Diesel Cycle with Fixed Compression Ratio – Effect of Cut-off Ratio on Mean Effective Pressure For a diesel (constant-pressure heat addition) cycle at a fixed compression ratio, how does the indicated mean effective pressure (IMEP) change when the cut-off ratio decreases?

Introduction / Context:
The cut-off ratio in a diesel cycle is the ratio of cylinder volume at the end of heat addition to that at the start of heat addition. It determines how much heat is added at approximately constant pressure. At a fixed compression ratio, changing cut-off alters net work, thermal efficiency, and mean effective pressure (MEP), which is a convenient load-independent performance metric.

Given Data / Assumptions:

• Ideal air-standard diesel cycle model.
• Compression ratio r is fixed.
• Cut-off ratio rc is varied; other losses neglected for the trend analysis.

Concept / Approach:

IMEP is proportional to net cycle work divided by displacement volume. With smaller rc (shorter heat addition), the amount of heat input per cycle decreases. At constant r, less heat added yields lower expansion work and therefore reduced net work, causing IMEP to fall. Although diesel-cycle thermal efficiency improves when rc is lowered (less late-cycle heat addition at high temperature), the absolute work output diminishes, reducing MEP.

Step-by-Step Solution:

Verification / Alternative check:

Air-standard cycle equations show net work proportional to the area of the P–V loop, which shrinks as rc decreases (at fixed r), even though efficiency (work/heat-in) rises. Practical engine maps exhibit similar trade-offs between load (MEP) and combustion phasing/duration.

Why Other Options Are Wrong:

“Increase” reverses the trend; “remain the same” ignores altered heat input; “first increase then decrease” lacks basis for monotonic rc reduction; “cannot be predicted” is false—trend is well established.

Common Pitfalls:

Confusing thermal efficiency (which improves with lower rc) with output (which decreases). High efficiency does not necessarily mean high power or MEP.

Final Answer:

decrease