Mamod Ceramic Gas Burner

[Chris Cairns]

With the recent discussion concerning the failure of the Thomas Telford gas burner I thought some info on these burners may be of interest.

The style of the burners has basically remained the same although the type of ceramic material has changed.


William burner at the top, William II burner at the bottom

The ceramic material used in the Golden Jubilee & William burners was quite rough and was mounted quite high in the burner box. The ceramic material used in the William II, Mark I & II, and Brunel burners is of a better quality and mounted closer to the top edge of the burner box.


My Mark II burner (the staining on the left side is caused by the exhaust condensate getting everywhere)

The ceramic material in my Brunel burner has been sealed in around the edges with Instant Gasket material, and I now use this when I refurbish my burners.


Example of a cut edge that needs sealing in

The gas jet holder does not allow much movement to adjust the gas/air mix, and one assumes that Mamod have decided the mix is right with this holder pushed all the way in as they have no instructions for checking the correct gas/air mix. The jet has no number marked on it so not sure what size it is. The holder is also a very loose fit into the burner which I believe was the cause of a rather scary blow back on one of my locos. So I wrap the jet holder with PTFE tape so it is a snug fit into the burner.



The old nylon gas pipes have an olive fitted to the jet holder end to effect the seal (I assume the copper gas pipes are the same). At the other end the nylon/copper gas pipe is held into the gas tank with a knurled nut compressing an 'O' ring.



However the face that the 'O' ring compresses onto is rather rough from the machining, and needs some filing to smooth it out (I suffered a leaky 'O' ring on my William burner probably due to this rough surface).

I've just finished refurbishing this Mark II burner and I've replaced the nylon gas pipe with some copper tubing. I think that the combination of air slots in the chassis frames plus 4 holes on each side of the combustion chamber walls is allowing to much air in or heat out and causing the burn pattern to drift when the loco is running. I'm going to look into fitting a metal strip to block off those 4 holes each side to see if I can improve the burner's stability.

Chris Cairns.

[mikewakefielduk@btinterne]

When I discussed the Thomas Telford burner issue with Peter Johnson he said they'd been surprised to find their burner used a jet size 20 while the trial burner from Roundhouse was much smaller (a 5 or an 8, I forget which). Of course the burner design is fundamentally different so presumably the much bigger jet is required for a ceramic burner.

Do let us know whether closing the air holes in the Mk11 chamber walls makes any difference.

Mike

[Chris Cairns]

Now that is a very interesting piece of information.

I'm not aware of anyone else using a No.20 size gas jet, the popular sizes being 5, 8, 12 & 16. I did find a website which quoted the flow rate of gas that the different sizes produces but I cannot find it again at present.

So something else to experiment with when I have some time available - next week after I finish the OO Live Steam demonstrations.

Chris Cairns.

[dougrail]

Um, I'm no expert but...yea. size 20? Is that necessary? My Bixer was a size 5, and my new burner [a PPS superburner, bigger ceramic area] is still only a 5....

I know is going to sound either simple or naive but...gasjet size? Too rich a gas mix is making a poor flame?

[mikewakefielduk@btinterne]

Yes, I agree. More to the point perhaps is what's the size of the jet in a Mk11. If that too is 20 then OK, its the size necessary for the type of burner. If its a fair bit smaller than 20 then I guess it points to what Mamod have been trying in order to get the Thomas Telford to run reliably.

[Chris Cairns]

Found it - here is some test results from a ceramic gas burner.



So increasing the jet size needs a similar increase in air flow to get the right burn - the current Mamod jet holder does not allow for much movement to change the gas/air mix.

I have a couple of different jet holders so I'll experiment with my William II/Mark II burners although not sure what size of jets I have available.

Chris Cairns.

[mikewakefielduk@btinterne]

Will be very interested in the results of your experiments Chris, but I don't envy you. One of the (very few) things I dislike about the Mk11 is the amount of hassle it takes to remove the gas jet compared with other locos. It was better when there was a nylon gas pipe but now this has sensibly been replaced by copper I reckon a bit more thought needs to be given to redesigning the whole thing. Hopefully Mamod will seen how Roundhouse do it on a Millie and produce something similar.

Mike

[Chris Cairns]

The Burner Trials

Sorry no photos or video but it was not really practical to do that safely (really needs to be done in subdued lighting and my kitchen is not clear enough for that at present!).

Mamod Mark II Burner & Gas Tank, plus Bix Gas Tank & Gas pipe with No.5 jet, Cheddar Models/PPS horizontal gas tank & gas pipe with Nos 3, 5 & 6 jets. All tried using GoSystem Butane gas. The Bix jet holder & Cheddar jet holder were in slightly different positions when pushed fully into the Mamod burner due to the different designs.

