I guess it's naive to expect steam to follow any angle that we choose to put on the inside of a pipe. It's the expansive nature of the steam that's going to control that as much as anything. I would have thought that the outside shape would cause a smoother pick up of the gasses around it, but in reality those effects are going to be swamped by the huge blast up the chimney. I wonder how much smoke actually gets ejected along with the steam? Clearly whatever is above the nozzle and in the chimney when the blast begins is going to exit North pretty quickly, but when that blast is established I wonder what happens then.
Jos's english needs a bit of getting used to and a bit of background knowledge helps, but he quotes all the quantity of gas figures you refer to.
i should perhaps add that the Greenly rules are a starting point and many designs fail to accord with them . Jos has taken things up to a much higher level of perfection (as you will see from his comments on Speedy).
hi jordan, what on earth would be at all 'wrong' with the back end? do you mean grate and firebox? any improvement at the 'front end' on a poor design can work wonders and transform a loco! this applies both to fullsize as a result of draughting alterations carried out to many designs particularly in BR days that are well documented, and in miniature! have a look at how the GWR Manors were transformed and the LNER V2s and the BR class 2 2-6-0 locos by small adjustments to the draughting. cheers, julian
A Prime Example of Not getting it right is 4472, when the cylinders and draughting where vastly improved they didnt take into consideration the extra wear and tear on the Firebox, other locos have had the same treatment and tend to lift the fire off the grate less than ideal if you expect to get a long life out of the tubes and firebox .
As Julian says, many locos have been improved by better drafting, including some I am aware of in the antipodes. On the Victorian Railways just about all 20th century railways finished up with Kylchap like drafting. I can think of one class on the SAR that could have done with such treatment which might have made it a really useful engine; on that same system drafting modifications certainly improved performance.
I suppose you can have too much of a good thing, there is a limit to how hard a grate can be pushed before it becomes in effect a fluidised bed. The grate area does have to match the cylinder volume.
One way of overcoming fire lifting problems would be rosebud grates which control the air through the fire rather than rely on fire bed thickness.
jordan, apart from rectifying a badly steaming loco, one of the major benefits of good draughting is that you can actually get away with a softer blast and therefore less pull on the fire because the loco is working much more efficiently with less back pressure. i was shown when 16 the considerable advantages of less back pressure on a steam engine, and it is something ive never forgotten! virtually all research in this field is not how to increase the smokebox vacuum - this is easily done by throttling the blast with a smaller blast nozzle - but the research has instead been how to reduce back pressure and produce a free steaming loco with LESS smokebox vacuum and LESS consequent pull on the fire! cheers, julian
This is an interesting subject, and if you are inclined to delve deeper into the practice and theory of model stack design there has been a thorough in-depth discussion over several years with Jos Koopmans and other cogniscenti on the whole subject including on the diameter of blast nozzles etc. at: www.chaski.org/homemachinist/viewtopic.php?f=8&t=82682
A few refinements to the (suboptimal?) 1/3, 1/6 and 1/7 (blastpipe) ratios have been proposed.
i had read the above awhile back (think it ran to 20 pages eventually) and noticed you and ian had contributed. it is rather a shame that (with a few notable exceptions) our american cousins dont seem to be as receptive to analyzing these things and incorporating improvements.
i have never followed the 1/7 rule for blast pipe nozzle diameter (1/7th of cylinder bore). Bob Sanderson demolished this bit of theory in ME in 15th June 1984. Brian Hughes proposed a formula which i always use to check my own thoughts in ME 2nd Nov 1979.
notwithstanding the fact that many miniature designs have bores that are too big IMHO, the 1/7 rule still gives a figure for blast nozzle diameter far too small. a quick check on all but 1 of my own locos indicates they all have a blast nozzle diameter between 1/4 and 1/5th of the cylinder bore, not that i take any notice of the cylinder bore size anyway.
oh dear, the pics have all disappeared! i will have to try again!
