Jumper blast pipe?

You have the exhaust pipe from the cylinders set to blow up the venturi-shaped chimney and reduce the pressure in the smokebox. This draws air throught the fire.

The problem is to get the end of the exhaust pipe (the blast nozzle) the right size. If it's small you get a bigger suction effect than if it's large.

But when the engine is working hard you also get a bigger suction effect.

So you want a bigger nozzle when the engine is working hard than when it's not - so it doesn't suck lumps of coal right up off the fire.

So you have a "jumper top" - if the blast gets to fierce it jumps up and effectively increases the size of the nozzle.

Wilfrid Underwood

Hi Geoff

I've not seen it tried. But I have known models that have been transformed by adjustment of the fixed nozzle size - to improve poor steam production, or reduce constant blowing off.

Some brave soul must have had a go, though.

Wilfrid