Compound steam engines: When the high-pressure (HP) and low-pressure (LP) cylinders are arranged in line and operate with a common piston rod (one rod connecting both pistons), what is this configuration called?

Introduction:
Compounding expands steam in stages across multiple cylinders to improve efficiency and reduce thermal stresses. Different mechanical arrangements exist depending on how the cylinders are aligned and interconnected.

Given Data / Assumptions:

• Two cylinders: HP and LP.
• Common piston rod indicates in-line mechanical connection.
• Shared crank or colinear arrangement depending on design.

Concept / Approach:
In the tandem compound engine, HP and LP cylinders are on the same axis and their pistons are mounted on a single, continuous piston rod. Steam exhausts from HP directly into the LP cylinder without an intermediate receiver volume in some variants. This is distinct from cross-compound (separate cylinders on different cranks), receiver type (explicit intermediate receiver between cylinders), and Woolf type (a particular form of compound without a receiver, but the defining feature asked here is the common piston rod aligned tandemly).

Step-by-Step Solution:
1) Identify key mechanical feature: common piston rod for HP and LP.2) Map to configuration names: “tandem” implies in line, same rod.3) Therefore, the correct term is Tandem type compound engine.4) Other names correspond to different physical arrangements.

Verification / Alternative check:
Classic steam engine texts categorize tandem (same rod), cross-compound (separate rods/cranks), receiver (with receiver), and Woolf (no receiver) types distinctly.

Why Other Options Are Wrong:
Receiver type: emphasizes an intermediate volume, not a common rod.

Woolf type: lacks a receiver but does not necessarily imply a common piston rod configuration as the defining trait.

Cross-compound: cylinders drive different cranks; not a common rod.

Common Pitfalls:
Mixing functional steam path terms (receiver/Woolf) with mechanical alignment terms (tandem/cross-compound).

Final Answer:
Tandem type compound engine