Articulated Steam Loco Exhaust

[betamax]

blue streak 1

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.

[timz]

Alan Robinson

dedicating one stack for each engine would give very poor operation indeed

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.

[blue streak 1]

The picture along Hwy 169 shows the drivers 45 degrees out of phase. That would provide the smoothest total power strokes.

[BigJim]

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.

[CSSHEGEWISCH]

blue streak 1

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 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. 

[wjstix]

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.  

[selector]

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

 

[timz]

wjstix

when we talk about them being 'out of phase' we're only talking about a tiny difference, maybe 1 or 2%

wjstix

...so they can't be too far out of phase

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.

[timz]

wjstix

I believe the engineer on a simplex could adjust the steam going to just the front or just the rear cylinders

The engineer on a simple Mallet has two throttles, or two reverse levers? Or both?

[BigJim]

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.

[CSSHEGEWISCH]

timz

wjstix

I believe the engineer on a simplex could adjust the steam going to just the front or just the rear cylinders

The engineer on a simple Mallet has two throttles, or two reverse levers? Or both?

A Mallet is by definition a compound articulated locomotive.  That being said, Mallets were often equipped with a starting valve.  This allowed live steam into the low-pressure cylinders while the locomotive was first starting up and exhaust steam from the high-pressure cylinders had not yet reached the low-pressure cylinders to a significant degree.