# Thermal Efficiency

## 5AT Thermal Efficiency

The terms “Drawbar Thermal Efficiency” and “Cylinder Efficiency” are defined on a  separate “Thermal Efficiency” page of this website.

In the Fundamental Design Calculations (FDC 1.3), Wardale calculates that the thermal efficiency at the cylinders (i.e. comparing indicated power output with heat input) at maximum drawbar power will be approximately 15% and that the drawbar efficiency under that same condition will be approximately 11.5% – a substantial reduction due in part to the resistance losses of the very large tender that the locomotive has to haul.

These figures do not represent the maximum efficiency that the locomotive can achieve, and Wardale has not attempted to calculate a figure for maximum efficiency.  His statement that the 5AT’s efficiency will represent a 80% improvement over heritage steam is also subject to interpretation as he discusses at length in his response to a question on the subject in the FAQ Section.

The efficiency of a steam locomotive can be seen as the sum of the efficiencies of its various components.  Wardale provides examples of these in his book “The Red Devil and Other Tales from the Age of Steam” where (in Table 78, page 457) he quotes figures for standard and (proposed) modified Chinese Class QJ locomotives, and where (on page 501) he suggests what might be achieved from the further development to the level of “Third Generation Steam” traction.

The figures from these pages are combined in a single table below, however it is recommended that the qualifying texts from both Table 78 (page 457) and page 501 of Wardale’s book be read in association with them.

 Item Standard QJ Modified QJ Third Generation Steam Boiler combustion efficiency 78% 87% 95% Boiler absorption efficiency 78.2% 80% 90% Auxiliary efficiency factor 93.1% 94% 96% Cylinder efficiency 16.4% 19.05% 22% Transmission efficiency 89% 93% 94% Drawbar efficiency 94% 95% 96% Overall drawbar thermal efficiency = product of all the above 7.8% 11.0% 16.3%*

* Note:  Wardale’s estimate for TGS drawbar efficiency differs significantly from the figure of 25% that he quotes as being Porta’s estimate for condensing third generation steam locomotives – see Second Generation Steam page of this website.  However Wardale makes it clear (on page 501 of his book) that his figure applies to non-condensing locomotives and that “higher efficiency could only be obtained by expanding the steam to sub-atmospheric pressure and low temperature by means of condensing to counter the negative effect on the cycle efficiency of the restricted inlet steam temperature as done in stationary steam plant”.