Compound steam engine.

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

Joined
Apr 2, 2020
Messages
2,678
Reaction score
810
Location
Sunderland , UK.
Thanks Jason, but I am curious and confused. The original "idea" for amusement was about a compound twin - 180 degree crank I think? I have one in my garage - so I'll go and check. But it seems to me to have a direct feed from one exhaust directly (almost) into the opposing inlet just via an internal passage. I'll have to strip it to see... I think the configuration along the cranks is "drive coupling" main bearing, HP crank (datum zero), HP valve eccentric (normal 80 degrees or whatever), main bearing, LP valve eccentric (180 degrees to the HP eccentric), LP Crank (180 degrees to the HP Crank) Main bearing...
Whatever. It means the exhaust valve on the HP is opening just before "BDC" for that stroke, and simultaneously (or very closely) the inlet valve on the corresponding LP cylinder is just opening...
But I may be completely wrong?
Excuse me while I go and check?
Why would you have a 90 degree crank for a compound? I can understand for a reversing engine with 2 x HP cylinders.... but that wasn't the original post by Richard 1... or so I understood? "an engine with HP 1" bore x 1" stroke, LP 1" bore x 3" stroke ". But maybe you have a very good point as Richard goes on to describe a 3 cylinder compound - that I guessed would have a 120 degree crank - but my assumption was for no useful reason than 360 degrees divided by 3.....
With the small and larger pistons, compound engines are not so easily balanced as non-compound multiple cylinder engines where the masses are all the same per crank...
K2
 
Joined
Apr 2, 2020
Messages
2,678
Reaction score
810
Location
Sunderland , UK.
Aha! Sometimes I am wrong, but just for a change I got it partly right.
Error: There are 5 main bearings on the compound twin in the garage, not 3 as stated above, but otherwise I was right.
Correct: It is a 180 degree crank, with eccentrics as described. See pictures attached. Hope this helps?
Kz.
 

Attachments

  • P8052322.JPG
    P8052322.JPG
    306.5 KB · Views: 209
  • P8052323.JPG
    P8052323.JPG
    243.8 KB · Views: 221
  • P8052325.JPG
    P8052325.JPG
    247.3 KB · Views: 203
  • P8052330.JPG
    P8052330.JPG
    239.9 KB · Views: 195

Jasonb

Project of the Month Winner!!!
Project of the Month Winner
Joined
Mar 30, 2008
Messages
3,094
Reaction score
783
Location
Surrey, UK
Self starting would be one of the main reasons for a 90 degree crank on a compound and plenty are made that way, Take the Stuart compound for example, not much good if you stop to stick it in reverse and it gets stuck at TDC or BDC. And just about every compound traction engine I can think of has a 90deg crank excepting SSC's
 

GreenTwin

Well-Known Member
HMEM Supporting Member
Joined
Jul 2, 2021
Messages
1,468
Reaction score
686
Location
MidSouth
Why would you have a 90 degree crank for a compound? I can understand for a reversing engine with 2 x HP cylinders.... but that wasn't the original post by Richard 1... or so I understood? "an engine with HP 1" bore x 1" stroke, LP 1" bore x 3" stroke ". But maybe you have a very good point as Richard goes on to describe a 3 cylinder compound - that I guessed would have a 120 degree crank - but my assumption was for no useful reason than 360 degrees divided by 3.....
With the small and larger pistons, compound engines are not so easily balanced as non-compound multiple cylinder engines where the masses are all the same per crank...
Take this all with a grain of salt; it is not coming from a "compound expert" by any measure........

Having three cranks at 120 degrees theoretically will produce a constant torque (or an approximation of constant torque) on the crankshaft, which is why 3-phase electric motors are in widespread use.
Constant torque means almost no vibration, and much easier on the parts.

The engine-generator sets I think were typically non-self starting with 180 degree cranks, for smoother running.

