Draughting

A very popular starting point for Live Steam. With their low cost comes a number of problems which can be discussed here
Post Reply
User avatar
dougrail
Driver
Driver
Posts: 1532
Joined: Mon Mar 14, 2011 5:11 pm
Location: Salopia; on the edge of Arcadia

Draughting

Post by dougrail » Wed May 01, 2013 12:49 am

I have a question over draughting on a Janet/upgraded loco.

Originally my engine had a Summerlands Chuffpipe, but I removed this on several people's advice and did notice a little performance improvement.

Supposedly the Chuffpipes shouldn't affect performance, and so I wondered whether fair enough, larger locos such as RH and Accucraft have more power with which to voercome any 'resistance' whereas the oscillators, being small as they are, would have felt any resistance more acutely?

I removed mine and ran fine without.

Recently however, I exchanged the chimney cap. I used to have a wide open cap as per Dream Steam :-

http://www.dreamsteam.co.uk/mss-mamod-e ... -cowl.html

Combined with an exhaust pipe that reached just to the base of the funnel. I then extended this with some silicon piping and it worked brilliantly - not only did it run well but the steam drifted realistically.

The new chimney cap appears to have the chimney hole about 80% blanked off, with a hole in the centre. The hole leaves into a little tube, part of the cap which hangs down about 10mm.

I ran the engine on Saturday and started to get a strange feeling it wasn't working at optimum. Call it weird, call it gut instinct, but I just had this weird feeling that I had about part of the loco before - and was ultimately proven right about.

Could it be that this new chimney cap with its small release hole is restricting my engine somehow?

User avatar
Chris Cairns
Driver
Driver
Posts: 2364
Joined: Mon Oct 29, 2007 7:25 pm
Location: Glasgow, Scotland

Post by Chris Cairns » Wed May 01, 2013 11:25 am

Doug, surely draughting is the wrong phrase (that affects the firing method used, in either a coal burner or a flue mounted gas burner). What you are discussing is changes to the exhaust and thus affecting back pressure.

When I first started with Mamods I used a piece of silicon tubing to extend the supplied short exhaust pipe up towards the top of the chimney. This had 2 effects, firstly it gave a better exhaust plume, and secondly it allowed me to inject steam oil to be sucked back into the reverser/regulator valve & cylinders.

On my 1st IP Jane I fitted a piece of 5/32" copper tubing over the exhaust pipe which had the top blanked off and a hole drilled into the side to form a simple chuff pipe. It did not affect this loco's performance and is still fitted. However when I fitted it to my 2nd IP Jane it did affect the performance of that loco so it had to be removed to allow proper running.

Summerland Chuffers are a nice piece of kit but an expensive option if you start fitting them to all of your locos. So I bought an SCGP1-25, and the intention is to slowly modify all my exhaust pipes to a standard length and central position within the Mamod/MSS/IP Eng smoke boxes so that this chuffer could be easily moved between locos. So far I have only fitted it to my Mamod Diamond Jubilee locomotive (which has a smaller diameter chimney than the others above, so I should really have used a thinner diameter SCGP4-25 ) and it helped to improve the slow running as you can see in my YouTube video.

Sounds like you have introduced a restriction in your exhaust with this new chimney cap. Have you fitted your silicon tubing into the small pipe extending down from the cap? If the cap is easily removed it should be easy enough to do a comparison, run it with the cap fitted and note running performance (regulator position & pressure setting), then remove cap and at the same regulator position and steam pressure note any difference in running.

As you will be aware tubing is available in different thickness's (SWG) which will affect an exhaust pipe, and thus the back pressure. I prefer to use thick tubing for my steam pipes (easier to bend without fracturing/breaking) and thin tubing for the chuff pipe.

Chris Cairns.

User avatar
DolwyddelanLightRail
Driver
Driver
Posts: 2579
Joined: Tue Sep 29, 2009 7:27 pm
Location: Lost

Post by DolwyddelanLightRail » Wed May 01, 2013 11:48 am

Thinking about it, the chimney cap won't be creating any back pressure at all. If there was backpressure from it then steam would begin to leak from around the edges of the smokebox (i.e. where the boiler inserts into it) and also from any bolts/holes that go into it. The smokebox is not steam tight so if there is enough back pressure you'd see the steam leaking from around those areas.

What Chris has said is perfectly correct, which mentions towards the exhaust pipe and any modifications towards it. As the exhaust pipe has not been altered with (?) then there won't be any difference in the performance of the loco, as the hole in the chimney cap wasn't small enough to create enough backpressure.

Remember Doug, not every performance is the exact same. Some days a loco will run better than other days so it looks like it may be something to do with this more than anything else.

User avatar
dougrail
Driver
Driver
Posts: 1532
Joined: Mon Mar 14, 2011 5:11 pm
Location: Salopia; on the edge of Arcadia

Post by dougrail » Wed May 01, 2013 11:31 pm

James - the exhaust pipe itself was exactly as seen at Butterley on 3/3/13 : copper pipe > silicon pipe to halfway up funnel. The only difference was the chimney caps. That day it was a belter. Pboro seemed good but a wee bit sluggish at times hence my eyebrow lifting. But I'll remember your bit about no two runs being the same.

Then again at Butterley, it had had more 'wake up' time [a full two turns on the dual-branch] [like engine, like owner] whereas Pboro had had half an hour steaming Friday evening to quickly check it ready for running.

~

Chris - silicon piping -yup, that's what happened. I don't need to drip oil into it but the drifting and plume was far far better than before.

As for chuffers - I managed to scavange mine for £0.00 [I'm good] and originally fitted it to the engine.

Having had more people than fingers to count recommend against it [sound wasn't as good, and suspected performance choking] I removed it and it appeared that the loco did improve so it;s remained removed. When I have time I'm going to craft a new exhaust pipe that reaches up 2/3 the height of the chimney and crimp it for a more natural sound - the loco had earnt the nickname "Thundercracker" when fitted with the SC, due to the rather loud sound it made. Still, it made for easy to determine whether it was moving or not...

I've not hd opportunity to hook it so the silicon tube connects the Mamod exhaust pipe to the pipe of this odd cap - yet -as it's simply far too short and awaiting a supply.

My plan I suspect will be to steam up the loco...once 'woken up', one with the old, one with the new and see what happens. If worst comes to worst I could just carve out the new cap so it's opening is wider, no?

Post Reply

Who is online

Users browsing this forum: No registered users and 2 guests