Effect of compression ratio on thermal efficiency of an internal combustion engine For the same cycle type and similar operating conditions, how does increasing the compression ratio of an I.C. engine affect its thermal efficiency?

Introduction / Context:
Compression ratio is a dominant design parameter that influences efficiency in both spark-ignition (Otto) and compression-ignition (Diesel) engines. Higher compression generally leads to higher thermal efficiency, within the limits set by knock (SI) and mechanical stress/emissions (CI).

Given Data / Assumptions:

• Cycle comparison within the same ideal cycle family (Otto or Diesel).
• No change in combustion phasing assumptions except those implied by higher compression.
• Materials and fuel quality are sufficient to tolerate higher compression without abnormal combustion.

Concept / Approach:
For the ideal Otto cycle, thermal efficiency = 1 − 1/(r^(gamma−1)); as compression ratio r increases, the term 1/(r^(gamma−1)) decreases, so efficiency rises. For the ideal Diesel cycle, increasing compression ratio also tends to raise efficiency for a given cut-off ratio. In real engines, SI efficiency gains are limited by knock; CI gains are limited by NOx and mechanical constraints.

Step-by-Step Solution:
Consider the ideal relation showing monotonic efficiency increase with r.Recognize real-world constraints but preserve the general trend.Therefore, increasing compression ratio increases thermal efficiency.

Verification / Alternative check:
Engine families offered in multiple compression ratios show improved part-load fuel economy and peak efficiency with higher r, provided fueling and timing are properly calibrated.

Why Other Options Are Wrong:
Stating decrease or no change contradicts both ideal cycle analysis and empirical data for similar designs.

Common Pitfalls:
Assuming that raising compression can be done indefinitely; knock limits in SI and emissions/stress limits in CI constrain practical implementation.

Final Answer:
Increase