7 1/4 Dyno car

[peterseager]

EIM seems to run this sort of technical article.

Peter

[chris vine]

Hi Paul,

Were your two test routes obtained by running in different directions round the same track?!

Chris.

[pault]

Hi Chris
No, the GCR has a number of different routes which can be run continuously, however my choices were made by joining up gradients to produce the longest climb or the steepest. It’s a bit difficult to describe the layout, I will try and find a track plan to post.

Regards
Paul

[ejparrott]

Dec 22, 2012 11:26:50 GMT peterseager said:

EIM seems to run this sort of technical article.

Peter

Its the better bet

[pault]

Hi All
here is todays offering
No attempt has been made to measure what is sometimes called “negative draw bar” or in other words the work done by the train pushing the loco on the downhill sections. I have seen it stated that the “negative draw bar” should be taken away from the work done by the loco. My view on this is that it is not relevant in this context for the following reasons. If you start a run by climbing a hill and then the route is a mixture of up and down hill sections the energy in the train which it uses to push the loco has been put into the train by the loco when it pulled the train to the top of each hill. If you take this to an extreme, if the gradients were right you could end up saying that the loco had done little or no work whilst hauling the train which clearly is not the case. What I am interested in is the work done by the loco, after all that is what is being tested.

In the same way I have not made any attempt to come up with coal consumption figures during the trials. One of the main reasons behind the trials was to come up with facts and I feel that the measurement of coal consumption is too subjective to be called fact. At the start of a run you have an amount of coal burning in the fire box, but how much heat energy does the coal have left to give up? During the run you put a known weight of coal on the fire but at the end of the run again how much heat energy is left in the coal? No matter how much care is taken with regard to the depth of the fire and how burned through it is we cannot be sure about the energy released. Remember on our locos a half hour run may only use a pound or two of coal which is added in say three or four rounds. It’s not hard to see how a significant error could occur. When full size locos were tested it was done over a period of hours using tons of coal, so expressed as a percentage of the coal used, the remaining coal/energy in the fire box would be far less than on a miniature loco.

It this point it is probably worth mentioning that my background is a practical one and my knowledge of thermodynamics and a lot of the laws of physics can be written on the back of a rather small postage stamp. As a result some of my analysis, terms or theories maybe overly simplistic or I may miss something blindingly obvious to someone with a more theoretical knowledge. That said I am happy to make the raw data available should anyone want to do a more comprehensive analysis, if someone could give me a place to post it.

The first loco to be tested was the GWR Hall, fitted with the coax superheaters. It should be noted that this loco was fitted with a new boiler during the last winter and had been steamed about eight times before the trials took place, so as far as heat transfer goes this boiler is about as efficient as it can be. Before the trial the tubes, superheater flues and the inside of the fire box were cleaned as much as possible bearing in mind the limited access available to some areas. The instrumentation was fitted as listed below.



Item Parameter
1 Speed
2 Drawbar
3 Temp pre superheater
4 Temp post superheater non radiant element
5 Temp post superheater radiant element
6 Cylinder bock temp
7 Gas temp fire box end of flue tube
8 Gas temp smoke box end of flue tube
9 Exhaust temp
10 Water used
11 Event marker
12 Boiler pressure
13 Steam chest pressure
14 Distance travelled

For those that know the Great Cockcrow the test route was Hardwick to Everglades via the down
main (first run only), then repeated as many times as required Everglades, Cockcrow Hill, Green Lane, Everglades, Down Spur, Everglades. As the Loco has a pole reverser it was driven on the regulator, pulled back one notch from full forward gear for all of the trial running. This was to try and remove a variable in the way the loco was being driven.

The data acquisition system was set up to log each channel once a second. Since the changes in temperatures happens relatively slowly, and ideally we are looking for stable conditions, logging the data any quicker would just result in much larger data files with no improvement in the quality of the data.

Speed and drawbar pull were measured by the dynocar transducers. Temperature pre superheat was measured by a thermocouple fitted into the wet header. Temperature post superheater radiant/non radiant was measured by small dia thermocouples just entered into the ends of the inner tubes where they entered the dry header. Cylinder temperature was measured by a washer style thermocouple mounted on one of the rear cylinder cover studs, so it was not the true cylinder block temperature but still of interest. Item 7 the gas temp at the fire box end was measured by a thermocouple which ran down one of the tubes in the center of the top row of tubes and protruded into the fire box by about 5mm. Similarly the gas temp at the smoke box end was measured by a small thermocouple set in the gas flow at the smoke box end. Exhaust temperature, this is slightly compromised by less than ideal fitting, but was measured by a very small dia thermocouple fed down the blast pipe to a location just above where the two branches of the exhaust join together. Water used was measured by the turbine flow meter which was in the water feed to one of the injectors. A cable tie was used to mark a suitable level on the gauge glass and the water was raised to this mark before the run started. The runs finished at the same point, and the water level in the boiler was brought back up to the same mark. The total used could then be read of the total meter on the dynocar. Boiler pressure was measured by a pressure transducer connected to the top nut of one of the clacks. This clack associated injector could be used but were not during the running, as the flow meter was on the other injector. Steam chest pressure is not strictly the correct term as the pressure was measured at the wet header via the snifting valve connection.

