Factors increasing diesel knock tendency Which of the following fuel properties/conditions increase the probability of knocking in compression-ignition (diesel) engines?

Introduction / Context:
Diesel knock relates to the rate of pressure rise when a significant amount of fuel ignites almost simultaneously after a long ignition delay. Fuel properties that lengthen ignition delay or degrade spray preparation exacerbate knock tendency. Identifying these properties is essential for fuel selection and injector design.

Given Data / Assumptions:

• Compression-ignition engine with direct or indirect injection.
• Constant injection timing and in-cylinder conditions for comparison.
• Focus on fuel properties and their effect on ignition delay and mixture preparation.

Concept / Approach:

High self-ignition temperature fuels resist auto-ignition, increasing the ignition delay. Longer delay allows more fuel to accumulate before ignition, producing an abrupt heat release and stronger knock. Low volatility hinders fuel evaporation, slowing mixing with air and contributing to non-uniform combustion onset. High viscosity degrades atomization, leading to larger droplets, slow vaporization, and poor penetration–mixing patterns, all of which can prolong delay and promote knock intensity.

Step-by-Step Solution:

Verification / Alternative check:

Empirical correlations of ignition delay include temperature, pressure, and fuel cetane quality. Laboratory tests demonstrate reduced knock with higher-cetane, properly volatile, lower-viscosity fuels within injector design limits.

Why Other Options Are Wrong:

“None of these” contradicts established combustion science; each listed factor is known to worsen diesel knock when considered in isolation.

Common Pitfalls:

Assuming only cetane number matters; spray atomization and volatility are equally important for ignition and mixture formation dynamics.

Final Answer:

all of these