Pictures I have of simple articulated steam locomotives show two exhaust stacks. The captions say, “Double exhaust stacks, one for each engine, is a characteristic of a simple articulated”. Overhead pictures show smoke coming from both stacks so both engines must be exhausting into the smokebox.
How is this double exhaust accomplished? If both the front and rear engines exhaust into the smokebox how can the individual exhausts be separated?
Nick
Two blastpipes in the smokebox. One for each engine.
As the picture shows, there are two stacks, and the blast pipes for the engines are positioned below them. The blast pipes create the draft by exhausting the steam from the engine up and into the stack, drawing the combustion gases with it, and providing draft for the fire.
The reason for using two stacks is simple: velocity. If the stack is too large, it would be difficult to draw the comubustion gases up and out. Which would mean a poor draft.
The blast pipes are what creates the familiar "stack talk" or chuff. Especially when they start to move, you will hear the bark and a large plume of smoke shoots out of the stack.
It isn't just articulated engines that have double stacks. Some fixed-frame engines have them, too. Notably, UP's own 844.
-Crandell
But perhaps the front and rear engines of an articulated are out of phase to some degree. Pictures of articulated locos at rest may show front engine drivers up while the rear engine drivers down. Is this true?
On most simple articulateds, there is no mechanical link between the two engines so they are rarely synchronized.
Even compounds, those relying on expanded steam routed forward from the simple steam cylinders rearward, are often "out of phase", but it doesn't seem to affect them. If each set of cylinders has its own intake and exhaust porting, and each of those has pressure or vacuum as intended, then each bank of cylinders acts independently as an engine responding to changes in pressure. When a given valve passes a port in either engine, the condition of the volume at the port determines what happens to the piston ported. In the case of the exhaust, each cylinder exhausts spent steam which is routed through a conduit. In compounds, two of those ports route expanded steam, and two upward through the stack (single, as it happens, on the Y Class Mallets of the N&W). On Big Boys and Challengers, all cylinders route expanded steam directly up the two sets of blast pipes, converging to rise up a single pipe in each case.
CSSHEGEWISCH and selector,
Okay, I got it!! Thank you very much.
nick
CSSHEGEWISCH On most simple articulateds, there is no mechanical link between the two engines so they are rarely synchronized.
Paul,I think that you will find that a well maintained articulated will more often than not attain syncronicity on it's own. Obviously, if a slip occurs or one set of driver's tire diameter is different from the other then the engines will have that double-lick sound. But, there is plenty of video and audio tapes around that prove that after starting, articulated locos will come in-sync and stay in-sync for long periods of time. Even after a bad slip, if the two wheel sets are well maintained, the engine will shortly find it's way back into sync.
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nicknoyesThe captions say, “Double exhaust stacks, one for each engine, is a characteristic of a simple articulated”.
BigJim there is plenty of video and audio tapes around that prove that after starting, articulated locos will come in-sync and stay in-sync for long periods of time.
timzBy in-sync, you mean the two engines are both front dead center at the same time? Or does 90/180/270 degrees out of phase count as in-sync, since the exhausts are sort of in sync?
No articulated locomotive in North American practice (the conventional swiveling engine in front and the fixed to frame engine in back) had any mechanical means to synchronize the two engines. Nor did any of the duplex designs such as the Pennsylvania T1 have means to synchronize the two engines. They often operated out of sych. I have a whole disk of Union Pacific articulated locomotive recordings that illustrate this very well.
A blast pipe is a steam driven eductor device to produce a vacuum condition inside the smoke box. It consists of an inlet funnel section narrowing to a venturi and then widening to the exhaust opening we see as the upper end of the chimney. The exhaust nozzle shoots the exhaust steam out in a high speed jet where the momentum of the steam is transferred to the hot exhaust gases in the smoke box with the combination forced into the entry of the blast pipe. The shape of the blast pipe in modern engines is carefully designed to perform this mixing and momentum transfer as efficiently as possible so as to produce the most draft for the fire with the least consumption of steam and the lowest back pressure on the cylinders.
