>Wendy Diver (diver.wendy.wd@bhp.com.au) wrote:
>>I am interested in the performance characterictics of steam locomotives,
>>particularly in relation to diesel electrics both DC and AC traction. I
>>would appreciate any information about generic speed/TE curves for steam
>>locomotives and how these are affected by the number of driving wheels.
>As speed increases for diesel and electroc locos, the tractive effort
>decreases. For steam, it increases with speed.
Such a thing bids fair to lead to perpetual motion, a wonderful
concept, but one which does not hold. See my previous answer.
>The class 26 in South Africa is vastly more efficient than conventional
>steam locos, but lack of government enthusiasm prevented further work.
>The Alfred Country Railway has done similar things to its NGG16 locos.
Well people, like Dusty Durrant, reckon it was "vastly" more
efficient. The major modifications were to the fire-bed, to increase
combustion efficiency. But, there really was not much scope for
increased combustion efficiency under good conditions: 84% fuel ----->
boiler water was pretty easily achievable in any loco.. The 26-class
also adopted the Chapelon principles as far as steam passages and
valve events goes. This does tend to increase efficiency a little,
but essentially it increases utilization (i.e. power, not necessary
efficiency.... there was less restriction on the development of power
at speed)
>There are some highly efficient steam generation plants (e.g. power
>stations), but these use water tube boilers which are unsuitable for
>locomotive use.
The principal reason for increased efficiency (which can be over 40%,
compared with an average steam loco of 8%) is the increase in pressure
(and hence boiling temperature), plus a high degree of superheat.
Maximum possible efficiency is given by the famous Carnot equation
(T2-T1)/T2. The higher T2 and the lower T1, the higher the
efficiency. Steam temperatures can be over 1000 Centigrade and
pressures above 200 atmospheres in a modern thermal power plant. A
water-tube boiler is necessary to sustain these pressures, of course,
but they can be less combustion efficient than a fire-tube boiler
with a radiative-absorptive firebox. Another, and dominating, reason
for a power plant's high efficiency is low exhaust pressure- less than
0.01 psi (practically a vacuum) can be acheived with turbines, but
never with a reciprocating steam engine (about 20 psi). Low exhaust
pressure means low T1.
Geoff Lambert