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"Building a steam locomotive is like a love affair..."

Thus wrote Ing da Porta in his 1966 paper to the Manchester University Soc. of Engineers. And in a way he is truly right. He was probably the most successful steam engine designer of the 20th Century but as far as I know -I am am the only person to put some of his practices to work in a model loco. Having "grown up" on the works of LBSC and HG in the Libraries of my Gymnasium and later my Grammar school -it does come as a shock...

The rules as expounded by HG and LBSC simply do not apply as they are based on the idea that there should be a small amount of steam that expands into the cylinder after the port has closed. The standard HG and LBSC ideal of the slide valve and its sizes in shown in the article below, and YES they will work perfectly well.

What da Porta brings in this mix is the idea of continuous expansion and gas flowing of ports. What I am about to discuss now may be seen by some "traditionalists" as pure heresy. There can be no idea of "cut off" as the cylinder very rarely achieves enough steam in it to make this a viable option. Far better to have full admission and waste some steam than to have the steam condense in the cylinder from the work that it does. In order to do this successfully the steam has to ingress and exhaust through successively larger and larger ports compared to classical designs.

My cylinders are 22mm bore with a stroke of 30mm which makes them very much larger in bore and stroke than would be seen in a classical design of 12.5mm by 25mm. The steam chest has twice the volume of the cylinder and the ports connecting the steam chest to the piston valve are 8mm bore, these then feed two 4mm bore connections to each end of the cylinder. There are two super heaters each feeding one cylinder fed from a "Wet Header", these spears continue all the way through the two upper super heater tubes and project well into the firebox cavity. They are of 8mm bore as well. The steam exhaust ports from the piston valves are 10mm bore and they feed the base of a 7 hole Lemaitre ejector. This then blasts air, steam and exhaust gasses through the base of the LemPor exhaust stack. This is rather like looking at the intake of an AMAL carburettor in that it has a fat "lip" and two conical sections with a venturi between them. The following is written in the "Spanish English" as used by da Porta...

Now that you can close your mouth... The question is, WHY DO IT? Well the idea of having a steam loco is that it should run for a long time between stopping. If we look at the running time of the model as being a lollipop then the more of it we have -the better!

The initial work on the exhaust and steam circuits are only half of the story. If an steam injector and pump driven feed water heater are used then this results in a 10% increase in the thermal efficiency of the water heating side. This translates to more time. I have stated elsewhere that I consider gas to be the perfect fuel for a steam locomotive. There exist both electronic and mechanical pressure regulators that will reduce the amount of gas to the burner dependant on the boiler pressure. Unfortunately none of these will take the 125% pressure test of "The Orange Book"requirements on connected fittings with the std G3 pressure of 80PSI -as they are too new for it. BUT it has been agreed that you may remove the regulator, plug the connection then do a 125% pressure test and reconnect the regulator fitting afterwards.

Here I have to state a personal belief. The Stephenson Multitube Boiler is NOT the best for G3. What I would say is the best is the Brotan boiler. This is a combination of fire tube and water tube. It avoids all the complexities of staying and has VERY large heating surfaces. My boiler is the "Design Number 14" from K.N.Harris. With the number of holes and tubes it does require some knowledge of step soldering.

There are very few books dedicated to the art of boiler making and those of Tubal Cain, Martin Evans and K.N.Harris would be your first stop.


This is the use of the remaining expansive energy of the used steam to power another set of cylinders. The normal accepted ration of the cylinders is one volume High pressure cylinders to that of twice the volume of the Low pressure. Chapelon found that with some work in the streamlining of the ports that this could be increased to 1:2.25 and 1:2.5 with "reheating" of the used steam.

Gas Equipment.

I have a BIX large gas tank and gas regulator with two Clevedon gas burners with No.5 jets. I did try No.8 initially but the amount of air required would need some form of forced induction into the burner plate.

Gas Jet sizes.

No.3 = 0.15mm hole.

No.5 = 0.2mm hole : 55grammes per hour for butane and propane burners. Thread size is 1BA.

No.8 = 0.25mm hole : 69grammes per hour for butane and propane burners. Thread size is 1BA.

No.12 = 0.3mm hole.

No.16 = 0.35mm hole.

The Clevedon Steam 28mm fire tube burner fitted with a No. 5 burner has been tested by myself and is rated at 1.5Kw on the time taken to boil 1 litre of water from 15 degrees C. The 30mm pan burner is roughly 2Kw.

Based on feedback from friends who have converted to gas... The following empirical rule seems to hold. Ceramic radiant burners are better with fire tube boilers, Flame based "Bunsen burners" are better with water tube boilers.

Note: a gas tank will require a certificate to be allowed to run at a G3 event. If you opt to use a connection to a commercial supply canister then this will not require a certificate as it is seen as a disposable item. Connections from a tender tank will require flexible hoses. Commercial micro air fitting are suitable for a "dry break" connection.


Spirit burning is a fairly simple method of raising steam. However the main problem is that it is literally "fire and forget" and thus basically uncontrollable. The main basic designs are shown in the article below. As a guide Meths is more suitable to water tube boiler systems as the flame and combustion products have more surface area for their low temperature. Water tubes also have a very good circulation rate that takes the best advantage of this.

Solid Fuel

The G3S produces its own guide to solid fuel firing. Basically a small lump of meths soaked charcoal is lit under the pile of coal and an centrifugal exhaust fan plunged into the smoke stack draws the air through the boiler tubes until enough steam has been raised for the "blower" pipe to perform this duty. It is I have to admit an amazing thing to watch!!!

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Page last modified on January 05, 2020, at 09:21 AM