In the early days of steam locomotives when the boilers were small in diameter, most of the blast pipe length appeared above the top of the smoke box. You can often see the narrowest portion (the venturi) located just above the top of the smoke box. The entry funnel is inside the smoke box where the exhaust nozzle would shoot the exhaust steam into the entry funnel.
As steam engines became larger and the need to increase efficiency grew, there were innovations in the design of the exhaust system. Since the exhaust nozzle and stack could only be made so long while still fitting within the clearance envelope, the whole thing was first sunk inside the smoke box with only a bit of the stack sticking out the top. The next step was to enlarge the diameter of the stack but, with only a single exhaust nozzle shooting steam into the stack, efficiency suffered as the spacing between the large exhaust nozzle and the entry funnel was not optimal. Making the exhaust nozzle larger in diameter allowed too little space for steam and exhaust gases to mix before they entered the blast tube and there was too little velocity to cause the required vacuum.
The next step was to put in multiple smaller exhaust nozzles shooting into a single large diameter stack. This improved the efficiency by creating more mixing zone for the steam jets to entrain exhaust gases and blast them up the stack. This arrangement also allowed closer spacing between the shorter exhaust nozzles and the lower end of the blast tube.
When even this was too small for the horsepower of the engine, two stacks were installed with multiple exhaust nozzles blasting into each stack. UP 844 as well as many other high horsepower simple locomotives had this arrangement. Both cylinders would exhaust into all nozzles and both stacks at the same time. This prevents air from flowing back down the unused stack, thus destroying the vacuum in the smokebox, which is what would happen if both weren't used at the same time. The most extreme embodiment of this technique is the installation of four stacks and four exhaust nozzle groupings in order to improve efficiency.
It required power to create the vacuum in the smoke box, and that power was extracted from the exhaust steam. This caused back pressure in the cylinders which represented a power loss and an inefficiency. Thus, it was to the advantage of the railroad and the locomotive designer to optimize the exhaust system. In steam's last years,, this was a little known but important element of the high horsepower superpower movement in steam design. A well designed exhaust system could save a lot of coal and water and mean the difference between a successful design and a failure.
Articulated locomotives, whether they had one or two stacks and whether they were simple or compound, all worked in a similar manner. (Whether an articulated was simple or compound had nothing to do with how many stacks it had. For example, N&W class A and Y both had a single stack but the A was simple and the Y was compound.) A simple articulated exhausted all four cylinders into the same exhaust manifold and all exhaust nozzles and stacks worked simultaneously. A compound, of course, when operating as such, exhausted steam from the rear, high pressure cylinders into the larger front low pressure cylinders and from thence into the exhaust system. When starting, compounds could operate as simple locomotives using live steam at reduced pressure in the front cylinders. This required a starting valve to be used with the engineer switching the locomotive to compound operation once the locomotive was up to around 4 or 5 miles per hour. Some compounds (N&W Y6B class, for example) could operate as simple at much higher speeds.
Any steam engine using two cylinders had the two cranks set 90 degrees apart rather than 180 degrees, as is sometimes supposed. This is necessary to allow the locomotive to start no matter in which position the locomotive drivers are positioned. If the cylinders and pistons were cranked 180 degrees apart, then when the pistons were in the frontmost or rearmost position the engine could not produce any torque. Locating the cranks 90 degrees apart means that there is always a piston or pistons able to produce starting torque. Three cylinder locomotives would usually have the cranks at 60 degrees or nearly so for the same reason.
Alan RobinsonNo articulated locomotive in North American practice (the conventional swiveling engine in front and the fixed to frame engine in back) had any mechanical means to synchronize the two engines.
The two engines of an articulated locomotive drifted into and out of phase constantly.
If they are only a little out of phase, the human ear will not be able to recognize the difference.
At speed it all sounds like one loud noise.
One thing (which someone may have touched on and I missed it) is that in a true compound Mallet, the steam would first go into the smaller rear low-pressure cylinders, then be exhausted into the larger forward low-pressure cylinders. After being used by the low-pressure cylinders, the steam would be exhausted out of the stack.
