Effect of pressure ratio on volumetric efficiency: As the overall compression ratio decreases in a reciprocating air compressor with clearance, the volumetric efficiency generally __________.

Introduction / Context:
Volumetric efficiency measures how effectively a reciprocating compressor fills its cylinder with fresh charge. It depends on clearance and the re-expansion of trapped clearance gas, which is sensitive to the pressure ratio. Understanding this trend guides stage selection and intercooling strategy.

Given Data / Assumptions:

• Nonzero clearance volume at top dead center.
• Ideal valve action; negligible pressure losses in suction.
• Comparison at the same speed and ambient conditions.

Concept / Approach:

During suction, part of the stroke is used to re-expand the compressed clearance gas from discharge pressure back toward suction pressure. A higher pressure ratio means a greater expansion requirement, reducing the net fresh air admitted and hence lowering volumetric efficiency. Conversely, lowering the pressure ratio reduces clearance gas expansion work and increases the effective filling of the cylinder.

Step-by-Step Solution:

Verification / Alternative check:

Indicator-diagram geometry shows longer effective suction for lower discharge pressure, matching the analytical trend.

Why Other Options Are Wrong:

“Does not change” ignores clearance gas effects; “decreases” is opposite of the real trend; “negative” is unphysical for normal operation.

Common Pitfalls:

Confusing volumetric efficiency with isentropic efficiency; forgetting that zero-clearance (ideal) would make η_v approach unity regardless of ratio.

Final Answer:

increases