Understanding the Cross Compound Steam Engine Arrangement

A compound steam engine is a type of steam engine where steam is expanded in two or more stages, or cylinders, to extract more useful work. This process involves the steam first passing through a high-pressure (HP) cylinder and then exhausting into a low-pressure (LP) cylinder (or cylinders).

Compound engines can be arranged in several ways. The question asks specifically about the cross compound arrangement.

Types of Compound Steam Engines

Common arrangements include:

• Tandem Compound: The high-pressure and low-pressure cylinders are placed in line, on the same piston rod. The piston of the HP cylinder is on the same rod as the piston of the LP cylinder.
• Cross Compound: The high-pressure and low-pressure cylinders are placed side by side, driving separate cranks on the same crankshaft.
• Angle Compound: The cylinders are arranged at an angle, typically 90 degrees, often to save space.

Details of the Cross Compound Steam Engine

In a cross compound steam engine:

• There is one high-pressure cylinder and one low-pressure cylinder.
• These cylinders are positioned next to each other (side by side).
• Both cylinders drive the same crankshaft.
• The cranks connected to the pistons of the HP and LP cylinders are typically set 90 degrees apart. This crank arrangement provides a more uniform turning moment on the crankshaft compared to tandem arrangements where both pistons move together.

Let's evaluate the given options based on this understanding:

• Option 1 describes: "one high and one low-pressure cylinder are set side by side, driving the same shaft, cranks being set 90° apart". This matches the definition of a cross compound steam engine.
• Option 2 describes: "two cylinders are centred on the same piston rod, the l.p. cylinder being placed nearest the crankshaft". This describes a tandem compound engine, not a cross compound.
• Option 3 describes: "two cylinders are set at 90°, usually to save floor space". This describes an angle compound arrangement, which is different from a cross compound setup where cylinders are side-by-side and *cranks* are 90° apart (though angle compounds also have cylinders at 90°). The key characteristic of cross compound is the side-by-side arrangement driving the same shaft with 90° cranks.

Therefore, the description in Option 1 accurately defines a cross compound steam engine.