[steam4ian]

Paul

I suggest you contact Allan Wallace, search Avocet Engineering.

Allan has a dyno car which we use for efficiency trials at our club. Allan generally wins. A reason for Allan's success is that he places a somewhat consistent load on the engine by keeping the riding car brakes applied continuously once the train is in motion. As a professional engineer Allan applies a scientific appraoch to the testing and optimises the working of his loco, not all the drivers do this.

One problem with efficiency trials is those locos with ride on tenders; the work done hauling the driver is not included because the load cell coupling is behind the driver.

Surely an electric loco could be modified for dynamic braking to act as a load; a 4QD controller can regenerate.

Good luck
Regards
Ian

[pault]

Hi Ian,
Strangely enough I had email contact with Alan about 12 years ago regarding his valve gear simulator and an inside valve gear outside cylinder arrangement I was designing at the time.
You are right an electric loco can provide braking however there are two main problems. The first is heat dissipation, or battery state. “Regen” in normal use as a brake is not used continuously as a result temperatures drop when regen is not in use/ batteries are used and so discharge. If you use regen as an endurance brake (continuously) problems will occur with heat build up somewhere, and the results will probably not be good.
The other problem is control, ideally you want a control system which will maintain a condition, normally speed or drawbar. When working with a diesel engine vehicle on flat ground the load applied by the dynocar is fairly fixed. When working with a steam loco running over a hilly route the load applied by the motors will vary from very little when running up hill to fairly high when running down hill. Manual control is generally not very satisfactory.
Regards
Paul

[pault]

Just to recap quickly the purpose of the runs was to commission the dynocar and start learning what goes on in a loco hauling a real load on a real railway, this is as opposed to a loco running under controlled conditions or a test rig simulating part of a loco. The main area of interest at this stage was superheating. The loco being tested had a coax superheater fitted with 3 non radiant elements and one radiant element.

There were a number of issues with the early runs, mainly due to work overload and brain fade. In order to get uninterrupted running we ran during the week with me driving and my son as dynocar operator. Unfortunately as my son is not a “railway” person and there was only the two of us there I had to think about everything from unlocking, getting the test load in the right place, checking the calibrations, fixing issues, steaming the loco, setting the road for the test running, driving and instructing my son on what he was doing. As a result especially on the first runs things were forgotten. Which did leave some holes in data for instance when the water total meter was not zeroed at the start of the run. When the wrong route was set, and we had to reveres to reset the route.

The next two days we attempted to run were abandoned due to rain. As a lot of you will know aluminium rail becomes like ice when it rains, there is no point in attempting to run in those conditions. The last day we ran was on the first day of our gala weekend so we were there at 6 am to get some running in before the hordes arrived. Surprisingly enough people started to get out on the track by 9.30 so any hope of running nonstop went out of the window so we gave up at that point.

The first run was a very slow one at about 2 ½ to 3 mph, which is about as slow as I could run sensibly. The route was from Everglades, down the Spur which is steeply downhill and then a steady climb up Piggery back to Everglades. The steam chest pressure gives the best indication of how the loco was being worked.

The data from the runs was pasted into a rather large spreadsheet which gave a basic analysis, tabulated highs, lows and averages, graphs and some other numbers. Since the main focus of these runs was to determine what a coax superheater does I was interested in trying to quantify what the improvement in efficiency is. I reasoned that there must be a formula which was easy to find which would say something like a saturated locomotive will use X % less water if it is given Z degrees of superheat. I Emailed lots of people, posted on forums, brought 1920’s books by the Superheater Company off E Bay, and started reading up on thermodynamic. Nowhere could I find a simple formula which would give me what I wanted.

As I read I learned and it became apparent that what I was looking for would be an inaccurate simplification because it took no account of condensation particularly if you were talking about a saturated loco. However I was keen to get some feel for what sort of economies were possible. As a result part of the spreadsheet looks at the increases in volume and heat energy created by the superheat temperatures and compares that to the saturated steam values. My thought was that even although the figures are certainly not 100% accurate they would be of value for comparative purposes. When I looked at the figures that it came up with they seemed reasonable if a little on the high side.

