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An engine where the bore diameter (the diameter of the cylinder) is smaller than the length of the stroke (the distance the piston travels up and down within the cylinder). It is also known as a “long stroke engine.”

Key Points:

  • Bore: The diameter of the cylinder.
  • Stroke: The distance the piston travels within the cylinder.
  • Configuration: The bore is smaller than the stroke length.

Characteristics:

  • Torque: Typically produces more torque at lower RPMs.
  • Efficiency: Generally more fuel-efficient at lower speeds.
  • Power Band: Suited for applications requiring sustained low to mid-range power.
  • Engine Height: Engines tend to be taller due to longer stroke.

Advantages:

  • Higher Torque: Longer stroke allows for greater leverage on the crankshaft, increasing torque output.
  • Fuel Efficiency: Often more fuel-efficient at lower speeds compared to over square engines.
  • Better for Low-Speed Operation: Ideal for vehicles or machinery that operate at low to mid-range RPMs, such as trucks, tractors, and motorcycles.

Disadvantages:

  • Higher Friction Losses: Longer stroke can result in higher friction losses due to increased surface contact.
  • Lower RPM Limit: Generally has a lower maximum RPM, limiting peak power output compared to over square engines.
  • Potential for Greater Wear: Increased piston travel can lead to more wear over time.

Applications:

  • Commercial Vehicles: Trucks and buses that benefit from high torque at low speeds.
  • Agricultural Machinery: Tractors and other equipment that require strong low-end torque.
  • Motorcycles: Certain motorcycle designs, especially those focused on low-speed performance.

Comparison with Over Square Engine:

  • Over Square Engine: An engine where the bore diameter is larger than the stroke length. These engines typically produce more power at higher RPMs and are commonly used in performance-oriented vehicles.

Summary:

An under square engine, or long stroke engine, is designed with a smaller bore diameter than stroke length, resulting in higher torque at lower RPMs and better fuel efficiency for low-speed operations. It is commonly used in commercial vehicles, agricultural machinery, and some motorcycle designs, providing advantages in applications where low to mid-range power is essential. However, these engines may face limitations in high-speed performance and can experience greater wear due to increased piston travel.

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