5at Train

The 5AT Group - Steaming Ahead with Advanced Technology

Enhancing Performance -- Improving Reliability -- Reducing Costs -- Controlling Emissions

Wardale's Philosophy behind the 5AT Design Specification

The following is reproduced from David Wardale's book "The Red Devil and Other Tales from the Age of Steam". It originally formed part of the test report on the South African Railways 26 Class locomotive no. 3450 (The Red Devil), and describes the essence of how steam locomotive performance can be transformed from that associated with the locomotives of yesterday.

Significant improvements to locomotive performance can be made by optimizing basic components whilst retaining the inherent simplicity and accessibility that is considered essential for reasonable locomotive availability even with the best prevailing standards of mechanical design and maintenance. The following factors are of particular importance:

  • Maximize the boiler pressure.
  • Minimize the boiler - steam chest pressure drop: requires the largest flow area throughout the live steam circuit, particularly through the [superheater] elements.
  • Minimize the steam chest - cylinder pressure drop: requires the largest steam chests, the largest diameter valves of longest lap and greatest flow coefficient, and valve liners with the largest ports of highest flow coefficient.
  • Minimize the exhaust steam back pressure: requires the largest exhaust passages of highest flow coefficient, the best possible exhaust system and the lowest possible boiler gas flow resistance compatible with efficient boiler operation and high superheat.
  • Ensure that the boiler pressure / cylinder volume / coupled wheel diameter allow high power to be developed in the normal speed range at economical cut-offs.
  • Minimize the cylinder clearance volumes.
  • Maximize the steam temperature: requires the largest superheater and the highest fraction of the total combustion gas flow sweeping the elements.
  • Maximize the feedwater temperature: requires the largest feedwater heater of maximum heat transfer coefficient.
  • Minimize the boiler unburnt fuel loss: requires the minimum burning rate, achieved by maximizing the cylinder and drawbar efficiencies and hence minimizing the boiler energy output rate for the required power, and the best combustion system.
  • Minimize the combustion excess air: requires the best fuel / air mixing in the combustion zone.
  • Ensure that the draughting and combustion systems guarantee good steaming so that the maximum boiler pressure can be maintained in practice.
  • Maximize the locomotive's power : weight ratio: requires the maximum boiler energy output rate and highest cylinder efficiency, the smallest boiler, the lightest superstructure, the minimum number of carrying wheels, and the lightest tender consistent with operating requirements.

If the above are carried out, not to an 'acceptable' degree but to the maximum extent possible within the constraints of the available technology, steam locomotive performance will be in a different class from that hitherto known. This does not exclude other factors from being improved, but the above, which simply apply sound thermodynamic principles to the steam locomotive without any frills, are the key to enhanced performance.

See also Principles of Modern Steam section of this website.