A compound Garratt

[Moonraker]

Eight years ago, a friend and I built the K1 Garratt shown in the photo. We used two Roundhouse Billy kits fed from a locally sourced boiler with a single burner. It looked great but was a poor performer. After raising steam until the safety valve blew, it would start off okay but would then run out of steam after about fifty metres. The problem was that the single burner was not generating enough steam for two sets of cylinders. The obvious solution was to add a second burner (the new Accucraft K1 has two burners) but this was seen to be just too difficult. So the loco was put on the shelf and there it has stayed for eight years.

Then, this week, a chance conversation with Gordon Watson of Argyle Loco produced the suggestion from Gordon that I modify the loco to make it a compound....just like the prototype. This is just a matter of connecting the exhaust of the high pressure cylinders in the rear with the inlets of the front cylinders. That way, the burner is generating steam for only one set of cylinders. Okay, the front cylinders should be about twice the size of the rear cylinders due to the lower steam pressure. Mine, being Roundhouse standards, will be the same size so that I won't get anywhere near full power out of the front cylinders. But it has got to give a better result than I had before.

So my question is, does anyone have any experience of building a compound loco for G Scale? Are there any timing or other issues which need to be considered?

Regards
Peter Lucas
MyLocoSound

 

[Rhos Helyg Loco Works]

My answer to your question is "No, I have no experience" but some thoughts spring to mind...

- If the exhaust from the rear unit is so fully expanded then the front unit is not going to do very much with the residual pressure.

- If the front unit does not rotate then it will, in effect, act as a block on the rear unit's exhaust and create exhaust back pressure.

- If the back pressure is sufficient it will prevent the rear unit from working.

- The full-size K1 has a Simpling Valve which causes it use live steam (no compounding) in both sets of cylinders and is used when pulling away from a stand.

However, and as I said, I have no experience so I will be interested to see how this works out. Best of luck!

 

[James Day]

The race is on! Will you get your K1 working before the real one is running again?

Love that loco! Mine is an electric professionally built from a GRS kit.

James

 

[Neil Robinson]

Another less than ideal solution would be to use only one pair of cylinders. The other pair would be purely decorative' that's to say run it as a 4-4-0 or 0-4-4. If you remove the D valves from the decorative pair and leave the inlet and outlets open to atmosphere the pistons may offer little resistance allowing the wheels to rotate O.K.

 

[G-force1]

Another less than ideal solution would be to use only one pair of cylinders. The other pair would be purely decorative' if you remove the D valves from the decorative pair and leave the inlet and outlets open to atmosphere they may well offer little resistance allowing the wheels to rotate O.K.

but will have less pulling power than a standard Billy because of the extra dead weight it has to pull.

I have built a Garratt, from scratch, only in 7 1/4" gauge. There is little substitute for enough steam pressure in the right places. There will be some power (expansion) left in the steam from the first cylinders, and until the whole thing warms through a lot of that will be lost transferring to the second set, the best cure for that would be a higher working pressure. Bigger second cylinders would certainly be best, but I think it's worth a try. Perhaps a bigger burner could be made which would also help.

 

[AustrianNG]

Bigger boiler, bigger burner, more steam..........sorted.