uniflow/unaflow design question

Hi all
I am (slowly) moving towards building some unaflow engines and am in need of some advice.

1. In a (simple) unaflow, the exhaust gas that is not evacuated through the exhaust ports is compressed to a maximum at TDC. Presumably this pressure exerts a braking force on the piston returning, and is undesirable, though I've read that some say its useful to reduce shock ie on big end. (I know some designs used an 'auxiliary' exhaust valve.) Presumably that pressure can get up to steam supply pressure, in which case, steam would not flow until the piston had dropped till internal pressure was lower than steam supply pressure. Presumably also, a cavity must be left above the piston, because if not, exhaust gas would be compressed infinitely - and the engine wouldn't work. I'd be very grateful to hear any opinions or experiences on this matter.

2. Clearly, having a condenser on the exhaust ports would assist in drawing the maximum exhaust gas out at BDC. Any suggestions on condenser design?

3. A simply exhaust port would be a round hole about BDC. But that would take up stroke length. More, smaller holes makes better sense, maintaining or increasing cross section area of port and minimising stroke used up. Optimal exhaust ports would...presumably... occupy as little length in the cylinder at BDC as possible, allowing maximum length power stroke. By the same token, the should occupy maximum of the cross section. But of course there has to become surface for the piston/rings to move on. So what proportion of the circumference should be left solid? And what should be cut through to function as ports? I'm imagining long slots, occupying about 1/3 of the cross section of the cylinder we all. Does that sound about right? (I hope that makes sense).

For a start look at the steam tether hydro posts here. Windy has other posts on flash steam. Do a search on flash steam under his name. In a boat I would consider spraying water into the exhaust as a crude condenser. That's called a barometric condenser. Do a Google search on that for pictures.

Lohring Miller

A uniflow engine does need some 'clearance volume'. Compressing the exhaust is of value in cushioning the reciprocating mass in a full size engine, especially as the usual double-acting uniflow engine needed to have a piston length equal to stroke minus port width, which made it unusually heavy even though hollow. In a model the bearing loading due to inertia is much less.

However, compression is also desirable in achieving efficiency. By having the pressure in the cylinder near to steam-chest pressure when the inlet valve opens, you avoid a sudden, wasteful, inrush of steam. Uniflow engines often had trip gear on the inlet valve, and tended to work at very short cut-off. Another reason for their economy is that steam goes one way only: you are not cooling the inlet passages in the cylinder by shoving the relatively cool exhaust out through them.

Don't forget that a condenser will not help unless you have an 'air-pump' to pump the condensate (and air) out of the condenser to maintain a vacuum.

Your reasoning on the exhaust porting is good. The exhaust porting in full size would be a full ring of rectangular holes. These ports would be separated by port bars with a width about 1/3 the width of the ports, so 3/4 of the circumference would be open. The holes would be about square or a bit wider in the circumferential direction. Too wide, and they risk a piston ring catching on the edge of the port.