

===============================================
MANUAL FOR VALVE GEAR SIMULATORS
===============================================
Modified 14 July 2007

CONTENTS
========

1. WHAT'S THIS PACKAGE ALL ABOUT?
2. FREEWARE
3. INTRODUCTION
4. HARDWARE REQUIRED
5. FILES
6. RUNNING THE PROGRAM
7. OBTAINING HARD COPY
8. DATA INPUT & EDITING
    Mechanics of data entry
    Entering a new valve gear
9. DATA STORAGE/RETRIEVAL
A. INTERPRETING THE OUTPUT
B. WHAT DOES ONE AIM FOR?
C. "Known issues" (=Bugs)

===============================================
===


(1). WHAT'S THIS PACKAGE ALL ABOUT?
==============================

...FOR THE UNINITIATED

A valve  gear is  a mechanical  linkage  used  on  steam 
engines, particularly railway locomotives.

Such steam  engines are  based on  a piston  being 
pushed back and forth in a cylinder by steam which is 
controlled by a valve.  The task of the valve gear is to 
operate the valve.

The valve  gear primarily  needs to ensure correct timing 
of steam  admission and  exhaust from  each  end  of  the
cylinder.   However, it  also needs  to provide means for 
the engine  driver to  reverse  the  locomotive,  and  to 
control the  quantity of steam admitted at each stroke of 
the piston.

The nineteenth  century engineers who designed valve 
gear mechanisms used  such  techniques  as  models,  
graphical construction, intuition,  trial-and-error and 
empiricism. Valve  gears   became  the   source  of   more  
argument, misunderstanding, mysticism, patent activity 
and lawsuits than any other aspect of the steam engine.

This package  is a  new chapter in this ongoing (and some 
would say obsolete) debate.  Today's robot technology 
has provided  mathematical   techniques  and   hardware   
for analysing the  kinematics of  mechanisms at  the 
computer console.

...AND FOR THE INITIATED

These programs enable you to analyse the kinematics of
some common form of valve gear.

Strictly, it is not a design package, although it can be used 
as  a design tool.  There are a number of references 
available which  explain how  to design  a  motion.  This
software then  allows you  to check  your  design  before
cutting metal.   It  is also a useful diagnostic tool for 
studying  existing motions and experimenting with 
modifications.

Its great feature is that it shows you EXACTLY what is 
happening to  the valve,  because it  calculates  on  the 
basis of  linkage geometry,  not by  assuming  sinusoidal 
motion between extremes of amplitude.  The effects of rod 
angularity, for instance, are taken into account.

Essentially, you enter the key dimensions of the 
mechanism.   You can  then have  the  motion  plotted  on 
screen for  any chosen crank angle, showing in detail the 
position of the valve relative to the ports.

The program can repeat this calculation automatically for 
one complete  rotation of the wheels, and present results 
in  the  form  of  a  plot  of  valve  motion  vs  piston 
displacement (oval diagram), or as an indicator diagram.

The facilities  exist for you to "notch up", and to check the
sensitivity  of the  motion to  displacements of  the driving  
wheels   due  to  suspension  movement  or  worn 
axleboxes.

Finally, the change in timing of valve events as the 
expansion gear can be plotted, so that you can see how 
even the beats will be.


(2). FREEWARE
==================

This package is FREE. You should not have to pay anyone for it.

In the  interests of  supporting  the  model  engineering 
fraternity,  any   correspondence  is   welcomed.      If 
sufficient feedback  occurs, further  development of  the 
package will occur, and others may be released also.

Address for correspondence:

                    Dr A K Wallace
                    Email: allanw@avocetconsulting.com.au

(If  the   above  address  does not work,  try  c/-  The  South
Australian Society of Model and Experimental Engineers).

For another valve simulation program, written independantly,
contact:
Charles J Dockstader
PO Box 3111
California Valley, CA 93453-3111



(3). INTRODUCTION
=================

Choosing  the   "best"  valve   gear  involves  making  a 
multitude of  judgements over  a wide  range of  factors, 
including such  things as robustness, ease of
manufacture and  maintenance,   sensitivity  to  wear  and  
backlash, optimum thermodynamic efficiency, expected
duty, etc.

This simulator  primarily  lets  you  examine  the  valve
events produced by the motion that you specify.  The best
way to  express this  information is  in the  form of  an 
indicator diagram.   The  steam  engine,  after  all,  is 
primarily a  machine to  expand high  pressure steam  
and extract the  maximum possible  mechanical work  per  
unit mass of  steam.  (Well, not always, because when 
starting we often  want to  extract the  maximum possible 
work per unit volume  of cylinder.)  The indicator diagram 
is most fundamental in clearly showing how we are 
extracting work from the steam.  This topic is expanded in 
the section on interpreting the output.

