Steam engine indicator study: definition of missing quantity per stroke The “missing quantity per stroke” in a steam engine condensation/indicator analysis equals:

Introduction / Context:
When analyzing steam engine performance with an indicator diagram, we often compare the total steam supplied to the cylinder (cylinder feed) with the amount of steam accounted for in the diagram (indicated mass), considering condensation, re-evaporation, and leakage. The unaccounted difference is termed the “missing quantity.”

Given Data / Assumptions:

• Cylinder feed measured per stroke (from boiler to cylinder).
• Indicated mass of steam calculated from the indicator diagram using state properties.
• Effects of clearance, cushion steam, initial condensation, and re-evaporation are implicit in the balance.

Concept / Approach:
The missing quantity represents the steam that entered the cylinder but is not represented as live (dry) steam on the indicator diagram, typically due to initial condensation and wall effects that later may partially re-evaporate. It is the difference between what went in and what the diagram shows as present.

Step-by-Step Solution:
Let m_feed = cylinder feed per stroke.Let m_ind = indicated mass deduced from the PV diagram and steam tables.Define missing quantity m_miss = m_feed − m_ind.Interpretation: m_miss mainly covers condensation during admission and early expansion not counted as dry steam on the diagram.

Verification / Alternative check:
Experimental indicator work often shows m_feed larger than m_ind in wet-steam engines, consistent with measurable condensation loss, later partly offset by re-evaporation toward exhaust.

Why Other Options Are Wrong:
Adding masses or reversing the subtraction misrepresents the physical balance. Terms involving “mass of cushion steam” mix clearance effects with the basic definition and are not the standard expression for missing quantity.

Common Pitfalls:
Confusing missing quantity with cylinder condensation rate at a specific crank angle; overlooking that some condensed steam re-evaporates and shows later in the cycle.

Final Answer:
cylinder feed − indicated mass of steam