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Post by Deleted on May 7, 2015 10:28:42 GMT
Steam turbine and later gas turbine designers were always way ahead of steam locomotive designers in use of advanced materials .
I wondered for a long time why there was so little transfer of technologies . Perhaps the answer is the same one as mentioned in earlier postings - the steam locomotive was at it's best when kept simple . Use of advanced materials only gave very small improvements to engine performance and was very costly .
Incidentally all the main industries found stainless steels almost unworkable in the early days . Difficult to machine , cast or forge and nearly impossible to weld . It was quite a few years before all the manufacturing problems were solved .
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Post by joanlluch on May 7, 2015 12:01:29 GMT
Stainless Steel, is something that must be understood. To some extent is it not as forgiving as cooper for equipment subjected to heat.
- The inferior thermal conductivity calls for controlled or slow uniform heating and cooling of the boiler to prevent spots of excessive mechanical load. The chances of high temperature spots due to slow thermal diffusion are higher but the material stands much higher temperatures than cooper, so the problem is mostly mechanical rather than thermal. So just take your time to heat up the boiler.
- Chemical resistance of cooper and S.S is very different. For example, S.S will sustain with no problems both highly oxidising acids such as nitric acid, and highly alkaline chemicals such as caustic soda. S.S is a chemically nobler material than cooper.
- A S.S boiler can be thoroughly cleaned with no mechanical intervention whatsoever, just by using nitric acid and caustic soda (not together) followed by successive washes with water. Chemical cleaning is simply not possible with cooper.
- One important difference is that cooper does not get affected by sulphuric acid (needs the presence of an oxidant). Stainless Steel can be affected by inter granular corrosion when it is put in contact with *wet* sulphuric acid, so this must be taken into account on coal fired boilers. Just be sure that the fire tubes remain completely dry at all times and particularly avoid any water condensation in the smoke box after stopping the engine. Just keeping the smoke box door open to the air after you stop and while you store the loco should be enough.
- Gas fired locos are not affected by sulphuric acid issues, so no special care must be taken in this case.
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Post by Deleted on May 7, 2015 17:59:33 GMT
That's enough about heat transfer for the present - I know it's not very interesting to most people .
I would like to discuss the actual engine parts now - cylinders , valves , valve gears and anything related .
Any questions to start us off ?
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Post by Deleted on May 9, 2015 16:47:16 GMT
Obviously not .
I am going to leave any further discussion of these topics to the experts .
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Post by joanlluch on May 9, 2015 18:30:14 GMT
Hi Michael, Maybe some people just do not feel confident enough to make some very detailed questions, as not all of us are into the technical or theoretical aspects of the hobby and the steam engines in general. I am certainly one very interested in everything related though. So I would like you to go ahead and start yourself with one of these topics if you feel like doing so. I am sure that some questions will come up after an initial background is provided.
Nonetheless, I have one question about the space (or gap) that we should leave at the end of piston travel. It is generally considered that the less gap the better. In other words, we only set a gap to leave allowance for the eventual extra travel of the connection rod caused by the locomotive suspension. I would want to understand better which is the real importance of having a really tight gap. My question is because I think that if we work the engine in an expansive way, the fact that there were a little more steam at the beginning of the stroke should help to get the expansion further. I am a bit confused on why this is not desirable.
Joan.
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Post by ejparrott on May 10, 2015 7:52:15 GMT
I've just been too busy!
