is connected with the crankshaft via a connecting rod, because of which the crankshaft starts rotating. This stroke is called a power stroke.

At exhaust stroke again piston comes from the BDC to TDC and pushes the burned gases out of the cylinder.

So this is how a reciprocating engine work. In a 2-stroke engine Suction and Compression happens in one movement of the piston, i.e. from BDC to TDC and Power stroke and Exhaust stoke happen in another movement of the piston, i.e. from TDC to BDC.

I will highly recommend you to check out these article for complete knowledge about engines.

•	Internal Combustion Engine
•	Two-Stroke Engine
•	Four-Stroke Engine
•	Spark plug vs Fuel Injector

FAQ's

What is a Reciprocating Engine?

A Reciprocating Engine is a classification of the internal combustion engines in which the piston moves up and down. The top portion of the piston where valves are located is called Top dead center, and the opposite extreme down portion of the piston is called Bottom dead center.

How does a Reciprocating Engine Works?

The reciprocating engine has four types of strokes.
1	Suction Stroke (Where charge (Air+Fuel/Fuel) get inside the cylinder)
2	Compression Stroke (Charge are compressed and burned)
3	Power Stroke (The burned gas push the piston down words)
4	Exhaust Stroke (The burned gas is removed from the cylinder)

What are the Parts of a Reciprocating Engine?

A reciprocating Engine consist of the following parts:
1	Piston
2	Piston Cylinder
3	Connecting Rod
4	Valves (Inlet and Outlet) or Port (Inlet and Outlet)
5	Spark Plug or Fuel Injector
6	Piston Ring
7	Cooling jackets