Tim Stevens | 29/09/2018 16:27:54 |
![]() 1779 forum posts 1 photos | I am impressed by the recent chart of performance figures from IMLEC, and I hope someone can help: If all the carefully measured coal goes into the empty firebox (does it?) and you light the fire (adding energy), some of the dust will fall out of the bottom at once. As the fire gets going, some of it will be carried over up the chimney. As the fire progresses, some of the remaining coal will be turned to ash, and some of that to clinker. Some of the ash will go up the chimney with unburnt and coal debris, some will fall downwards. Some of the coal will be part burned - perhaps roasted, giving off its gaseous content, and leaving coke, perhaps just partly consumed leaving a grey mix of coal and ash. To be certain (ie to within 1%) you need to know how much of the coal escaped unburnt, how much extra energy was added as ignition gas, kindling, etc, and how much of the remaining solid fuel is fully burnt clinker, part-burned coal, accidental stone inclusions, and ash. I am interested to know, on the small scales we are involved with, how these calculations are carried out and verified. In industry, a similar test might take hours and involve hundredweights, and so the variations added from the kindling, and the amount of clinker counted as unburnt coal, could be determined fairly well, but I need to understand how this is done on the small scale. After all, the coal consumption is recorded for us in pounds to three places of decimals, ie to the nearest half a gram. (That is the mass of one dried pea). I note too that the time taken is recorded and the coal consumption calculated. My understanding is that the work done is force time distance, whether done quickly or slowly, so my second question is: Are the Lb/hr figures charted merely for interest, or do they form a part of the final calculations of position? And, why is no listing of power output given (for further interest)? My interest is prompted by noting that the Specific Consumption figures and the Efficiency figures are closely related mathematically*, and as one includes time and the other shouldn't I am doubly confused. * ie multiply one by the other and the result is the same for all entrants (17.55) Regards - Tim
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Clive Brown 1 | 29/09/2018 18:03:53 |
1050 forum posts 56 photos | Tim, May I contribute my take on the points raised, based on reading about IMLEC, not participating. Steam raising prior to the measured run is done with fuel that is not measured, so what is in the firebox at the start of the run is a bonus, cancelled out to some extent by what remains at the finish. AFAIK, charcoal is used initially, its lowish density presumably minimising the bonus.The before and after weights of fuel are not included in calculations. Although inexact, this seems to be a practical approach. Fuel losses during the run contribute to loss of efficiency of course. The average draw-bar HP is listed in my latest copy of ME. Specific fuel consumption and efficiency are different ways of measuring the same property, one is the inverse of the other and efficiency is of course dimensionless. That being so, multiplying the two will produce a constant, the value of which depends, amongst other quantities,on fuel calorific value etc. etc. Clive
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GordonH | 30/09/2018 12:31:32 |
64 forum posts 5 photos | Tim, I am a member of the BSME who organised and ran this year's IMLEC, and I also acted as one the official observers. Prior the competion run, the drivers choose the quantity and grade (size) of coal that will be required and the number of passengers that they wish to pull during the test run. The chosen number of passenger tickets and the amount of coal, pre-weighed and bagged is then collect and verified. In the Steaming Bay, charcoal is supplied for steaming the engine. At the start of the competition run, the engine is taken to the traverser, for transfer to the running track. At the traverser, the driver must stop adding charcoal and begin to use his coal supply Once on the running track, the dynamometer car (loaned by Leyland SME) is connected and calibrated, together with the chosen number of passenger trucks. The train is then driven to the station where the selected passengers board and the test run begins. At the start of the test run, the timekeeper starts the clock and begins the lap count. Each run must be for a minimum of 25 minutes. After this time, the the driver may stop or carry on. After 30 minutes,the driver must not begin another lap but if part way around is able to complete the lap. At the end of the run, the unused coal (any coal which has not entered the firebox) is weighed. The difference between the starting and finishing weights is then used in the efficiency calculation. No allowance is made for any energy remaining in the firebox, or for coal which may have passed through the firebox without being burned. The former is down to the skill of the driver, the latter is part of the engines efficiency. In practice, many drivers finished with no fire in the firebox, and little residual boiler pressure. This was down to the driver judging when to stop coaling. The overall efficiency is calculated from the speed and load data from the dynamometer, the distance travelled and the overall time , the amount of coal used and the calorific value of the coal. This calculation wash previously discussed in a thread on this forum be David Clark in January 2012. https://www.model-engineer.co.uk/forums/postings.asp?th=64687 Gordon Halliburton |
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