Compression-ignition (diesel) knock and compression ratio For a given fuel in a CI engine, how does changing the compression ratio influence the knocking tendency?

Introduction / Context:
Diesel knock arises from rapid pressure rise caused by accumulated fuel that auto-ignites after a long ignition delay. Compression ratio directly affects in-cylinder temperature at the end of compression and thereby influences ignition delay and knock severity.

Given Data / Assumptions:

• Fuel quality is fixed.
• Engine speed and injection timing are within normal ranges.
• We examine the qualitative effect of compression ratio on ignition delay and knock.

Concept / Approach:
Higher compression ratio increases end-of-compression temperature and pressure, reducing chemical ignition delay. Shorter delay means less fuel accumulates before ignition, thereby mitigating the abrupt energy release that causes knock. Conversely, lowering compression ratio reduces temperature, lengthens ignition delay, allows more premixed fuel to accumulate, and heightens knock intensity.

Step-by-Step Solution:
Increase CR → higher T and p at TDC → shorter ignition delay.Shorter delay → less premixed charge → smoother pressure rise → less knock.Decrease CR → lower T at TDC → longer delay → more premixed fuel → stronger knock.Therefore, knocking tendency is enhanced by decreasing compression ratio.

Verification / Alternative check:
Engine calibration practices exploit higher CR in diesels to obtain quick self-ignition and smoother combustion, within mechanical stress limits.

Why Other Options Are Wrong:

• Enhanced by increasing CR: opposite of the typical diesel behavior.
• Dependent only on injector size/Unaffected by CR: both ignore the dominant thermodynamic link between CR and end-gas conditions.
• Exhaust backpressure effect is secondary here and does not reverse the fundamental trend.

Common Pitfalls:
Transferring SI engine knock intuition to diesel; in SI, higher CR can increase knock, but CI relies on auto-ignition by design.

Final Answer:
Enhanced by decreasing compression ratio