|David Clark 1||31/01/2012 19:32:00|
3357 forum posts
I am working through the IMLEC efficiency computer formula and have a couple of queries
Load done and distance in feet are not in the formula.
Would this equal work done (Ft lbs) as measured with the Dynamometer car?
What is the constant 18.0176?
1980000 = foot pounds per hour
33000 = 1 horsepower
Edited By David Clark 1 on 31/01/2012 20:32:20
Edited By David Clark 1 on 31/01/2012 20:38:14
|Stub Mandrel||31/01/2012 19:56:02|
4315 forum posts
work done = force x distance moved.
So the 'area under the curve' of a plot of drawbar pull against distance travelled gives the work done. Note that if the train pushes the engine the pull is 'negative' and the work done is 'negative'
It's not average pull x distance travelled...that only works if the speed stays constant.
I imagine that any practical calculation takes something like the average pull over each foot fo track and makes a running total. Certainly better than wrestling with calculus...
... and for once I have to admit it would be lot easier in metric units!
|David Clark 1||31/01/2012 20:36:53|
3357 forum posts
CALCULATION OF RESULTS
(With acknowledgement to the Bristol Society of Model and Experimental Engineers)
As used at Leeds in 2002 - thanks to Colin Abrey
The dynamometer car measures and gives a direct reading of Total Work Done (in ft lbs) and Total Distance Travelled (in feet). Additionally the Overall Run Time (in minutes) and Weight of Coal Used (lbs) are recorded.
|David Clark 1||31/01/2012 20:37:03|
3357 forum posts
|From these parameters the following calculations can be made.
Average Drawbar Horsepower = Total Work Done (ft lbs) DBHP -(1)
Coal Consumption Rate = Weight of Coal Used x 60 lb/hr -(2)
Specific Fuel Consumption (SFC) = Coal Consumption Rate -(3)
Substituting 1 & 2 in 3: SFC = Weight of Coal Used (lb) x 1980000 lb/DBHP.hr-(4)
Overall Thermal Efficiency h = Work Output x 100 percent
Overall Thermal Efficiency h = 1980000 x 100 %
Where K is the calorific value of the coal in BTU/lb and 778 is the number of BTU per HP.
The locomotive returning the highest Overall Thermal Efficiency will be the winner.
From Colin Abrey
|65 forum posts|
Just an addition to Neil's contribution. The dynamometer car has an integrator which multiplies draw bar pull by the distance travelled and gives the result in ft lbs (or Nms) - i.e. work done. Neil is mistaken in assuming that when the train pushes the locomotive the dynamometer subtracts the work done by the train. This should not be the case, and certainly wasn't in full size practice. The intention is to determine the efficiency of the locomotive when it is working and therefore the work is only recorded when the locomotive is doing it and the drawbar should have a stop to prevent the recording of negative work.
If the dynamometers IMLEC has been using don't have this facility then it may explain the low efficiencies recorded.
|Stub Mandrel||01/02/2012 21:33:01|
4315 forum posts
Tom Walshaw set off a huge and extended letter campaign in 1990 abouth whether or not "negative drawbar pull" should be taken into account. It's a fact that when the train pushes the loco down a slope it is, at least theoretically, possible to shut the regulator and build up some pressure whilst benefiting from regaining some of the potential energy expended going up the hill.
Others advanced the view that the ups and downs cancelled out...
The final analysis, I think, was that the truth or whether this really mattered or not would never be known until someone actually did the experiment instead of just talking about it like greek philosophers!
|65 forum posts|
I'm neither Greek nor a philosopher, but I am an engineer. I am, like most engineers, interested in the efficiency of the machine when it is doing what it was designed to do i.e. work. The machine only uses fuel when it is working and efficiency is the ratio of useful work out to energy in. What the machine does when it is not working is of no interest, therefore what it gets back from the train when it is running down hill or slowing down for that matter should be discounted. When full size locos were road tested they were only recorded on uphill or level stretches which is why the Settle & Carlisle was a favoured line. 15miles of 1 in 100 could give them 20 to 30minutes of hard work. The North Eastern had a counter pressure locomotive to keep the load steady on undulating railway for test purposes and BR built load absorbing vehicles to run with the dynamometer to allow extended constant load test runs. No, we must only record the positive work.
Yes, on a continuous railway the work done to climb the hills is returned when going down the other side. The resistance of the train is, however, more or less constant save the effects of curvature and speed. The dynamometer should take all of the train resistance into its accumulated work and the work done climbing because that is the reason that fuel is consumed. If the driver needs to recover the pressure and water then the down grade is where he can do it. That's not cheating as you seem to imply, it's just good management, as is easing off or stopping for a blow up if the loco has been worked too hard and the fire and water are low.
