Cylinder Efficiency

Cylinder efficiency is calculated by dividing the work done by the steam in the cylinder by the heat drop in the cylinder.

In Line 84 of FDC 1.3, Wardale calculates the isentropic cylinder efficiency of the 5AT at maximum drawbar power output to be 81%.  He gets this figure by dividing the actual specific work done by the cylinder in kJ/kg (calculated from the indicator diagram) by the isentropic heat drop in the cylinder (also in kJ/kg) measured from an h-s chart (click on link to see chart and explanation).

Cylinder efficiency is governed by the shape of the Indicator Diagram and in particular by the losses that are evidenced by it – most especially expansion losses, condensation losses and leakage losses.

Note: in Line 80 of FDC 1.3, Wardale estimated that the steam flow to the 5AT’s cylinders needed to be increased by 5% above theoretical requirement to allow for “heat transfer to the cylinder walls during steam admission”.  He goes on to point out that this low value results from using “all practical features to reduce it, such as:

  • very high superheat,
  • long stroke:diameter ratio,
  • optimum cylinder insulation,
  • high rotational speed at normal train speed,
  • low clearance volume,
  • special engine component design, etc.”

All of these features serve to increase cylinder efficiency.

Limit of Cylinder Efficiency:  It should be noted that even with no losses, there is an upper limit to cylinder efficiency governed by Carnot’s equation which states that the maximum theoretical efficiency of any heat engine is governed by the temperature difference between its heat source and its heat sink.

Note: Isentropic efficiency is another (but very different) measure of cylinder efficiency.  Instead of describing the ratio of work output to work output, it describes the ratio of work output with maximum possible work output based on steam conditions – see Thermodynamics definitions.