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
I am willing to accept that this is what happens, but I don’t understand why it happens.
Alan,
Don’t get your panties in such a wad. No one ever said there was a mechanical means! The fact that since there is no mechanical means brings up your next thought;
That common connection would be the exhaust pipe. My guess is that the scavenging of gases would be the root cause of the separate engines coming and staying in-sync for long periods of time.
Also note that I never said that they stayed in-sync forever. It just happened. And as I said, if the locomotive was well maintained, then it would stay that way until a slip or curvature or whatever caused the engines to go out of sync. After that, gaseous pressures being what they are would seek an equalibrium and cause the engines to come in-sync again.
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 is absolutely right on this subject. The front and rear engines on an articulated locomotive function from a physics/engineering as two separate steam locomotives on a common frame. They may run at times in sync, but it is purely from a statistical relationship. 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,
I don’t know why you threw the compound mallet into the mix. While operating in compound you are only going to get four exhausts per revolution no matter how you cut it.
More correctly, if the front engine slips, it runs out of steam because the rear engine can’t supply it fast enough. If the rear engine slips it get choked from all of the back pressure steam that the front engine can’t use fast enough. Ask Mr. Mellin and ALCO.
And that is what we are talking about here. And instead of flat out stating that a simple articulated locomotive could in no way ever run with both engines synchronized for any length of time
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
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?
No, you still have EIGHT power strokes. You may only hear 4 beats because they are close enough in phase to appear as one even though there are two beats happening.
Again, since the two engines are not physically connected in any way, they will slowly drift in and out of phase. There is no way you can guarantee that all the drivers are exactly the same size, have the same amount of wear, and slip at exactly the same time, for the same period of time.
You also cannot believe video or film soundtracks, as many of them were recorded with silent cameras, and the sound was recorded on a tape recorder, then dubbed in for the appropriate effect on the finished product. The sound may even have been recorded at a different point in time.
FWIW, Kratville’s book on the UP 4-6+6-4s says the later twin-stack engines did use one exhaust stand for each engine. No idea whether it’s true.
On the sync question: if the phenomenon exists, still pics will show it just as well as video-- or better. Most pics aren’t broadside enough to give a clear answer-- often we can’t tell whether the engines are 90 degrees out of phase or, say, 120 degrees. But try these
http://www.flickr.com/photos/2719/sets/72157607619367396/
You’d think there’d be lots more pacing (i.e. broadside) shots out there somewhere.
The picture along Hwy 169 shows the drivers 45 degrees out of phase. That would provide the smoothest total power strokes.
Trying to understand some of the hardheadedness that has made its way onto this thread, I went back to reread my original post. I can see how I didn’t make myself perfectly clear on this sychronicity deal.
Let me see if I can make my point clearer.
I know that there is no mechanical means of synchronization between the two engines of an articulated locomotive.
I know that the two engines of will go in and out of phase over the course of time. The frequency will vary due to wheel diameters, slipping and the like.
I also know that at times the two engines will sychronize themselves for some length of time, meaning that you hear four exhausts when there is actually eight. This is most apparent at a slow speed with heavy throttle and lasts for sometimes quiet a few minutes.
What I am not certain of is why this happens. I have offered up my idea as to why this could occur. It may be wrong or it could be right. The fact is, it happens.
I would like to know the true answer, but it may never come, at least not in this lifetime.
And Alan,
GP40’s comments do stink of diesel fuel. [;)]
You just didn’t see the “smiley” or the humor of injecting diesels into a steam thread.
No matter how the opposite sides are offset, you will still get only four power strokes per revolution on each engine since the two engines are not mechanically connected with each other.
I also read in a back issue of TRAINS that Lima’s proposal for the T1 included driving wheels of different sizes (80" on the front engine, 76" on the rear engine) to specifically prevent the engines from getting in sync.
One thing to keep in mind is when we talk about them being ‘out of phase’ we’re only talking about a tiny difference, maybe 1 or 2% because of a slight wheel slip or something. It’s not like the front drivers are going around at 30 RPM and the rear set are going around at 20 RPM.
On a true compound Mallet, the front driver’s cylinders get their steam from the exhaust of the rear cylinders, so they can’t be too far out of phase simply because the timing of the front drivers are determined by the timing of the rear driver’s exhaust steam.
What you hear is the steam ‘chuffing’ up the stack. On a true compound Mallet, the exhaust from the rear hi-pressure drivers are exhausted into the front low-pressure cylinders, not out the stack, so the rear drivers don’t “chuff”. On a simplex engine, all four drivers get steam from the boiler and exhaust it out the stack, so you could hear an out of phase “chuff chuff…chuff chuff…” on occassion. I believe the engineer on a simplex could adjust the steam going to just the front or just the rear cylinders, so I’d think could make slight adjustments to put the drivers back in phase if they slipped out due to a tiny wheel slip.
The passage of steam between engines on a Mallet does not really control the synchronization, nor is synchronization desired or needed. If it were desired and needed, such engines would routinely stall when trying to start a train or when trying to haul a train up a substantial grade when they get out of ‘synch’, which they do routinely…all the time, hour in, hour out, minute in, minute out.
View this video after the 2 minute mark.
http://www.youtube.com/watch?v=so7-Fu2psjc
Each engine has valve gear which controls the inputs and outputs in each case, not the puffs, not the chuffs, not the “pressure waves”, but the motion of the valves sliding back and forth in their own cylinders. What matters is the volume of steam available at the the intake ports when the spindle valve clears the first edge of the port during its baring stroke. Since the exhaust from the simple engine to the rear is done in a fraction of a second, each expanded steam stroke is largely the same as the one before it and the ones that will follow because the volume of spent/expanded steam is only slightly variant from stroke to stroke. If the valves and the ports were shaped differently, and timing different, then a much slower intake process to the main cylinders would very certainly have a profound effect on the steam available at the forward engine. But as they are, the larger cylinders at the front have a full charge of steam available to them, ableit expanded, each time their valves clear the leading edge of the intake ports.
So I take it to be. I am enjoying the discussion and discovery here. [:)]
-Crandell
When the rest of us talk about them being “out of phase”, we’re talking about the positions of the two engines at the moment, not their speeds. If the left crankpin on the front engine is at 9:00 when the left crankpin of the rear engine is at 10:30, then the two engines are 45 degrees out of phase, even tho they could be running at the exact same speed.
The engineer on a simple Mallet has two throttles, or two reverse levers? Or both?
There is only one throttle and one reverse quadrant.
The power reverse is connected to the rear engine’s reverse shaft lever ( Walschaert ) or reverse yoke ( Baker ) on the right side. This reverse shaft lever/yoke is attached to to the left side lever/yoke by the reverse shaft, so that they both move at the same time. On an articulated locomotive, another lever attached to the reverse shaft inboard connects to a rod going forward to a like lever and the reverse shaft on the front engine.