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
Thank you; cprted, betamaz and selector; from you answers it seems I am missing a critical fact in understand what is happening; do the front and rear engines work in unison?I know that for a single engine the left and right side power strokes do not work in unison. The alternating power and exhaust strokes are timed to be one quarter turn out of phase ensuring the four power strokes are evenly distributed.
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?
Nick
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
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.
Lots of simple Mallets had circular stacks; in the case of the PRR 2-8+8-0 that was because it had four exhaust stands, but I suspect the others had one.
By 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?
What I mean is that you will hear only four exhausts per revolution. Not eight beats or the double-lick or odd exhaust beat.
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
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.
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.
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
Not necessarily so, and I’ve got over eight hours of tapes to prove it.
Depends on wether or not one engine slipped or not. There was no mechanical connection between the two engines, so their beats could/would drift in time. If the drift was very slow, you wouldn’t notice it, especially over only a few minutes.