Estimated Water Savings for first run
Saving Using Non Radiant Superheater Over Saturated 9.93 %
Saving Using Radiant Superheater Over Saturated 30.35%
Saving Using Radiant Superheater Over Non Radiant 20.41%

Highs lows and averages for first run.
Parameter
Temperature Post Superheater Non Radiant (°C) Max 196.46 Min125.89
Average172.33

Amount Of Superheat Non Radiant (°C) Max 93.19 Min -1.11
Average 42.41

Temperature Post Superheater Radiant (° C) Max 263.86 Min 168.62 Average 229.77

Amount Of Superheat Radiant (°C) Max 155.71 Min 30.78
Average 99.85

Temperature Pre Superheater (°C) Max 166.79 Min 90.60
Average 134.94

Temperature cylinder Block (°C) Max 122.34 Min 80.83
Average 109.25

Temperature Exhaust (°C) Max 117.95 Min 87.77
Average 98.88

Temperature Firebox (°C) Max 952.08 Min 651.71
Average 826.47

Temperature Smokebox (°C) Max 410.70 Min 194.43
Average 310.43

Boiler Pressure (bar) Max 6.06 Min 5.08 Average 5.56

Pressure Steam Chest (bar) Max 4.68 Min 0.00 Average 1.94

Drawbar (n) Max 646.75 Min 0.00 Average 197.19

Speed (km/h) Max 7.40 Min 0.00 Average 3.86

Drawbar Power (w) Max 619.95 Min 0.00 Average 215.67

“Amount of Superheat” is the temperature of the steam leaving an element minus the temperature of the steam entering the element.
The traces for speed and drawbar pull are quite “peaky” as I was trying to maintain a continuous low speed which was not easy, requiring constant adjustments of the regulator. The graph posted below is plotted from the raw data and is not easy to decipher.

Attachments:

[pault]

The following one is “smoothed” using a 6 point moving average. This shows the trends more easily however the maximum values on the graph do not agree with the tabulated maximums as a result of the smoothing. The graphs I post may or may not be smoothed depending on what shows the data most effectively.

Attachments:

[pault]

Reading up about coax superheaters (thanks Julian) and from some peoples comments it would seem that the general consensus is that the steam should go down the inner pipe and return to the header via the outer pipe. Long before reading these things I decided that the easiest way from the manufacturing point of view was for the steam to go via the outer tube and return up the inner tube, in other words the opposite way. This does improve efficiency as it makes it contra flow in both directions.

From the graph below it can be seen that both element types do create superheat and unsurprisingly the radiant element does produce significantly more. Interestingly the amount of superheat seems to be relatively independent of most things that you would think would influence the temperature. It can be also be seen that the superheat temperatures rise and stabilise over a period of about 3 ½ minuets. This is probably due to the fact the elements are relatively thick walled and so take some time to warm up. The fact that they have thick walls mean that they will also be able to give up heat for a period of time

Attachments:

[pault]

I have included steam chest pressure on most graphs to illustrate how the engine was working. The graph below shows the gas temps and boiler pressure

Attachments:

[pault]

this one just shows the cylinder block and exhaust temperature

Attachments:

[pault]

last graph from the first run which shows the superheat temps along with the gas temps and steam chest pressure.

taking a breather and will then sort the next higher speed run.

Attachments:

[uuu]

This is fascinating stuff. I had begun to wonder if I should change the design of my loco from coaxial superheatrrs to the hairpin type, but this analysis is demonstrating that these ones are working sucessfully.

I look forward to the next instalment.

[alanstepney]

It might be worth comparing the actual results, with those from (the late Prof) Bill Halls software.
Bill spent a lot of time producing it, based upon his and others model loco's, plus the results from some full size locos.

[chris vine]

Hi Paul,

This is a most fascinating thread, thank you!!

You mention about the direction of the steam in the coax superheater being contra-flow in both directions. In a way this is true, but not in the sense normally meant with a heat exchanger. In your case, the steam returning up the centre tube is already heated to an extent, but as it goes back up the centre tube (towards the front of the engine) it is passing steam on the outside which is cooler and cooler, the further it goes. So this is not contra-flow.

I think it is this reason that some people have wondered about the coax type of superheater. However they have just wondered while you have measured!!

Of course the Hairpin type is also contra flow in one direction and not in the return. However it all depends on the temperatures of the steam in the tube and the flue gasses around it. If the flue gas at the smoke box end is still hotter than the steam in the pipe, then you are still adding heat!

Can't wait to see what you discover...

Chris.

