Author Topic: SS Xantho John Penn trunk engine  (Read 16919 times)

PatJ

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Re: SS Xantho John Penn trunk engine
« Reply #30 on: January 18, 2014, 07:06:55 PM »
Here is my "get in the ballpark" layout for the engine, at 1/8" scale.

I drew the passages at the bottom of the cylinder, but be aware that you would only see those when looking down on the top of the engine, not in the side view I am showing.

Bore is 2.625", stroke is 1.5".

Due to the angularity of the rod, the piston is not quite at mid-stroke.
The dashed lines show the piston at TDC.

Peter-
Nice video.  Some terrific engines in that place.

Edit:
Sometimes the valve face has to be extended outward to align with the eccentrics.
« Last Edit: January 18, 2014, 07:15:59 PM by PatJ »

Offline Pedro

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Re: SS Xantho John Penn trunk engine
« Reply #31 on: January 18, 2014, 11:52:31 PM »
Very nice it is too! No way a criticism, looks like there's a typo (ID/OD) on the trunk dimensioning. I'm still hopeful for input from the museum, will let you know asap. Well in the ballpark there I'd say. Well done.
« Last Edit: January 18, 2014, 11:56:14 PM by Pedro »

PatJ

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Re: SS Xantho John Penn trunk engine
« Reply #32 on: January 19, 2014, 04:52:24 AM »
Thanks Pedro-

Yes, you are correct, I did miss that dimension.
I laid out the bore dimension (in haste), and offset out from that in the wrong direct.
That sketch is an just an hours work, but the geometry appears to be correct for a functional engine, ie: everything has the correct clearances in all the extreme positions of piston and crank/rod travel.

You can transpose a few lines inward to correct the bore dimension.

Nice engine though.  I like it.

I was surprised to see the steam chest design that extended upward for the steam pipe in one of the engravings for another engine.
It appears that it was standard practice of the day for trunk and direct connected engines?

These engines are somewhat rare, and probably the class of engines I know the least about.

It would really be nice to get some plans from the museum, but I have only heard of one individual having success with that.
I think museum people are very protective of what they have, and sometimes to the detriment of the flow of technical information.
Museums should at least photograph the drawings they have and publish them, but I don't think they do that.

Pat J

Edit:
There are some striking similarities between the Xantho engine and the engine below.
If you moved the condenser? on the right from the engine below, it would appear to be almost exactly like the Xantho engine, with the excepting of the engine below using crossheads instead of trunks.
The wheel with the worm gear would be how the engine was reversed, or the link position changed.
And they actually got the link suspension point correct on the engine below, which is at the center of the link, slightly inward of the link centerline radius.

The engine below gives a good idea of the alignment between the steam chest/valve and the eccentrics.
And like the Xantho engine, the cylinders on the engine below appear to be cast as separate units and then bolted together.

I am guessing from how shallow the steam chest on the engine below is, that the valve was a balanced type, and the top of the valve rode against the steam chest cover?
« Last Edit: January 19, 2014, 05:11:44 AM by PatJ »

Offline Pedro

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Re: SS Xantho John Penn trunk engine
« Reply #33 on: January 19, 2014, 09:51:51 AM »
I'm no expert, but that steam chest design is common on the engravings that I've seen. It could be that considering the low pressures of the time (about 2 atm + vac) it was the most reliable steam tight connection. Though the Xantho engine is reported as designed for 90psi (elsewhere 60) they could have continued it out of habit. Being a small engine, the forces on it would be much lower than with some of the monsters they built.
The original print that I have from The Engineer 24 Dec 1897 p623 (graces guide has it online) appears to be from a later mid-sized engine, though that's just my judgment. The drawing clearly shows a leaf spring preloading the valve, so this one at least isn't balanced.
Another point of interest is that the steam passages are partly in the cylinder covers. This may have reflected difficulties with shifting cores,or just contemporary thinking on getting the steam in the right place behind the piston but whatever it's an elegant detail. though it looks like the Xantho was more orthodox.
There are 3 original prints, all off the same plate by the look of it, on UK ebay, either labelled "John Penn trunk engine" or just "Trunk engine" The resolution on the photos is quite good, and worth a look, though they are of quite a large engine I think. 
Re the museum, I'm hoping that they're not being jealous and just haven't been asked. Also policies can change in 20 years,  Drawings are potentially an IP asset though so I can perhaps understand their possible reluctance to publish on the net, but I'd be surprised if they refused more photos and a few vital statistics. I guess it depends on whether there's someone interested enough to help. Alex Kilpa at the museum has published a recent thesis on the boiler safety valve, so it's not as if the original crew are all retired and "no-one knows nuffin".
« Last Edit: January 19, 2014, 11:07:10 AM by Pedro »

