Dynamometer

I'm in the process of building a Dynamometer for testing loco's. Broadly following on from the WDMES "Pullometer" articles published in ME a few years back. Using strain gauges from cheap digital scales.

I have a question regarding the load measurement. As well as recording the pull on the train but also recording the push from the train on downhill sections. I've had it argued to me to only record the pull. I'm currently doing both push and pull with push and distance being recorded as negative work.

What is 'normal' practise, if there is such a thing?

Pete

As one of the team behind the Warrington pullometer can I point out that as currently set up it won't record push. Sounds as though you've altered the circuit could so that it does. Could you send me a copy of the circuit diagram?

Having said that unless there is some way that a loco can store the enrgy from being pushed for use later, it does seem sensible to only record pulling. On our track which has 1:80 gradient, you don't get a lot of push in any case. One doubt is that if you had a spring loaded drawbar you could get some kind of oscillation set up where the loco was doing a little work, but the dyno would record a lot. I've noticed that with lightly loaded trains there is an oscillation in the drawbar making it rapidly go tight/slack. It's one of those things that I meant to get round to investigating, hopefully Doubletop is going to do it for me!

Hi Duncan

You had sent me the details of the WDMES version some time back. Actually the only thing mine has in common is the use of the strain gauges from cheap scales which was really the catalyst for me making start on my project.

I'm using dual strain gauges and INA126 op amps and a single PIC micro controller with bluetooth connection to a mobile phone or tablet for logging and display. Dual strain gauges as its for 7.25"gauge and 50Kg or 110lbs didn't seem enough as the theoretical drawbar pull on my Dart is around 76lbs and some of our club locos are way larger. Maybe I could have used one strain gauge but erred on the side of caution.

The main thing is the whole lot will eventually go into a single box on the drawbar with only the wheel sensor connection to it and a single on/off switch. The whole thing only draws 65ma so battery power in the box is also feasible.

Early days yet but I've had a prototype running. The only issue at the moment is the mobile phone is just about unreadable in bright sunlight.

You've touched on the reason I'm asking this. Going up the hill the train is storing energy from the loco and feeding back to the loco on the way down. Do we ignore this energy transfer or record it?

Pete

I think it would depend on how accurate you want the result to be, and whether the refinement is justified given the other sophistications that would be needed to maintain accuracy. (For example, Duncan's comment about oscillation is interesting.)

One problem with a dynamometer towed by an engine is the track. Curves, up gradients, down gradients, track condition, and points will all have an effect that could be allowed for.

I'm not sure it's worth it though, and in the case of running round a loop the effect of uphill and downhill sections would tend to cancel out.

If the test were run on a straight track it might be easier to survey the rise and fall of the track and use the data apply a correction to the dynamometer results.

The GWR had a rolling road dynamometer at Swindon for which they claimed great things. The rolling road made it possible to eliminate most track variables to get more accurate results. It's not the whole story though, although a rolling road gives cleaner data, it is still useful to measure locomotive performance on a real journey where a wider range of conditions might show something else up.

With a steam engine, the fuel, skill of the stoker, and skill of the driver are an awkward set of variables. You might have fixed the track and built the ideal dynamometer only to find that the driver is a badly hungover clown. (Surely not!)

Analysing the reasons for wanting a dynamometer in the first place might help. For example:

• A requirement to measure output and efficiency for design purposes implies good accuracy for which a standardised track or rolling road might be essential
• A dynamometer that can be pulled by any engine on any track at any time and still give accurate results at all times is a worthy challenge.
• Comparing the power output of two engines or two runs of the same engine on the same track is much less demanding.

Please let us know how you get on. It's an interesting subject.

Cheers,

Dave

PS Meant to say, if you can easily capture both pull and push data you might as well.  Post-processing would let you ignore it or take it into account, or both. 

Edited By SillyOldDuffer on 30/08/2016 12:08:58

Edited By SillyOldDuffer on 30/08/2016 12:12:59

Edited By SillyOldDuffer on 30/08/2016 12:13:50

I knew I'd sent it to someone!

