Typical Compression Ratio of a Petrol (SI) Engine Select the compression ratio that most closely represents a conventional petrol engine.

Introduction / Context:
Compression ratio strongly influences thermal efficiency, knock tendency, and combustion quality. While modern engines may vary, a baseline value is helpful for quick estimates and comparative questions.

Given Data / Assumptions:

• Conventional, naturally aspirated petrol engines without extreme high-efficiency or forced-induction tuning.
• Historical textbook ranges (older designs 7:1 to 9:1; many modern engines 9:1 to 12:1 with knock mitigation).

Concept / Approach:
Petrol engines are limited by knock; thus their geometric compression ratios are moderate compared with Diesels. A representative book value used in many basic problems is about 8:1. Diesels, by contrast, commonly use 15:1 to 25:1 to ensure autoignition of injected fuel.

Step-by-Step Solution:
Identify engine type: spark-ignition petrol.Recall typical range: approximately 7:1 to 10:1 in many classical references.Choose the nearest listed value: 8:1.

Verification / Alternative check:
Contemporary direct-injection engines may use 10–12:1; the question asks for a “nearly” value, for which 8:1 is the standard textbook approximation.

Why Other Options Are Wrong:
4:1 is too low; would yield very poor efficiency.15:1 and 20:1 are Diesel territory.30:1 is unrealistic for light-duty reciprocating engines.

Common Pitfalls:
Assuming turbocharged or Atkinson-cycle numbers; the question targets a basic, conventional SI engine.

Final Answer:
8:1