Notes on FDC 18
Notes on FDC 18 – Performance Predictions
In this retrospective set of calculations, Wardale (with the assistance of Dr David Pawson) used two software packages, Perform and Perwal written by Professor W. Hall to produce performance predictions for the 5AT and compares the results with his own hand calculations. Wardale introduces the calculations with a cautious qualification:
“As there is no transparency in the computer programs, their validity cannot be directly verified. They require the use of data, such as discharge coefficients, that has to be estimated, and however good the theory behind the programs, the results are clearly only as accurate as the estimation of this data. Good correlation between performance figures obtained from these programs and test results for B.R. locomotives is said to exist, depending on the values used for the various coefficients (i.e. the coefficients have been chosen such that there is agreement between calculation and test). This would point to an indirect verification of the programs’ validity when applied to First Generation Steam (FGS). However when using them to predict performance where there are no test results available for comparison, as in the present case, there is no way of confirming that any estimated input data is correct.
Although (with the reservation given above) the programs appear applicable to FGS, this does not necessarily guarantee applicability to SGS such as the 5AT, where engine design shows a number of advances over that of FGS designed to improve steam flow and reduce heat transfer, i.e. to make the power generation process approach closer to the isentropic ideal. Such advances are, however, at least partly accounted for in Perwal by the input data (in respect of such items as discharge coefficients, expansion and compression indices, valve motion and cylinder cover temperature). Steam flow and heat transfer processes in an engine are complex, and there must be doubt that any analytical approach is better than an approximation. However as the calculations concerned are not design calculations, which of necessity must be accurate, a degree of uncertainty can be accepted, i.e. the present work can be taken as giving at least a reasonable guide to the expected 5AT performance. It also follows that any error will tend towards under-estimating performance of SGS, i.e. the 5AT should perform as well as or better than Perwal predicts.”
The concluding lines of calculation include the following non-consecutive paragraphs:
Comparing the 5AT target Maximum Indicated Power-Speed curve from the data of FDC 1.1, which was estimated from the performance recorded by SAR 26 Class No. 3450 shows that the two curves agree well above about 100 km/h, where the maximum difference is only some 3%. Below 100 km/h the 5AT performance predicted by Perwal is significantly better than the target curve, being some 15% higher at 70 km/h and 33% at 40 km/h. ….. To summarise: Perwal supports the previous estimation of the 5AT maximum power above about 100 km/h and predicts significantly higher power below this speed.
The difference in maximum drawbar power predicted by the two methods is within the accuracy of the calculations.
The highest cylinder thermal efficiency according to Perwal is at the highest speed (200 km/h) and lowest cut-off (10%) of the ranges analysed. This is attributable to the good cylinder internal streamlining and high superheat.
The indicator diagrams given by Perwal for each condition analysed are uniformly good, ‘fat’ diagrams being produced even for the extreme condition of 200 km/h and 10% cut-off. The diagrams are extremely regular, with little difference between front and back of the cylinders. Also as speed is increased at constant cut-off, successive indicator diagrams are drawn one over the other and show relatively little diminution in area with increasing speed. All this is testament to good internal streamlining, which not only gives good ‘breathing’ at high speed but also determines the lowest speed at which maximum evaporation (i.e. the full capacity built into the boiler) can be used. For the 5AT Perwal predicts this to be around 36 km/h in full forward gear, at which the indicated tractive effort (155,3 kN) is 99% of the nominal starting tractive effort (157,0 kN). Because maximum indicated tractive effort is effectively constant below 36 km/h, the indicated power is approximately proportional to speed, making the power – speed characteristic a straight line below 36 km/h.
The indicator diagrams and ‘more detail’ diagrams show no evidence of compression loops at any speed and cut-off (which, if present, might have been attributable to the long exhaust lap). The ‘more detail’ diagrams show a little too much lead at short cut-offs and low speeds, showing as the cylinder pressure rising to steam chest pressure just before dead centre. At all likely running Cut-off-Speed combinations the lead achieves steam chest pressure in the cylinders at exactly dead centre, i.e. is perfect. No evidence of lack of lead, showing as a triangular loss on the indicator diagram near dead centre, is evident at any combination of speed and cut-off within the locomotive’s rated maximum cylinder steam flow…… All diagrams do show the admission line starting at steam chest pressure at dead centre. From the above it is considered that the lead chosen (which is fixed for all engine working conditions) is correct.
It is suggested that little, if any, improvement in the diagrams would be achieved by using poppet valve gear.
As Webmaster, I hope I will be excused for taking the liberty of copying a line from the letter from Wardale which accompanied these calculations. I think it makes a fitting epilogue to the FDCs by providing an glimpse of Wardale’s motivation in undertaking the huge task that these calculations represent:
“Perhaps no-one can imagine as well as I the experience that the 5AT would give as it accelerates at full power (according to Fig. 2. above) from low to high speed. In my own mind I can see it, and hear it. The ‘stack talk’ would be out of this world, and that’s what it is all about. Forget about economics and efficiency. How would you apply these to the Mona Lisa or Shakespeare or Salisbury Cathedral?”