Two cylinder, double acting, horizontal engine???

[tel]

Probably not Brian, but I like my engines to look like engines.

 

[Brian Rupnow]

What a way to spend a Saturday morning. One of the things that is making me crazy is trying to figure out how long the valve actuator rod should be center to center. There is a distinct relationship between where the piston is in its cycle, compared to where the valve is in its cycle. This means there is a distinct angular relationship between the eccentric lobe and the throw on the crankshaft. If a wiser head than mine can jump in and exlplain these relationships clearly, I certainly would appreciate it.---Brian

 

[Brian Rupnow]

So---add some bearing stands and caps, a baseplate and a spacer block, and we're nearly finished.

 

[Brian Rupnow]

So-------

 

[SandyC]

;D

Hi Brian,

It looks like a workable design so far, great work with the 3D.

One major thing though....... with such a long stroke you will definitely require a cross head arrangement, otherwise you will find that the piston rods will be deflected sideways thus rapidly wearing out the lower cylinder bearing, which, in turn, will cause the pistons to foul the cylinder walls. :'( :'( :'(

You will also require some sort of steam seal (o-ring or graphite yarn) in the lower cover, around the piston rod, for a double acting engine..... which will also need the help of the crosshead guides if it is to remain steam tight for more than a few hundred revolutions.

I attach some valve timing diagrams which should give you enough info for the various locations at the critical points of the cycle.

They show a D slide valve on the (1st) main one, however, this can be exchanged for a piston valve as per the second diagram.
The timing relationship for both valve types is identical.... at least it is for a line on line valve arrangement....i.e. one with no steam lap and/or lead.

They are extracts from a series of articles I did....I am considering turning the whole lot into a small book at some time in the future. :-\ :-\ :-\ ;D ;D

Hope this helps.  :big: :big: ;D ;D ;D

SandyC

Edit..... I should add that the diagrams are for an OUTSIDE admission valve... with the exhuast in the centre.

For an INSIDE admission...everything remains the same as far as the valve timing goes, however, the direction of crank rotation will reverse.
The only other thing that would change for a slide valve type would be the need for a BALANCED slide valve...to prevent the valve lifting of the port face....NOT a problem with a piston valve.

View attachment Fig 10 slide valve sequence with no Lap or Lead.pdf

View attachment Fig 12 Equivalent Piston Valve.pdf

 

[Brian Rupnow]

Sandy--Thank you--That information is exactly what I needed. I did not use any rod seal on the "beam engine at double scale" that I built a couple of months ago, so I'm not 100% sure a seal is necessary The plate that bolts on where the rod exits is more thn just a 1/8" thick plate---It has a 1/2" diameter boss that extends 5/16" into the cylinder to give lots of bearing surface for the rod. I'm sure you are right about the crosshead guides, so I will look into adding them.---Brian

 

[Brian Rupnow]

Sandy--Thank you--That information is exactly what I needed. I did not use any rod seal on the "beam engine at double scale" that I built a couple of months ago, so I'm not 100% sure a seal is necessary The plate that bolts on where the rod exits is more thn just a 1/8" thick plate---It has a 1/2" diameter boss that extends 5/16" into the cylinder to give lots of bearing surface for the rod. I'm sure you are right about the crosshead guides, so I will look into adding them.---Brian

 

[SandyC]

;D ;D

Hi Brian,

Ok I thought they may help.

The reason I suggested the lower seals and cross head on this engine are that it will experience far greater sideways forces than your beam engine did, since this had a much straighter push pull from the beam..... the angular forces from the crank are a great deal higher.

Something to keep in mind.

Best regards.

Sandy. ;D

 

[Philjoe5]

Brian,
Sorry for the late post to this thread but what you are building sounds almost exactly what I've started. It's in Work in Progress "A two cylinder mill engine under construction ". I'm also pondering the crankshaft design. Unfortunately for some reason I cannot view any of the jpeg's you've posted (anyone else having this problem?) so I can't comment on how close your design is to the one I'm working on.

Cheers,
Phil

 

[Brian Rupnow]

Philjoe--I don't know why you can't see the .jpg's . I did look at the engine in your post, but without plans, I could not build it. Seeing as I had such great success with the "beam engine at double scale", and seeing as I design machinery for a living, I thought I would go ahead with my own design, basically "from scratch", but incorporate the things I know work well from the beam engine build.---Brian

 

[Brian Rupnow]

In deference to SandyC who seems to know what he's talking about--I have extended the bolt-on rear piston guides (pink) to act as "crosshead guides" to help remove the bending moment from the piston rod and to aid in sealing steam (or air) from escaping around the rod at that end. I have also added a third bolt to help hold it squarely against the cylinder body. Since the "throw" on the eccentric that runs the valves is considerably less than the "throw" on the crankshaft, I am not too concerned about crosshead guides on the valve rod---Besides which, if I did that I would have to make the entire base longer and move the crankshaft centerline out farther away from the cylinder body.

 

[Brian Rupnow]

Sandy--I have a question for you. In the .pdf attachment "fig.12 equivalent piston valve" the diagram half way down the right hand side in the center states that the "eccentric offset (rad)=steam port width", and the diagram at the bottom of the page shows that the length of the large diameter on the steam valve should be "same as steam port width". When I designed the "beam engine at double scale" a few months ago, I based my design on one of Elmer Verburgs engines http://www.john-tom.com/ElmersEngines/24_beam.pdf I followed the relationships given in the Elmers beam engine .pdg, and his are not equal. In his engine designed incorrectly, or am I missing something here. I am going to use the same valve arrangements and relationships in this new engine as I did on my beam engine, and if making a small change can make it run better/more efficiently then I will make that change in my design. This whole science of valve timing is a bit of a "black art" to me right now, but I am learning.--Brian

 

[SandyC]

;D ;D

Hi Brian,

Well now, that puts a different light on the subject....... HOPE YOUR A GLUTTON for PUNISHMENT..... :big: :big: ;D ;D

The answer lies in the the valve on the ELMER engine.

It is designed with both OUTSIDE and INSIDE LAP.... I was hoping to keep away from that at this stage cos it complicates things for you....not that you won't grasp it, it's just a bit more difficult to understand what is going on.

The valve and eccentric dimensions shown on my previous drawing relate only to a slide valve or a piston valve which has neither INSIDE or OUTSIDE LAP, in other words, the steam and exhuast edges are in line with the cylinder port edges with the valve in its MID position.

I took a quick look at Elmers drawings and it would appear that his eccentric is slightly less in throw than I would have made it initially, given the port and valve land dimensions, however, I have not studied the effects of the angular linkage at this stage and this will most certainly make a difference.

Normaly, for a directly driven valve, the eccentric offset would be = to port width + the LAP, since this is the distance the valve would need to travel to fully open the port.
The total valve travel, to achieve the identical, but opposite position for opposing piston ends, would be 2 x port width + 2 x lap.
For an indirectly driven valve, (i.e. one not directly connected to the eccentric strap) then the length ratio of any lever arms or links will have an effect, as will any angularity effects, and therefore these must be taken into account. This could lead to the eccentric offset being larger or smaller than for the direct connection dimension.

The other major effect of LAP is to alter the angular advance of the eccentric relative to the crank.

The INSIDE LAP effects the exhaust timing and compression phases.

I have attached a few more pages from my articles which explain the terms and also shows the timing of a valve with lap.
This should get you thinking. ;D ;D ;D ;D :big: :big: :big: :big:

Hope you can get to grips with this.

Best regards.

Sandy  ;D

Edit.... re-worded eccentric offset and valve travel statement to make more clear.

View attachment Fig 11 Slide valve sequence with lap and lead.pdf

View attachment Fig 11a Slide valve sequence with lap and lead ctnd..pdf

View attachment Fig 9 Slide valve and eccentric.pdf

View attachment PDs_ 5.pdf

 

[SandyC]

And one more attachment which might help understand the diagrams and documentation a bit more.

Sandy.

View attachment Hyp ind diag..pdf

 

[Brian Rupnow]

[size=10pt][size=10pt][size=10pt][size=10pt]Awright!!! Enough of this development phase. I have moved over to the "A work in progress" thread and started to build this thing.---Brian[/size][/size][/size][/size]