Simple and Compound Expansion

The term “Simple Expansion” refers to the single use of steam in powering a steam engine. “Compound Expansion” refers to the multiple uses of steam in powering a steam engine.

In a “simple” engine, the steam enters the cylinder at high pressure, expands as it pushes the piston through its stroke, and is then exhausted to atmosphere as the piston returns, whereas in a “compound” engine, the exhausted steam is reused in a second “low pressure” cylinder where it expands further as it pushes the low pressure piston through its stroke. In the case of a double expansion compound, the steam will then exhausted to atmosphere. In a triple-expansion compound (mostly used in marine applications) the steam is reused again in a third (even lower pressure) cylinder.

In a compound engine, the steam will pass from the high pressure cylinder into a “receiver” (a pipe or pressure vessel of adequate volume) before being admitted into the low pressure cylinder. In some cases, the receiver will incorporate means of re-superheating the steam to raise its temperature to minimize the risk of condensation.

In historical texts, proponents of compound expansion are reputed to have claimed that greater use is made of the steam by expanding it twice (or more) thereby increasing the work it does and the efficiency achieved, whereas proponent of simple expansion are reputed to have claimed that the simplicity and lower cost of simple engines outweight the efficiency gains offered by compound expansion.

In fact the arguments for and against compound expansion are more complex – and too complex to expound on a website such as this and are more adequately covered elsewhere – for instance

However a few simple and pertinent points are worth summarizing here:

  • Use of compound expansion allows longer cut-offs to be used, thereby delivering more uniform wheel-rim tractive effort;
  • The ability of compound engines to operate efficiently at longer cut-offs increases their α-coefficient and thus their power-to-weight ratio.
  • The reduction in vibration (or knocking) achieved from the use of longer cut-offs removes the incentive to operate a locomotive with a throttled (partially opened) regulator, thereby allowing full boiler pressure in the steamchest;
  • More uniform torque delivered by compound locomotives reduces the propensity for initiation of wheel-slip at moments of (transient) peak torque. This renders compound locomotives better suited to heavy haulage;
  • Reduced temperature differentials between steam entering and leaving a cylinder, minimizes heat losses and reduces or eliminates condensation, especially where the low-pressure steam is re-superheated.

Notwithstanding the above, Wardale has adopted simple expansion for the 5AT for several reasons. most of which are outlined in an FAQ on the subject:

  • Wardale has no personal experience of compound locomotives to draw on or upon which to base an “assured” locomotive design;
  • Wardale believes that for a high-speed locomotive such as the 5AT simple expansion – using all the cylinder refinements that are now possible, but which are not common knowledge – is the right choice, and that the 5AT will define ‘state of the art’ for 2-cylinder simple locomotives and may serve as a reference level to which the performance of all other types of locomotive (including compounds) can be compared.
  • Such improvement in thermal efficiency that might be gained by compound expansion cannot guarantee to justify the extra design complexity and higher manufacturing costs involved.
  • The limited low pressure cylinder volume possible within the British moving structure gauge, with a conventional layout of the cylinders, is an important limiting factor on compound design and performance.”