Mark II burner burns with a slight yellow on top of flame and moving the jet out of the holder seems to make no difference to the burn. Ceramic only starts to glow at edges of burner.

Bix combination into the Mamod burner produces the same flame pattern and moving the jet out in the holder causes the increase in gas noise which indicates it is not burning right. Ceramic glow at edges of burner only as above.

Cheddar combination into Mamod burner. No.3 jet does not burn as hot, No.5 burnt well and more of the ceramic material started to glow, No.6 jet burnt with a weaker flame (not enough air for gas flow).

Whilst I had these all out I tried my new RWM circular ceramic gas burner (for my PPS De Winton prototype) and it glowed very red with a No.5 jet, having a different ceramic material to the Mamod one.

So Mamod seem to have chosen the right jet size and their strange jet holder has been optimised for a good gas/air mix. What lets it down is the type of ceramic material used. However you cannot fit a standard gas jet as they have not cut a long enough thread into the jet holder so a standard jet will not screw in tight (and I was not prepared to try the burner that way having recently had a leaking burner which ended up burning behind the gas jet).

Currently struggling to get 2 brass strips fitted to blank off the combustion chamber side wall holes, and the spacing washers have come off my burner again which will make refitting interesting!

Stay Tuned!

Chris Cairns.

[Chris Cairns]

Combustion Chamber Trial

Whilst not conclusive I've just run my Mark II once with the air holes blocked off in the combustion chamber side walls, and I found that I had to have the gas turned down a bit more to stop the flames licking over the boiler, and it definitely got hotter to touch.

When things have cooled down I'll give it another run. Certainly with the gas turned down a bit it lasted longer and the gas ran out just as the water got to the low level.

I've managed to break the Mamod coupling on my converter open wagon (centre buffer coupling on the other end) when the oversize Guard's Van toppled over due to the high speed on a curve, so I need to clean up the axles on my other rolling stock first before another run.

The Thomas Telford with a No.20 gas jet must have been glowing red!

Chris Cairns.

[dougrail]

Interesting research indeed Chris, I do believe that my new[?] 'rare burner' in SB is a no 5 Gasjet.

[mikewakefielduk@btinterne]

That's a very piece if research there Chris and at least we know Mamod seem to have got the jet size right with their Mk11.

Steamie1 felt the holes in the side wall are to improve combustion and the like. Did you notice any difference to the flame colour or shape?

As you said, a 20 jet size on the Thomas Telford and the burner must have been almost incandescent. I shall be nice and say they've simply been experimenting although I suspect its a case of trying anything to up the steam output.

Mike

[Chris Cairns]

Mike,

All my testing with the burner and jet combinations was done with the burner out of the loco. As you know you cannot really see the ceramic when the burner is mounted in the loco and by blocking off those air holes you see less of the burner flame pattern.

That said it appeared to me that there was a more pronounced pattern of cones above the burner and a slight increase in the glowing edges of the ceramic. The flame colour seemed the same blue. Unfortunately I had just burnt some toast which was still in the air around my kitchen so it was not possible to do a smell test for richness above the running loco.

(edit) - I managed a 2nd run this morning this time running the loco backwards. It was not running rich although I did notice the burner pattern was a little unstable within the combustion chamber, but setting the gas carefully meant it did not stray over the side tanks & boiler (first time I've run this loco backwards and watched the burner very carefully).

So I guess the best combination is to blank off the air holes and mount the side walls using your washers or possibly some ceramic blanket.

Doug,

I found a previous posting where you stated that your Bix & PPS burners both had a No.5 jet. In my tests the Cheddar No.5 jet burnt better than the Bix No.5, but as their jet holders are a different shape & thickness there was probably a better gas/air mix with the Cheddar one. As the Bix jet appears to have been held in with thread lock I could not remove it without damaging the holder, so I was limited to changing the 3 jets around in the Cheddar holder.

Cheddar Models made some very fine burners and locomotives. The same burner was used in a set for the Mamod mobile Traction Engine & Steam Roller.

Chris Cairns.

[mikewakefielduk@btinterne]

As already mentioned on the Thomas Telford thread, I called into Mamod today and heard the ceramic burner issue has been resolved to provide greater heat.

Apparently the original burner had a restriction at the entrance designed to work as a venturi and speed up the gas/air flow to the underside of the ceramic block. This has now been found to increase the air flow too much, the gas and air weren't having time to mix properly, and poor combustion resulted. This has been resolved by opening up the restriction and the burner is now said to be some 100% more efficient than before compared with the burner from a Millie which was felt to be only 60% more efficient. The loco now reaches pressure in about 2 minutes compared with about 4 minutes before.