As  with   all  mathematical  models,  the  situation  is 
idealised in  order to keep the task down to a manageable 
size.  The main assumptions are:

1. There is no valve or piston leakage
2. No condensation occurs during the expansion
3. The expansion is adiabatic and reversible (isentropic)
4. No  pressure loss  over the  valve ports  and cylinder
passages
5. Sonic velocity in steam is infinite (no delays)
6. Steam chest and exhaust pressures are constant.

Notwithstanding  the  above  simplifications,  the  sheer
quantity of  numeric  processing  required  is  enormous.



(4). MINIMUM HARDWARE/SOFTWARE REQUIRED
==========================================

The simulator  requires an  IBM compatible  computer running
Microsoft Windows 3.11 or later.


 (5). FILES
==========

The files comprising the simulator include:

WVALVE.EXE   The simulator program file
*.L?       Data files (optional)
WVALVE.TXT   Help file (text)


(6). RUNNING THE PROGRAM
========================

The only  requirement is  that the valve.txt file
resides in  the current  directory with VALVE.EXE.

You can run the programs by double clicking on the .EXE
file in File Manager or Explorer.

There is  a model installed ready to run when the program
first starts, even before you load a data file from disk.

You can modify this freely, writing your modified version
to a  disk file  for later  retrieval if  you wish.   The version
within  the program  will not  have changed  next time you
start the program.


(7). OBTAINING HARD COPY
========================

To get  a printed  copy of  this manual,  drag the VALVE.TXT
file into an icon for Notepad or a word processor, and
print it from there.

You can capture any screens to the Clipboard for storage or
subsequent printing via Excel, Word, paintbrush etc. by pressing the
PrintScreen key, which copies the entire screen to the Clipboard.
You can then paste it into the other application.

*********HINT!**********
However, the preferred way to copy screens into other applications
is to copy to clipboard using the right mouse on most screens.

The main menu gives a direct print-to-printer option. You can change
the size on the printer by resizing any of the windows on-screen.


(8). DATA INPUT & EDITING
=========================

THE MECHANICS OF DATA INPUTTING

All input  information is  given to the simulator via the Edit
menu.  This information can be named and permanently
stored on  a disk  file for  later use. This operation is done
under the File menu.

*********HINT!**********
A shortcut to the edit screen is available by double clicking
the main window.


ENTERING A NEW VALVE GEAR

For economy  in referring  to valve  gear features,  they
have been  given letters.   You can see an annotated view
of the  gear by clicking the Annotate button on the main screen.

To enter  a new valve gear, you will need the dimensioned
drawings.   There may be minor variations in
arrangements.  If you have  this, then  you  will  have  to
live  with  an approximation.   (There is  a reasonable
chance that  an updated software version will be written to
cope with the more common variations.  Correspond with
your supplier.)

Undoubtedly there will be cases where dimensions
required by this  simulator are not explicitly shown on the
plans. You  may   have  to   do  some   auxiliary  scaling
and calculating.   This is  particularly true if the cylinder
axis is  inclined, because  the main coordinate system of
the simulator is aligned with the cylinder axis.

There are also cases where apparently critical
dimensions are not  required by  the simulator.  This
occurs because the  simulator   makes  certain
assumptions,  which  are inherent in  the proper  design of
the valve  gear.  For instance, on Walschaert's gear the
length of the union link (length UD on the annotation) is
not required.  The simulator calculates it by assuming
that the  combination lever  (TVU)  must  be vertical when
the crosshead is at mid stroke. However, if you want to
simulate an existing gear that does not conform with this
convention, you can opt to turn off the automatic assumption
and enter your particular geometry.

When entering dimensions, you  may use any system of
units, so long  as you  are consistent.  Most of the
input data is  displayed to 3 or 4 decimal places, it is
appropriate to use mm, cm, inches or feet.   You  can enter  the
data  to more decimal places,  in which case that value
will be used in the calculation,  but you  will only  see it
rounded when displayed.

When you first enter data, it is common to get invalid
indicator diagrams or chaotic plots for the event timing vs
gear plots.  This will be due to the valve events
being wrong.  For example, a steam port may not be
opening at all.  Solutions near mid gear are also prone to
fail due to no steam admission.  Also check that the
direction of rotation is correct (some simulators set this
automatically).

Note that the Walschaert's simulators will automatically set
the return crank and eccentric rod each time you edit the
input data, unless you turn off AutoSetting.  The other
simulators must be manually set up.  One method is to look at
Messages to get the extremes of valve motion, then use the
AdjustValvePosition variable.
This is best done first in mid-gear, when the valve motion
should only just crack each port.  It is not uncommon to
fine tune Adjust Valve Position to get the best result in the
most common setting of the expansion gear. Note that at the
very bottom of the Messages listing the extremes and mean
of the valve travel are given. You can correct
AdjustValvePosition by the given mean value to centralise
the valve exactly.

The recommended procedure is to get the static plot
looking right first, then get the movie working, watching for
valid valve events, then try a single indicator diagram,
and finally proceed to the family menu for timing variation
analysis.

