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Discussion in 'General Engine Discussion' started by chucketn, Jun 14, 2018.
I found this page a while ago, seems useful, I wrote to the author but communication petered out.
This is a wonderful resource - Cornell has one of two existing sets of Reuleaux's late C19th encyclopedic set of mechanism models. The other is in Germany I think. http://kmoddl.library.cornell.edu/rx_collection.php
I'd love to see it.
They also have CAD models and STL files of many mechanisms.
I've found a few tidbits of info. Like relative size of piston in relation to bore and stroke. Mostly from old engineering books describing real engines. On the other hand, a lot of the old texts on steam engine design discuss the history, or a brief description of improvements. I've not found much that describes the relationships of stroke to port angle or crank diameter...
Brian, how did you do it?
With simple wobblers in a lot of cases they are simply designed to run with little thought
A few designers the likes of K N Harris, James Senfit and Harry Wedge have developed the engine
The stroke and the crank to cylinder pivot have an effect on the porting as you need to fit 3 diameters as in two in the upright and one central in the cylinder within the swing of the cylinder. I tend to sketch it up with a rule and pencil just to see what will fit in . I try to keep the dead area to a min and have tried a little overlap on the exhaust and find they run a bit better
I use the simple wobbler for most of my steam launches on steam and only bed in on air
Another factor often forgotten is the forces acting on the cylinders pivot especially with some of the rather hefty offerings about
Steam tables may also be useful
Thanks again, fcheslop. The names you mention are familiar. As I mentioned before, I figured out a simple drawing to find the trunion ports. Not sure I understand what you mean by dead area, or overlap.
Do you mean that there is a short period when the cylinder port is open to both inlet and outlet? - that for that time, steam supply goes directly to exhaust? This seems counterintuitive. Do you have an idea why this is advantageous?
I love the simplicity of the wobbler, but surely the symmetry of valve timing (inlet and exhaust being open for same amount of time (number of degrees of crankshaft rotation) is not optimal. Theres no way - as far as I can see - to have inlet open for a short time after TDC and exhaust open all the time between BDC and TDC.
I've looked into the theory that steam engines won't run on air, or won't run well on air. The main difference is that steam is hot and air is cold, so from thermodynamics, at the same pressure, hot gas has more energy than cold gas, so you get more power out of the hot steam than you get out of the cold air, but if you didn't let the air cool while compressing it, it would have about the same energy as steam at the same temperature. As far as the notion that air won't supply power to the engine during expansion phase, that doesn't compute, the only thing we know is that it won't supply as much power as hot gas, but to say that it won't supply any power is simply incorrect.
The timing of the steam engine.
Interesting, I've never seen such a diagram for a steam engine (I'm not a steam guy). What's the reason for the compression region? Isn't it just wasted engine power and efficiency? I would have thought exhaust til almost TDC would work better.
The purpose of compression in addition to the steam pressure emitted into the cylinder a bit earlier before TDC, we utilize the steam as much as possible when the steam expands in a closed cylinder. The purpose is reduced steam consumption and better power out of steam engine.
The wobbling steam engine is a poor economy in steam consumption and difficult to get more power out of wobbling steam engine due limited by spring who are holding the cylinder to valve plate.
I'm afraid I still don't understand the compression purpose - it uses engine power (from steam of last stroke) to compress air before the inlet opens. Once the inlet opens, the amount of steam admitted depends on difference between steam pressure and cylinder pressure, so having increased pressure from compression reduces the amount of steam admitted to the cylinder. So in my mind it uses power, then restricts available power as well. It seems very inefficient to me.
In fact steam pressure is higher than pressure of compression. The advantage of compression in the cylinder: Less loss time to create cylinder pressure when the vapor pressure is admitted into cylinder. Then the cylinder has got more pressure to transfer power to the crankshaft just after TDC has been achieved and the steam can continue to expand after the slider valve is closed.
See at the indicator diagram for steam engine..
Dead space the angular movement before the port starts to open
On some of the better toy engines made in the 1920s the exhaust port is drilled slightly larger dia then the inlet allowing it to open to exhaust sooner
Circular ports although easily machined are not the most effective does it have a lot to offer making them as per K N Harris design I simply dont know or should say can prove other than the engine performs better on the water
Anyone can build a wobbler that will run it takes a bit more care to make one that runs well
In his books either 1 or 2 Building Simple Steam Engines the author Tubal Cain(not the guy on utube) actually suggests allowing a slight overlap of inlet and exhaust ports allowing slight bypass and again from experience it seems to help and once the engine is running how much steam do you really loose giving the time it takes the wee cylinder moves fully left to right probably a lot less than from an ill made engine
All the above is simply from experience building wee wobblers for the last 40+ years and sailing them in my toy boats
The reason some slide /piston valve have also later exhaust to de accelerate the mass before going over centre another is to use the desaxe principal as found in petrol engines.
Just my two bobs worth from a rough ar@ed model maker
Oscillating engines have been used in full size practice powering ships and factories and are still available today for steam launches Reeves sell castings .These engines have little resemblance other than the principle of operation to the rather nasty tinny things made for children to set fire to carpets or lino with or that maybe my ill spent youth
Thanks pp2076 - that makes sense, I knew it couldn't be about power.
Jens - I realise the steam pressure is higher than compression but the total pressure can only be as high as steam pressure. So by raising the cylinder pressure before admitting steam, the amount of steam admitted is reduced, and available power is thus reduced, for a given cylinder pressure. The energy of the compressed air is generated by the engine so costs more energy than it produces, which is why I was confused. It is thermodynamically impossible for the compression to create more power.
You may find this usefull for what you want to do , I did
Thank you, thank you, thank you...
Glad to be of help. If you just change one number at a time you will soon see how it works
Separate names with a comma.