Steam & Excursion > Even These Steam Locomotives Were Tested From Time To Time!

During the age of steam, there were occasions that the shop forces needed to perform dynamometer testing on certain locomotives.  This often resulted in the installation of a shed on the pilot of the locomotive for the dynamometer operator to ride in while the tests were being conducted.  The full testing equipment would be contained in the dymamometer car coupled directly behind the locomotive.

We see one such application here on Southern Pacific Daylight 4-8-4 #4418.  She is parked at the Los Angelas roundhouse along with some of her newer Daylight locomotives that show the grime of their most recent runs.  #4418 appears to have been freshly shopped.  This may account for the need to put her through some testing to see if the modifications added during the shopping have increased the power she is producing.\

Back in the day, when you saw one of these sheds bolted onto a locomotive's pilot, you knew that she was undergoing testing.

Martin



Edited 3 time(s). Last edit at 04/09/20 03:37 by LoggerHogger.

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The main purpose of the wind deflector ‘shielding’ was to
Protect the guys collecting
“Indicator diagrams”...

Indicator diagrams made a ‘trace’
of the pressure developed on
One side of the piston, during a single stroke.

The indicator instruments were common in calibrating low-RPM
stationary steam plants in factories ,
etc.

Stationary steam plants were slow, smooth, low RPM one or two cylinder affairs, commonly regulated by a Corliss valve gear.

These engines typically operated at well below 100 rpms...

Locos engines ran much faster.

The indicator made a ‘plot’ on a sheet of graph paper ——- tracing
A pencil line on an X-Y axis...

The vertical (X) axis was piped to the front cylinder head...so that the pressure would be relayed to a small cylinder on the indicator instrument.... so the pencil moved
Vertically, up and down the rotating
Drum...

The drum contained a single sheet of graph paper.... the drum was internally spring loaded to its resting
Position... it was connected by a string to the crosshead and piston rod of the engine. As the engine was running, the drum rotated a partial ‘turn’, then revolved back to its resting position as the piston reciprocated in it’s cylinder...

The left and right rotation continued as the piston travelled in the cylinder... turning a single sheet of graph paper...

The pencil trace jumped up and down,vertically,
replicating the pressure at the cylinder head...

The test engineers would trip the pencil holder to draw a single-line Trace on the graph.... at the end of the piston’s Travel the pencil was automatically lifted from the drum & paper- but still jumping up and down with the cylinder pressure. The single-line Trace happens in milliseconds..

The drum continued to oscillate as the crosshead reciprocated... but a single trace was the only line on that test sheet...

The loco test crews, riding in the housing ( to protect them from the wind), were tasked with taking multiple tracings from the indicator
during the test runs..

In stationary plant use ( factories) with a steady, typical ‘plant load’
( like plant powering multiple
weaving looms, etc) RPMs were
Low to modest speeds and the indicator diagrams’ tracings’
resembled the profile of a high-top
‘Sneaker’ ( shoe).. from the heel of the shoe to the top (ankle covering)
pressure trace was a vertical rise
reflecting the steam admitted to the piston face— as the drum began to rotate ( powered by the piston/crosshead) the trace would
Stay at the maximum pressure for a portion of the stroke—- until the intake valve rings ‘cut-off’ the boiler steam from continuing to push on the piston...

The piston was still under the trapped pressure in the cylinder...
But the pressure would decrease as the cylinder volume expanded during the single stroke...

During the ‘expansion phase’, the drop in the cylinder pressure would follow the laces of our sneaker simile, and would drop towards the sneaker toe.. as the piston approached the end of the stroke.
At the end of the stroke, the valve’s
Exhaust rings open the port ( to the stack) and the cylinder pressure drops to its lowest level...

The resulting pencil trace looks like the outline of a sneaker, in profile..

The entire area surrounded by the trace corresponds to the power delivered to end, of one cylinder, for one stroke... a drafting device called a ‘planimeter’ would calculate
Total area enclosed by the “sneaker trace”.... That calculated area together with the axle RPMs can generate the total HP developed at the selected cylinder pressure...

On locomotives, the sneaker-like traces were readable up to about 25-mph... above that axle speeds,
The traces were virtually squiggly lines on the graph paper...

HP readings generated from those
Tracings were functionally un-readable...

“String and pencil” Indicator diagrams in modern modern locos don’t tell you very much, or don’t really tell to the power in the cylinders...

The Dynamometer car will give you
The strain on the car’s draft gear, and the track speed will give you
‘Time’ .... so you can calculate the
Gross HP from the performance..