Rembmer that it's the sound of the steam coming out the stack - not the cylinders - that you hear. So on a compound engine, you'd only be hearing the steady chuff...chuff...chuff of the exhaust from the front cylinders. On a simplified articulated, all four cylinders got high-pressure steam, and all four exhausted directly to the stack. So on those engines, if the rear and front sets of drivers got a little out of synch, you could hear the out-of-phase chuff-chuff...chuff-chuff...chuff-chuff until they sycnhed up again since both the front and rear cylinders exhaust directly out the stack.
BigJim Alan RobinsonNo articulated locomotive in North American practice (the conventional swiveling engine in front and the fixed to frame engine in back) had any mechanical means to synchronize the two engines.That is what is facinating about this entire thing. That two separate engines under one boiler could synchronize themselves not very long after not being sychronized at all.
And would go right back out of synch just as quickly!
This matter was discussed about a year ago. An Australian engineman named Mark Newton (since retired from the forums) went into considerable detail as to the reasons, but what it all boiled down to was that, absent some mechanical linkage between the two sets of drivers, even tiny differences between the two engines would result in different revolution counts while traveling the same distance down the track.
Also, twin stacks were NOT a characteristic of simple articulateds (note the N&W Class A, or the Clinchfield's Challengers - the latter had the twin stacks removed!) Steam exhaust and stack geometry was an evolving science right up to the end of reciprocating steam loco operation, and some rather odd configurations were developed and used (Like the Geisl - a long, skinny 'slot' stack and seven exhaust nozzles.) Therefore any 'one size fits all' pronouncement should be viewed with considerable skepticism.
Chuck
tomikawaTT And would go right back out of synch just as quickly!
BigJim tomikawaTT And would go right back out of synch just as quickly! Not necessarily so, and I've got over eight hours of tapes to prove it.
I don't have as many hours showing that they do, but I do have such footage. On a Yellowstone of the DM&IR, no less.
I repeat, no articulated or duplex locomotive had any mechanical means of synchronizing the two engines. That being said, let's examine possible mechanisms to account for sometimes "seeing" synchronization in such locomotives.
The true mallet, a compound locomotive wherein the exhaust steam from the rear engine was reused at a lower pressure in the front engine, exhausted the steam from the rear cylinders into a common pipe manifold directing that steam to the front engine, where it then supplied both front cylinders. Because of this, there was a steam pressure connection between the engines, not a mechanical one. The connecting manifold experienced four pressure pulsations per driver revolution of the rear engine. These pulses were in turn applied to the front engine, so this supplied a means for synchronizing the two engines. However, this synchronizing mechanism wasn't very strong and could easily be overcome by heavy loading or positioning of the throttle or reversing lever. Any difference in driver diameter would usually cause the two engines to move slowly in and out of synch continuously. The front engine of most mallets was more prone to slip than the rear, especially on early machines, as it was difficult to equalize axle loading, weight transfer and piston forces in these machines. A lot depended on the skill of the engineer, too.
The mallet was a good design for one reason, however. If the front engine began to slip, it would automatically transfer more of the load to the rear engine by quickly exhausting the steam in the connecting pipe. This increased the pressure drop across the rear engine to force it to do more work while simultaneously reducing the amount of work the front engine was doing until the engines were brought back into balance. Similarly, if the rear engine slipped, the front engine would be supplied with a higher pressure and quantity of exhaust steam, forcing it to assume a greater share of the load while the rear engine would produce less power due to the lower pressure drop across the rear engine, again moving them into balance. But this balanced condition would rarely amount to something we could speak of as being absolute synch as would be obtained with a mechanical interconnection of the two engines.
There were two ways to vary the power the locomotive was producing. One was to vary the throttle setting and the other was to vary the cutoff by means of the reversing lever. These compound engines, because of this pressure connection between the front and rear engines, didn't take kindly to operation with the throttle wide open and the reversing lever notched way back to reduce power. This caused maximum pressure variation in this connecting pipe, very rough running and the real possibility of engine damage including broken frames or connecting pins between the front engine and the rear. Reducing power by throttling and keeping the reversing lever away from the center provided for steadier pressures in the connecting pipe and much smoother running. This method of operation alone greatly weakened the tendency for the two engines to achieve perfect synchronization since it minimized the amount of coupling by means of those pressure variations.
