Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 3: Internal Combustion engines introductions. working of four stroke petrol engine.

Four Stroke Engines

•In four stroke engines, piston perform four different strokes to complete all the operation of the working cycle.

–Suction stroke

–Compression stroke

–Power stroke (Expansion stroke or Working stroke)

–Exhaust stroke

Stroke	Position of the piston		Inlet valve	Exhaust Valve	Crank rotation
	Initial	Final
Suction stroke	TDC	BDC	Open	Close	00 – 1800
Compression stroke	BDC	TDC	Close	Close	1800 – 3600
Power stroke	TDC	BDC	Close	Close	3600 –  5400
Exhaust stroke	BDC	TDC	Close	Open	5400 –  7200

Four Stroke petrol engine

•The Working principle of four stroke petrol engine is based on theoretical ottocycle. Hence it is also known as a otto-cycle engine. A four stroke petrol engine performs 4 different strokes to complete one cycle. The working of each stroke are discussed below.

Suction stroke:

At the beginning of the stroke, piston is in TDC and during the stroke , piston moves from TDC to BDC. The inlet valve opens and exhaust valve will be closed. As the piston moves downwards, suction is created in the cylinder as result, fresh air-petrol mixture(charge) is drawn in to the cylinder through the inlet valve. As the piston reaches BDC, the suction stroke completes and inlet valve closes. The suction stroke is represented by the line AB on PV diagram.

Compression stroke:

At the beginning of the stroke, piston is in BDC and during the stroke the piston moves BDC to TDC. Both inlet and exhaust valve are close. As the piston moves upwords the air-petrol mixture in the cylinder is compressed adiabatically. The pressure and temperature of charge increases and this is shown by the curve BC on the P-V diagram. When the piston reaches the TDC, the spark plug ignites the charge. The combustion of fuel takes place at the constant volume and is shown by a line CD on the P-V diagram. The compression ratio in petrol engine raging from 7:1 to 11:1.

Power stroke/Expansion stroke/Working stoke: 

At the beginning of the stroke, piston is in TDC and during the stroke the piston moves form TDC to BDC. Both inlet and exhaust valve remain close. The combustion of fuel liberates gases and these gases starts expanding. Due to  expansion, the hot gases exerts a large force on the piston and as result the piston is pushed from TDC to BDC. The power impulse is transmitted down through the piston to the crank shaft through the connecting rod. This causes crankshaft to rotate at high speeds. Thus work is obtained in this stroke. Hence, this stroke is also called an expansion stroke.

The expansion of gases is adiabatic in nature and this is shown by the curve DE on the P-V diagram. As the piston reaches the BDC, the exhaust valve opens. A part of the burnt gases escape through the valve out of the cylinder due to their own expansion.

Exhaust stoke: 

At the beginning of the stroke piston is in BDC and during the stroke the piston moves from BDC to  TDC. The inlet is closed and exhaust valve is opened. As the piston moves upward, it forces the remaining burnt gases out of the cylinder to the atmosphere through the exhaust valve. this is shown by the line EB and BA on P-V diagram. When the piston reaches the TDC, the exhaust valve closes and this completes the cycle.

In the next cycle the piston which is at TDC moves to BDC there by allowing fresh charge to enter the cylinder and the process continues.