Air motion in CI chambers – what is swirl? In compression-ignition engines, the term swirl refers to which characteristic motion of the gases within the combustion chamber?

Introduction / Context:
Mixture formation and combustion in diesel engines rely on controlled air motion. Swirl, squish, and tumble are engineered flow patterns that enhance fuel–air mixing and influence ignition delay, heat release, and emissions.

Given Data / Assumptions:

• Compression-ignition engine with an appropriate intake port and chamber design.
• Fuel injected as a high-pressure spray into hot, compressed air.
• Objective: rapid and uniform mixing for efficient combustion.

Concept / Approach:
Swirl is a coherent, rotary motion of the bulk charge about the cylinder’s axis, generally induced by helical or tangential intake ports and shaped chambers (e.g., toroidal bowls). It persists into the compression stroke and interacts with squish to intensify turbulence, promoting mixing of fuel with air.

Step-by-Step Solution:
Identify flow pattern: rotation around axis → swirl.Relate to purpose: improved atomization utilization and diffusion combustion.Outcome: better mixing reduces ignition delay variability and soot formation, aiding efficiency and emissions control.

Verification / Alternative check:
Flow bench tests and CFD visualize swirl number and swirl ratio; engine maps correlate optimal swirl with injection strategy and chamber shape.

Why Other Options Are Wrong:

• Random motion is turbulence, not specifically swirl.
• Purely radial or purely axial motions describe other patterns, not swirl.
• “No organized motion” contradicts the deliberate design of swirl in many CI engines.

Common Pitfalls:
Confusing swirl with tumble (rotation about a transverse axis) or squish (radial squeeze near TDC). These may coexist but are distinct.

Final Answer:
a rotary, organized motion about the cylinder axis