7 1/4 Dyno car

[pault]

Hi All
Ok i have a bit of time to spare and some silly people have asked for it so here goes

Why build a dynamometer car? The answer to this question has its roots back in 1988 when I was employed by the MOD to work in diesel engine test cells. The work undertaken fell mainly into two categories, testing to asses the suitability of an engine for an application, or development of an existing engine. The engines would be mounted on a test stand and coupled to a dynamometer. As well as the services that an engine needs like fuel cooling water exhaust etc they were fitted with instrumentation to measure things like pressure, temperature, speed, movement, and flow.

The dynos used in test cell come in a number of different forms, but the ones we used were Froude water brakes. Put simply they are a large water pump which is coupled to the crankshaft of the engine being tested. It is fitted with a valve on the outlet of the pump which can be closed to increase the resistance to turning of the pump. The more the valve is closed the greater the resistance. The body of the pump is mounted so that it is free to turn, and an arm is fitted to the body which presses down onto a load cell. From the length of the arm and the load on the load cell the torque being produced can be calculated, from this and the engine speed the power being produced by the engine can be calculated. This is then expressed as brake horse power (BHP) and is the power available at the crank shaft of the engine. This allows the engine to be run under very controlled conditions. In railway terms the brake horse power produced by the engine, motors or cylinders can be significantly higher than draw bar horse power (DBHP). The power available at the drawbar is what is left of the BHP after losses in transmission of the power, moving the loco etc have been deducted.

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[pault]

The early 90’s saw me move from testing engines to providing instrumentation for the testing of complete vehicles, and while doing this I came into contact with my first dyno car, which I ended up looking after and running. It was based on a Scamell S26 heavy haulage tractor unit which had been modified with amongst other things extra cooling, a hydraulic retarder on the gear box and a Jake brake on the engine. The reason for the extra endurance braking systems is that whilst a significant draw bar load could be created with the normal foot brake it is not designed for continuous heavy braking. The heat build up in the brake drums and shoes would quite quickly result in the brakes fading to the point where they would become ineffective. Add in the wear on the brakes and the trials would be short and expensive. It also was fitted with data acquisition equipment for recording the all important speed and drawbar load. Other recording equipment could be added to record data from the test vehicle, but normally the test vehicle would have the data acquisition equipment on board. One of the most common trials carried out using the dyno car was known as towed cooling and was a test to look at the cooling of engines and transmissions of a vehicle whilst running in a sustained full power condition. The vehicle under test would be attached to the front of the dyno car by a steel cable via a load cell it would then tow the dyno car at full throttle, whilst the Jake brake and retarder were used to pull the speed down or drawbar up to a per determined value. This condition would then be held for an hour or until the trial vehicle overheated. The dyno car was capable of maintaining about 18 tons continuously on the draw bar between the two vehicles. Whilst working in this environment I often thought that it would be very interesting to instrument a miniature steam locomotive and find out what is really going on inside it.

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[pault]

Jump forward to I think it was 2008 and I saw a letter to Model Engineer about superheating, which was followed by a number of others. Most of the letters fell into one of two categories, either the subjective opinion i.e. I fitted one to my engine and it did nothing/went like a rocket, or the theoretical based mainly around assumptions . Whilst interesting what I really wanted to see was facts, and this desire reawakened the thought of instrumenting a miniature loco. With fairly modest intentions I started looking on the net for a 4 or so channel temperature logger with which to measure superheat and flue gas temperatures. What I found seemed to be a bit expensive for what was just a bit of curiosity, but I kept looking. I then found a 16 channel data logger with a built in amplifier rack on eBay, which I ending up buying. What I got was a data logging and amplifier system originally produced for the military by a company called Micromovments who I was familiar with from my time with the MOD where we used their amplifiers extensively.

Getting the system suddenly moved the goal posts for me as it would be a waste to use it just to measure a few temperatures. My thoughts then changed to building a 7 ¼” gauge dynamometer car with which we could really learn a lot. With this in mind the system was put to one side and I started to scour the net for calibration equipment, transducers and amplifier cards for the system. As I don’t have an unlimited budget this took some time.