PatJ

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Re: SS Xantho John Penn trunk engine
« Reply #34 on: January 19, 2014, 04:21:00 PM »
Pedro-

That is some interesting information.

I found an engraving of a Penn trunk engine on the open internet, and I will post that in a minute.

"The Engineer" article is very interesting, although a bit misleading perhaps.
The article mentions that the crankshaft is counterbalanced, as is the valve.
More correctly, they should have said "the valve is balanced", as there is no counterbalance for the valve mass.

The leaf spring affair is actually the inside shape of the valve which helps turn the steam as it flows into the exhaust port.
You can clearly see that the valve is balanced (at least I can, do others see that?).

The recess in the cylinder heads is only at the ports, and this allows for a higher efficiency engine since you reduce the clearance space between the cylinder head and the piston at TDC which would otherwise have to be filled and emptied with every stroke, but adding no additional power to the piston.

And you can see that the piston over-rides the end of the cylinder, which was common practice to keep from wearing a ridge in the end of the clyinder bore.  Valves also over-rode their seats for the same reason.

And the article mentions 6 psi, which sounds more like the old style pressure levels.
I am more familiar with the modern "high speed" steam engines; ie: the ones that ran from 150-300 rpm at the higher pressures, perhaps 50-100 psi.

Apparently the trunk engine was the next step in steam engine evolution from the large oscillators used for the sidewheelers? (check me on that).

And the offset of the steam chest towards the head end of the cylinder is interesting.
Apparently the engine was made as compact as possible, and then the steam chest position adjusted for an asymetrical location to allow the eccentric arms and link to fit.
This arrangement makes for unequal passage lengths, which would affect the engine performance to a slight extent, and perhaps compensated by valve gear adjustment.

And the article mentions that the trunk and piston are cast as one unit, but at best only the trunk on one side and the piston could be cast as one unit.  You can clearly see the bolts holding one trunk to the piston and other trunk.
This allowed installation of the wrist pin, and also made for a convenient installation of the piston ring or rings.

Offline Jasonb

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Re: SS Xantho John Penn trunk engine
« Reply #35 on: January 19, 2014, 04:53:10 PM »
I'd say there is a leaf spring holding the valve to the port face, there is certainly something with a shallow eliptical shape fixed to the valve chest cover. Its certainly not the curved cavity within the valve.

It would not be a leaf spring a swe imagine them on a car or truck, more just a thin brass shim with a curve to give some "spring" and just enough to hold the valve to the port. Probably fixed at one end and slotted at the other to allow it to get longer as it is flattened.

 The porting within the cylinder end covers would just be a small area scalloped out adjacent to where the port exiys the cylinder, much the same way as we mill or file out a small are aon models to get the same results.

I also don't think the piston over runs the cylinder, the only place that is look as if it may do is where the steam port is, the rest of th eway the bore is parallel as it all the way with no increase in dia towards the ends

For some ideas about sealing the trunk on a model it would be worth looking at Anthony Mounts "Bodmer's sliding cylinder" engine, as that has a central tube or trunk with fixed piston that moves withing the cylinder although in this case the trunk is fixed and the cylinder moves but sam esort of setup.

