Condensing Steam Engines — Exhaust Pressure Relative to Atmosphere In a condensing steam engine connected to a surface or jet condenser, the cylinder pressure during the exhaust stroke is typically __________ the atmospheric pressure.

Introduction / Context:
Condensing engines discharge exhaust steam into a condenser where it is condensed to water, creating a vacuum. Recognizing how this affects cylinder back pressure is crucial for understanding the efficiency advantage of condensing over non-condensing engines.

Given Data / Assumptions:

• Properly operating condenser with adequate cooling water.
• Air extraction (air pump) maintains low absolute pressure in the condenser.
• Exhaust stroke connects cylinder to condenser during release.

Concept / Approach:
By reducing the exhaust (back) pressure below atmospheric, the condenser increases net work per cycle: the expansion can proceed to a lower terminal pressure, and the negative loop due to back pressure is reduced. Hence, during exhaust, the cylinder pressure is typically below atmospheric (i.e., vacuum conditions exist on the exhaust side).

Step-by-Step Solution:
At release, exhaust port opens connecting cylinder to low-pressure condenser space.Condensation of steam rapidly lowers absolute pressure, sustaining sub-atmospheric conditions.Therefore, during the exhaust stroke, cylinder pressure remains lower than atmospheric, reducing pumping loss.

Verification / Alternative check:
Indicator diagrams for condensing engines show the exhaust line (back-pressure line) below the atmospheric line, confirming sub-atmospheric operation during exhaust.

Why Other Options Are Wrong:

• Equal to / higher than: describe non-condensing or throttled exhaust cases, not a functioning condenser.
• Always above atmosphere: contradicts the purpose of a condenser.

Common Pitfalls:
Assuming condenser vacuum equals complete vacuum; in practice, some residual non-condensables and temperature lift remain, but pressure is still below atmospheric.

Final Answer:
lower than