In Sept 2009, David Wardale wrote a lengthy letter in response to a question about the thermal efficiency of the 5AT in comparison with other "modern steam" designs. He wrote as follows:
- FDC 1.3.F.: Drawbar thermal efficiency of the 5AT at maximum drawbar power = 11.4% Representative best corresponding figure, simple expansion FGS (BR 7MT) = 7.7%. Increase = 48%. This is correct.
- From "The Red Devil ..." Page 501: My estimate for maximum possible drawbar thermal efficiency, SGS. = 16.3%. Page 492: Representative best corresponding figure, simple expansion FGS (BR 7MT) = 9.2%. Increase = 77%. This is also correct. Here the SGS figure assumes compound, the FGS is for simple, but the comparison is taken as valid because globally FGS compounding did not find favour, whereas it may be assumed that it can be successfully applied to future SGS (but for various reasons not to the 5AT.) If you don't like that then consider that FGS compounds peaked at about 10% drawbar thermal efficiency which would give a SGS/FGS drawbar thermal efficiency increase of 63%. This is also correct. And Porta estimated maximum SGS drawbar thermal efficiency as 17 - 18% ("The Red Devil ..." page 501, footnote 2) which would give a SGS/FGS drawbar thermal efficiency increase of 70 - 80% based on all-compound, or 85 - 96% based on compound-SGS/simple-FGS. If you prefer Porta's estimate to mine, all these figures are correct.
- From "The Red Devil ..." Pages 217-8: Increase in drawbar thermal efficiency for the SAR 26 class over the 25NC class (i.e. modified FGS over standard FGS) at maximum power for the 25NC class = 150%. This is also correct.
- Drawbar thermal efficiency of Stephenson's 'Rocket' running light engine = 0%. Drawbar thermal efficiency of state-of-the-art 3-phase AC electric locomotives running light engine = 0%. For this case drawbar thermal efficiency increase from about 200 years of traction development = 0%. This is also correct.
Conclusion: 1. So many different figures, and they are all correct - for the particular conditions of each comparison. 2. Therefore thermal efficiency figures cannot be arbitrarily stated without giving the exact basis of the comparison. 3. The absolute maximum 5AT drawbar thermal efficiency cannot be deduced from the FDCs. Nor is it readily calculable and cannot therefore be stated. It will most probably be less than my 'Red Devil' estimate of 16.3% for various reasons such as the large tender/engine mass ratio and being optimized for fairly high speed.