J
« Last Edit: January 19, 2014, 05:40:56 PM by Jasonb »

Offline Pedro

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Re: SS Xantho John Penn trunk engine
« Reply #36 on: January 20, 2014, 12:57:13 AM »
   As I understand it, the origin of the trunk engine (and the back-acting engine) was due to the strict military requirement for a screw engine that remained below the water line, out of the direct line of shot.  As such, it had to be horizontal, and could not afford the space taken by a crosshead. Given the Admiralty specifications, there aren't many other solutions. They are reported as being mechanically problematic, to the extent that there was a "right" and "wrong" main direction of rotation; the power pulses were arranged so that con rod angularity pushed and pulled the piston upwards against gravity, because they found this reduced wear. (source Xantho project). These problems  would have made them unsuitable for merchantmen and it seems that they were rare outside warships; Xantho's was war surplus after all.

    Pat, I must disagree with you on a couple of points. My copy of "The Engineer" article is an original, and the spring is clearly visible.between the valve and the cylinder cover, with a fastener to the cover at one end, and a radiused skid at the other.
    Re pressure, the 6psi quoted was for the previous decade. There are too many references to the gunboat engines working at  90psi and 180 rpm  to be baseless and we must remember that they are famous for being the world's first high pressure ship engines and were cutting edge technology for the world's only industrial and Naval superpower. 

    Jason, how do I find out how Mr Mount did it? I do believe that trunk engines could perhaps be viable modern steamboat engines with up to date seal and rod guide technology.
« Last Edit: January 20, 2014, 01:11:48 AM by Pedro »

PatJ

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Re: SS Xantho John Penn trunk engine
« Reply #37 on: January 20, 2014, 10:36:21 AM »
I believe it was common practice to have the piston over-run the bore slightly.

I know for sure the D-valves over-ran the seat.

The piston should not extend into the port area, this is documented in the book by Charles Porter; it caused piston slap and knocking.

I have looked at all the engravings I have, and the only spring I see is in the condenser.
I don't see a spring at the valve, just the valve up against the flat interior surface of the ribbed steam chest cover.
Maybe you have a better or different engraving than I do.

The trunk engine has other problems even if it is rotated correctly.
Some of the documentation mentions that the large trunk thermally conducts too much heat out of the cylinder.
The trunk engine was an early design, and inferior to the crosshead-type engine.  It would not be a good modern engine even if the seal problem could be overcome.

It is a very interesting engine type, but just as it obsoleted the oscillatiing engine, the crosshead type engine obsoleted the trunk engine, the high speed engine obsoleted the low speed engine, and the compound obsoleted everything previous to it.  It was just progression of knowledge and design experience.

Offline Jasonb

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Re: SS Xantho John Penn trunk engine
« Reply #38 on: January 20, 2014, 12:40:57 PM »
Pat, this is from the 1897 Engineer


Red arrow shows that the piston overruns the port.

Blue arrows show that the rest of the way around the circumference there is no over run of the piston

Just inside the valve chest I can see the curve of the leaf spring and a fixing at one end sketched in green just above. I have also sketched what I think the valve spring looks like

Link to the engineer here
« Last Edit: January 20, 2014, 12:49:08 PM by Jasonb »

PatJ

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Re: SS Xantho John Penn trunk engine
« Reply #39 on: January 20, 2014, 01:45:25 PM »
Jason-

I see what you are saying about the lack of counterbore in the bore.
That is a bit odd, but it is as you say.

For a slow speed engine, I guess it is not significant.

The article mentions that the crank and valve are "counterbalance", and I assume that means the valve is balanced.
And so if the valve is balanced, then there is a hole in the top of the valve which keeps the top of the valve at nearly the exhaust pressure, and with a spring as you show, then steam would be admitted into the hole and out the exhaust at all times, so something does not add up with that.

