In 4 stroke cycle engines, one working cycle is completed in four stroke of the piston or two revolution of the crankshaft. Hence, it is called as four stroke engine. In 4 stroke engine, 2 valves are placed instead of port as that of two stroke engines. These are termed as inlet and exhaust values. The schematic diagram of four stroke cycle engine is shown in fig.
Working of 4 stroke cycle (S.I Engines)
It consists of the following four strokes
- Suction stroke
- Compression stroke
- Power or expansion stroke
- Exhaust stroke
At the beginning of the stroke, the piston is at the top most position (TDC) and is ready to move downward. As the piston moves downwards, vacuum will create inside the cylinder. Due to this vacuum, air fuel mixture from the carburetor is sucked into the cylinder through inlet valves till the piston reaches bottom most position (BDC). During the suction stroke, exhaust valve remains in closed condition and inlet valve remains open. At the end of the suction stroke, the inlet valve will be closed.
During the compression stroke, both the inlet and exhaust valves are in closed condition and the piston moves upward from BDC to compress the air fuel mixture. This process will continue till the piston reaches TDC as shown in fig. The compression ratio of engine varies from 6 to 8. The pressure at the end of compression is about 600 to 1200kN/m².
The temperature at the end of the compression is 250 to 300°C. At the end of this stroke, the mixture is ignited by spark plug.
It leads to increase in pressure and temperature of the mixture instantaneously.
Power or Expansion stroke
Both the pressure and temperature range of the ignited mixture are 1800 to 2000°C and 3000 to 4000KN/m² respectively. During the expansion stroke, both the valves are remains closed. The rise in pressure of the mixture exerts an impulse on the piston and pushes it downward. Therefore, the piston moves form TDC to BDC. This stroke is known as power stroke Fig.
During the exhaust stroke, the piston moves from BDC to TDC, the exhaust valve is opened and inlet valve is closed. The burnt gases are released through the exhaust valve when the piston moves upward. As the piston reaches the TDC, again the inlet valves will open and the fresh air fuel mixture enters into the cylinder for the next cycle of operation.
It is obvious from the above operations; only one power stroke is produced in each and every four stroke of the piston or two revolution of the crankshaft. Hence, it is termed as four stroke engine.
Valve timing diagram for a four stroke cycle S.I engine
The exact moment at which each of the valves open and close with reference to the position of piston and crank can be shown graphically in a diagram. This diagram is known as “Valve timing diagram”.
Theoretical valve timing diagram
In theoretical valve timing diagram, inlet and exhaust valves open and close at both the dead centers. Similarly, all the processes arc sharply completed at the TDC or BDC. Fig. shows theoretical valve timing diagram for four stroke S.I engines.
Actual valve timing diagram
Fig. shows actual valve timing diagram for four stroke S.I engine. The inlet valve opens 10-30° before the TDC. The air-fuel mixture is sucked into the cylinder till the inlet valve closes. The inlet valve closes 30-40° or even 60° after the BDC. The charge is compressed till the spark occurs. The spark is produced 20-40° before the TDC. It gives sufficient time for the fuel to burn. The pressure and temperature increase. The burnt gases are expanded till the exhaust valve opens.