Triple expansion double acting steam engine conundrum

[gbritnell]

I'm building a miniature triple expansion steam engine. I have a thread in engines of your own design. I know quite a bit about steam engines but I'm a little at a loss on the configuration of this engine.
Here's my question. High pressure cylinder is at TDC, steam is being admitted into the cylinder and pushing the piston downward. Exhausted steam is moving from the bottom of the cylinder into the Intermediate pressure cylinder but with the crankshaft at 120 degree spacing the I.P. piston is already on it's way upward. Exhaust from the I.P. cylinder is moving to the L.P. cylinder but the piston in that cylinder is already 120 degrees into it's stroke. When the engine gets reversed the steam entry seems to be more convoluted.
What part of the engineering am I missing here?
I have searched the net and all of the information I have seen is that the cranks throws are at 120 degrees apart from each other. On the other hand my Stuart compound launch engine has the crank throws at 90 degrees to each other.
gbritnell

[MJM460]

Hi George, I am another of those keenly following your triple expansion build.  Thank you for letting us all look over your shoulder as you work.

The simple answer to your question is that so long as the volume in which the steam is confined is expanding, the steam is still doing useful work.  While the HP cylinder travels up the volume of steam on the exhaust steam is  reducing, but the volume of steam in the IP cylinder is expanding.  Because of the relative diameters and while the HP exhaust and the IP inlet is open, the net effect is expansion, and the steam is doing more work in the IP cylinder than it absorbs in the the HP cylinder.

Similarly, the exhaust from the IP cylinder will expand into the larger volume of the LP cylinder.

When the HP piston reaches bottom dead centre the HP exhaust valve starts to open, however if it opens a bit earlier, early release, it will be close to the point at which the IP inlet valve opens so the transfer begins more or less smoothly.  The piston speed is quite slow near the top and bottom of its travel, so I assume there is a bit of tolerance on those valve opening points.

I find it very tricky to visualise all the valve events on a double, let alone a triple.  Even a single is hard when I try and understand where the bottom side of the piston is going at the same time as I look at the top.  It becomes a bit easier if you make little sketches showing the piston positions at each 60 degrees of crankshaft position, which gives you a snapshot of the relative piston positions each time any piston gets to top or bottom dead centre.  It may be easier to do this on CAD, it’s a bit tedious by hand with a pencil.  It is helpful to draw in the valve positions as well so you can see the accompanying exhaust sequence.

It’s also hard to visualise without carefully sketching the valve positions as well.  I don’t pretend to have ever persisted through far enough to see clearly the whole cycle, and I have not built one, so  I will leave it to others who have set up these engines to add some information on the valve sequence.  I am not sure how lap and lead affect the events.  And of course to visualise it all you need to look at the other side of the pistons as well.

I hope that makes sense, otherwise I will have to tidy up my sketches so they are fit to post.

MJM460

[Stuart]

Having just completed a reeves triple expansion engine

Timing crank at 120 deg. I then timed each cylinder as I do for a single eg port cracked at TDC BDC as per

Then I disconnected the pistons in turn and got each section running as well as I could

Then I ran it with HP and IP Ran ok
Then all three still ok

Now this is on air don’t cringe it runs ok at 100rpm at 35 psi note I have the condenser bypassed to save getting it oily

Hope that helps still got a knock to sort out

[Admiral_dk]

There has been at least one earlier thread here about this subject on this site and the easy answer is that no full size expansion engine has 120 degrees between throws. The debate concerned what angles then should be used and why + difference between air and steam in this application.

I hope this will point your attention in a usefull direction, George.

Best wishes

Per

[Stuart]

Per

I think it’s the scaling factor with compound engines as they do not have inter cylinder chambers to allow for the temp storage

With mine and by the look of Gbritnell one the HP exhausts directly into the IP steam chest


I wold like to see the steam diagram for a 12 inch to the foot one


But I do think it’s a problem when scaling we cannot scale steam nor pressure

Stuart

[Jo]

Here's my question. High pressure cylinder is at TDC, steam is being admitted into the cylinder and pushing the piston downward. Exhausted steam is moving from the bottom of the cylinder into the Intermediate pressure cylinder but with the crankshaft at 120 degree spacing the I.P. piston is already on it's way upward. Exhaust from the I.P. cylinder is moving to the L.P. cylinder but the piston in that cylinder is already 120 degrees into it's stroke. When the engine gets reversed the steam entry seems to be more convoluted.
What part of the engineering am I missing here?

The eccentric for the HP valve is at a different angle to that of the IP and LP cylinders to alter the admission timing. On the Stuart it is 30 degrees (HP) and 15 degrees (IP & LP)… Its a bit of a fudge 

Jo

[steamer]

Here's my question. High pressure cylinder is at TDC, steam is being admitted into the cylinder and pushing the piston downward. Exhausted steam is moving from the bottom of the cylinder into the Intermediate pressure cylinder but with the crankshaft at 120 degree spacing the I.P. piston is already on it's way upward. Exhaust from the I.P. cylinder is moving to the L.P. cylinder but the piston in that cylinder is already 120 degrees into it's stroke. When the engine gets reversed the steam entry seems to be more convoluted.
What part of the engineering am I missing here?

