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Too large overlap = more high compression?

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Jens Eirik Skogstad06/03/2020 21:09:05
394 forum posts
22 photos

Hi folks!

When i started up the compound steam engine, there is not easy to run over TDC both up and down. The piston stop long way from TDC either the piston is in up or down. The steam pressure is high enough 6 bar.

I suspect there is a bit large overlap in the slide valve compared to the valve opening in cylinder block. There is not fault with timing, the excenter is set to 20 degree (i has tried with 0 degree with same result). And the slide valve is centered in right place in both way.

Resolving the problem of reducing overlap by shortening the slide valve a little?

Howard Lewis07/03/2020 17:08:03
3154 forum posts
2 photos

If it can be turned over with the cylinder covers off, it may be that the stroke is too long, so that tone or both pistons hit the covers, rather than valve events.

Once you are confident of the mechanical integrity, then others who are much more knowledgeable can advise you on the valve events for the H.P cylinder and for the LP cylinder. to prevent any compression between exhausting from HP into the LP.

As a non steam man, I would expect the LP inlet to open at the same time as the HP exhaust, if not a little before. But I could be wrong.


Jens Eirik Skogstad07/03/2020 17:21:28
394 forum posts
22 photos

The piston are not hit the covers, good clearance between piston and covers. The stroke is correct measured when i created the crankshaft. There is not hydraulic lock there. 

I'm thinking if the exhaust is close too early then compression will be too high to get turn over TDC.

Can the length of the slide valve be a bit larger to close a bit sooner to create low compression a bit before TDC? The excenter timing is still correct at 20 degree as in drawings.

Edited By Jens Eirik Skogstad on 07/03/2020 17:22:25

Paul Kemp07/03/2020 18:29:55
437 forum posts
18 photos


A little difficult to be definitive on the small amount of info provided. However assuming the port face is conventional with 2 inlets one either end and the exhaust in the middle (rather than some balanced type of valve that sends the exhaust through the valve and out the top) try measuring the recess in the valves (exhaust recess) and comparing the width of the recess to the distance between the inner edges of the cylinder ports, it should be the same or a little more (exhaust clearance or negative exhaust lap). If the distance is less (not uncommon on fast running engines) you will be developing compression in the ends of the cylinders because the exhaust will close before the piston reaches TDC. With a compound it may be designed as such to maintain a higher exhaust pressure from the HP to deliver to the LP, I would expect the LP valve cavity to have clearance though to get the spent steam out as easily as possible.

Hope that makes sense and gives you some ideas?


Jens Eirik Skogstad07/03/2020 19:13:28
394 forum posts
22 photos

I will check again and find out if the slide covering the exhaust port is shorter or the same length between the two cylinder ports. I'll add more news if there is improvement with the twin compound steam engine.

It is not easy to measure exactly in such small dimensions in the steam machine so I take color on the slide and color print on the cylinder block to find out if it is larger or smaller in relation to the two inlet ports of the cylinder.

JasonB07/03/2020 19:39:05
17874 forum posts
1954 photos
1 articles

just a thought have you got the slide valve the right way round? I have seen a few posts where people have put the valve in but rotated 90degrees on several different engines.

Jens Eirik Skogstad07/03/2020 23:45:01
394 forum posts
22 photos

Jason, the slide valve was not rotated in 90 degree.

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