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[pault]

At this point it may be worth me explaining the basic layout of the system. Transducers are devices which change a real world event or phenomena into an electrical signal. They come in many different forms, depending on what you want to measure, the environment it needs to work in and the sort of output you want. The ones that I have for the dyno car are as follows. To measure the drawbar pull a commercial load cell has been fitted to the draw hook of the dynocar. This gives a voltage proportional to the load applied to the cell. The voltage is created by a network of 4 strain gauges which are bonded to the load cell its self. As the load changes so does the resistance of the strain gauges giving the output voltage. Speed is measured by a home made transducer which gives 10 pulses per revolution of the wheel of the dynocar. By the way it takes 9 magnets on the disc to give 10 pulses per rev you have to think about it, well I did. Since I accurately know the diameter of the wheel the speed can then be calculated. The pulses are created by magnets on a disc passing a Hall Effect device which sends out pulses as each magnet passes. Thermocouples used for the measurement of temperature, the ones I am using are known as K type thermocouples which are a general purpose industrial device. Again they come in many shapes and sizes, but you can get them down to 0.5mm in diameter. They consist of a stainless steel or inconel outer sheath which has two dissimilar wires inside which are welded together at the tip of the probe. This creates the hot junction, of the system, the cold junction is inside the acquisition system and the difference in temperature between the two junctions creates a small voltage which is proportional to the temperature difference. Pressure transducers work in the same way as the load cell, but the strain gauges are bonded to a thin diaphragm which is exposed to the pressure being measured, again giving an output proportional to the pressure applied. I also have a flow meter to measure water used, this has a turbine which will revolve a known number of times as a given volume of water passes through the meter. The passing of each blade of the turbine triggers a Hall Effect device in a similar way to the speed transducer.

The transducers are connected to the amplifier/signal conditioning part of the system. In a lot of cases the voltage from the transducers is very small indeed so the signal is amplified to improve resolution. In other cases the signal requires some sort of conditioning to make it of value. A case in point is the speed transducer and flow meter whose outputs are just pulses, with changes in the frequency giving the information required. These are put through a frequency to DC converter which gives an output voltage proportional to frequency of the signal. The flow meter and speed transducer outputs are also put through counters to give total values.

The output from the amplifiers and signal conditioning then goes to the data recording part of the system where it is converted to engineering units, displayed on the screen and stored for later retrieval. In order to make the data accurate it is necessary to calibrate the system, whilst the system is capable of providing calibration voltages and frequencies to its own amplifiers I had a number of external bits of signal conditioning which needed calibration. I also prefer to put a calibration voltage or signal in at the transducer end of the cabling, that way any losses in the cabling are taken into account and any faults in the cabling show up. As a result a number of bits of kit like accurate voltage supplies and a frequency generator were required and found again on eBay.

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[pault]

this is the finished speed transducer

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[pault]

This is the load cell used to measure drawbar pull. That all for today more to follow if anyone is still awake

Regards
Paul

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[Deleted]

this looks very interesting Paul... I love things like this.. thanks for sharing

Pete

[jma1009]

keep going pault!

having met jim ewins years ago and avidly followed everthing he'd written, and having a particular interest too in miniature loco efficiency and reliability, i am very interested in your research... which could be the next big landmark since brian lee's indicator tests of a miniature loco and jim's discovery (seems obvious now) that miniature loco fires get as hot as fullsize.

cheers,
julian

[pault]

Ok a few pictures to start the day, first one of the K type thermocouples used to measure temperature. These are available down to very small diameters 0.5mm or 0.020” and lengths up to 2 metres off the shelf, so it is feasible to say feed one down a superheater element to find the temperature at the fire box end. They are also available incorporated things like washers and jubilee clips and various probes.

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[pault]

This is one of the pressure transducers used with an adaptor to allow a 1/8” pipe to be used to connect to the loco. Quite long pieces of pipe like pressure gauge siphons were used to isolate the transducers from the heat of the steam.

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[pault]

As most of the bits of equipment were coming together my thoughts turned to the actual vehicle that the kit was going to be put on. It was apparent that it would not be practical to fit it all to one of our coaches just when it was wanted and remove after any trials, a dedicated vehicle was needed. A non standard coach was picked to be the dyno car and work started on modifying it for the job.

The floor was cut away over what was to become the leading bogie and extra steel work welded to the frame of the coach to provide a mounting point for the load cell for measuring the drawbar pull. The load cell has a flexible mounting so that it always sees a straight pull even when going round curves. The other end of the load cell is attached to the coupling hook on the end of the coach.

An aluminum disc was made with 9 magnets embedded in it, to give 10 pulses per rev, which was fixed to one of the axles of the leading bogie. A housing was made which enclosed the disc and provided a mounting for the circuit board on which the Hall Effect devices were mounted. Because of the difficulty in getting to the Hall Effect devices two were fitted to the board so that should one fail swapping a few wires round would allow the other one to be used. The housing is mounted on the axle on ball races and an arm goes from the housing to the bogie frame to prevent the housing from turning. From memory the transducer gives a pulse every 33 ish mm that the coach travels so the distance recorded is fairly accurate.

