[N&W] Re: Baker valve gear
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nw-mailing-list at nwhs.org
Tue May 4 23:18:27 EDT 2004
[Gary Rolih writes:]
A thermodynamic comment about valve gear in general... The purpose of the
design of the locomotive is to capture and put to use the expansion of the
high pressure gas or steam. In thermodynamics, the steam cycle is called
the Rankine cycle. The most efficient Rankine cycle is one in which water
at "room" temperature and atmospheric pressure is raised by the addition of
heat energy (from burning coal) to high pressures and temperatures- 300
psi and a temperature probably around 800-1000 degrees F. This energy is
converted into mechanical energy by expanding the steam in the cylinders.
The objective is to expand the steam back down to atmospheric pressure
again to capture to maximum amount of energy put in as heat. Naturally,
the losses in a steam locomotive are very high. Only around 4% is
recaptured as mechanical work. ( A very well designed stationary steam
plant for electricity, running essentially at constant conditions may only
recapture 10 to 20%- with some serious exhaust heat recovery systems the
best plants may get up to 30%.)
One of the key items in the cycle is to expand to atmospheric pressure at
the end of the piston stroke. Of course in a locomotive this could not be
done. Some pressure had to remain to force the steam out of the cylinder,
through the exhaust duct and through the nozzle into the smoke box to cause
the forced draft.
The reason the N&W persisted in developing the Mallet engines was that the
steam is expanded TWICE in the high pressure and low pressure cylinders.
The Y classes were more thermodynamically efficient than the Class A's with
the simple, single expansion.
The valve gear had a role in this expansion. Not like automotive motor
valves with their fixed timing and lift or opening, the steam locomotive
with the Johnson bar or Power Reverse had the ability to change the valve
opening and closing time. The engineer could control the expansion process
to fine tune the cycle- and reverse the direction of the locomotive, too.
Each type of valve gear had some method of converting the rotary motion of
the driving linkages- it takes some of the heat energy to drive the valve
operating system- to an oscillating, linear motion to slide the valves back
and forth and open and shut. Naturally, there are drawbacks and
inefficiencies for each system. Some had better motion which improved the
energy conversion, some were easier to operate and maintain, like the Baker
type. Some had superior steam flow characteristics, like the Franklin
Poppet valve designs.
The N&W engineering group chose to use the Baker based on life-cycle
costing principles. Its motion was not quite as good as the later
Walshaerts, but the ease of maintenance around the rotating pins was
superior to anything else. Plus the Baker components fit on more than one
type of locomotive whereas the Walschaerts parts were essentially unique to
each class. Consequently , the N&W did not have to stock a lot of
different parts at each service terminal.
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