IMLEC results

I know it's sad, but I've been doing some comparisons on the IMLEC results published in latest ME. I would expect there to be some correlation between number of passengers (NoP) and the mean drawbar load (MDL). I can work out MDL by dividing the published work done by the published distance travelled, and NoP is published passengers + 2 (driver and observer added). I would expect some scatter because passengesr are equally heavy, but with up to 19 passengers you'd expect this to average out.

What I find is that MDL/NoP varies form 1.006 (run 4) to 1.900 (run 16). One might expect longer trains to have higher rolling resistance per person because of curving, but the 1.9 figure comes from the smallest number of passengers (4), and the 1.006 comes from one of the highest (18). Anyone got any good ideas to explain this?

I should have said not all passengers are equally heavy

My ME hasn't arrived yet so I can't look at the numbers, boo hoo, but could the cause be due to bearing differences?

Regards,

Dave

Well it could be, but comparing 2 runs with 18 passengers, both presumably using the same trucks, run 4 was 1.006, run 1 was 1.344. were the passengers on run 1 34% heavier on average than run 4 passengers? Doesn't seem likely

Driving style? A very steady driver will aim to maintain his momentum and minimise the need for acceleration, reducing the MDL compared to someone who is less skilled or has a loco that struggles to sustain a steady pull. Also with fewer passengers the load has less momentum so speed may vary more.

Finally, speed has an influence as well, just going 10% faster will require considerably more pull.

Neil

Why not pull a defined load

As I read the figures I realised how disappointing it must be to spend all that time making a complex and clever device, only to find that 98% of the energy available goes to waste.

Yes, I know that the best steam engine can only manage about 20% efficiency, and I know about the problems of scale ...

I also wonder what is the object of this sort of test when there are so many variables, and so little control of many of them. All your best efforts could come to nothing if granny at the back has her belt dragging round an axle and no-one notices.

I think I will stick with making bits for full size motors.

Cheers, Tim

Edited By Tim Stevens on 03/09/2016 09:50:14

I think the efficiency would be nearer to 5% than 20%. For testing, the train on the engine should be equal for engines of similar power, and should be ballasted if required to equalize the weight, the rolling resistance also needs to be the same. Just my ideas.

Ian S C

mean speed on run 1 which has MDL/NoP = 1.344 is 7.05 mph, for run 9 which has MDL/NoP = 1.006 is 6.58 mph. However, for run 2 which has MDL/NoP = 1.226 is 8.11 mph. If you plot MDL/NoP vs speed there is no discernable trend

This is even sadder...

The trouble with looking at outliers is that they can be given excessive weight, we need to look at the whole dataset so it's graph time!

I also did a slightly different calculation that gives two comparable interpretations of the work done, although this doesn't change the relationship between teh two sets of values.

Assuming the force required to pull the train is proportional to the number of passengers then as work = force x distance moved, the number of passengers x distance travelled (passenger-feet) should be proportional to the work done - indeed this is the way that the work done was calculated in the halcyon early days of IMLEC).

So I plotted work done (Y axis) and Passenger feet (X axis). I assumed that when passengers were dropped this was at the half way point. There's huge scatter which we might expect, (especially if some drivers do 'negative work' holding the train back on downhills), but the r-squared value is 0.6. When I added in the driver and observer (I forgot them so not on this graph) teh fit is even better and has an r-squared of about 0.7 which perhaps isn't bad given the high number of variables and idosyncracies involved?

Edited By Neil Wyatt on 03/09/2016 16:09:34

Another observation ... the two dots furthest to top left above the curve had relatively small numbers of passengers (4 & 7). The outlier furthest off the line to the bottom right had 28 passengers. Clearly some trains with small and large numbers of passengers were on the line, but maybe this is another confounding factor.

Neil

Neil says: Assuming the force required to pull the train is proportional to the number of passengers

Well, to me that is a very dangerous assumption. It means that if there are no passengers then no work will be done - either the train will not move or there will be movement but no drawbar pull. And this means zero efficiency, or infinite efficiency.

In the best traditions of Francis Bacon - there is a flaw in the argument.

Regards, Tim

No flaw really, the apparent paradox is caused by ignoring other factors - the weight of the train and the rolling resistance provide a load even when there are no passengers.

If the bearings had zero friction and the train was in a vacuum then there would indeed be no drawbar pull unless the train was accelerating. Once moving the train would carry on at the same velocity for ever.

Personal experience of measuring 'work' is counter-intuitive. When I hang by my arms from a climbing frame, the maths says that I'm not doing any work because I'm not moving. Why do I get so tired then? It's because muscles don't simply take my weight as a rope would. Instead they constantly tighten and relax slightly, doing work even though I think I'm still. No work is done when I sit on a swing suspended from the same climbing frame.