Don’t get a mystified about locomotive valve gear indicator
Diagrams —- they’re way too inaccurate to be relied upon.

In today’s world of electronic testing, much more accurate
readings could be obtained ...

From all four piston volumes...
Up to the maximum RPMs of
500 to 600 revs... After that, you’ll
Be in the ditch..

W.

(Not proofed, yet)

Posted from iPhone



Edited 5 time(s). Last edit at 04/09/20 07:15 by wcamp1472.

Thanks Wes. Interesting lesson. With a locomotive with 73 inch drivers approx. how many strokes/minute would the pistons be making at 80 mph?? 

  Thanks, Ross Rowland 

80 mph = 368.55 rpm with 73"drivers

Each complete revolution equals
2 strokes per piston.....
Out AND Back..

So, that speed would be about
6.14 driver revs per second😃

There’s four power strokes per revolution—- so, at that speed,
The sound at the stack is a steady
ROAR...

At such speeds, it’s superheating
that gets you there, not the specific boiler operating pressure... and with a decent weight train, your throttle would probably not be fully wide-open, you’d be running on a short cutoff, too.


80” inch drivers would be a little fewer strokes..

W.

Posted from iPhone



Edited 4 time(s). Last edit at 04/09/20 06:36 by wcamp1472.

On a modern, computerized
‘Locomotive indicator Trace’...
on a modern, oscilloscope-like
tracing device, you would have NO
challenges with, strings, pencils and
Paper.... the electronic systems would have much greater frequency capabilities than the “slow” events of steamer’s valve gears....


The Pressure/Volume Trace
would be an angled oval-like
Trace, with very little width between
the ‘power’ portion of the the stroke and the back pressure trace—- from trying to squeeze out through the narrowed Exhaust ports...
( you would expect to see high
back pressure on the pistons,
but a back-pressure gauge at the exhaust nozzle {smokebox} might
Not give you the true BP at the piston face)

As you squeeze off the intake ports,
You also reduce the size of the exhaust port openings 😡

Poppet valves don’t have that problem... poppet exhaust valves
are separate from the intake ports...
and the exhaust valves are larger diameter.. 😃

The net result is that poppets get
You a wider “oval” on your P/V diagram... The wider the P/V oval,
The more power into the train..

W...

Posted from iPhone



Edited 4 time(s). Last edit at 04/09/20 07:23 by wcamp1472.

You guys amaze me with all of the detail of steam ops that you can muster.  The average person looking at a steam engine would likely think that it is just a big steam kettle, so to speak.  For those more familiar, the science of steam locomotion seems every bit as complex and sophisticated as anything in our "modern" world of transportation.

Greg Stadter
Phoenix, AZ

Put a crescent moon on it and you'd have the first train crew toilet on a SP steam locomotive.

For anyone who wants an in-depth discussion of indicator measurements on steam locomotives, I recommend reading the book Locomotive Performance, by Williams Goss.  It is available for free reading or downloading from Google Books, as the copyright has expired.  At the time, Goss was Dean of the Schools of Engineering, Purdue University.

All of the test data was acquired during stationary testing of a locomotive on a dynamometer, or test plant, located on the Purdue campus.

The book presents a discussion of the problem of measurement inaccuracy introduced by a long pipe required during road measurements with indicator instruments.  They did this by using two setups, one for reference with a very short connection from cylinder to instrument and another with a longer pipe of different lengths, simulating what might be used in a cabin mounted on the pilot beam.  This was done at different speeds and valve cutoffs.  The artificial phase lag introduced by some pipe lengths resulted in indicator diagrams distorted in such a way as to overstate power outputs, particularly at higher speeds. 

These results are cited by Ralph Johnson in his 1944 book, The Steam Locomotive.

This effect may have resulted in ALCO's ambitious claims of indicated horsepower, such as 6680 HP for a Niagara at 85mph, while the same locomotive produced only 4600 drawbar horsepower at the same speed.  (Thanks to poster sgriggs for alerting me to this apparent inconsistency).

A sample of the data from Locomotive Performance is shown below.
 

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This effect may have resulted in ALCO's ambitious
> claims of indicated horsepower, such as 6680 HP
> for a Niagara at 85mph,

The "operative words" are "indicated horsepower" ...

Remember this means results are from an engine 'indicator device"...the "string and pencil rig"..
Any trace obtained at 6 or more revs per second ....is totally unreadable from a rigorous 
mathematical perspective.

DO NOT PAY ANY ATTENTION WHEN YOU SEE THE WORDS "INDICATED HP"...

