Steam Engine Clearance

[Deeferdog]

Can anyone explain to me the effect of increasing or decreasing cylinder clearance in a steam engine? If I increase the clearance, and by this I mean the distance from the top of the piston (at TDC) to the cylinder head, will the engine consume more steam? If it does, is more power the result? Conversely, does reducing the clearance have the opposite effect? Finally, how does a designer of a steam engine determine what the clearance should be? At the moment I'm building a twin cylinder steam engine and I've had to fiddle around with the plans to such an extent that I'm starting to wonder if its going to work. Probably not the best time to ponder these things as it is over half finished. Help would be much appreciated. Peter

[Jasonb]

It will use more steam as the larger "void" needs to be brought upto pressure each time the valve is opened to that end of the cylinder but you will not get any more power out of it. This is why you often see the larger engines with quite intricately shaped piston faces so that they closely follow the contour of the cylinder end covers to keep this "dead" volume to a minimum.

[Jo]

In a model the power difference is going to be very small, on full size it is worth worrying about. On very larger models as JB says they do design those to minimising the dead volume as they are intended to be working engines.


The amount of energy (Calorific value) available in the steam is all to do with pressure, moisture content of the steam and swept volume. The amount of steam that can get in is due to the swept volume (and a very small bit due to reheating that dead area) . All this interesting stuff was published as Steam tables... which I cannot find at the mo 


Having a "space" that is going to absorb some of that power is not ideal but is it worth worrying about? We need to understand more about your model design: Do you have a bore of >50mm and more than 2.5mm gap at either end?

Jo

[Deeferdog]

Thanks for the interest and the quick replies. The model is based very loosely on a twin cylinder engine with reversing gear, scratch built, plans by Inventor Wizard. I am not too concerned with efficiency or power as it is intended to be a display model with a few runs on air. The bore of each cylinder is 40mm, stroke 36mm and the clearance is 7mm which I suspect is way too much for efficient output, hence this post. Cheers, Peter

[MJM460]

Hi Peter, as already stated, the tiny extra power will not be noticeable, but the extra steam required will be.  I can go into more detail if you are interested.

But how to determine what the clearance should be?  I suggest that it should be the minimum practical.  Zero is not a good choice as it will make setting up very difficult.  I generally aim for about 1 mm as something I can achieve within the tolerances I can achieve, and not too difficult to set the piston (assuming it is adjustable).  It also means there is allowance for wear, which will always increase the piston travel.  However, my latest engine, the Bolton no 5, I achieved a bit less than this, and no problem running at up to 2000 rpm on air.

At the other extreme, I have been involved in the commissioning of a compressor, where similar considerations apply.  It was 600 mm bore x 300 stroke.  The clearance of 1 mm each end was demonstrated by squashing lead wire inserted through the valve openings.  I will come back shortly with ideas on how to adjust it.

MJM460

[MJM460]

Hi Peter, I wanted to add that any of cylinder length, stroke or piston length could be altered to alter the clearance at each end.  Depending on the piston rod detail, the rod may also have to be remade.

Of these, I suggest the easiest would be to make a longer piston if all three are already committed.  You could perhaps make a piston from two or three parts, with a hollow section or an aluminium section to minimise the extra weight.  A longer piston would allow machining of more grooves which will tend to reduce the leakage flow past the piston.

As to pondering these things when half finished, it is not always immediately obvious what effect the various alterations you have made will have until you get to this stage, unless you already knew what to look for.  You will understand better now how the dimensions of so many parts relate to each other.  The alterations are inevitable when you try and make best use of the tooling you have available, and the materials you are able to source.  But inevitably, an alteration in one dimension will require a corresponding alteration in something else.  Half way is a major milestone in understanding.

MJM460

[paul gough]

Sometimes it is worth considering the volumes under the port face and the passage to the cylinder and add them to the clearance volume in the cylinder if you want to make a reasonably close estimation/calculation
 or are interested in the highest possible performance. I don't bother with absolute accuracy but an easy workable method of measuring the total clearance volume, ie including passages, is to have your piston at the end of its stroke, valve chest cover removed with valve face horizontal and introduce water from a suitable size syringe until the level fills to the valve face. Reading off the volume used in the syringe will give you a good enough total clearance volume. Regards, Paul Gough.

[Jasonb]

If you have 7mm each end and it is mostly made I would turn up a couple of 6mm thick spacers and fit them to the inside faces of the cylinder covers, this would not entail any remaking of parts. Just make sure you notch an edge so the steam passage is not blocked or they can be turned to say 38mm and the annular gap will let the air/steam in again much like full size practice.

The next easiest would be a longer piston but than can start to affect balance though again negligable if just a slow running display model.

Doing one of these will at least stop the compressor kicking in so often even if you are not worried about efficiency and putting th eengine to actual work.

[Jo]

The model is based very loosely on a twin cylinder engine with reversing gear, scratch built, plans by Inventor Wizard.
...
The bore of each cylinder is 40mm, stroke 36mm and the clearance is 7mm which I suspect is way too much for efficient output, hence this post.

A couple of observations:

The stroke is less than the bore - the implications is this is going to be a fast running steam engine. Is that your intent?

Clearance: do you mean 3.5mm at each end of the stroke or 7mm at each end?  If the latter then that is huge and needs reducing.


If the clearance is 3.5mm on the figures you have given then I would reduce it to 1.5mm.  On an existing cylinder set you could increase the depth of the boss on the covers by 2mm or as MJM suggested increase the length of the piston. Remembering that the weight of the piston will increase but if this is a fast running engine that would not be desirable.

Is there any reason for not increasing the stroke of the engine? If you did the finished engine would run slower and would make better use of the available steam as the additional swept volume is providing for greater expansion.