Dave

Edited By SillyOldDuffer on 03/09/2016 18:19:15

It should be simple enough to check the mean drawbar pull. The rolling resistance of the riding vehicles used on our miniature railways is around 1 percent of the weight of the train, increasing slightly due to flange friction on curves with correctly profiled wheel sets and dramatically with parallel treads and square cut flanges. Resistance also increases slightly with the square of the speed and may increase with increasing bearing load. Drawbar pull also increases with acceleration and with climbing inclines. For Neil's benefit the negative effect of drawbar pull does not affect the work done as the dynamometer is designed to record only the positive work done by the locomotive. Given the gradient profile for the track and the weight of the train it is simple to calculate the work done climbing inclines. Estimating the weight of the trains at IMLEC is difficult since we don't know the weight of the vehicles or the passengers. A reasonable guess would be passenger cars at 120lbs for a four seater and 14 people to the tonne, but since the passenger weight is more than 80% of the load, it bears considerable significance in determining the load on the drawbar. If the actual MDL is sighificantly different from the estimate then there must be something influencing the MDL that is not accounted for. This could be passenger weight e.g lots of children on board or a high percentage of heavyweights. Drivers can often be seen leaning on the locomotive despite rules to forbid this. Additional drag on the train may be possible from various sources, although the drivers are not allowed to use the brake at anytime. So Neil is heading in the right direction - drawbar pull is close to being proportional; to the weight of the train - not just the passengers. Incidentally I can recall only one IMLEC where the actual train weight was recorded (Tyneside 1972).

I don't think IMLEC ever used the simple formula of load x distance / fuel consumed to determine the winner, I think there has always been a dynamometer, but we do use this formula for our locally held annual Stephenson Memorial trials (63rd event to be held next weekend). Used against estimated loads for IMLEC the formula gives broadly the same finishing order.

So there are flaws in the WD/NoP analysis but it could be modified to give a more rational check on the results. The more interesting analysis though would be to determine what makes one engine more efficient than another when working a similar train at similar load and speed, which really is what the competition was intended to stimulate.

Eddie

It seems rather odd to me, with a vague memory of A-level maths, when the measurement of efficiency is based on guesswork (Estimating the weight ... we don't know the weight ...) that the result is quoted to four significant figures.

Surely this implies confidence in the accuracy of every number in the calculation at least to the same degree? Or don't the rules of mathematics apply to steam locomotives?

Regards, Tim

Eddie and I have both entered IMLEC. In my case it was probably the most nerve racking thing I've ever done.

In my case in 1995, I produced more work done at the drawbar pulling just 6 passengers plus the heavy dynamoeter car than Alan Crossfield who came 2nd pulling more passengers. However Alan burnt less coal than me. So I came 3rd. However with a lighter load I recorded a higher drawbar reading than did Alan with a heavier load!

I did a great deal of study of IMLEC results pre 1995.

The classic was Jim Ewins with his 9F 'Iron Duke' that pulled a tremendous load but didnt win. Poor old Jim spent many years building special locos to win IMLEC but didnt. Percy Wood built bog standard LBSC designs and won 3 times.

Cheers,

Julian

No, if there are zero passengers they will weigh nothing and no force at all will be required to make them move at the speed of light.

Neil

There is at least one glaring oddity in the table:

Paul Pavier ran with 15 passengers for 27 minutes 30s and covered 18,397 feet.

John Cottam ran with 16 passengers for 30 minutes 10s and covered 17,435 feet.

The table says that Pavier only did 136,949 ft/lbs while Cottam did 316,068.

However as the figures also state that Pavier used 1 1/2 times as much coal to do less than half the work of Cottam, but give him a higher efficiency figure, I have to conclude that there are some errors in the table!

Neil

An idle 'wet Sunday morning' thought:

Is there anything to be learned from the sport of Tractor Pulling ?

http://www.uktractorpulling.co.uk/whatistractorpulling.php

... It seems to me that the essence of the two competitions is similar, but the method of assessment is different.

MichaelG.

Ah, tractor pulling. For those who don't know, a tractor pulls a trailer which has wheels at one end and a flat sledge at the other. A large weight is carried, and is winched automatically, as the trailer is pulled, from the wheeled end to the sledge end, effectively increasing the load steadily and measurably (at least in comparison terms). Thus the distance travelled before the tractor can pull no more gives a fair comparison of the pulling capability.

This eliminates any need to count passengers. A difficulty would be in providing the sliding surface, but it would be simple to have an extra strip of metal along each side of the rails, and arrange for the increasing friction to be applied by a pair of shoes pressed down onto the strips. The increasing load could be done as with the tractors by winding a weight from the wheeled end to the sliding end.

No attempt is made to measure the efficiency of the tractor, as the measurement of small amounts of fuel is not easy. But it is a lot easier than measuring the solid fuel used in a locomotive as it covers a short distance, which seems at least as prone to error and guessing as the notion that all passengers weight the same.

A change in the weather would influence results, as it does in tractor pulling, of course. I expect it would also tend to reduce the numbers of willing passengers.

PS Tractor pulling has been described as 'ironing for farmers'

Regards, Tim