Typical compression ratio for spark-ignition (petrol) engines Within standard automotive practice (unboosted SI engines), which range best represents the usual compression ratio adopted for petrol engines?

Introduction / Context:
Compression ratio (CR) is the key geometric parameter influencing thermal efficiency, knock tendency, and emissions in spark-ignition engines. While modern engines may push slightly beyond historical values with direct injection and knock control, the classical range remains informative for fundamentals.

Given Data / Assumptions:

• Conventional, naturally aspirated petrol engines without extreme knock-mitigation or high boost.
• Regular fuel octane ratings, typical passenger-car operation.
• Comparison based on geometric compression ratio.

Concept / Approach:
Theoretical efficiency of the Otto cycle increases with CR, but real engines are limited by knock (auto-ignition). Historically, SI engines have used CR values around 6–10. Direct injection, cooled EGR, and high-octane fuels can allow somewhat higher CR, but the foundational range taught in thermodynamics courses is 6–10.

Step-by-Step Solution:

Verification / Alternative check:
Engine specifications across decades show many unboosted SI engines with CR within 7.5–10.5; teaching examples standardize 6–10 for simplicity.

Why Other Options Are Wrong:

10–15 and higher ranges are typical of diesels or specially optimized SI with DI and high octane; 15–25 and 25–40 suit CI engines or special cycles.

Common Pitfalls:
Confusing geometric CR with effective CR under valve timing effects; the geometric number still falls in the indicated range for typical SI engines.

Final Answer:

6 to 10