Working Of Four Stroke Spark Ignition Engine (SI) with PV Diagram | Mecholic

Working Of Four Stroke Spark Ignition Engine (SI) with PV Diagram

4-Stroke si-Engine-with-airflows animated

🔗Basic components of IC engine and their functions
🔗Comparison between SI engine and CI engine

SI engine is an internal combustion engine, where the fuel is burned with the help of spark plug. SI engine working the principle of the Otto cycle. As the name suggests, in a four-stroke engine, the one cycle of combustion operation is completed in four strokes. Each stroke consists of 180° rotation of the crankshaft and hence four stroke completed in 720° rotation of the crankshaft. The engine takes two revolutions of crankshaft to complete this four stroke. The four-stroke for a spark ignition engines are

  • Suction/ intake stroke
  • Compression stroke
  • Combustion/ Expansion/ power stroke
  • Exhaust stroke

Various stroke and the corresponding position of piston and valves for a vertical IC engine, PV diagram are described below.
pv diagram for si engine
PV diagram for four-stroke spark ignition engine

🔗Working of four stroke compression ignition engine with PV diagram
🔗Working of two stroke engine with PV diagram

Suction or Intake stroke

Process 0→1.
SI engine suction

This stroke starts when the piston is at top dead centre (TDC) and about to move downward. While the piston moves downward the intake valve is open and the exhaust valve is closed. The movement of piston produces a vacuum inside the cylinder and charge consisting of air-fuel mixture drawn into the cylinder from the carburetor. When the piston reaches the bottom dead centre (BDC), the inlet valves close. During this process, the volume increases but the pressure inside the cylinder remains constant (atmospheric pressure).

Compression stroke

Process 1→2.
SI engine compression

The piston moves from BDC to TDC. During this process, both the inlet and outlet valve is closed and the charge previously drawn into cylinder get compressed. At the end of this stroke, ie piston at TDC, the whole charge is compressed into clearance volume. The pressure increases with a decrease in volume. The compression ratio of SI engine is between 6 and 10.

Combustion/ Expansion or Power stroke

Process 2→3 and Process 3→4.
SI engine combustion

At the end of compression stroke, the charge is burned using spark plug at the cylinder head. In ideal SI engine, it is considered as the instantaneous process. The temperature and pressure inside the cylinder increase dramatically at constant volume due to the burning of fuel (process 2→3). This is the only stroke in which actual power is produced. The high pressure burnt gases push the piston from TDC to BDC. During this process, both intake and exhaust valves remain closed. In this process, temperature and pressure decrease while the volume of gas increases. At the end of the power stroke, the exhaust valve is open, the pressure drops to atmospheric pressure (process 4→5).

Exhaust stroke

Process 5→0.
SI engine exhaust

The piston moves from BDC to TDC while the inlet valve is  closed and the exhaust valve is open. The piston pushes out the burnt gas from the cylinder to the atmosphere. At the end of exhaust stroke, some residual gases get trapped in clearance volume. This residual gas gets mixed with fresh charge drawn into the cylinder during the following cycle.

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