Compounding of steam expansion and its effect on piston stroke length When the expansion of steam is compounded in two or more cylinders (e.g., high-pressure then low-pressure), the length of stroke of the engine generally:

Introduction / Context:
Compounding divides the total pressure drop of steam across multiple cylinders to improve efficiency and reduce excessive pressure ratios in any single cylinder. A common question is whether such thermodynamic staging alters the mechanical stroke length of the reciprocating mechanism.

Given Data / Assumptions:

• Reciprocating steam engine with fixed crank radius and connecting rod geometry.
• Compounding via two or more cylinders sharing the same crankshaft (e.g., HP and LP).
• Normal valve gear operation and typical load ranges.

Concept / Approach:
The stroke is a purely geometric quantity determined by crank radius (twice the crank throw). Compounding rearranges where the expansion work happens but does not alter the kinematic geometry of the mechanism. Therefore, stroke length remains the same unless a different engine or crank geometry is chosen.

Step-by-Step Solution:
Identify what sets stroke: stroke = 2 * crank radius.Recognize compounding affects cycle thermodynamics (pressure-volume distribution) but not crank geometry.Hence, with the same engine hardware, compounding does not change the stroke length.Conclusion: “does not change.”

Verification / Alternative check:
Historical compound, triple-expansion, and quadruple-expansion marine engines retained the same stroke across cylinders (often identical for balance), confirming that compounding targets efficiency and smoothness, not stroke alteration.

Why Other Options Are Wrong:
“Increases” or “decreases” would require a new crank and rod geometry—not a thermodynamic staging change. “Zero” and “random variation” are physically meaningless in conventional operation.

Common Pitfalls:
Confusing indicated diagram changes (PV curve shape) with kinematic dimensions; assuming multi-cylinder layouts imply different strokes by necessity (they typically do not).

Final Answer:
does not change