Compression-ignition (diesel) engines – why does part-load efficiency drop less than in spark-ignition? As the load on a compression-ignition engine decreases, its efficiency deteriorates less sharply (and may appear to “increase” relative to a throttled spark-ignition engine). This behavior is primarily due to which governing method used for load control?

Introduction / Context:
When comparing part-load behavior of compression-ignition (CI) and spark-ignition (SI) engines, CI engines usually retain better efficiency at reduced loads. The key reason is how load is controlled. This question tests understanding of governing methods and their impact on pumping losses and thermal efficiency.

Given Data / Assumptions:

• Engine type: four-stroke CI (diesel) versus SI as a reference.
• Load reduction from a medium operating point to lighter load.
• No exotic technologies (e.g., variable compression ratio or advanced turbo compounding) are assumed.

Concept / Approach:
CI engines operate unthrottled; fresh air is admitted freely and load is regulated by the injected fuel quantity. This is called qualitative governing. Since intake is not throttled, manifold pressure remains high and pumping work is small, so part-load efficiency remains relatively better. In contrast, SI engines often throttle the intake (quantitative governing), which raises pumping losses and reduces indicated efficiency at small loads.

Step-by-Step Solution:
Identify governing in CI: fuel-quantity control at nearly full air flow (no throttle).Relate governing to pumping work: unthrottled intake → low pumping losses.Connect to efficiency trend: smaller losses → less deterioration of efficiency as load drops.Therefore, the observed advantage at light loads is due to qualitative governing.

Verification / Alternative check:
Brake specific fuel consumption maps and pumping-loop p-V diagrams show that CI engines avoid the large negative pumping loop area typical of throttled SI engines at light loads.

Why Other Options Are Wrong:

• Higher maximum temperature at light load is not the cause; peak temperatures usually fall with load.
• Quantitative governing refers to throttling, characteristic of SI engines, which hurts efficiency at part load.
• Hit-and-miss governing is for small constant-speed engines, not modern automotive diesels.
• Variable compression ratio is not the standard reason for the trend described.

Common Pitfalls:
Assuming CI engines “increase” absolute efficiency at light load. In practice, efficiency still drops, but much less than in throttled SI engines due to the absence of throttling losses.

Final Answer:
qualitative governing (varying the fuel quantity without throttling the air)