Jo

[Jo]

The more I think about the dimensions you gave Peter the more I am getting suspicious that the throw dimension on the original drawing is wrong.

Jo

[Jasonb]

Would help to know the plans you are using and how much "fiddling about" you have done to them. Is it the one on GrabCad etc?
https://grabcad.com/library/vertical-twin-steam-engine-with-reverse-gear-6/details?folder_id=1502739

Looks like it is that one.

63mm O/A cylinder length
cover spigots 3mm each end giving 57mm internal length
10mm thick piston
18mm crank throw = 36mm stroke

57 - 10 - 36 = 11mm so should be 5.5mm each end which is still a lot.

22mm throw crank would cure the problem provided their is enough room for it to turn with the big ends in place.

[Jasonb]

Digging deeper this looks to be a scaled up and redrawn version of Jean Luc Soumard's design and whoever did the drawings rounded some down and some up.

http://jean-luc.soumard.pagesperso-orange.fr/bicylindre.en.htm

Original engine was 13mm bore and it looks like things should have been scaled up 3 times

13mm bore should be 39mm not 40 (not too critical)
6.5mm stroke should be 19.5 not 18 so that's 1.5mm each end
5mm piston should be 15mm not 10mm so thats 2.5mm each end

So if it had been scaled up well you would have 1.5mm each end not 5.5mm

One of the problems with building from drawings by people who probably have not made the actual engine but just drawn it in their Armchair.

[Deeferdog]

Thanks again for the interest and help. Yes the plans are https://grabcad.com/library/vertical-twin-steam-engine-with-reverse-gear-6/details?folder_id=1502739, I downloaded them about three years ago but the project did not fare well as medical issues forced a halt. When I resumed it was only to make a major blunder with the cross heads so the model ended up under the bench. Every time I looked at I felt guilty about giving up so earlier this year I picked it up again for another attempt. I changed the crosshead design to accommodate my ineptitude, worked out a way around machining the cylinder blocks and it appeared to all be going quite well until I noticed the clearance issue. I know very little about steam engine design but I did think that the bore being larger than the stroke, (we used to call that "oversquare" when I was messing about with petrol engines), was unusual, I thought steam engines were mainly long stroke slow revving chuggers. I think the last comment by Jasonb hits the nail on the head. It sure is a difficult engine to machine as it is drawn, for me anyway. The cylinder block is an example of that and is one of the things I changed. This is was meant when I said I was fiddling with the plans. I will try to post some pictures but so far I haven't had any success with that either. On the upside, I know a lot more about steam engines because of this model and because of the interest and helpful attitude of the members here, I intend to be a much more regular contributor. Cheers, Peter.

P.S. The pics refused to upload, I keep getting an error message.

[MJM460]

Hi Peter, the pictures need to be less than 1 Mb each, and no more than 8 pictures in a post.

Try reducing them to around 300 kB if you are using the forum attachment method.

MJM460

[Deeferdog]

Thanks MJM460, here is another try.

Reduced images (4) to 300 Mb each. That didn't work either. 

[MJM460]

Hi Peter, at least one came through, and is showing a great job.

Are you using Apple to get those file names?  Make sure you are selecting the correct size for each additional attachment before you use it.  It’s like so many things that are easy when you know how.  Many of us have trouble initially but most soon get the hang of it.

MJM460

[Deeferdog]

Another try. I'm using a PC. Pics taken on Android phone, resized from 2.3Mb.

[Jo]

You are a long way into building that engine Peter it would be a shame not to get it running  Being a vertical engine you have a number of options as mentioned earlier that would recover its performance. Lets look at the challenges of each:

• Lengthening the stroke of the crankshaft would mean a replacement crankshaft would be required but it would also bring up clearance issues around the big ends and the soleplate.
• Replacing the two end covers (I am not sure if the lower one is integral with the guide from your pic  ) and making new ones with additional bosses is do-able and no one would ever see the changes.
  
• Making a longer piston is much quicker and easier than either of those, but it adds additional moving weight but as you have made a vertical engine this weight is not pushing against the bore. There are various ways of reducing the weight of the piston - making it in two parts so it can be hollowed out or more like a piston valve/cotton reel with a reduced centre width

Which ever method you choose I suggest you aim for 2.5mm clearance at either end 


If you are feeling overwhelmed by this engine Peter may I suggest that you initially make it without the reversing gear with a single eccentric rod driving the valves on either end.  In that way you can test the engine on air and see it running. And from what you have already done it looks like you are not far off from being able to do that and that may encourage you to finish off the reversing gear or leave it as a simpler working engine 

Jo

[Jasonb]

If you want a simple solution to solve the clearance issues and get the engine running ( may well do without altering anything) I'll stick with my earlier suggestion of filling the voids. Next option up the list would be a new piston about 18mm long which will compensate for teh under size piston and shorter crank throw at one sitting. Lastly if you do go for remaking the crankshaft it looks very slender in those images so think about increasing at least the main shaft, pins can stay the same.

Looking at the drawings there is plenty of thickness in the end covers to drill and tap a couple of small holes say M2 to M3 so that a suitably drilled and CSK infill can be added, for the top cover the plug can be JBWelded into place to fill the excess piston rod nut recess. Make the infills 38mm diameter so air/steam can flow all round and get to the face of the piston.

Talking of air getting to the piston make sure you notch between the cylinder wall and steam passage as per drawing, as it is now the cover will fit tight against the end of the passage.

Looking at a couple of videos of the original JLS engine if notched up it looks to run at a fairly modest speed maybe 50-60rpm so you can see the bits moving and it's not all a blur so there is hope yet.