Compression ratio – typical range for Diesel (compression-ignition) engines In automotive and stationary Diesel engines (compression-ignition), which of the following ranges best represents the usual geometric compression ratio?

Introduction / Context:
Compression ratio strongly influences peak pressure, efficiency, and ignition characteristics of internal combustion engines. Diesel (compression-ignition) engines rely on high compression to raise air temperature sufficiently to auto-ignite injected fuel, so their compression ratios differ markedly from spark-ignition engines.

Given Data / Assumptions:

• Compression ratio r = V_max / V_min in the cylinder.
• Typical modern Diesel engines operate with much higher r than gasoline Otto-cycle engines.
• We seek the broadly accepted range rather than a single value.

Concept / Approach:

Diesel engines commonly use r in the ballpark of 15–20 (sometimes slightly lower for turbocharged engines or as high as ~22 in some designs). Gasoline spark-ignition engines usually have r around 8–12. Extremely low ratios (3–8) are unrealistic for Diesel ignition and efficiency, while 20–30 is higher than standard automotive practice and can impose excessive mechanical and thermal stresses for many designs.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturer specifications for many Diesel engines (automotive and generators) list compression ratios in the mid-to-high teens, confirming the selection.

Why Other Options Are Wrong:

3–6 or 5–8: Too low for compression ignition; more in line with early low-efficiency engines or not applicable.20–30: Above typical; while possible in special cases, it is not the usual range.8–12: Typical of modern gasoline engines, not Diesel.

Common Pitfalls:

Confusing boost pressure (turbocharging) with geometric compression ratio; boosting alters effective compression but not the geometric ratio r.

Final Answer:

15 to 20