Working Of Four Stroke Compression Ignition Engine with PV Diagram

 Compression Ignition Engine running

CI has no carburetor and spark plug, but has a fuel injector. This engine work on the principle of diesel cycle. Similar to SI engine CI engine also has the four stroke in one complete burning cycle. The four strokes are known as intake, compression, expansion and exhaust. These four-stroke or one cycle of operation completed in two revolutions of crankshaft.

🔗Working of four stroke spark ignition engine with PV diagram
🔗Difference between SI engine and CI engine

PV diagram for CI engine

Intake or suction stroke

Process 0→1.
ci engine suction

The piston is about to move from the top dead centre (TDC) to bottom dead centre (BDC). The inlet valve open and the exhaust valve is closed. In CI engine, air is alone drawn into the cylinder. The pressure remains constant (atmospheric pressure) during the process.

Compression stroke

Process 1→2.
ci engine compression

Both the intake and exhaust valve is closed. The piston moves from BDC to TDC. The previously drawn air inside the air is then compressed into clearance volume. For CI engine the compression ratio is about 16 to 20. The fuel is injected into the cylinder at the end of compression stroke. The temperature at the end of compression is very high enough to self-ignite the fuel (that is the reason this engine called compression ignition engine).

Combustion/ Expansion stroke or Power stroke

Process 2→3 and 3→4
ci engine combustion

Both inlet and exhaust valve remain closed in this process. When the piston moving from TDC to BDC, the fuel is injected in such a way that combustion maintains the constant pressure inside the cylinder while volume increase (process 2→3). After the complete injection of fuel, the pressure decreases as the volume increase (process 3→4). At the end of this stroke, the exhaust valve opens and pressure reduced to atmospheric.

Exhaust stroke

Process 5→1.
ci engine exhaust

The exhausts valve is open. The piston travels from BDC to TDC and expels the burned gas from the cylinder. Some residual burned gas get trapped in clearance volume, this will later get mixed with fresh air during the next suction stroke.


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