Select the typical compression ratio range for a diesel (compression-ignition) engine.

Introduction / Context:
Compression ratio is a defining difference between SI and CI engines. Diesels require high compression to heat air sufficiently for fuel auto-ignition, while petrol engines are limited by knock. Recognizing typical ranges is basic engine design literacy.

Given Data / Assumptions:

• Conventional road and stationary diesel engines.
• Modern trends may slightly shift ranges but fundamentals hold.
• Compression ratio defined as V_max / V_min in the cylinder.

Concept / Approach:
Diesel engines commonly operate around 15:1 to 20:1; heavy-duty or cold-start-friendly designs may be higher, while some modern light-duty diesels use ratios a bit below 16:1 with advanced injection to meet emissions and NVH. Values far below 15:1 struggle with reliable CI ignition; values above ~25:1 are unusual due to peak pressure and stress limits.

Step-by-Step Solution:

Verification / Alternative check:
Reviewing manufacturer data (trucks, generators) confirms ratios in the teens to low twenties, validating the selection.

Why Other Options Are Wrong:

• 6–10 or 10–15: Typical for SI, not CI.
• 25–40: Extremely high; structural loads and emissions constraints make this rare.

Common Pitfalls:
Confusing geometric compression ratio with effective compression in turbocharged engines; boost changes effective compression temperature but not geometric ratio.

Final Answer:
15 to 25