Philosophy of Modern
Steam Locomotive Design
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 highcapacity boiler
2. Roller bearings on all engine and tender
wheels
3. A onepiece cast steel bed frame
4. Improved counterbalancing
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 (highcapacity 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 
 A highefficiency exhaust system.
 High degree of superheat.
 Streamlining of the Steam Circuit.
 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 postWWII 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 
 Efficient combustion system
 Maximization of adhesion
 Advanced tribology
 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 
 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
chest, 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 cutoffs.
 Minimize the cylinder clearance volume.
 Maximize the steam temperature: requires
the largest superheater and the highest
fraction of the total combustion gas
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 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 overriding 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.