It was decided to house the data acquisition system in an enclosure to protect it from cinders soot and oil from the loco under test. The enclosure takes the form of a box mounted on top of the coach which is not elegant but serves its purpose. Across the top of the enclosure there is a panel which has switches for selecting the power source, and powering up various bits of equipment. I was felt that there was no point in using up battery life powering up equipment that was not being used ok and as a kid I loved things with lots of switches and lights. There is also a battery voltage meter, a speed and distance meter, a flow rate and total meter for use with the water flow meter, and an event marker button. The event marker button puts a spike on one channel of the data, which can be used to mark locations on the railway or an event taking place. There is also provision to plug in a remote event marker button which the driver can use to mark an event or condition. A speed readout can also be plugged in for the driver, this is slaved off the dyno car operators speed meter. I thought I may have got a bit carried away but there is also an intercom fitted to allow the driver and dyno car operator to talk without having to shout at each other. As it turned out this was a very worthwhile feature as we found out when I left the headsets at home. Shouting to someone sitting 6 feet behind you with their head in a box over the noise of a loco working hard is not conducive to good communication

Two 12 volt batteries are mounted on the floor of the coach and they power all the equipment, there is also a mains power supply which again can power everything. The reason for having a mains power supply is that setting up and calibrating the equipment can take some time and it would be a shame to set it all up and find the batteries were flat when it came to running a trial. A battery charger is also mounted in the coach so charging the batteries is simply a case of plugging the coach in and flicking a switch.

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[pault]

Towards the end of the winter a new boiler arrived for one of our locos and the frames came into the workshop to be united with the new boiler. The loco was built in 1952 and is based on a GWR Hall class although it is slightly over scale. It has slide valves I don’t know the cylinder size but they are fairly large compared to most of our locos so they are probably around the 2 3/8” 2 1/2” mark. This was one of the engines that I had fitted with a concentric or coaxial superheater some years ago.

I have liked the design for some time, for a number of reasons. Firstly I believe that they are less likely to fail than the hairpin type. The hairpin type effectively has a hot and a cold leg, which makes them want to bend like a bimetallic strip. I have struggled to remove some radiant superheaters because they have been up against the crown of the fire box due them being bent by the unequal leg temperatures. Clearly this puts significant strain on the elements themselves and on the plumbing in the smoke box which may lead to premature failure. I have yet to see a coax superheater element bent like this, and the element cannot exert any forces caused by expansion, on anything in the smoke box. I have yet to have need to repair a coax however as most of the bits are screwed together taking one apart is easy and repairing it is easy. It is certainly easier than trying to shorten and re-weld a hair pin element when the spear point has gone. I also thought it may have some thermodynamic advantages over the hair pin style element. When you look at the area of a radiant coax element that is directly facing the fire it is significantly greater than the normal vertically stacked hair pin element. I did also make up an inner tube that was bent into a sine wave in two planes to create turbulence in the flow in the outer tube to break up the boundary layer in the outer tube however this has never been fitted.

The outer element is 20mm 316 stainless steel tube with a 7/16 copper inner tube. I sought and received the owner’s permission to modify the superheater to allow trials to be carried out. Three 1/8”BSP holes were tapped in the superheater header to allow thermocouples to be fitted to measure temperature pre superheater and the temperature of the steam coming out of two of the elements. The two tapings for measuring the steam coming out of the elements are positioned to allow a long small diameter thermocouple to be fed down the inside of the elements. The idea being to measure the steam temperature at the fire box end to see if the steam is further heated or cooled on its way back to the header.

I could not get my head round whether the steam would be heated or cooled on its way back to the header. The problem is that there is a temperature gradient as the steam travels up the outer pipe and a different one as it travels back down the inner part of the element. I argued and debated the effect of the second part of this with myself and could convince myself that the steam would continue to pick up more heat or that it would lose heat. The bottom line was that I had no idea what was happening. However if you want the maximum amount of superheat it is an important point. To start with the inner part of the elements were made from copper, my thought being that the steam would pick up more heat from the steam in the outer part of the element so increasing the overall amount of superheat. However should the steam be being cooled it would probably be better to make the inner tube from stainless steel which is a much poorer conductor of heat than copper.

