Engine Geometry — What do we call the small volume at T.D.C.? When the piston is at top dead centre, the volume remaining above the piston within the combustion chamber is termed as:

Introduction / Context:
Compression ratio and combustion characteristics depend on the relationship between the swept volume (displacement) and the small residual volume at top dead centre. Correct terminology is key to engine thermodynamics.

Given Data / Assumptions:

• Piston at T.D.C.
• Four-stroke engine, simple chamber shape.

Concept / Approach:
The total cylinder volume at bottom dead centre equals swept volume + clearance volume. At top dead centre, only the clearance volume remains. Compression ratio CR is defined as (swept + clearance) / clearance. The clearance volume affects turbulence, flame travel, and detonation resistance.

Step-by-Step Solution:
At T.D.C.: piston occupies the swept space.Remaining gas space = clearance volume.Therefore, the correct term is “clearance volume.”

Verification / Alternative check:
Engine design texts consistently define CR using clearance volume Vc and swept volume Vs.

Why Other Options Are Wrong:
Cylinder volume: ambiguous; often refers to total or swept volume.Exhaust volume: not a standard term in this context.Swept volume: the displacement moved by the piston from T.D.C. to B.D.C., not the residual space at T.D.C.

Common Pitfalls:
Confusing total cylinder volume with swept volume; total at B.D.C. includes clearance volume.

Final Answer:
clearance volume