In answer to David Eaton’s query about the Stoker Engine, the piston rod grooves appear to create a labrynth seal where the rod passes through the inside cylinder cover in the absence of any form of gland. Some form of seal is needed here to separate high and low pressure steam in the two cylinders.

These seals function in accordance with the basic gas law “Pressure by Volume over Temperature is a Constant”. It is assumed that the change in temperature across the seal is very limited, hence gas pressure is directly proportional to volume. HP steam will bleed through the rod running clearance from the top cylinder and expand into the first void, or groove on the piston rod – i.e., as its volume increases, its pressure will decrease. This is repeated in each groove over the rod length contained in the seal area reaching equilibrium at exit into the LP cylinder. These seals although never totally gas tight therefore minimise steam leakage along the piston rod.

Not the easiest thing to explain – will try to clarify if needed.

Regards,

Mike Johns.

Hello, it’s Lee Kimber here up in bonny Jockland. I thought that I would just drop you a line to let you and all the “fellas” know how I am getting on with my Traction Engine. Well, I’ve finished it. I worked on it for four weeks solidly so that it was complete before I had to return to Scotland. What a great feeling it is having accomplished my first Traction Engine model. Because I am as keen as mustard as you well know, I am looking for my next project. If you hear any talk of a Traction Engine or a Loco coming up for sale, incomplete or just castings, then please let me know. I hope to be at a Meeting soon—it’s just a nightmare having the Club meet up on Tuesdays when I am up here during the week. Never mind, I hope all is well and I will see you guys soon.

Lee Kimber

I was interested to read about Dave Eaton’s “Stoker” Engine in the last “Oily Rag” although I am still at a loss as to how it actually functions. Live steam entering on top of the small piston will produce a power stroke in the normal way. However when the upper side is connected to the lower, the area on the top is bigger than the bottom by the section of the piston rod so there is still a net downward force. As the piston moves up, the piston rod enters the cylinder and the steam is actually compressed! Similarly for the LP cylinder. When the HP exhausts to the LP then there is a downward power stroke but when the upper side of the LP piston is connected to the lower, the net force remains downward due to the relative piston rod areas. It therefore appears to me to operate as a single acting compound engine. Perhaps I am missing the point somewhere. If so, then I would be delighted to hear an explanation!

One question with which David left us is why the piston rod is grooved. May I suggest that this forms a labyrinth seal where it passes through the centre cover? To apply a gland with packing here would be quite difficult but the labyrinth would limit the leakage with no maintenance required!

With kind regards,

Steve Gosling