The locomotive was fitted with non radiant elements which ended about 3/8” short of the fire box. Whilst this arrangement is probably not the best, when the engine was being prepared for the trials it was found that the copper inner tube of the element had been discoloured by heat for 3 to 4 inches at the fire box end so something was happening. To enable a comparison between radiant and non radiant elements one of the existing elements was replaced with a radiant element which reached almost to the back of the fire box. Two of the tapings in the superheater header are positioned so that small diameter thermocouples can be positioned just inside the end of the inner, return pipe of the element. This allows the temperature of the steam from that particular element to be measured before it enters the header. Another advantage of the coax superheater as far as testing goes is that it is a relatively easy job to remove the inner tube of the element and blank the outer one off so that the loco runs on saturated steam. This allows a direct comparison between a super heated and saturated loco to be easily carried out using the same loco. As a result the data from superheated and saturated runs will be directly comparable without having to allow for differences between two different locos.

To allow a comparison between the coaxial and hairpin elements another engine with a hairpin superheater, which was in the workshop at the same time was fitted with with thermocouple tapings and one radiant element, the rest of the elements being non radiant The loco was a WD 2-10-0 built in 1948 fitted with piston valves and from memory 2 5/16” cylinders.

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[chiptim]

Wow this was a major major project! Can't wait for the results.

I guess it's good that you were testing a steam loco because that wouldn't give off any electrical noise to mess up the readings.

Regards
Tim

[chris vine]

Hi Paul,

thank you for taking the time to put this information up. I can't believe that you haven't told us about it before, it is fascinating and amazing!

This is real engineering. Keep going....

Chris.

[pault]

Hi All
Thanks for the comments, guess that makes you lot as sad as I am. SWMBO has just arrived home having finished for Christmas so the speed of the updates will probably slow up somewhat.
Your right Tim steam locos do not suffer with electrical noise. Most of the fighting vehicles were not a problem as their electrics are heavily screened, however cars and civilian trucks could cause big problems because of the amount of noise floating around.
I had not bothered to post it because it is still work in progress, and probably will be for some significant time to come.

Regards

Paul

[peterseager]

Very good work Paul. I hope you will write it up for a Magazine when you have finished, it deserves wider circulation.

Talking about the simplest things confusing, I can't get my head around 9 magnets for 10 pulses per rev. At work we used to talk about this sort of problem by referring back to school days counting ladders and telegraph poles.

In the simplest case, my bike computer, it has one magnet which counts once per rev not twice.

The only way I can conceive of 10 counts from 9 magnets is if the first magnet is taken as producing count 1 and then, the same magnet one rev later, count 10. This shows one revolution of the wheel but is not the same as 10 counts per rev from a continuously revolving wheel. That requires 10 magnets to my way of thinking.

Peter

[pault]

Hi Peter
you are totaly correct in what you say, I was suffering from a mixture of brain fade and cross talk I will correct that.

Regards
Paul

[springbok]

Hi PaullT
Facinateing would you consider presenting a series of articles on this for ME I am sure that David C would be delighted.
Bob

[pault]

Hi Bob
I did look into it, but ME seem to make difficult rather than easy to give them articles and showed very little interest. A shame, but probably why they are struggling for articles and just reprinting old LBSC builds.
Regards
Paul

[pault]

Hi All
just a short item for today

My idea for the testing was that the locos would run a test route for 30 minutes hauling a train which was comfortably towards the top end of their respective haulage capacities. One of the problems that the dyno car suffers from is the fact that the load on the drawbar cannot be controlled. This means that a steady state cannot be maintained, as the drawbar load is created by the rolling resistance of the train, the weight of the train, and the gradient etc of the track. Some of the full size railways overcame this problem by the use of counter pressure steam engines or load bank cars to provide sustainable endurance braking. The use of these allowed the loco under test to be run at a constant power output regardless of the gradients encountered on the test route. Whilst building the dyno car I did consider adding a hydraulic or electric endurance braking system with a control system which would maintain a steady drawbar pull or speed. After considering it for some time I decided that it would be a large and costly project of questionable value so that is on the back burner for now.

Whilst running in a sustained stable condition is an important development tool it is not representative of what most if not all locomotives do when they are hauling trains. A loco may reach a stable superheat temperature after say 10 minutes of continuous running at a fairly high power output. If your loco never runs at that power output or runs continuously for that long it will never reach that temperature. As I was interested in seeing what the locos actually did I was content with running with varying power outputs for now.

With this in mind two choices of test route were available to me, one with short steep climbs and fairly long downhill sections or one with easier but much longer climbs and relatively short steep downhill sections. Whilst the short steep climbs would probably give higher power outputs it was decided that the latter option was the best. The longer climbs and shorter downhill sections give a better chance of the super heat reaching a stable condition which could be sustained. This is a much more meaningful state than that which would be found on the short steep climbs where the engine would be eased in all probability whilst the temperatures were still rising.