[pault]

Hi Chris
you make a good point and it was why i could not get my head round what the best material would be for the inner tube, in the end I went for copper, but if the steam is being significantly cooled on its way back to the header then stainless may be a better material to use, we shall see

Regards
Paul

[jma1009]

absolutely fascinating, paul, and you are to be congratulated on your efforts.

the late jim ewins carried out a very extensive series of testing of his famous 5"g 0-6-2T in 1965. the results were published in the SMEE journal, and later in ME in march 1966.

paul's results posted so far have some interesting comparisons. generally they are in line with jim's results.

one might expect paul's testing of a large 7.25"g loco to show results closer to fullsize than jim's.

however, this isnt the case! whether this can be attributed to jim's loco being generally a more effecient unit, or further benefitting from the cube laws that govern many aspects of miniature locos i cant say. however having driven jim's loco some years ago when it was mechanically a bit rough it was still a cracking loco and did everthing you would expect of a loco built by jim.

in particular the firebed temperature of paul's loco is lower (poorer coal or a grate not being pressed to it's optimum or incomplete combustion or firehole door left open?).

the smokebox temperature is surprising high...much higher than on jim's loco...nearly double... perhaps indicating a tube arrangement that isnt making the most of the available heat transfer of the flue gasses.

the superheated steam temperature measured by paul is also approximately half that obtained by jim with radiant superheaters of return bend type. this would support jim's theory that co-axial superheaters are not as efficient as return bend superheaters. (my personal view is that jim went a bit overboard with his radiant superheaters, as there are other considerations to be taken into account when considering the amount of superheat advisable in miniature, but there are obvious reasons why a radiant double element is more efficient than a radiant co-axial superheater).

cylinder temperature is about the same.

paul's exhaust temperature is lower. this is inline with his other results.

why does any of this matter? consider a car that doesnt have 4th of 5th gear. this is the effect of using 'wet' steam in the cylinders of a steam locomotive. if the valvegear is well designed, made, and set, to allow expansive working with a short cut off then 'hot' steam and steam which has been considerably increased in volume by superheat can work expansively. it also benefits in miniature from being more fluid and able to negociate sometimes restricted ports and passageways. it also makes life a lot easier for the driver as a result of the efficiency savings as the injectors dont have to put on so often, and the fire burns less coal.

paul's very high smokebox temperature is a bit beyond me to explain... perhaps you could comment please paul!

cheers,
julian

[pault]

Hi Julian
I will try and answer your questions. The first thing to remember is that the data posted so far was from a low speed run that averaged about 3 ½ mph, the engine was not being worked very hard.
The “fire bed” temperature is not really fire bed but flue gas temp. The thermocouple was located in the centre, top row fire tube, poking out of the tube into the fire box by about 5mm. The idea was that this would be the temperature of the gases entering the superheater flue. This would probably give a significantly different temperature to a thermocouple mounted in contact with the fire.
The Smokebox temperature was measured at the Smokebox end of one of the top row fire tubes with end of the thermocouple being roughly central to the tube. Do you know where Jim measured his smoke box temperature? I would imagine that there can be significant differences in temperature within the smoke box. I do know other people who have measured what comes out of the chimney with infrared thermometers and found it to be in the 200/250 degree C range. From that it would appear that the exhaust steam is being significantly heated by the flue gases (which is being cooled) which would point to them being somewhat higher in temperature than the 200/250 range. The boiler on the loco was a new one which had only been steamed about 8 times since it had been fitted so heat transfer through the tubes would be about as good as it could be for that boiler. Next time I get the chance I will have a measure up of the tubes etc and put them through Jims? Software.
Interestingly when you look at the BR report on the static testing done on a Merchant Navy at Rugby some of the figures are very close to mine. Depending on the firing rate the fire box temperature ranged between 843 deg C and 1038 deg C. The Smokebox temperature ranged from 304 deg C to 421 deg C at the exit of the small tubes and between 293 deg C and 382 deg C at the exit from the large tubes. The superheat was between 48 degrees C at the low end of cylinder steam consumption rising to 143 degrees C as the steam consumption increased. I have to say I was surprised at how little superheat was being created, however when you look at the steam temperatures going into the cylinders (up to 371 deg C) I guess that they did not want much more then again it was only the Southern Railway (takes cover)
Why does any of this matter, I just wanted to know the answers cos I’m a curious sort of person. It will be interesting to see what comes out of the higher speed runs although I don’t think that there were any dramatic differences.

regards

Paul

[jma1009]

hi paul,

many thanks for the further info. i will dig out my copy of jim ewins' test results again and try and reply to the specific points you've raised.

i should add that at this stage i quoted the lower of jim's results when his loco was tested without a load... quessing that perhaps you still had something 'up your sleeve' so to speak!

many onlookers (and model engineers) are fascinated why a miniature loco can perform as well as a fullsize loco, with scale cylinders and externally a scale boiler, yet operating at a much lower boiler pressure. part of the answer lies in the results paul and jim ewins discovered.

cheers,
julian