The eccentric for the HP valve is at a different angle to that of the IP and LP cylinders to alter the admission timing. On the Stuart it is 30 degrees (HP) and 15 degrees (IP & LP)… Its a bit of a fudge 

Jo

Possibly Jo.   The whole drive for a good multi expansion engine is to have equal work done by each cylinder.    The is usually accomplished by balancing the steam cut off between cylinders such that the PV diagrams have equal area's.     Now, That is affected by the cut off, and more directly, by the cylinder ratio.     D²/d².    There is one other variable in this that George has circled around.   That would be the Receiver volume.    The LP cylinder is getting steam from both ends of the IP cylinder  ( partial stroke from each side) so the volume of the receiver will have a large effect on the admission line of the PV diagram.

With my compound, I have a cylinder ratio of 4   ( 3²/1.5²) and to get as near as I could manage equal power from each cylinder, I have equal cut off for each cylinder  at 3/4 in full link.   The volume of the receiver to affect a minimum drop between the cylinders was a volume equal to the swept volume of the LP cylinder...   

This was all calculated by the then "bleeding edge" analysis method as outlined by Cecil Peabody in his textbook "The Thermodynamics of the Steam Engine".    Peabody used this book at the Massachusetts Institute of Technology for the classes that he taught on the subject, and I am lucky to have an original copy ....

https://archive.org/details/thermodynamicsof02peab/page/n2


I have not done the calculations for the triple, but I did do it for the compound.    14 equations, 14 unknowns, solved simultaneously. 
The later half of this presentation has a theoretical PV diagram as created using the method outlined by Peabody

http://www.neme-s.org/The%20Construction%20of%20the%20Steamlaunch.pdf


 ....but as we have already alluded to.....we're not using steam!....

Compounds and Triples don't run great on compressed air.   That's because the expansive properties of the two fluids are dramatically different.    Air just wont expand like steam does.     The result is in a multi expansion engine, the air gets over expanded, and by the time it comes past the LP valve, it's usually at a pressure below atmospheric.    To get around this, when I run my compound on air at a show, I leave the condensate valves open on the LP..it runs much smoother.

So......to answer Georges question directly, each successive stage gets some of it's steam from each end of the previous stage.    Keep your receiver volumes as large as you can, and keep them well insulated.    Expect a bit of lumpiness with multi expansion model engines running on air...especially if the air supply is at a low pressure.....like below 100 psi,   To smooth out the running of these engines on air, keep the IP and LP drains on the at least partially open, or raise your compressed air pressure, or both.

Dave

[gbritnell]

Thanks everyone for the useful comments. I can understand the receiver being large enough to store the exhaust steam from the previous cylinder then when that cylinder'I haves valve opens there is steam on hand to push the piston. I have made layouts and drawings of the valve events and just couldn't get my head around the operation when the successive cylinder was already 1/3 along it's stroke when any exhaust from the previous cylinder was flowing.
gbritnell

[steamer]

Generally speaking George, when the IP exhaust opens there will be step in the admission line.    A large reciever minimizes this step, but it will be there.....just part of the beast.

Dave

[crueby]

There has been at least one earlier thread here about this subject on this site and the easy answer is that no full size expansion engine has 120 degrees between throws. The debate concerned what angles then should be used and why + difference between air and steam in this application.

I hope this will point your attention in a usefull direction, George.

Best wishes

Per

Well, I am currently looking at my copy of the blueprints for the Holly Mfg Co water pumping engines used at the Ward station in Buffalo NY - these are the original builders blueprints (were left in the building, found recently). These are 60' tall triple expansion engines, and they have 120 degrees between cranks, and Corliss valves on the cylinders. There are valve motion diagram pages, can post those if they would help? These engines (there is a row of 5 of them in the pumphouse) do have large receivers between the cylinders, and there is a condensor on the output exhaust pipe to get the most work out of the LP piston. Maybe having the receiver tanks allows/requires them to change the timing more?

Chris

[steamer]

The large receiver minimizes the drop in the admission line.    The cylinder ratios drive the valve timing

[Zephyrin]

In many engines, HP cylinder have piston valve and slide valves in the IP & LP cylinders, hence valve settings are obviously different.
And some engines don't have a receiver in its own, but simply large piping and steam chest !

[steamer]

In many engines, HP cylinder have piston valve and slide valves in the IP & LP cylinders, hence valve settings are obviously different.
And some engines don't have a receiver in its own, but simply large piping and steam chest !

If the Cranks are disposed at 180 degrees, then receivers are not required.    Otherwise they are.   A receiver can take the form of a large pipe, a large steam chest, ect.

A multi expansion engine with cranks at 180 is called a "Woolf" engine.     Tandem compounds are an example with the HP on top and the LP below.


Dave

[ChuckKey]

There has been at least one earlier thread here about this subject on this site and the easy answer is that no full size expansion engine has 120 degrees between throws.

<a href="https://www.youtube.com/watch?v=iw8VoDeugL4" target="_blank">http://www.youtube.com/watch?v=iw8VoDeugL4</a>

<a href="https://www.youtube.com/watch?v=KhlJp1VZMB8" target="_blank">http://www.youtube.com/watch?v=KhlJp1VZMB8</a>

This one is 3 cylinder HP, but impressive

<a href="https://www.youtube.com/watch?v=CcoLiTFzXAg" target="_blank">http://www.youtube.com/watch?v=CcoLiTFzXAg</a>

[Admiral_dk]

Yes I was there a few months ago - but it is NOT an expansion engine (actually it is, but not the kind we're talking about here), as all three pistons are the same size ....