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[kno3]

Hello,
I'm trying to find out what the operating pressures should be for (model) compound steam engines with 2 or 3 cylinders. I'm especially interested in how much pressure drop is at the exhaust from the HP cylinder and the rest, and what the relationship between cylinder volumes (bore/stroke) should be to get a compound to work as intended.
Do you know any (easy) literature on this topic?

[gondolier88]

Always a minimum of 150psi to the HP piston to get true compounding, the higher the better, and superheat adds more power. The pressure drops are not standard for the size of the cylinders but varies on EVERY engine to a degree, valve efficiency, port size/shape/length, entry to pistons, steam speed all have their hand in making it a none exact science- you could do individual experiments on two engines of the same design and not find exactly the same results.

You building a miniature compound or a triple????

Greg

[kno3]

Hello and thanks for the answer. I'm just considering building a double expansion engine. I doubt a triple would really justify the effort in model size.
But before building I'd really like to know what the operating parameters need to be so that the second cylinder does useful work too. There's no point in building a compound and having only the HP cylinder do actual work...

That's why I need to know the relationships between cylinder volumes, recommended pressures etc.

[gondolier88]

You can't use compounds in models because they don't self start. There are three ways of getting round this- simpling valve, make it a three cylnder engine- two HP and one common LP or make it a steeple compound.

Cylinder volumes of the LP vary from between 1.5 times HP volume to 3 times HP volume- there doesn't seem to be any correlation really- though most manufacturers tended to go with twice HP volume.

Greg

[kno3]

I'm thinking about using a simpling valve to make it self starting. Seems simple enough 

I read the LP vol = 2 x HP vol on a French forum too.

[Steamer]

Cylinder and expansion ratio's will determine the most efficient initial steam pressure.

A cylinder ratio of 4 to 1   ( LP diameter = 2 x HP diameter) will result in cylinders with very near the same cut off.   An engine of this ratio will do well at 150 psi and condensing.

The receiver volume is important here too.   Never less than the volume of the LP cylinder


The Stuart 6A has a lower expansion ratio and is meant for lower pressure....like 100-125 psig.

Dave

[kno3]

Dave, what is the receiver?
I am not considering condensing engines, as this isn't very practical for model engines.

[gondolier88]

Nothing like a bit of repetition for emphesis eh steamer

I'm very interested in your simpling valve idea- how are you going to shut it off once the engine is going then make sure it automatically re-engages when the engine stops?

Kno3- Reciever is the intermediate valve chest between the HP and LP, in later designs of compound and triple engines the first valve was usually a piston valve recieving minimum of 200psi, superheated to Loco' standards with slide valves on the IP and LP recievers- though surely the reciever wants to be the same volume as the cylinder precluding the IP and LP cylinders Steamer? If the reciever is the same size as the next cylinder it supplies isn't the steam already going to be expanded?

Greg

[kno3]

My thought with the simpling valve was to run a thin steam line from either the main supply or from the HP valve chest to the LP valve chest. Interrupted by a valve on the way. I could control it manually with a servo, whenever the engine stops.
I guess there must be people who did this before, so maybe there's a more practical solution.

[gondolier88]

I can't see there being a problem with that. Hope you can make it work.

Greg

[benjaml1]

In discussions with Stuart Turner Models, their model compound launch engine has a maximum working pressure of 120 psig & an operating pressure of 80psig. I was wondering if there was any benefit ( or detriment ?) of running it ( this engine) uncompounded ? IE Exhausting the first stage to atmosphere & supplying the second stage with full line pressure ?

[gondolier88]

Why would you? Just get the twin high pressure version!!!

Greg

[benjaml1]

Why would you? Just get the twin high pressure version!!!

Greg

Currently there is a compound on e-bay which is quite nice. Plus I'm a stickler for scale, a compound being more accurate for the type of model I am building. Only a steam engineer would spot the different ( incorrect) pipe layout 

[steamboatmodel]

Currently there is a compound on e-bay which is quite nice. Plus I'm a stickler for scale, a compound being more accurate for the type of model I am building. Only a steam engineer would spot the different ( incorrect) pipe layout 

Just add some extra piping and valves in the correct position for compounding that are blanked off, and clam your simpling valves are stuck.
Regards,
Gerald.

[benjaml1]

Thanks, I may go that way if the bidding doesn't get too silly on the compound launch. If not, there is a nice D10 or a Stuart Sun that may fit the bill....

[Steamer]

The only problem with a compound in a RC boat is that they MAY not be self starting.....a  starting valve, which is just a smaller version of a simpling valve would be a good idea in a RC boat....the other way is to shift the links back and forth as you will change the valve timing and dump some live steam into the receiver.  That will usually be enough pressure to get the HP off TDC or BDC.

Watch your expansion ratio as most of you run at 100 psi or lower.....The LP can over expand if your not careful and you will waste more power than your generating........it all depends as they say...

Dave

[benjaml1]

Well, After all my dithering, I scored a nice D10 instead, boring I know but for my service functional... Thanks for all the input, I learned a lot from this thread...

[Steamer]

Ahhhh  A prudent choice.....you won't regret it....

Good Luck!

Dave