Simple articulated locomotives were a different story. Each engine was supplied with full pressure steam from the boiler and the exhaust steam from each engine was routed separately to the exhaust nozzles and stack or stacks. The only possible mechanism that might account for prolonged periods of synchronized running would be the fact that both engines shared a common connection pipe from the throttle valve, through the superheater and then from the superheater outlet to the inputs of all four cylinders. There was a pressure drop across the throttle valve and superheater and the connecting pipes. This means that the pressure at the inlets of the four cylinders would undergo small pressure variations caused by drawing of steam by all four cylinders. Apart from this weak connection, these locomotives had no more tendency for the two engines to run in synch than if they had been mounted underneath separate locomotives coupled to the same train. Any small disturbance such as driver diameter differences, valve settings, bearing friction, weight distribution differences and operator skill would cause the engines to operate at slightly different speeds. Even fairly modern engines were prone to front engine slippage. Union Pacific's first run of Challengers was especially noted for this, a problem greatly improved in their second batch.
Again, for both types, any cause of coupling between engines would be extremely weak and easily overcome by the factors discussed above.
BigJim That is what is facinating about this entire thing. That two separate engines under one boiler could synchronize themselves not very long after not being sychronized at all.
That is what is facinating about this entire thing. That two separate engines under one boiler could synchronize themselves not very long after not being sychronized at all.
I am willing to accept that this is what happens, but I don't understand why it happens.
Alan RobinsonI repeat, no articulated or duplex locomotive had any mechanical means of synchronizing the two engines.
The only possible mechanism that might account for prolonged periods of synchronized running would be the fact that both engines shared a common connection pipe
Big Jim, my undies remain nicely fitted, thank you. If there is any pique in my message it is directed at your constant repetition that the two engines of an articulated locomotive will "come into synch and remain that way for prolonged periods" and that you have over eight hours of tapes to "prove it". Your tapes prove nothing other than that your watching them has convinced you that you are seeing something that others don't necessarily see. You also offer no mechanism or reason why this synching might or should occur other than that it happens in a "well maintained locomotive". In the absence of any more rational explanation, or even theory why such synching might happen, I tend to doubt the assertion.
In contrast to your posts, my recent post posited possible reasons why such synchronization might occur, how strong those synchronizing forces might be, and why they might or might not account for what you say you observe. It is my observation and opinion, as well as the opinion of others who have actually operated such locomotives, that the synching mechanisms at play in either simple or compounded articulated locomotives simply aren't strong enough to cause the two engines to run in synch with any degree of certainty and, indeed, they don't.
I've got many recordings of articulated types, both simple and compound, and much more often than not I see and hear them slowly drifting in and out of synch for the reasons and in the manner I mentioned in my post. However, rather than relying on this as "proof", I've attempted to understand and present how these locomotive types were actually built and operated and how the details of their construction and operation might or might not account for any possible synchronized running of the two engines.
This is a fascinating subject worthy of study, but not worth fighting about. Enough said.
Alan RobinsonThe true mallet, a compound locomotive wherein the exhaust steam from the rear engine was reused at a lower pressure in the front engine, exhausted the steam from the rear cylinders into a common pipe manifold directing that steam to the front engine, where it then supplied both front cylinders.
Simple articulated locomotives were a different story.
I should have said that my observations come from the single stack N&W Class A. Double exhaust stand locos such the UP Challengers, I would assume, would have a greater tendency to run out of sync due to their lack of a common exhaust stand.
An analogy is that there would be statistically times when the diesel engines on a multi-locomotive consist would be firing all cylinders exactly at the same moment, but most of the time they are going in and out of sync.
Alan et al.,
There is very little point in arguing with Jim on any aspect of steam locomotive operation. He thinks he knows far more than he actually does.
