« Back to Dictionary Index

In the context of a two-stroke engine, trapping efficiency refers to the effectiveness with which the engine retains the fresh charge (air-fuel mixture) in the combustion chamber during operation. Here’s a breakdown of the concept:

Trapping Efficiency Equation:

Trapping Efficiency (%) = (Mass of Fresh Charge Retained / Mass of Fresh Charge Supplied per Cycle) * 100


  • Mass of Fresh Charge Retained: This refers to the amount of air-fuel mixture that remains in the combustion chamber after the scavenging process is complete. In a two-stroke engine, scavenging involves the displacement of the exhaust gases from the previous cycle and the filling of the combustion chamber with a fresh charge of air-fuel mixture.
  • Mass of Fresh Charge Supplied per Cycle: This represents the total mass of the fresh charge (air-fuel mixture) introduced into the combustion chamber during each cycle of the engine.


  • Trapping efficiency is a critical factor in determining the performance and efficiency of a two-stroke engine. Higher trapping efficiency indicates better utilization of the fresh charge, leading to improved combustion and power output.
  • Efficient trapping helps minimize the loss of fresh charge through the exhaust port before combustion, maximizing the engine’s power and fuel efficiency.

Factors Affecting Trapping Efficiency:

  1. Scavenging System Design: The design of the scavenging system, including the intake and exhaust ports, plays a significant role in determining trapping efficiency. Effective scavenging ensures thorough removal of exhaust gases and optimal filling of the combustion chamber with fresh charge.
  2. Cylinder Head and Piston Design: The shape and configuration of the cylinder head and piston influence the flow of air-fuel mixture into and out of the combustion chamber, affecting trapping efficiency.
  3. Engine Speed and Load: Trapping efficiency may vary with engine speed and load conditions. Optimal trapping efficiency may be achieved within specific operating ranges.
  4. Fuel and Air Mixture: The composition and quality of the fuel-air mixture can impact trapping efficiency. Proper fuel atomization and mixing promote efficient combustion and trapping.
  5. Compression Ratio: The compression ratio of the engine affects the density of the air-fuel mixture in the combustion chamber, influencing trapping efficiency.

Importance in Two-Stroke Engines:

  • Trapping efficiency is particularly crucial in two-stroke engines due to their unique scavenging process, where the intake and exhaust phases occur simultaneously. Maximizing trapping efficiency helps optimize combustion and power delivery in these engines.

Trapping efficiency in a two-stroke engine measures the effectiveness of retaining the fresh charge in the combustion chamber during operation. It is a key parameter that influences engine performance, efficiency, and emissions.

« Back to Dictionary Index