Dual strain gauges is interesting. Being a clanky I'd have used a second order lever, then by moving the connection to the strain gauge you could change the fsd, but I've no doubt yours will work.

If we assume that the train is more free running than the loco, when it is going downhill, the train pushes the loco, and so reduces the amount of steam the loco needs to progress down the hill, but I don't think that affects the argument, it has to use this steam and more to get the loco back up the hill. Even if the loco gains speed, and so stores kinetic energy, when you get to a flat or uphill, the rolling resistance of the loco will not allow this KE to be used to do drawbar work. With the exception of oscillation, I think you should just measure pull, or preferably measure both for information but just use pull in the calculation of work done, after all the purpose of the loco is to pull

A rolling road where you can apply some resistance and measure pull as described by Dave is on the bucket list, but so are lots of other things

I can't resist a thought experiment.

Say the test track is 1000m long from start to finish and rises 10m over that distance.

If the engine pulls the dynamometer and train up the incline, the engine will have to:

1. overcome the rolling resistance of the train,
2. lift the mass of the train 10m up,
3. lift the mass of the engine itself up 10m.

I suggest that because the dynamometer measures drawbar pull it won't 'see' the work done by the engine lifting itself up the incline. If that's true (?) then the dynamometer will read low because part of the total work being done by the engine is invisible to the dynamometer.

Moving on, what happens when the engine pulls the train down the same incline?

Once again the engine has to overcome the rolling resistance of the train but, as seen by the drawbar, the rolling resistance will be apparently lower. This is because the train now has 10 metres worth of potential energy available. I suggest that in this case the dynamometer reads true but we get a result lower than the engine is actually capable of.

An extreme downhill scenario is that the potential energy of the train is high enough to push the engine down the track with the engine's wheels skidding. Dynamometer readings wouldn't mean much if that happened.

Going back to my "is that true" question, I remember getting confused at school about 'every force has an equal and opposite reaction' when applied to a train or horse and cart in motion. I'm very happy to be corrected if my assumption about what the drawbar 'sees' is wrong, which it could well be.

A nice level track would eliminate these questions. How sad that we live in the real world and have to work with what we've got!

Dave

As I suspected it’s a bit of a conundrum with no clear answer. I had always intended that the logging would be raw data, time, distance and drawbar load allowing any detailed calculations being done offline. With microcontrollers there is a limit to the accuracy the floating point calculations can go so a potential source of error with the cumulative nature of the results. A challenge I am dealing with at the moment, or will have dealt with once the new chips get delivered from Microchip and the Mk2 prototype being built.

So I’ll log the raw data and a have a control on the real time display for “Pull only” or Push ‘n’ Pull to be used in the calculations.

A for the oscillation reported by Duncan in the spring loaded draw bar. One of our members made a very fancy one and had the same problem. It gave a very spongy feel to the ride and resulted in the oscillations. I suspect caused by the draw on a steam loco not being constant through each wheel revolution and the impact of passengers micro compensating for the slight variations in movement setting up a harmonic oscillation in the spring.

Thanks for your input

Pete

I can't help thinking that for efficiency competitions the accuracy of a dynamometer car contrasts with the need to make sure the fire is exactly the same at beginning and end of the run (of course boiler pressure and gauge glass reading should be identical as well...) Of course the longer the run the smaller these errors become.

Neil

Neil

Likewise; Not having been involved in an efficiency trial I have always wondered at what point do you start measuring the amount of coal used? What was the ambient air temp and temp and volume of water in the boiler before the fire was lit?

Of course there is always the point of view taken by LittleLEC that it’s a bit of fun not to be taken too seriously. Same track, same time, same measuring gear and the results are all relative to each other and not necessarily absolute.

Pete

Hi Pete,

I have made a bit of a study of the UK IMLEC reports, and entered IMLEC once in 1995 coming third.

