Working of the Two-Stroke Engine with P-V Diagram


In a two-stroke engine, the cycle is completed in one revolution of the crankshaft. In this engine, two unproductive strokes suction and exhaust strokes are eliminated.

Construction and working of the two-stroke engine

Fig shows the simplest form of a two-stroke engine. The cylinder is connected to the closed crank chamber. Instead valves they use the port to transfer air or fresh charge and to remove exhaust gases. During the upward motion, (from Bottom Dead Center to Top Dead Center) of the piston gas in the cylinder gets compressed. At the same time, the pressure inside the crankcase is reduced; air-fuel mixture or charge is inducted to the crankcase through the spring loaded inlet valve.

Read: Port-Timing diagram of two-stroke engine

When the piston move from TDC to BDC (expansion stroke) the charge in the crankcase get slightly compressed. Near the end of this stroke, the piston uncovers the exhaust port burnt gas to escape through the port. Cylinder pressure drops into atmospheric. Further movement of the piston uncovers transfer port, slightly compressed charge in the crankcase then flows to the cylinder.

The piston head has a hump or deflector, which deflect the fresh charge to upward. The incoming fresh charge helps to drive away the burnt gas from the cylinder. The piston again moves from BDC to TDC, transfer port close first and then exhaust port and charge get compressed. Thus, the cycle is repeated.

Following fig shows the PV diagram of the two-stroke engine.
Actual_and_theoretical_PV_diagram_of two_stroke_petrol_engine

  • The incoming fresh charge pulls out burnt gas. This process is known as scavenging.
  • In direct fuel injection two-stroke engines, the fuel is injected after exhaust port closed near to TDC. This solves the problem of high fuel consumption.
Scottish engineer Dugald Clerk invented the two-stroke engine in 1881.

- Difference between Two Stroke and Four Stroke Engine
- Advantages and disadvantages of two-stroke engine over four-stroke engine

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