Those figures are pure 'advertising';  whereas, the numbers generated from those devices are
the results of actual measurements.....

W.
 

Wes,

The Purdue indicated power data measured with the instrument closely coupled to the cylinder actually looks quite believable.  In the table below, you can see for a particular run, indicated horsepower was about 12% higher than the dynamometer horsepower (which is equivalent to drawbar horsepower, neglecting the effect of locomotive aerodynamic drag).  Also, looking at multiple test runs in Table XXV, the difference between indicated and dynamometer horsepower is in the range of 5% to 18%.  That seems plausible to me for friction losses in the locomotive and the dynamometer machinery.

The obvious problem with indicated horsepower in the Purdue tests is with the long-pipe setup simulating road measurements, where the frequency response is corrupted.

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The SP test report from GS3 locomotive 4418 exists and may be downloaded from the files section of the Facebook group Steam Locomotive Builders and Users, Documents and Procedures.  I quickly skimmed through the summary of tests and it doesn't look like the railroad took indicator readings from the 4418 during the tests,  They did measure drawbar pull and horsepower with a dynamometer car, as well as boiler, back pressure, steam chest pressure, fuel and water consumption, steam chest and exhaust temperatures, smokebox gas composition,, etc.  It's been a while since I reviewed these SP test reports in any detail, but as I recall the engines were being operated well below their capability, with the GS3 locomotives making less than 4000 hp while hauling their Daylight trains,  I've tried to attach a link to the GS3 report below. If it doesn't work, I encourage anyone interested to join the Facebook group.  They have a lot of interesting stuff in addition to the SP tests on locomotive and burner performance.

Scott Griggs
Louisville, KY

https://lookaside.fbsbx.com/file/Southern%20Pacific%20Test8%20%20GS-3.PDF?token=AWzimkgEwDzB0kMWbIzBoMqkslrSZs3px2OA5dfexsgEvwWtViXV0tred3Lz85kchXmjL1OcmHsDYDjB7B2fOyQjBZ_0j2D0qevcYVCE-KPfKdWKkBfm-wd5gqOEdrWeaKem0_IOGGJXBiTL5LJKSXvt_Fx-MqS_cLqAIA_Ss99i2EMdKq6y6eGSSpGEhjlAkDEDdyt8zxajifyGuJUGmSbjeWCH1rv7-WrmgMbge7Xs4g

Scott,

Thanks for digging up this record of the test.  Wonderful information to go with the photo.

Martin

The trouble with indicator diagrams is that some can be trusted, others can’t. Hundreds of Indicator tests were done on the Altoona plant a hundred years ago now. Most all results look perfectly sensible, and if you analyse them against a modern computer programme, they largely are. Looking at the diagrams, you do wonder how this can be- but it is so! Well done Altoona. Oddball results include a couple of short cut off/high speed runs on the K4 where engine efficiency- generally not very good- shot up to best in class- obviously not true. Such short cut off at 75-85mph diagrams are very ‘thin’ and accurate area measurement can be difficult, a problem which beset United Kingdom test results even in the 1950s- results up to 15% out.  One proud and successful company there who believed they were best at everything had indicators which magically proved (wrongly) they had top efficiency in this regime.

The above largely refers to indicators of the older ‘mechanical’ type. By the 1940s, the PRR- and others in the USA I suppose, and to my knowledge, France, were experimenting with electronic devices which were more reliable. So as WCamp1472  says reliable ‘indicating’ should not be a problem today, though since indicator diagrams can now be computed to within reasonable experimental accuracy, this is not so much of a need.

Looking at the NYC test report, there’s no doubt that the Niagara did develop something like 6600IHP on test, as did the PRR T1 at Altoona. The PRR Q2 developed nearly 8000IHP. Analysing the behaviour of the engine says this is not at all surprising- the issue is whether the boiler can continue to provide the steam! However, such figures are far removed from what the locos would be expected and able to deliver on a daily basis. The NYC said 4000IHP would be ‘above average passenger working’ for a Niagara. So some claims, whilst true, take you into fantasy land when it comes to daily performance.

Indicating is fundamental to understanding the power and efficiency of an engine. Even 100 years ago, indicating was a perfectly respectable technique in good hands. There may be some wrong results around – that is, more than a few percent out- and some extreme unrepresentative results may have been used for publicity purposes, but nowadays we can work out what is and is not possible, and what is seriously wrong.  
 

Thank you for sharing the link to that report Scott. That group on Facebook has many good reports, including many from the SP.