Condition at the start of compression (pressure): At the beginning of the compression stroke in a reciprocating air compressor, the cylinder air pressure is typically __________ the local atmospheric pressure.

Introduction / Context:
Correctly identifying the starting pressure for compression is foundational to calculating pressure ratios, work input, and volumetric performance in reciprocating compressors.

Given Data / Assumptions:

• Air is drawn from the atmosphere during the suction stroke.
• Minor intake losses are neglected in the basic idealization.
• No pre-pressurization devices at the intake.

Concept / Approach:

In the ideal model, suction fills the cylinder with ambient air, so the pressure at the instant compression begins equals atmospheric. Real systems may have small pressure drops across filters/valves, but conceptually, p_start ≈ p_atm is used for introductory analysis and indicator-diagram interpretation.

Step-by-Step Solution:

Verification / Alternative check:

Textbook examples take p1 as ambient in work and temperature ratio formulas, confirming the standard assumption for baseline calculations.

Why Other Options Are Wrong:

“Less than” or “more than” would require throttling losses or supercharging intake; “indeterminate” ignores the model assumptions; “twice” is physically unjustified.

Common Pitfalls:

Confusing start-of-compression with mid-stroke states; overemphasizing small intake pressure drops in conceptual questions.

Final Answer:

equal to