Two-Stroke Spark Ignition Engine Process

Two-Stroke Spark Ignition Engine

• The power stroke is produced in the following sequence:
  • Piston is forced downward from TDC, opening the exhaust port and allowing burnt gases to escape into the atmosphere.
  • Further downward movement of the piston opens the transfer port, allowing the partially compressed mixture to reach the combustion chamber.
• The piston head has a special shape that deflects a fresh change of fuel mixture up into the cylinder.
• The process of scavenging occurs, where the fresh mixture flows down and pushes the burnt gas out through the exhaust port.
• One power stroke is obtained in two revolutions of the crankshaft.

First Stroke (Suction and Compression)

• The piston moves up from BDC, closing the inlet port, exhaust port, and transfer port.
• The upward motion of the piston creates a partial vacuum inside the crankcase, drawing the air/fuel mixture into the crankcase through the inlet port.
• The exhaust and transfer ports remain closed during this operation.

Four-Stroke Spark Ignition Engine

• The sequence of operations is:
  • Suction stroke: Piston moves from TDC to BDC, creating a vacuum inside the cylinder, and the inlet valve opens while the exhaust valve remains closed.
  • Compression stroke: Piston moves from BDC to TDC, compressing the charge and increasing pressure and temperature.
• The four operations take place in the four strokes of the piston, giving one power stroke in four strokes, i.e., in two revolutions of the crankshaft.
• Three strokes are idle strokes, requiring a heavier flywheel due to non-uniform load on the crankshaft.

Comparison between Four-Stroke Engine and Two-Stroke Engine

• Four-stroke engine: Four operations take place in four strokes, giving one power stroke in two revolutions of the crankshaft.
• Two-stroke engine: Four operations take place in two strokes, giving one power stroke in one revolution of the crankshaft.
• Four-stroke engine has a heavier flywheel due to non-uniform load, while two-stroke engine has a lighter flywheel due to more uniform load.

Two-Stroke Diesel Engine

• The sequence of operations is:
  • First stroke: Piston at BDC, scavenging port and outlet valve open, and a root blower sucks in pure air and presses it through the scavenging port into the cylinder.
  • The air is brought into a turbulent motion, and the cylinder is completely flushed out and filled with fresh air.
  • The exhaust gases flow out towards the outlet valve.
  • Second stroke: Piston at TDC, scavenging port and outlet valve closed, and fuel is directly injected into the cylinder, vaporized into an ignitable fuel-air mixture, and automatically ignited and burned.