I hope David can get IMLEC back on track. I haven't entered for many years, partly because of some of the strange rules that have appeared (like not being allowed to use the brake) and partly because of the failure to force observance of or the dropping of some of the original rules. The formulae are not a problem - they were fourth form physics in my day - useful energy out over energy in. Making the competition fair again is the first step. There are a number of efficiency trials around the country at club level that are run on a fair and friendly basis each year, some of which preceded IMLEC by many years. David might do well to talk to the organisers for some guidance.
|Stub Mandrel||03/02/2012 20:12:12|
4315 forum posts
This is interesting Eddie,
I'm not rying to prove any particular point here, just curious really as it was obviously a major bone of contention 22 years ago. Here's a (Greek style) thought experiment:
Simple track - a climb, then down again then a flat.
Run to top of a hill, freewheel down the other side, and then coast as the inertia of the train pushes you to the finish.
Run to top of hill, come down with the brakes on to limit speed, pull to the finish.
Clearly different amounts of work done, but what sort of impact on fuel use?
Perhaps I am just re-counting angels on pins...
|65 forum posts|
I have to apologise to David for inadvertently pushing this topic off the rails. IMLEC has been of interest since I attended my first in 1972 and I've spent a lot of time over the years trying to make sense of the results.
I think what people forget when they talk about testing locomotives is that they are just an engine or 'prime mover' to use the modern term. When efficiency tests are done on a power source, be it a reciprocating or rotary engine driven by steam, internal combustion, hydraulics or electricity, the ideal situation is to put it on a test bed, couple it to a brake and do a series of tests at a range of loads and speeds and measure the power output and the fuel consumption. From these tests it is possible to determine the relationship between power, speed and fuel consumption and plot the efficiency of the machine relative to its speed and output. The important point about this type of testing is that the environment is controlled so the engine can work under steady state conditions. Once the steady state is reached the test begins and the results are only considered valid if the steady state is maintained and are found to be repeatable. At no time during a test would the engine be taken off power and allowed to be driven by an outside power source.
When a locomotive is road tested the engineers have all sorts of outside influences to contend with, not least the weather, and gradients. Typically the power output recorded by the dynamometer would be adjusted for gradient, so the work done to lift the locomotive would be added, converting the draw bar power to effective drawbar power on level track. The purpose of the exercise was to determine how much useful work a unit of fuel would deliver. At the same time the fuel was carefully measured and the firing rate was recorded so that this could be correlated against the output. Usually the tests were conducted at constant output, and to aid maintaining this the locomotives were fitted with a smoke box vacuum meter. Since the vacuum is directly related to output, the driver would adjust cut off and throttle to suit the requirement for the particular test thus keeping the vacuum constant. Once the train topped the summit of a climb it was impossible to keep the steady state and so the test would terminate. How much easier to put the loco into the roller test facility at Rugby or Swindon and do it under laboratory conditions. In fact locos tested at these facilities were road tested as well to confirm the results at some of the outputs.
IMLEC isn't meant to be a laboratory test but it is intended to fairly compare the efficiency of the competing locomotives and therefore all the work done by the locomotive should be used to calculate the result, and that means ignoring the effect of the train on down grades or when reducing speed, just as in prototypical testing practice.
As for the philosophical examples. In theory the second would use more fuel since it would require the locomotive to do some work to pull to the finish, and in doing that it could influence the run average efficiency either positively or negatively. One thing is certain though, when the engine isn't working it's efficiency is 0% and the run average will fall.
|Stub Mandrel||04/02/2012 12:11:26|
4315 forum posts
I've never been to IMLEC but having read many reports, I'm struck by two main things:
1 It is a test of driver skill as much as engine efficiency,
2 The accuracy of the efficiencies quoted always seems spuriously accurate - given the dependednce on a visual assessment of the fire before and after a run and the difference that a half-ounce of coal, a few PSI of pressure or a slight change in water level could make to the results.
Perhaps the event is best regarded as a test of the drivers, to work their engine close to its limit, while minimising slippage and well managing the boiler?
|Jonathon Williams||08/08/2012 21:56:06|
|1 forum posts|
I am organising a mini imlec for my club but unfortanatly our dyno car is an old one and has broken. is there any whay of working out a score and finsing a winner without it by using amount of coal used, weight pulled, time and distance traveled??
please reply asap
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