I suggest 'work done' can only properly be assessed when the loco is pulling. In the UK for the above IMLEC competition, the 'canny' drivers go up the gradients slowly so trying to achieve same steaming rate. There is a speed restriction in the UK IMLEC competitions. You then go downhill still with steam on pulling and registering a drawbar pull up to the maximum of the speed limit.

The 1995 results were published in ME. I came 3rd behind John Heslop and Alan Crossfield, who had both won IMLEC previously.

The drawbar problem is interesting. No slack in the pins, or on loco or tender sprung drawbars or sprung drawhooks.

Cheers,

Julian

Julian

I can see the argument that work done can only be calculated when the loco is pulling. However, when it isn’t pulling (being pushed) distance is being travelled and time is being taken so how would that be handled in the overall calculations or is that distance and time ignored as well?

Actually in writing this reply I’m erring on the side of ignoring ‘push’ as the work has already been done and is only being recovered. If a weight is carried to the top of a building and dropped back to the ground the system that carried the weight to the top of the building doesn’t recover the energy used.

This is all probably O level physics but that was a long time ago….

Pete

My understanding of full size dynamometer cars is that the car included a variable load (a friction brake, dynamo, etc) and a device to measure the draw-bar pull. No further carriages were used, and testing was done on the level (and on a calm day) and so the draw-bar results were related directly to the 'output' of the engine. But I may have it quite wrong (not for the first time).

So, I cannot see the point in worrying about the effects of gradient when towing a line of waggons. Could there be some confusion about the actual measurement of pull, and its interpretation in real-world scenes?

Cheers, Tim

Hi Pete,

The point I was trying to make from my own view of these things, and was of the same view of Sam Ell of ex-GWR fame, was that one ought to try and achieve a consistent steaming rate.

This optimises efficiency.

Also on a downward run the loco is still pulling, so negates your perceived problems.

To rush up a gradient overloading the fire, then coast down, is not the best way to run a steam loco whether in fullsize or miniature.

Cheers,

Julian

The dynamometer car doesn't generate the load. It is a measuring and recording device capable of collecting all sorts of data from the locomotive. Its primary function is to measure distance and drawbar pull and by integrating these to calculate the work done at the drawbar. With the introduction of time to the calculations it is possible to derive the instantaneous power being generated at the drawbar by the locomotive. Load is normally generated by attaching a train to the dynamometer car, although the North Eastern Railway testing department had counter pressure locomotives available and BR had a train capable of absorbing considerable power using electrical generators driven by the wheels, the energy being dispersed as heat through banks of resistors. The use of these devices allowed the locomotive the work trains down inclines at the same power output as they used up them.

The point of testing is normall the determination the efficiency of the locomotive in producing useful work at the drawbar. Given the amount of fuel used from the starting point of the test to the end and the work produced at the drawbar by the locomotive in the same period it is possible to calculate the efficiency. Only the positive work is measured. If the train pushes the locomotive then no work is being done by the locomotive at the drawbar, although it continues to use fuel, so there is no deduction from the useful work done by the locomotive and of course no fuel is recovered, and therefore the dynamometer does not need to record the negative work done by the train.

Locomotive testing was done by some very clever engineers and scientists, and a lot of written material is available on the methods and results of those tests, culminating with the series of BR Bulletins which were produced in conjunction with tests carried out on the static test plants at Rugby and Swindon, and supported by on the road dynamometer car tests. Well worth a read for budding dynamometer car builders.

Eddie

Julian/Tim

I think we are on the same page but different phases of the thought process. I’m seeking an understanding of what needs to be measured whereas you are both looking at how it would best be used. Both go hand in hand as the measurements and test conditions lead to the understanding of the results. No doubt we’ll get to the accuracy of the results at some point as well.

