Compression ratio sensitivity — effect of increasing clearance volume in I.C. engines If the clearance volume of an internal combustion engine is increased (with other factors constant), how is the compression ratio affected?
Correct Answer: decrease
Introduction / Context:Compression ratio strongly influences efficiency, power, and knock tendency. Understanding the geometric definition reveals how design changes alter this critical parameter.
Given Data / Assumptions:
- Swept volume Vs fixed.
- Clearance volume Vc is changed by head gasket thickness, piston crown shape, or combustion chamber design.
- Definition of compression ratio: r = (Vs + Vc) / Vc.
Concept / Approach:From the definition, increasing Vc (denominator and part of numerator) reduces the ratio because Vc grows faster relative to the total volume at bottom dead center.
Step-by-Step Solution:Let r = (Vs + Vc) / Vc.If Vc increases while Vs remains constant, the denominator increases.Therefore, r decreases.
Verification / Alternative check:Numerical example: Vs = 500 cm^3, Vc = 50 cm^3 → r = (500 + 50)/50 = 11.Increase Vc to 60 cm^3 → r = 560/60 ≈ 9.33, which is lower.
Why Other Options Are Wrong:
- Increase or be doubled: contradicts the formula.
- Remain constant: impossible if Vc changes and Vs is fixed.
Common Pitfalls:Confusing clearance volume with swept volume; only Vc change here affects the ratio inversely.
Final Answer:decrease