Compression-ignition (diesel) engines — effect of engine speed on ignition delay Consider an 'oil engine' (compression-ignition). As the rotational speed increases (other conditions broadly similar), what generally happens to the ignition delay responsible for diesel knock?

Introduction / Context:
Ignition delay in a diesel (compression-ignition) engine is the time between the start of injection and the start of combustion. It strongly influences noise (diesel knock), maximum pressure rise rate, and smoothness. Understanding how speed affects delay is essential for calibration and diagnosing knocking.

Given Data / Assumptions:

• Conventional diesel with proper injection timing and healthy atomization.
• Operation within the normal speed/load range (not extreme misfire or overspeed).
• Fuel of reasonable cetane number; coolant and intake conditions comparable.

Concept / Approach:
Ignition delay has a physical (chemical-kinetic) component governed by charge temperature/pressure and a mixing/physical component. As engine speed rises, charge temperature near the end of compression increases due to higher residual and reduced heat loss per cycle. This elevates reaction rates and shortens chemical delay. Although the crank-angle duration of delay can sometimes appear larger, the real-time (milliseconds) delay generally decreases with speed, which is what matters for knocking propensity since less fuel accumulates before ignition.

Step-by-Step Solution:
Higher speed → less time for heat transfer → higher end-of-compression temperature.Higher temperature/pressure → faster pre-flame chemistry → shorter chemical delay (in seconds).Less fuel premixed before ignition → gentler pressure rise → reduced knock tendency.Therefore, the claim “delay period increases with speed” is not generally correct.

Verification / Alternative check:
Empirical maps show shorter ignition delays (in milliseconds) at higher speeds for a given load and cetane rating. Engine ECUs often advance or retard timing to maintain optimal combustion phasing as speed/load vary, consistent with these trends.

Why Other Options Are Wrong:
“Agree” reverses the usual relationship. Statements limited to “only at idle” or “only above rated” are over-specific and ignore the broad trend explained by thermodynamics and kinetics.

Common Pitfalls:
Confusing time-based delay (ms) with crank-angle delay (degrees). At higher speed, the same time corresponds to more degrees; however the physical delay time typically decreases.

Final Answer:
Disagree