Compound Locomotive - Introduction

Introduction

The usual arrangement on a compound engine is that the steam is first expanded in one or two high-pressure (HP) cylinder(s), then having given up heat and losing pressure, it exhausts into a larger volume low-pressure (LP) cylinder, (or two, - or more), thus extending the cycle. Thus the cylinders can be said to work in "series" as opposed to the normal arrangement of a simple-expansion locomotive where they work in "parallel", the steam being expanded just once in any one cylinder. In order to balance piston thrusts of a compound, the HP:LP cylinder volume ratio has to be carefully determined, usually by increasing the LP cylinder diameter and/or by lengthening the stroke. In non-condensing engines, the HP:LP volume ratio is usually 1:2¼. On geared locomotives, cylinder volumes can be kept more or less identical by increasing LP piston speed. Strictly speaking compound only stands for double-expansion, but the term is loosely applied to any multiple-expansion engine. The instances where railway locomotives have employed triple-expansion with steam going from high pressure (HP) cylinder, to medium pressure (MP) cylinder and finally to a low pressure (LP) one are rare, but triple expansion engines were very common in steamships in the late 19th/early 20th centuries.