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uuu
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Post by uuu on May 10, 2015 10:29:59 GMT
Nonetheless, I have one question about the space (or gap) that we should leave at the end of piston travel. It is generally considered that the less gap the better. In other words, we only set a gap to leave allowance for the eventual extra travel of the connection rod caused by the locomotive suspension. I would want to understand better which is the real importance of having a really tight gap. My question is because I think that if we work the engine in an expansive way, the fact that there were a little more steam at the beginning of the stroke should help to get the expansion further. I am a bit confused on why this is not desirable. Joan. I thought I'd try and apply some maths to this question, and see if I could make any sense out of it. Now I'm using abitrary units here, and I'm not using steam tables (I've never figured out how), so the behaviour of my gas is theoretical. And I'm not accounting for any heat losses to the cylinder or anything like that. I'm also assuming that the clerance volume, where relevant, is completely exhausted - there's no exhaust back-pressure. Suppose we take a cylinder of volume 100 units, and admit gas at a pressure of 100 units, for 50% of the stroke. Then we expand the gas for the remaining 50% of the stroke. If we have no clearance volume at all, then we've used 50 units of gas. And our pressure for half the stroke is 100 units and the average pressure for the other half of the stroke is 75 units, since we start at 100 and end at 50. So our work done is (50% * 100) + (50% * 75) = 87.5 units. Now we add a clearance volume of 5 units. So we take an initial fill of 55 units of gas. And at the end of the stroke the pressure is 52.4 units as we're expanding from 55 units to 105 units. So our work done is (50% * 100) + (50% * 76.2) = 88.1 units. So we've got more work out - but a cost of using more gas. If you work out the proportions the efficiency is slightly reduced. From 87.5/50 = 1.75 to 88.1/55 = 1.60 (aribrary units, so not a percentage answer). And as you increase the clearance volume more, you continue to get more work out, but always at a loss of efficiency. Now suppose we amend the cutoff - saying that as we increase the clearance volume, we could reduce the cutoff, to maintain a contant work output. This would improve the efficiency, by giving more expansive working. So to get a work output of 87.5 units, with a 5 unit clearance, we set the cutoff at 48.8% - we only use 53.8 units of gas, instead of 55 - and our arbitrary efficiency quotient, moves to 87.5/53.8 = 1.63 - so we're still not matching our ideal starting performance of 1.75. I have a secondary question though. Suppose in the real world that conventional wisdom is correct, in spite of my shaky maths, and that it is desirable to minimise the clearance volume. Conventional widsom also has it that the passages between cylinder and valve chamber should be nice and big, to allow for a free exit of exhaust. But this passage adds to the clearance volume. So we have contradictory position. Poppet valves in the cylinder (IC engine style) would resolve this, but assuming we stick with slide or piston valves, there must be a compromise on the port sizes. Wilf
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Post by Deleted on May 10, 2015 10:58:27 GMT
Joan , Wilf and Ed :
Ok I'll have a go at explanation . May be later today or tomorrow .
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Post by joanlluch on May 10, 2015 13:03:36 GMT
Hi Wilf, Thanks for your thorough reply. Your maths have a lot of sense to me and they bring some light on what could actually be the case. Your secondary question is one that I planned to make after a reply to the first one was given. So you advanced me by posting it yourself. Possibly the right answer to the second question is that we have more to loose with small passages than what we are loosing due to the extra volume. IC engines have highly optimised intake and outtake passages and multiple poppet valves to increase flow without adding clearance.
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Post by Deleted on May 10, 2015 21:07:12 GMT
(1) Other things being equal increased clearance volume gives increased MEP and hence more work done per stroke .
(2) The increase in work done is at expense of a drop in efficiency .
(3) With realistic values of clearance volume loss of efficiency is quite small and not much can be done about it anyway in a practical design .
(4) There has to be clearance between piston and cover at the dead centres otherwise steam would see a very restricted passage to get steam on the working face of the piston .
(5) There has to be clearance between piston and cover also to prevent any possible contact in adverse running conditions due to such things as minor errors of assembly , axlebox movement , thermal expansion , wear in bearings , ingress of scale or slugs of water (due to priming) .
(6) With passages the general view of full size designers was always that having free flow of steam and exhaust was much more important than losing a tiny bit of efficiency due to clearance volume . In many cases anyway free flow of steam more than compensated and overall efficiency was actually higher .
(Incidentally that was where Gresey's great strength as an engine designer was founded - all his designs had exceptionally free flow of steam all through the engine . If you watch that video again you will see the inspector talking of running at just 15% cut off and that was at high speed and with a heavy load - that could only happen with very free flow of steam .
Many otherwise good engines are choked at higher speeds and longer cutoff's have to be used to compensate .)
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Post by Deleted on May 10, 2015 21:41:30 GMT
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(5a) An often overlooked part of the cylinder cycle is the compression phase at the end of the exhaust stroke . The exhaust gets cut off at some point in the stroke in the same way as for the live steam . This leaves a volume of exhaust steam in the cylinder which is being compressed by the continuing movement of the piston . This is done deliberately so that the rising pressure in the cylinder acts on the piston so as to help slow it down smoothly - the inertia forces on a full size piston , rod , crosshead and con rod are quite large at speed . An incidental benefit of this is that the clearance volume gets filled with steam at reasonable pressure and at least moderate temperature and to some extent you recover a bit of the efficiency loss .
(7) Area of steam passages between valves and cylinder is determined by the valve port area - ideally they would be the same .
Calculating area of port needed to pass required volume of steam/sec is relatively simple . Usually just based on a reasonably high steam velocity as determined by design experience or from published tables . Could be done now by more sophisticated methods but answer would probably be no different .