A Thomas Telford loco under development was filled with water from cold and came up to pressure in a couple of minutes, so it does look like a simple fix has hopefully resolved things. Mamod intend to introduce the change to all of their models that use a ceramic burner.

The gas jet size is a 5 and this is standard across all of their burners, so clearly the talk of a size 20 jet was part of their experiments to get to the bottom of the Thomas Telford running out of steam.

I forgot to ask by how much the burner restriction had been opened out.

Mike

[Chris Cairns]

One has to wonder if the William II, and previous models would have worked better with this modified burner?

Their failure will certainly have put a lot of people off buying another piston valve Mamod.

Chris Cairns.

[mikewakefielduk@btinterne]

This morning Peter said Mamod had bought in the expertise of a gas engineer to come up with the design of their original ceramic burner and it was he who specified the reduced aperture to give a venturi effect. Tongue in cheek I did ask if he was simply a fitter who worked for the likes of British Gas and he said no, it was guy who designed gas burners and understood the theory of gaseous flow etc. Hence they had assumed their burner was efficient.

Its only now, with the trouble they've been having with the Thomas Telford, that's made them question whether what worked with much larger gas burners was not necessarily the case when reduced to the size of a model and maybe the gas engineer hadn't realised this.

Does anybody have another make, but similar design, of ceramic burner and would like to compare the size of the aperture where the gas air mix goes under the ceramic to say whether the Mamod is significantly smaller? Unfortunately my other locos have completely different burner designs.

Mike

[Chris Cairns]

I'm not going to remove my Mark II burner again (trying to paint over the aluminium rivets) but I have a smaller Mamod burner that is mounted in a scuttle for the traction engine/steam roller. As the air/gas pipe goes into the body of the burner it does have a slight venturi going into a smaller diameter tube which seems to have a further restriction at the far end.

In the circular ceramic burner I got from RWM for my De Winton prototype the air/gas tube is straight to the edge of the burner box where there is a piece of stainless steel mesh across the mouth of the tube. There does not appear to be anything else under the ceramic (torch shines through to the bottom) and this burner glowed very red in my recent trials.

In the small ceramic burner for an internal flue Cheddar Models boiler the air/gas tube is straight but extends almost to the far side of the burner. Again there does not appear to be anything under the ceramic, and this ceramic has much larger holes than the other 2 above.

I do have a Bix burner but that requires dismantling my De Winton which I do not wish to do until I'm ready for that project to start again.

Hope this helps your enquiry.

Chris Cairns.

[mikewakefielduk@btinterne]

Thanks for the info Chris. Like you, I'm put off taking the burner out of the Mk11 unless I have to, its too much trouble and anyway it seems to work well as it is.

However I am tempted to have a tinker with my Brunel as currently turning the gas up beyond a tiny amount results in the flame lifting off the ceramic and then going out only to re-ignited with a bang a second or so later. Not really an issue but it means you have no fine control of the burner. Could this be caused by a venturi effect speeding up the gas/air mix?

Perhaps I'll wait a couple of weeks first though and then bother Mamod again and ask them about the Brunel burner and if it too has a venturi. I don't want to distract them now from anything other than the Thomas Telford.

At the same time I'll try to get some more details of what exactly they've enlarged in the burner. You never know, it might simply be a case of drilling a larger hole of a specified size.

Mamod are going to be at The Telford Garden railway Market this coming Saturday which I would normally attend but unfortunately I'll be in London visiting family. If anybody else is going perhaps they could do a bit of asking.

Mike

[Chris Cairns]

Mike,

Here is the ceramic burner air/gas tube on my Diamond Jubilee Saddle tank loco.



You can see the venturi effect at the bottom of the tube. Previous owner had an interesting leak on this burner which I'll reveal in a new Diamond Jubilee Topic soon.

Chris Cairns.

[mikewakefielduk@btinterne]

I've been doing a bit of reading up on venturi gas burners and all the information I've found so far points to them having a design something like the diagram below:



As you can see, the venturi accelerates the gas which mixes with air at the other side. The Mamod burner mixes the air and gas before the venturi so all it does is accelerate the whole mixture for no purpose as far as I can tell.

Mike

[mikewakefielduk@btinterne]

I spoke with Peter Johnson at Mamod today. He says they are drilling out the venturi in the ceramic burner with a 6mm drill. You have to be very careful as behind it is part of the ceramic of the burner.

Its Peter's view that the venturi is speeding up the air and gas to such an extent that they don't have time to mix properly. By removing the venturi the gases are slowed down, mix correctly, and result in more efficient combustion.

He also said the Brunel burner doesn't have a venturi.

Mike