Offline peatoluser

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Re: SS Xantho John Penn trunk engine
« Reply #40 on: January 20, 2014, 02:15:20 PM »
I'm no expert and bearly understand valves, so feel free to correct this, but sometimes the valve linkages were counterbalanced. metallurgy was in it's infancy especialy for bearing surfaces, and sometimes a counterweight was fitted to try and alleviate wear. as here

http://www.digitalis.uni-koeln.de/Matschossd/matschossd_index.html   page 650

I know it's a side lever engine but it does show the principle, and I think that is what is going on here as well
http://collections.rmg.co.uk/collections/objects/68060.html

perhaps when the article refers to a counterbalanced valve , it's refering to something like that?

on the plan view in the engineer (also here in matschoss :- http://www.digitalis.uni-koeln.de/Matschossd/matschossd_index.html    page 710 )
there does seem to be two weights jutting out on the right hand end

yours

peter

PatJ

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Re: SS Xantho John Penn trunk engine
« Reply #41 on: January 20, 2014, 02:25:30 PM »
I am no steam expert either.
I make comparisions between what I see in the old engravings, and what I read in the old books, and try and connect the dots.

Steam engines underwent a tremendous development in a very short period of time.

Here is an engraving for a balanced valve.

Offline Pedro

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Re: SS Xantho John Penn trunk engine
« Reply #42 on: January 20, 2014, 04:04:19 PM »
A couple more titbits. Xantho project states that the gunboat engines needed so much maintenance that there was if possible a fully equipped engineering support vessel accompanying them on deployment. One has to wonder if that experience was what persuaded the Admiralty to abandon the below water line rule and thus enable the introduction of crosshead engines.

Also, the 1/6 scale model at the museum was completed before the engine was dismantled and thus the internal design could only have been guesswork, so unless there's a more recent model out there, no one has yet made a dimensionally accurate one.

Pat, I wholeheartedly agree with you about reduced thermal efficiency, but efficiency isn't a huge consideration for small steamboat engines. I strongly feel that the major weakness of the design is the reliance on the packing as a bearing to take the side thrust from the con rod, and it may well be possible to fix it with modern bearing materials. Relying on direction of rotation to save wear is at best a bodge and highlights inadequate design
If it could be fixed, it would make an interestingly different  machine. 
« Last Edit: January 20, 2014, 04:10:08 PM by Pedro »

Offline Jasonb

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Re: SS Xantho John Penn trunk engine
« Reply #43 on: January 20, 2014, 04:20:10 PM »
Pat, as Peter says the valves were counter balanced on some engines not pressure balanced, if you look at this one that you posted you can see that the linkage to the expansion link/ die block has a counter balance weight in much the same way as a crankshaft is counter balanced. I suspect there is something similar on teh Penn engine.



PatJ

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Re: SS Xantho John Penn trunk engine
« Reply #44 on: January 20, 2014, 05:31:39 PM »
Pedro-

I have learned to never say never, since I have seem people suggest improbable solutions several times in the past, then they actually worked well.

There is always the possiblility of solving the sealing issue, but I also recall the Wankel car engine which was suppose to replace all other auto engines.
The Wankel did run very well, but its achilies heel was the seals, and that was never solved.

Edit:
I did see some special gland nut seals on and engine, but I forget which engine, and it seems like they were metalic rings perhaps sanwiched between regular packing.  But the article said that it solved all of the piston rod sealing issues.

I find it ironic that the first known engine (Hero's engine) is still the steam engine that is in common use around the world.
The uniflo had great promise and very good efficiency, but it came too late in the game.

Jason-

I see the weight on the valve gear to balance it out, and that may indeed be what the author is referring to, but the D-valve in the engraving looks a lot like the balanced valves I have seen, since it extends up to the face of the steam chest cover, and the shapes on either side of the valve could be seals.
I will try to research when the balanced D-valve came about, and perhaps that could rule it out as a balanced D-valve.

I made a 3D model of a Stanley 20 hp engine from the drawings from the Stanley museum, and noticed several things that I later found mentioned in the old books such as the valve often traveling only 80% of the port opening (sometimes the valve overtraveled in some engines), and the over-run of the moving parts such as the pistons, crossheads, valves, etc.  Generally anything that moved was designed to over-run if possible to prevent ridge buildups.
A ridge would destroy an engine as the bearings wore.

The Stanley also had roller bearings on the large end of the rods.  I had heard that these were ball bearings, but they were actually roller bearings.

I wonder if the museum could clear up any of these points?
Surely they must know.
« Last Edit: January 20, 2014, 05:37:17 PM by PatJ »