Internal Combustion Engine

4 Stroke Engine courtesy Wikipedia

The internal combustion engine is an engine in which air and fuel mixture in a combustion chamber is ignited under high pressure and temperature, the power of the explosive energy created is transferred to the piston which in turn is converted to mechanical energy.

The 3 major substance needed for internal combustion to occur in an engine is Oxygen, heat and fuel.

Bottom Dead Center. (BDC):  when the piston is at the lowest possible level in the cylinder, its position is at the Bottom Dead Center.

Top Dead Center (TDC): when the piston’s position is at the highest possible level within the cylinder, it is at the Top Dead Center.

Stroke: when the piston moves from top dead center to bottom dead center, it is called a stroke. For internal combustion to occur, the piston moves from TDC to BDC and from BDC to TDC, that is, it takes 2 strokes for combustion to occur. This is where the name 2 stroke engine is derived.

Basic Process of the Internal Combustion in a 4 Stroke Engine.

Basically it involves 4 processes namely;

  1. Suction
  2. Compression
  3. Ignition
  4. Exhaust


The piston starts by taking a downward stroke and in the process, sucks in Air and fuel mixture into the cylinder until it reaches BDC.


 At the BDC, air/fuel intake supply is cut off and the piston now takes an upward stroke. In the process, the piston compresses the air/fuel mixture until it reaches TDC.


At TDC, the spark plug ignites the mixture causing an explosion within the cylinder. The force of the explosion forces the piston back down.


As the piston moves back up after the impact of the explosive energy, the exhaust valve is opened to expel burnt air fuel mixture.

When the piston reaches TDC, the exhaust valve is closed and air/fuel intake valve is opened and the whole process repeats.

The up and down piston is converted to circular motion which is used for instance to rotate the wheels of cars, generate electrical energy and so on.

 The 2 Stroke Engine

 An engine is called 2 stroke engine if its takes just the piston to move 2 strokes for internal combustion to occur. In a 2 stroke engine, the piston moves from TDC to BDC sucking in air and fuel mixture into the combustion chamber. It then moves from BDC to TDC and in the process, compressing the fuel mixture, followed by ignition then exhaust of burnt mixture as the piston moves down to BDC.

The 2 stroke engine is not as efficient as a 4 stroke engine for this reason it is used for small engines. In the compression stroke of a 2 stroke engine, some air and fuel mixture escapes through the exhaust pipe. For this reason, power produced is smaller compared to a 4 stroke engine.

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