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Post by joanlluch on May 10, 2015 21:47:54 GMT
Another question that I asked myself sometimes is about valve lead. Lead helps full sized engine pistons to cushion the end of their travel just before reversing direction. For small engines the commonly accepted practice is to limit lead to the minimum (even set it to zero) as it is considered not necessary due to the minimal inertial forces involved, and reduction of efficiency. However, I wonder if allowing some lead would be positive as well for small engines. My gut feeling is that in practice lead does not affect that much efficiency because the piston is almost at his dead end point when applied, and it is the other piston the one that is giving real torque to the wheels at this time. However, having some lead makes the valve to open more at the onset of the stroke, which may be beneficial for smoother and powerful running even at short cut-offs. So what about some lead in a 5" loco engine?
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Post by Deleted on May 10, 2015 21:50:58 GMT
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(8) Steam at higher flow velocities doesn't like flow restrictions , rapid changes of flow area or going around sharp corners . Any of these things will inhibit free flow of steam .
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Post by Deleted on May 10, 2015 22:08:50 GMT
Another question that I asked myself sometimes is about valve lead. Lead helps full sized engine pistons to cushion the end of their travel just before reversing direction. For small engines the commonly accepted practice is to limit lead to the minimum (even set it to zero) as it is considered not necessary due to the minimal inertial forces involved, and reduction of efficiency. However, I wonder if allowing some lead would be positive as well for small engines. My gut feeling is that in practice lead does not affect that much efficiency because the piston is almost at his dead end point when applied, and it is the other piston the one that is giving real torque to the wheels at this time. However, having some lead makes the valve to open more at the onset of the stroke, which may be beneficial for smoother and powerful running even at short cut-offs. So what about some lead in a 5" loco engine? Lead is primarily used to get full pressure on piston as early as possible in stroke . At the start of piston stroke valve is only just open and steam flow is restricted . Opening ports a little earlier compensates for this initially restricted flow . Effect of lead varies with running speed . At slow speeds it is not very important . At higher speeds it definately is important . Lead can actually be positve or negative ie with steam admission before or after piston dead centre . Usually positive on majority of full size engines but GWR sometimes used negative lead . GWR use of negative lead was not much benefit in itself but it compensated for the fact that some of their valve gears increased the lead as engine was notched up and the lead actually achieved at normal running condition was positive and correct value . Correct use of lead is as beneficial on little engines as on big ones .
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Post by Deleted on May 10, 2015 22:25:21 GMT
(9) Remember that everything to do with steam flow takes time . Not often done even in text books but thinking about steam events in real time is much more instructive than thinking about them in terms of percentage strokes ,cutoff's and crank angles .
(10) Steam events in engines running fast are significantly different to steam events in engines running slowly .
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Post by Deleted on May 28, 2015 16:42:42 GMT
Steam revesring gear .
Mention of the violent antics of the reverser pole on some engines brought to mind the subject of steam reversing gear . Only a few examples to be found on UK steam engines but more common in USA .
There are several working principles possible but only two ever caught on :
(a) Conventional screw reverse operated by a small steam engine .
(b) Semi-servo steam cylinder and 'cataract' .
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steam4ian
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Post by steam4ian on Jun 4, 2015 23:20:25 GMT
Michael.
Power reversing was used extensively on larger engines in Australia; made much easier by the availability of compressed air. On the rail system I know best, Franklin type reversers were used with either a screw of lever type driver's control. The reach rod was operated by an air cylinder. The air admission/exhaust to both ends of the cylinder was controlled by a D slide valve controlled by a position difference between the driver's level/screw position and the reach rod; this gave a servo action which held the reach rod in position. In the screw type system the screw is linked to the power piston so that in the event of air loss the system can still be operated manually.
I have seen and actuated the Hadfield steam reverser used by Beyer Peacock. The power cylinder is steam operated by a slide valve linked for servo action for the difference between the drivers control rod and the reach rod. Because steam condenses it will not hold the position of the gear so an oil filled cylinder is also connected to the reach rod. A valve linked to the servo controls the movement of oil to/from one or other end of the cylinder.
Ian
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Post by Deleted on Jun 5, 2015 8:11:39 GMT
Ian ,
Thanks for that .
Some of the servo type systems used in UK had a bad reputation for drifting and needing fairly frequent adjustment . Most of them had some kind of hydraulic lock cylinder .
Strangely one of the the simplest and most reliable systems with the small vacuum/steam/air engine to operate a normal screw was the least used .
Mechanical stokers were also a rarity in the UK .
On most British engines firing was very hard work but within the capabilities of one fireman .
There were just a few big engines like the Duchesses that just kept developing more power the harder they were fired and one fireman could not sustain the required firing rate for long periods .
Regards ,
Michael .
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