BigJim, the reason that compound articulated locomotives are relevant to the discussion is that they have two engines. Looking at such a locomotive in operation, one would notice that the two separate engines would not turn at exactly the same rate. The steam pressure coupling means between the front and rear engine in a compound is, if anything, stronger than it is in a simple articulated. The pipe conducting steam from the exhaust of the rear engine to the inlet of the front engine provides a source of steam for the front engine (and a sink for steam from the rear engine) that is characterized by pressure pulsations. If there is any explanation for the two engines to run in synch with each other, it would be the fact that the pulsations within this pipe would be at a minimum when the two engines were running in absolute synch with each other. But such is not necessarily the case, as observation of such locomotives clearly shows. Even though you don't hear the two engines coming in and out of synch with each other because only the front engine exhausts into the stack (at least when operating as a compound locomotive), you see the lack of synchronization with your eyes is you watch the show.
As I stated, simple articulated are a different story. Part of the confusion here seems to be arising from an inference contained in a photo caption earlier in the postings that simple articulateds with two stacks use one stack for the rear engine and the other stack for the front engine. Clearly this is not the case. Your reference to the N&W class A (along with many others) as being a simple locomotive with a single stack puts the lie to that idea. If you think about it, dedicating one stack for each engine would give very poor operation indeed, as I pointed out in an earlier post. Suppose the rear engine is exhausting into only the rear stack, while the front engine is mid stroke and not exhausting at all through the other stack. This would mean that the draft drawn by the rear stack would be "short circuited' by air rushing downward through the unused front stack, greatly diminishing the draft through the tubes and flues. So, simple articulated locomotives with twin exhaust stacks operated exactly the same way as those with a single exhaust stack did. The exhaust steam from both engines was connected into a common exhaust manifold which was connected to all of the exhaust nozzles beneath all of the stacks so that all stacks were in use at the same time. Functionally, there was no difference between the N&W class A and the Union Pacific's Challengers and Big Boys. (The differences between the tendency of the two Union Pacific Challenger classes to exhibit slipping of the front engine was due to changes in springing and supporting mechanisms that loaded the front engine more effectively with it's proper share of the locomotive weight on drivers in the later design. It had nothing to do with changes in steam piping per se.)
The question remains whether the piping shared by the front and rear engines could provide enough of a synchronization mechansim to account for what you assert to be true. Both engines share a common connection from the steam dome, through the superheater and throttle valve. From that point, or a point quite close to it, the steam flow is broken up so that each engine draws live steam separately. Then the steam exhausts and is brought together again in the common exhaust manifold and exhaust nozzles. Pressure drop in the piping common to both engines will be the same no matter which engine is drawing/exhausting steam and in any case is very low as losses here would represent substantial drops in efficiency so the piping is sized with ample capacity. Suppose they both draw/exhaust at the same time. Is there any reason they should remain in that condition? The pressure available to the cylinders would be lowest in this mode of operation. Suppose the two engines stagger their operation so that one draws/exhausts steam when the other is in mid-stroke. In this case, the pressure available in the cylinders would be slightly higher, but the diffence between the two conditions would be quite small due to the large pipe sizes. However steam, being a gas, is compressible, and in all this piping there are pressure waves set up every time a valve opens or closes, mushing up this simple picture of things and swamping the frictional losses. What do we see in real life? Not a very strong coupling between the front and rear engines, nowhere near strong enough to overcome the ordinary differences in wheel diameter, valve settings, track conditions, etc.
GP40's comments don't "stink of diesel fumes", but there is a better way to think about it. Suppose GP40's diesel engine was built so that all 12 cylinders were connected to common intake and exhaust systems and common fuel supplies, but that each one had it's own crankshaft connected to its own load. What would the chances be that all would run at the same RPM even with careful attention to matching the loads? Not very great. Even with only two cylinders, the odds are that they would not remain in synch short of some kind of positive feedback mechanism. There has to be a common crankshaft to do that
In articulated locomotives, the most powerful feedback mechanism forcing the engines to run in approximate synch is their contact with the rail. That's it, really. All the rest is wishful thinking.
question: If they are in sync then you have only 4 power strokes per revolution of both sets of drivers; if they were 45 degrees offset per power stroke wouldn't you get smoother power with the 8 strokes per revolution?
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