« Back to Dictionary Index

Transfer Passage:
In a two-stroke engine, the transfer passage is a critical component responsible for transferring the air-fuel mixture or fresh charge from the crankcase to the combustion chamber. Here’s a detailed explanation:


  1. Air-Fuel Transfer: After the piston moves upward during the compression stroke, creating a vacuum in the crankcase, the transfer passage allows the air-fuel mixture to flow from the crankcase into the combustion chamber.
  2. Scavenging: In addition to transferring the fresh charge, the transfer passage also plays a role in scavenging the combustion chamber by expelling the exhaust gases through the exhaust port.
  3. Crankcase Compression: As the piston descends during the intake stroke, the transfer passage is sealed to prevent the air-fuel mixture from escaping back into the crankcase. This allows the crankcase to be pressurized, enhancing the efficiency of fuel delivery.

Components and Operation:

  • Port Timing: The timing and duration of the transfer passage opening and closing are crucial for optimizing engine performance, including power output, efficiency, and emissions.
  • Transfer Ports: These are openings or passages in the cylinder wall that connect the crankcase to the combustion chamber. They are strategically positioned to facilitate the transfer of the air-fuel mixture.
  • Piston Movement: The movement of the piston controls the opening and closing of the transfer ports. As the piston moves up and down, it uncovers and covers the ports, regulating the flow of gases.
  • Reed Valve or Piston Ports: In some two-stroke engines, a reed valve or piston ports are used to control the flow of air-fuel mixture into the transfer passage.

Design Considerations:

  • Port Shape and Size: Optimizing the shape and size of the transfer ports is essential for maximizing airflow and fuel delivery while minimizing turbulence and pressure losses.
  • Scavenging Efficiency: The design of the transfer passage should promote efficient scavenging of exhaust gases to ensure complete combustion and reduce emissions.
  • Crankcase Sealing: Proper sealing of the transfer passage during the compression stroke is critical to prevent leakage and ensure effective crankcase compression.


  • Engine Performance: The design and operation of the transfer passage significantly influence the performance characteristics of the engine, including power output, torque, throttle response, and fuel efficiency.
  • Emissions: Efficient scavenging and combustion facilitated by the transfer passage design help reduce exhaust emissions and improve environmental sustainability.
  • Reliability: Proper sealing and functioning of the transfer passage are essential for the reliable operation of the engine, minimizing issues such as air-fuel mixture leakage and incomplete combustion.


  • Two-Stroke Engines: Commonly used in small engines, such as those found in motorcycles, scooters, outboard motors, chainsaws, and lawn equipment.

The transfer passage is a vital component in two-stroke engines, facilitating the transfer of the air-fuel mixture from the crankcase to the combustion chamber while also aiding in scavenging and crankcase compression. Its design and operation have a significant impact on engine performance, emissions, and reliability.

« Back to Dictionary Index