Compound engines — torque smoothing and flywheel sizing A receiver-type compound steam engine (with cranks typically at 90°) requires a lighter flywheel compared to a single-cylinder engine. Is this statement correct?

Introduction / Context:
Flywheels mitigate cyclic speed fluctuations by storing and releasing kinetic energy. Engines with more uniform torque over the cycle can use smaller (lighter) flywheels for the same allowable speed variation. Compound arrangements modify the torque profile appreciably.

Given Data / Assumptions:

• Receiver-type compound with two cranks set typically at 90°.
• HP exhaust flows to a receiver before LP admission, phasing power strokes.
• Speed regulation target and allowable fluctuation remain the same for comparison.

Concept / Approach:
With cranks in quadrature and separate expansions in HP and LP cylinders, the combined indicated torque waveform has smaller peak-to-mean variation than a single-cylinder engine. The coefficient of fluctuation of energy is reduced; therefore, for the same permissible coefficient of speed fluctuation, the required mass moment of inertia (and thus flywheel size) can be smaller.

Step-by-Step Solution:

Verification / Alternative check:
Indicator diagrams and torque summation plots for cross/receiver compounds show reduced gaps between power strokes, confirming lower energy fluctuation and lighter flywheel requirement.

Why Other Options Are Wrong:

• “No”: contradicts standard design practice.
• “Only at very low speeds” or “only for Woolf”: the effect holds broadly; it is most associated with receiver compounds.
• “Cannot be judged”: the principle is established without needing individual cards.

Common Pitfalls:
Confusing receiver vs tandem arrangements; ignoring that governor action and load irregularity also influence flywheel sizing but do not reverse this general conclusion.

Final Answer: