piston/cylinder head clearance

[Deckel]

Hi all.
I tried to find some info about clearance between piston and cylinder head at TDC but no luck.
I guess cylinder ports size/position may affect the clearance and whether or not the engine uses exhaust lap.
Is there's any rule of thumb for this or some engineering must be done before setting the value/ratio?
If too much clearance is used to be "safe", I guess overall performance will be reduced due to the slow pressure build up in the chamber,
and may also result in higher steam consumption to achieve the engine rated power for it's size.
But if there's not enough clearance the engine will have a hard time to compress the steam left over by the exhaust lap (if any...).

Thanks, Deckel

 

[tel]

l usually aim for between 1/32" and 1/16" depending on the engine.

 

[steamer]

It was usually expressed as a percentage of total volume

2-3% on large stuff...excluding steam passages.

Small as you dare on the models

Tel's comment of 1/32 inch is good on the average .75" bore steam model

Watch your thermal expansion of the various parts and if your bearings take up on the connecting rod, does it move the piston farther or closer to a cylinder head?

You should also install condensate drains on each end of the cylinder on a slide valve engine.....The wobbler will take care of itself.

Dave

 

[Deckel]

Thanks guys

Steamer, can you tell me more about the condensate drains or give a link if it has already been discussed on the forum.
The engine I am working on is a double acting compound 1.75+3x2 ( or so, still thinking ) slide valve with Stevenson linkage for reverse.

Deckel

 

[steamer]

;D


You mean something like this?

I might be able to tell you something about that.
Mine is a 1.5 x 3 x 2.5"

Dave

 

[steamer]

Deckel,

Tell me what your looking for and I can help you.

The drain valves were ordered through PM as they work just fine. There are other alternatives, but mine are just globe valves.

I'm running about 1/16" clearance top and bottom on my pistons.

How are you determining your valve timing?

1.75x 3.0 x 2.0 has a cylinder ratio of 2.9 so your cut-off will be long on both cylinders..similar to a Stuart number 6A....do you plan to run condensing?

I assume this is a marine application?

I have a TON of information on designing a engine of this size...but it depends on your application, boiler pressure, ect.... with a cylinder ratio of 2.9, I would guess your looking at 100 to 125 psig range?

You usually determine the total horsepower by assuming your expanding all your steam in the LP. Steam consumption is by the volume of the HP...on engines this small you can ignore the cut-off as condensation losses are high....just use full stroke for volume calculations.

Bar stock or castings? Mine are a mix with the blocks and standards of castings, and the rest of bar stock.

There are designs out there on the web in this size, I can point you to those..

Let me know

Dave

 

[Deckel]

I love it !

Yes, ultimately it might be for a small boat (which my father would love to build, already done a wooden canoe)
and if the boat takes too long to build, I might use the engine on a mower tractor, could be fun.
Honestly, the specs of my engine were determined by doing an average of some engines I saw on the net or books.
(on the net it was mainly a ratio of 1.5 between hp and lp, but the book was more a ratio of 2 but it was for very large stuff, so I did an average, just as for stroke...)
Not too much science there, but might just work fine.
For the cylinder ratio of 2.9, what gives that value and how will it affect the cut-off (or what ever it will affect...)
I first tought not using exhaust lap in hp cylinder since the lp cylinder is somewhat the extend of the hp running expensively.
Honestly, I don't really understand why exhaust lap is used in compound engine, while in single cylinder it makes sense to me since it makes the engine much more efficient.
The engine is 100% made of bar stock, if I can avoid welding I will, or if I finally get my foundry done before the engine, I might use some casting.
For the boiler pressure, I tought of 100-125 as you said, but simply because air compressor are just about the same size, and don't looks to be that much thick or whatever fancy to handle the pressure, and also some web info, but also saw some engines using 200 and even 600, scared me a bit...

Deckel

 

[Marinesteam]

Hi there,

I'm glad to see some other 1:1 marine engine builders on the site.

I'm currently building a 3 x 5.25 x 3.75 compound. I've been working on a CAD model, to allow quick checks on dimensions and just for fun. I may post a movie of the animated valve gear in the near future.

Exhaust lap is to cushion the end in the cylinder stroke as well as retaining some pressure in the cylinder for the next intake cycle. I'm not sure if that is needed on a compound but I trust our forefathers as thay really had these details worked out.

Ken

 

[Maryak]

Exhaust lap

In a vertical engine our old friend gravity comes into play and exhaust lap is normally positive for the piston going down and negative on the upstroke. In a compound engine. The total weight of the LP components is more than the HP. IMHO exhaust lap is required on the LP. I am talking 12":1ft scale re the above.

Hope this helps ??? ???

Best Regards
Bob

 

[Deckel]

Sorry guys,
I mixed exhaust lap and cut-off on my reply, might mixed you also and just made me feel stupid as I read my own reply... :-[
Now you know this, my last reply might have more sense... hopefully

Deckel

 

[steamer]

Bob is right regarding full size practice, absolutely.

For engines smaller than Marinesteams, not so much. Exhaust lap which is a cushion is not really needed. There's none on mine and it will comfortably turn away at 600 rpm without any movement at all.

From 3 x 3 on up though.....you probably need some exhaust lap

Cylinder ratio is computed by D2 /d2 (LP diameter squared / HP diameter squared)

A very useful constant.

I have more, but PAPA duty calls at the moment.

Dave

 

[steamer]

You absolutely need steam lap

Talk more later


Dave

 

[steamer]

Deckel,

Steam lap will allow you to use steam expansively. Otherwise, your efficiency will be absolutely awful.

For a Marine simple, such as a launch engine, you should plan on a large cut-off with the valve gear in full link. Say 7/8 cut off ( 88% of stroke)

You can link up from there.



For a compound, it gets tricky as the cylinder ratio, which is really a measure of the ratio of cylinder volumes, comes into play

The purpose of a compound is to divide the expansion of the steam over two cylinders so that each cylinder sees a small temperature variation. Doing this reduces the condensation losses to each cylinder and allows the steam to be expanded beyond what can be reasonably done with a single cylinder alone. With Stevenson link the minimum cut-off is about 1/4 stroke, which is very short. Other nasty valve events happen when you try to get a Stevenson link to link up shorter than that.


In any case, the goal of a marine compound engine design is usually equal power from each cylinder. To do that the expansive work done on each must be the same. To do that requires that the cut-off of each cylinder be carefully designed with the cylinder size in mind.

Increasing the HP cut-off increases the power of the whole engine, while adjusting the cut-off of the LP will determine how the power is distributed between the cylinders

Increasing the cut-off of the LP will actually DECREASE the power produced by the LP and increase the power created by the HP...its counter intuitive, but think about it for a minute.

Designing in the right valve events relative to each other is KEY with a multi expansion engine.

The next thing is the receiver volume. Much has been made of it, but it depends first on what kind of compound your going to construct. Woolf or cross compound?

I will assume a cross compound like mine, with that a receiver volume equal to the swept volume of the LP cylinder at full stroke is a good starting point, but no smaller!

I'll stop there......but I could go on for a few hours......

Dave