As for braking on the test wagon as suggested by Tim that is indirectly in hand. Most of our owners drive trolleys have been fitted with water tanks and electric high pressure water pumps, they are invaluable when public running on a crowded track. Being held up by traffic and the ability to hit the ‘panic button’ helps a great deal with stress levels. As we have 12v on board this concept has been extended to electric/vacuum braking with cheap Chinese vac pumps, or even 12v tyre pumps. Tests have fully laden trains running at 8mph stopping in the length of one carriage. Therefore ability to apply the brakes in isolation of the loco could provide the load test conditions. (OK this may upset the purists but the elfns’ mob are happy.)

Pete

Ediie got me googling and there’s plenty on the subject including this page

https://books.google.co.nz/books?id=ygKio-Ks0doC&pg=PA213&lpg=PA213&dq=locomotive+dynamometer&source=bl&ots=AIa0qFDeuo&sig=kJr7RjpYvuSiVUiimi5EvyYRI3g&hl=en&sa=X&ved=0ahUKEwjN5YSslO3OAhWDF5QKHcu0AdEQ6AEINzAE#v=onepage&q=locomotive%20dynamometer&f=false

look at the drawbar pull. Its sinusoidal which would no doubt be the cause of the cyclic oscillations when using a sprung drawbar

Pete

Hi Pete, 5 years down the road did you get a dyno up and running ? I'm thinking of making a rolling road Dynamometer. I would be interested in what electronics, strain gauges Etc you used. Seasons greetings. Noel.

Noel,

I started to build a Doubletop Dyno, but it didn't quite get finished. I have the PCB and programmed chip (supplied by Doubletop), all soldered up. You're welcome to them if it will help.

I eventually built one based on Arduino with a lot of help from Silly Old Duffer. I used one of these as load sensor, but made my own amplifier as there is a suggestion that the one that comes with it is a bit iffy. Beyond my pay grade to know why.

If you want photos of my setup of the sensor just pm me your email, if you want the hardware I'll need your dirt mail address

Edited By duncan webster on 25/12/2021 17:47:06

Noel

It has been a while things have moved on and I can't recall what stage I was at 5 years ago so here is a summary of the current state

• Various micro controller upgrades to new bigger, faster, cheaper variants of the PIC
• Third version of commercially produced PCB
• Data logging to SD cards, now micro SD
• Real time clock to support the datalogging time, date
• Strain gauge replaced by commercial item. Replacing the baggage scale variant proposed by Duncan in his ME article that sterted all this off.
• LCD screen replacing mobile phone (phone was hopeless in bright light)
• 4 magnets on wheel for sensor to improve speed accuracy at low speeds
• GPS position and speed logging (needs Kalman filter code including to deal with random jumps)
• Gyro and accelerometer logs in 3 axis
• Speed limit detection and auto application of drive trolley brakes (Autobrake of my Auto Coach)
• Firebox temp and boiler steam temps logging
• Boiler pressure measurement and logging
• Moble phone alerts for temp and pressure (ideal for the times you and up chating and the loco needs attention)

As with any of these non commercal home projects there is a list of things that need attention or could be done better. I have been tinkering with Wifi modules for remote monitoring of the logs, Give me a few days and I'll see what I can pull together by way of more info and a parts list.

Pete

P.S It does occur to me that a rolling road has a different set of requirements. Draw bar pull can't be measured all work needing to be measured at the wheels. How you do that could result in interesting solutions and challenges.

Edited By Doubletop on 25/12/2021 20:16:16

Pete and Duncan, Thank you both for your replies ! As you will understand I'm thinking of building a static dyno so many problems of indicating what's happening should be easier ! Strain gauges for the brawbar pull/ push, so the loco can be run in both directions- yes ! RPM I was going to use a simple hall effect counter on the rear coupling rod ! power - the simplest way seemed another strain gauge on a band brake. Boiler pressure was going to be a much larger dial gauge. I had not at this stage thought of data logging, though it seems a good idea if my skills and knowledge will strech that far. The first task is to try and get the power from the wheels so it can be meassured without large amounts of slip - in some ways the rest would seem relativly easy - just electronics !

I will PM you both, Noel.