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Discussion Starter · #1 ·
HI,

After carrying out some of the modifications recommended for the alcohol-fired version of Accucraft's GS4 (incl. Gordon Watson's combination levers and nozzles and new wick material) I am still not REALLY happy. The performance is quite OK but could be better.
So I referred to some textbooks and articles on the subject, e.g. Martins Evans' books and J.v. Riemsdijk's articles in the G1MRA Newsletter.
As regards the front end, i.e. arrangement of nozzle, petticoat (chimney extension) and chimney, both authors recommend the same geometrical dimensions. I built two templates to check these dimensions on my Accucraft GS4, see first picture
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The template tapered 1:6 governs the height of the chimney top above the nozzle, which appears to be OK, see second picture
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However, after applying the second template tapered 1:3 which determines the extension of the petticoat, it appears that the petticoat is at least 1/4" too long!
[url="http://URL=http://img146.imageshack.us/my.php?image=petticoatrl4.jpg]
[/url]

How's this? This template has 8 stripes green/yellow each 7/64" wide. The template shall sit in the blast nozzle with the tapered part still inside the chimney extension or petticoat by about 3/16". So at least six of the eight stripes should show underneath the petticoat, however, on my GS 4 only three and a half are not covered by the petticoat. That means it is too long and most likely impedes draft!
Has anyone else observed this?
Has anyone unscrewed the petticoat? How is it fixed? I would like to shorten it to make another test.

Michael
 

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Michael,
               You dont state the reason for your search for an improvemnet in the Gs4 running since you are working in the draft area I assume its poor steaming?   I can see from the photos that you do not have a reducing bush in the central flue, without this bush the gas flow is unbalanced across the flue bank , effective heating area is smaller and steaming not optimum,, turn up and install the reducer bush , first and measure the change.

The petticoat unscrews from the stack body and can be shortened in the lathe .but there is no real need, if all else is correct.

                                                                                    Gordon.
 

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Discussion Starter · #3 ·
Gordon,

Thanks for the hint! Did I miss something here? What's the dimension for the reducing bush? Was this stated in your instructions for the combinations levers, too?

I changed the blast nozzle that was included in your package, but haven't modified the blower nozzle. I have no problems in raising steam, howver, while running I can't get over 50 psi with whatever train. I would like to see the safety valves release some steam at least once in a while to ensure to generate enough steam.

Anyway,  Aster locos raise the bar!

Michael
 

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Discussion Starter · #5 ·
Gordon,

The arch has yet to be installed...

That' s one of the reasons why I consulted the 'classical' sources: before venturing into 'fancy new areas', I wanted to make sure that the 'basic' design rules have been complied with.

Talking about 'fancy new areas': in one test run I had replaced the asbestos(?) wick sheet with fine stainless steel wire mesh - and nothing else! Each burner could only to be ignited by heating it with a miniature soldering torch from underneath until enough Meths had evaporated. This type of burner 'worked' , too, but the impact on the running qualities was at best marginal.


Michael
 

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Michael
I would guess that you have read our posts on GS4 modifications.  In the post your will see the arch in the firebox.  This one is a simple design held in place by the wick containers.  One could also do a baffle/arch as did Gordon (if I get a chance will post photo).
The reducing ring on the center flue in the smoke box was documented by Dave Hottman long time ago.  Recently we repack a burner set of wick using Gordon's layout of wick material- outstanding fire!  Might be the fuel flow, maybe need an extension adjustment on the sump.

Dave Hottman reference in archives(with contributions from Brooks, John and Andrew):
http://archive.mylargescale.com/forum/topic.asp?TOPIC_ID=35223&SearchTerms=GS4,restrict

Try original wick design with arch and restrictor in center flue.
 

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Discussion Starter · #7 ·
Charles,

Yes, I downloaded all the threads on the topic I could find, but it looks like I missed the one with the reducing ring (now, after re-reading the instructions provided with Gordon's kit, I see that he mentions the ring , too ).

I'll tackle these last mods over weekend.

Thanks to all for the help.

Michael
 

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RE: Accucraft GS 4 Chimney&Petticoat

Hi Michael,

My GS4 has the same problem as yours. Could not hold pressure above 50 psi while running.
Two fixes seem very important. 1) reducing the diameter of the blast pipe; It is not the exit volume that is important
but the exit velocity. 2)The fire box arch

It sound like you have done the first and should try the second.

After a year of fiddling these two changes made the difference.

jim
 

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Jim
Assumptions...given Gordon kit and the numerous ones we hae done; exhaust nozzle was spot on, so I never questioned that but it would effect the performance.  One can solder the hole and re-drill, 2mm
 
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