To see the effect of wear and backlash, set the backlash
variable under Edit/Conditions.  If you have non-zero backlash,
then you must ensure continuity of rotation when entering
[View/IndicatorDiagram] or [View/Family], otherwise
the initial valve position may be wrong because it is
floating within the backlash.  A convenient way to do this
is to run [View/IndicatorDiagram] twice.
You need do this only if you have changed the direction of 
rotation or disturbed the model using DrawMotion.


(9). DATA STORAGE/RETRIEVAL
===========================

The "File"  menu item allows you to store the current
system on  disk file  and recall  or delete  a previously
stored one.





(A). INTERPRETING THE OUTPUT
==================================

The simulator  lets you  see the performance of the valve
gear in  a number  of ways.  These are  summarised
below, with the menu path shown in brackets:

     - as a motion picture [View/Movie]
     - as a static plot (default)
     - as an indicator diagram [View/Indic]
     - as  a plot  of valve travel vs piston displacement
          [View/Indic]
     -  as   a  summary  text  listing  of  valve  events
          (message window)
     - as  a detailed  listing  for  one  complete  crank
          revolution (message window)
     - as family of indicator diagrams [View/Family]
     - as a plot of event timing vs gear [View/Family] etc


The motion  picture is  simply a  repetitive static  plot with
crank  angle stepping  around.  The direction is set on the
main window. The movie can be stopped by clicking or
pressing  any key.  You will notice that backlash
is represented by lost motion between the valve rod and the valve
as shown at the top of the valve.

The Static Plot is interactive, so that you can rotate
and change gear with the slide bars and instantaneously see the
result. 

Probably the  most  direct  indicator  of  overall  valve
timing is  the indicator  diagram.   This is derived from the
port opening and closing event timings and the piston
displacement.

A number  of valve  gear design  and thermodynamics
texts explain the indicator diagram and its interpretation.
In brief, the diagram plots the cylinder pressure vertically
and the  piston  displacement  horizontally.    A  normal
diagram has five identifiable components.
     1. admission (the top horizontal line)
     2. expansion (the descending curve)
     3. release (the sudden descent to exhaust pressure)
     4. exhaust (the bottom horizontal line)
     5. compression (the rising pressure)
The area  bounded by  the cycle curve represents the
work done per cycle by the steam on the piston.  In full
gear, a locomotive  may typically  admit steam  for 85%
of the stroke, which  will produce  a large area.  For
efficient running, the  admission is much shorter and the
expansion curve is longer.  The release is therefore
smaller, which means that  less energy  is being  dumped
up  the funnel. The kinematics is such  that  the
compression generally becomes higher as the admission is
reduced.

The valve  travel plot  lets you  look inside  the  steam
chest to  see the  bobbins of  the valve  moving over the
cylinder ports.   The  edges of  the ports are the dashed
lines, and  the trajectory  of the  edges  of  the  valve
bobbins are  the solid  curves.   The program  finds  the
valve events  by looking  for the  crossing points.  This
plot is  useful to  check how evenly the valve is moving.
Clearly, the  ideal is  to have equal port openings.  The
feature of  this simulator  is that  it shows you exactly
what is happening, because the valve motion is
calculated from  the   kinematics  of   the  mechanism
without  any assumptions such as sinusoidal valve and
piston motions.

The static  plots are  useful  for  checking  your  input
geometry.   If you  set up  the hard copy carefully, this plot
can help spot clearance problems.  The upper portion of
the  screen displays  a magnified  view of  the  valve
moving over the ports.

The listings  are provided for your detailed examination.
One of  the approximations  is  that  the valve  events  are
snapped  to  the nearest  subsequent calculation  step.
This is the reason  for the recommendation to  increase
the  number of solution steps when you  are fine tuning.
Should you want super accurate cutoff information,  you
could  interpolate manually from the listed information.

The Event timing vs gear plot indicates how even the
strokes are. You can directly compare cutoff at each end
of the cylinder.


(B). WHAT DOES ONE AIM FOR?
=============================
Essentially, one  attempts to  get the  valve events even
for both ends of the cylinder and for the widest range of
gear.  Other desirable factors are that:
     - the release is as late as possible
     - the  compression returns  the  pressure  close  to
     steam chest pressure at the top of the stroke
     - the port openings are as even as possible
     - mechanical  factors, eg  minimum die  slip in  the
          expansion link,  satisfactory  compactness  and
          layout on the locomotive etc

You may  find that  you develop  a lot of respect for the
old engineers  who achieved their compromises without
the convenience and accuracy of a computer.

(C) Known Issues
(This is what Microsoft calls bugs, in my language)
none


Version 2.2 created in Jan 2006.  Added a rocker option
to Stephenson's.
Version 2.4 added variable ARy to Wal1
Version 2.5 fixed bug in Ste1
Version 2.6 added admission angles, reported cutoffs from dead centres
version 2.7 fixed hints, renamed Tailpin
version 2.8 Added Slant to Stephensons2

[*** end of valve.txt ***]
