Steam Engine Testing — Interpreting the “Missing Quantity” Consider a steam engine heat and mass balance. The “missing quantity” (unaccounted steam/heat between admission and exhaust) is mainly attributable to which causes?

Introduction / Context:
During steam engine trials, the measured steam supplied does not always reconcile perfectly with the steam accounted for by the indicator diagram and exhaust conditions. This discrepancy is called the “missing quantity.” Knowing why it occurs is vital for diagnosing cylinder performance and improving economy.

Given Data / Assumptions:

• Saturated or slightly superheated steam supply to a reciprocating engine.
• Conventional slide or piston valves with typical clearances and leak paths.
• Heat transfer to cylinder walls and back from walls during the cycle (condensation–re-evaporation).

Concept / Approach:
The missing quantity is primarily due to cylinder condensation: hot steam enters and contacts cooler metal, a fraction condenses, and part of this condensate re-evaporates later during expansion/exhaust. A smaller part of the discrepancy is usually due to leakage of steam past valves and piston rings. Together these effects explain why indicated steam consumption differs from feedwater measurements.

Step-by-Step Solution:
At admission, steam at high enthalpy meets cooler cylinder surfaces ⇒ condensation stores latent heat in metal/condensate.During expansion, pressure and temperature fall ⇒ previously condensed water partially re-evaporates, altering measured expansion line.Minor continuous leaks bypass measurement at the ports/rings, adding to discrepancy.Net effect is an apparent deficit or “missing quantity” when balancing mass/energy across a cycle or test period.

Verification / Alternative check:
Lagged cylinders, steam jacketing, superheat, and higher jacket/insulation quality reduce condensation and therefore reduce the missing quantity, which is consistent with observed improvements in trials.

Why Other Options Are Wrong:

• Disagree: Ignores well-documented cylinder condensation mechanisms.
• Speed- or state-specific qualifiers: While speed and superheat influence magnitude, condensation and small leakage remain the principal root causes across typical conditions.

Common Pitfalls:
Attributing the entire difference to instrument error; while measurement errors exist, thermodynamic interactions within the cylinder dominate the missing quantity in most practical engines.

Final Answer:
Agree