IC Engines — Definition of Compression Ratio In internal combustion engines, the compression ratio r is defined as which of the following volume ratios?

Introduction / Context:
Compression ratio is a critical geometric parameter influencing efficiency, knock tendency, and emissions. A precise definition avoids confusion among swept, clearance, and total volumes in reciprocating engines.

Given Data / Assumptions:

• Total (maximum) cylinder volume V1 = swept volume Vs + clearance volume Vc.
• Minimum volume at top dead center (TDC) is V2 = Vc.
• Air-standard viewpoint for definition only.

Concept / Approach:
The compression ratio r is defined as r = V1 / V2. Since V2 = Vc and V1 = Vs + Vc, r = (Vs + Vc)/Vc = 1 + Vs/Vc. This connects to thermodynamic efficiency relations in air-standard cycles such as Otto and Diesel.

Step-by-Step Solution:
Identify total volume: V1 = Vs + Vc.Identify minimum volume: V2 = Vc.Compute r = V1 / V2 = (Vs + Vc) / Vc = 1 + Vs/Vc.Therefore, the correct descriptive ratio is “total volume to clearance volume.”

Verification / Alternative check:
Textbook definitions and manufacturer specifications express r this way; example: r = 10:1 indicates V1 is ten times V2 (the clearance volume).

Why Other Options Are Wrong:
Options A and B mix swept with total in the wrong order; option C (Vs/Vc) is a related quantity, but the formal definition requires V1/V2, not Vs/Vc alone.

Common Pitfalls:
Equating Vs/Vc with r directly; ignoring that r must be > 1 by definition; confusing geometric with effective compression in forced-induction engines.

Final Answer:
total volume to clearance volume