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Philosophy of Modern Steam Locomotive Design


Philosophy of Modern Steam Locomotive Design Gas Producer Combustion System Exhaust System Design Resources

Much thought has been given to the best form and characteristics for steam locomotives. Different countries and different designers had different ideas about this, some very valid and some with no basis in science or engineering! These philosophies were applied with varying degrees of success. More recently, with steam projects in the 1980's, alternative design philosophies were adopted to attempt to gain interest from commercial railways (i.e.- maximum compatibility with existing diesel operations). The following lists concentrate on the basic design ideas that should be applied to classical Stephensonian steam locomotive designs.

In the U.S., the Norfolk and Western Railway set forth a definition for a "modern steam locomotive" in the 1940's which still provides valid criteria for steam locomotives today.

Norfolk & Western Railway definition of a
"Modern Steam Locomotive"
1. A high-capacity boiler
2. Roller bearings on all engine and tender wheels
3. A one-piece cast steel bed frame
4. Improved counter-balancing
5. Complete pressure and mechanical lubrication

While this definition covers many important points, it is not complete and significant important design requirements are missing. It also reflects the U.S. emphasis on achieving power through high steam output (high-capacity boiler) as opposed to the alternative idea of maximizing power by minimizing steam consumption. Famous French steam locomotive designer Andre Chapelon incorporated other important design characteristics in his designs:

Andre Chapelon
  1. A high-efficiency exhaust system.
  2. High degree of superheat.
  3. Streamlining of the Steam Circuit.
  4. Compounding

Chapelon's ideas point to the maximizing of locomotive efficiency. Chapelon studied U.S. steam design and planned on incoporating the ideas stated in the N&W "Modern Steam Locomotive" definition in his post-WWII locomotive designs.

Following Chapelon, L. D. Porta's locomotive designs have incorporated the following additional features which would be desirable for any new steam locomotive:

L. D. Porta
  1. Efficient combustion system
  2. Maximization of adhesion
  3. Advanced tribology
  4. Careful attention to detailed design

In his book The Red Devil and Other Tales from the Age of Steam, David Wardale listed the following criteria, which incoporate all of the above ideas plus additional ideas of his own. Wardale's criteria capture many ideas previously incorporated in various designs, but never put to paper as a list of design criteria:

David Wardale
  1. Maximize the boiler pressure.
  2. Minimize the boiler - steam chest pressure drop: requires the largest flow area throughout the live steam circuit, particularly through the superheater elements.
  3. Minimize the steam chest - cylinder pressure drop; requires the largest steam chest, the largest diameter valves of longest lap and greatest flow coefficient, and valve liners with the largest ports of highest flow coefficient.
  4. 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.
  5. 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.
  6. Minimize the cylinder clearance volume.
  7. Maximize the steam temperature: requires the largest superheater and the highest fraction of the total combustion gas sweeping the elements.
  8. Maximize the feedwater temperature: requires the largest feedwater heater of maximum heat transfer coefficient.
  9. 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.
  10. Minimize the combustion excess air: requires the best fuel / air mixing in the combustion zone.
  11. Ensure that the draughting and combustion systems guarantee good steaming so that the maximum boiler pressure can be maintained in practice.
  12. Maximize the locomotive's power : weight ratio: requires the maximum boiler energy output rate and the highest cylinder efficiency, the smallest boiler, the lightest superstructure, the minimum number of carrying wheels, and the lightest tender consistent with operating requirements.

The over-riding design principle embodied in Wardale's list is that each individual aspect of the locomotive must be designed as well as is possible, a philosophy which was sadly absent in many previous steam locomotive designs.