hi Ed, you are quite right the 1:6 cone should be drawn where it intersects with the top of the blast nozzle. it doesnt make any significant alteration to the dimensions on a 'long' chimney, but would do on a short chimney of large internal diameter. cheers, julian
Oh dear--Oh dearie me !!......... Having read the above ( including that excellent link--thanks Rob )..I'm now getting those nasty, creeping, niggly doubts as to The Bear's draughting !!............. Which means re-visiting my master records ( Ugh !!)........ I'll also have to contact the good folk at Scunthorpe re}--- their models smokebox set-up....Anyone know how Mr Head in Torquay is getting on with his Bear ?? EDIT}---- Julian, try using Tinypics .... tinypic.com/?t=postupload .... it's very easy, why--even I can do it LoL !!
Hi Julian, interested by this thread I have looked at my copy of Martin Evans's Manual of Model Steam Locomotive Construction and he shows that the 1 in 6 cone fixes the position at top of the chimney. Am I correct in assuming that this establishes the position of the blast nozzle, and the petticoat pipe is then installed to give the 1 in 3 cone. He also mentions the 1/7 x diameter for the orifice but does quantify this by saying this is a rough guide for a 2 cylinder simple engine.(which mine is) Regards Paul
3 1/2" Juliet 2 (Shortly to be named GWR 101), 7 1/4" Bridget & Class 07 type electric.
Since my last post I have looked at the drawings for my build and assuming my calculations are correct based on a chimney bore of 0.75", nozzle bore of 0.157" (No22 drill) and cylinder bore of 1.00" I have the following results. The diameter of the orifice based on 1/7 of cyl. dia. should be 0.143" so that's not too far away, I was thinking I might go for this size as I can always open it out but the other calculations suggest that this is not helping the other ratio's. The top of chimney ratio is 5.59 which is fairly close but the petticoat pipe ratio is 2.21. which is a fair bit away, I suppose I could lower the blast nozzle by about 0.375" which would bring both values nearer to that mentioned. Decisions decision's ? Regards Paul
hi paul, you have the advantage of Juliet being 'freelance' so you can adopt whatever arrangement suits the loco best without worrying about keeping to scale for chimney length etc. im not too sure i understand your ratios. with say a 5/32" diameter blast nozzle draw a 1:3 cone fitting into it and same as for the 1:6 cone. the 1:6 cone should be just below the top of the chimney (ideally in 3.5"g i would suggest by 1/2"). the 1:3 cone must hit the sides of the petticoat pipe just above the 'choke' ie just above where the flare on the bottom joins the parallel tube. Stepney also has 1" dia bore cylinders though i expect the stroke is quite a bit more, though your smaller wheels will be turning faster. i decided on a 3/4" dia bore petticoat pipe for Stepney which by coincidence is the same as Juliet (the martin evans drawings for Boxhill show 7/8" but i though this a bit big and it didnt accord with the Brian Hughes formula that ive always used for these things very successfully). however your boiler is shorter and smaller grate area, so i would be inclined to use an 11/16" ID for the petticoat pipe, then with a very small taper of 1.5 degrees just above the choke till chimney top. in fact if you applied the Brian Hughes formula i expect it might suggest a petticoat internal diameter of 9/16". i can work it all out for you if you want. i can also double check with a great friend of mine who has that Juliet that performs excellently that was in the youtube clip i posted the other week - he isnt a slave to drawings and also works all these things out for himself! cheers, julian
Hi Julian many thanks if you could get some details off your pal like Pete says I am sure there are a few of us who would be interested. I based my calculations on Martin Evans drawing in the book with the cone being right to the top of the chimney. On the Juliet drawings the distance from the nozzle to the top of the chimney is 3.312" and to the petticoat pipe is 1.313" at the point where the petticoat pipe necks down to it's smallest bore. Obviously if I take the cone for the chimney down by 0.5" this would make the ratio 4.7 which is worse, so any information from your pal would be of great help. Regards Paul p.s. I have edited my previous post as I would need to reduce the nozzle height by 3/8" not increase it as I previously stated.