It would seem that you would balance a compound engine one cylinder at a time, with corresponding crank web weights for each cylinder? but I will have to go back and look at the old engravings, because I am not sure I recall seeing this.
Without balancing one cylinder at a time with corresponding weights on the crank webs at each cylinder, it seems like you could induce torsional forces down the crankshaft.

I recall reading about dynamic balance of multi-cylinder marine engines, but I can't recall the specifics.

The dynamic balance of locomotive wheels is also complex, and each wheel weight is generally a different size, and the weights are not necessarily symmetrical from side-to-side.

Check me on all of this.

Edit:
There was a story that Charles Porter told in his book about a person who purchased a Porter engine and could not get the governor to function correctly. Porter went to the site and noticed a long line shaft with a large flywheel on the end opposite the engine.
Porter said "remove the flywheel", and that solved the problem.
The line shaft was bending due to torque and feeding power back to the engine/governor, which upset the function of the governor.

.
 

Jasonb

Project of the Month Winner!!!
Project of the Month Winner
Joined
Mar 30, 2008
Messages
3,094
Reaction score
783
Location
Surrey, UK
Thats right a 180 crank compound or double high will be smoother running than the same engine with a 90deg crank but could get stuck at TDC/BDC.

Not such a problem on something like the genset shown as once started it will be running the same way but in something like a boat or vehicle which will be stop start use then the self starting of the 90deg is a big bonus and you just have to balance out the vibration.
 

GreenTwin

Well-Known Member
HMEM Supporting Member
Joined
Jul 2, 2021
Messages
1,468
Reaction score
686
Location
MidSouth
you just have to balance out the vibration
With a variable loaded device, and a device that does not require operating at full output continuously, I would think it would be easier to live with some extra vibration.

For a ship engine, or generator set, I think excessive vibration would start breaking things.

Obviously a tug engine that may have to be reversed a lot would want automatic stop-start, as well as traction engines, etc.

I will have to go back and start looking at old photos/engravings now and see which engines had which configuration.

Compound engines are a bit of a gray area for me, give that there are so many variations on how to compound an engine, with some compounds having two pistons on the same piston rod.
.
 

GreenTwin

Well-Known Member
HMEM Supporting Member
Joined
Jul 2, 2021
Messages
1,468
Reaction score
686
Location
MidSouth
I feel lucky having some understanding of a single-cylinder steam engine with D-valve and Stephenson's link.
Beyond that is pushing my knowledge.

Some classifications of steam engine types:

From "Steam-Engine Principles and Practice", Terrell Croft, 1st Ed., McGraw Hill Book Company, Inc.,
1922, p.19,
and
"Steam Power", C. F. Hirshfeld, M.M.E., T. C. Ulbricht, M.E., M.M.E., 1st Ed., John Wiley & Sons, Inc.,
1916, p.93,

(public domain books), we have the following classification of engine types:

Basis of Classification Primary Subdivision Secondary Subdivision
----------------------------------------------------------------------------------------------------------
(1) Cylinder arrangement (A) Single cylinder
(B) Tandem
(C) Cross
(D) Duplex
(E) Opposed
(F) Angle
*(G) Inverted
(H) V-configuration (added item, not original)
----------------------------------------------------------------------------------------------------------
(2) Longitudinal axis (A) Vertical
(B) Inclined
(C) Horizontal
----------------------------------------------------------------------------------------------------------
(3) Rotative speed (A) High speed
(B) Medium speed
(C) Low speed
----------------------------------------------------------------------------------------------------------
(4) Ratio of stroke to (A) Short stroke
diameter (B) Long stroke
----------------------------------------------------------------------------------------------------------
(5) Valve gear (A) Slide valve (a) D-slide valve
(b) Balanced valve
(c) Multi-ported valve
(d) Gridiron valve
(e) Piston valve
---------------------------------------------------------------------------
(B) Corliss (rocking)vvalve (a) Detaching
(b) Positively-operated
---------------------------------------------------------------------------
(C) Poppet valve (a) Detaching
(b) Positively-operated
----------------------------------------------------------------------------------------------------------
(6) Engine mechanism (A) Standard
(B) Back-acting
(C) Trunk-piston
(D) Oscillating-cylinder
*(E) Rotary cylinder
*(F) Direct acting
*(G) Beam
----------------------------------------------------------------------------------------------------------
(7) Steam expansion (A) Single expansion
---------------------------------------------------------------------------
(B) Multi-expansion (a) Compound
(b) Triple
(c) Quadruple
----------------------------------------------------------------------------------------------------------
( 8 ) Steam flow (A) Counter flow
(B) Uniflow
----------------------------------------------------------------------------------------------------------
(9) Steam conditions (A) Initial Pressure (a) High pressure
(b) Medium pressure
(c) Low pressure
---------------------------------------------------------------------------
(B) Initial temperature (a) High superheat
(b) Low, or no superheat
---------------------------------------------------------------------------
(C) Back pressure (a) Condensing
(b) Non-condensing
----------------------------------------------------------------------------------------------------------
(10) Use (A) Stationary engines
(B) Portable engines
(C) Locomotive engines
(D) Marine engines
(E) Hoisting engines
(F) Automobiles (added item, not original)
(G) Aeroplanes (added item, not original)
----------------------------------------------------------------------------------------------------------
*(11) Connection (A) Direct connected
(B) Geared
----------------------------------------------------------------------------------------------------------
*(12) Condensation (A) Jet condensing
(B) Surface condensing
----------------------------------------------------------------------------------------------------------
*(13) Designation by Name (A) Watts
(B) Corliss
(C) Porter
----------------------------------------------------------------------------------------------------------


------------------------------------------------------------

From "Steam Engines", Llewellyn V. Ludy, 1917 we have the following additional classifications:

Direct acting (with balance lever or beam)
Indirect acting (without balance lever or beam)

and from R. H. Thurston "A Manual of the Steam Engine":

1. Stationary mill engines.
2. Agricultural engines.
3. Portable and semi-portable engines.
4. Road locomotives.
5. Railway locomotives.
6. Pumping engines (crank and flywheel) and (direct acting).
7. Marine engines (paddle engines) and (screw engines).
8. Special types.

.
 
Last edited:

GreenTwin

Well-Known Member
HMEM Supporting Member
Joined
Jul 2, 2021
Messages
1,468
Reaction score
686
Location
MidSouth
Some of the notes I collected on compound engines.
I can't verify the accuracy of the information below, so do your own checking/verification and be sure it is correct.

28. Compound steam engine: Compound engines have two or more cylinders of differing sizes, typically
with a high pressure cylinder, a low pressure cylinder, and sometimes an medium pressure cylinder.
The exhaust from the high pressure cylinder is used to power the medium pressure cylinder, and the
exhaust from the medium pressure cylinder is used to power the low pressure cylinder.

The work produced by the cylinders is designed to be equal.

Often two low pressure cylinders are used on large engines, since the size of a single low pressure
cylinder would be excessive.

An engine with (1) high pressure, (1) medium pressure, and (2) low pressure cylinders is a 4-cylinder
engine, but is called a triple-expansion engine, since the steam expands three times as it passes
through the engine.

29. Cross compound steam engine: A cross-compound steam engine has two cylinders parallel and
side-by-side (called cross-compound since the steam has to cross over from one cylinder to the other).
A cross compound engine that has cranks at 90 degrees to each other can be self starting in any
position, but a receiver must be used between the cylinders to store the steam that is exhausted
from the high pressure cylinder until the appropriate time to introduce the steam into the low pressure
cylinder.

A cross compound engine with crank pins at 180 degrees to each other, or on the same crank pin does
not require a receiver, and the high pressure cylinder can exhaust directly into the low pressure cylinder.

30. Tandem steam engine: A tandem steam engine has cylinders connected on the same piston rod
(think tandem bicycle).
The two cylinder tandem engine has dead points, and is not self starting, but does not need a receiver.
The tandem-compound engine has only one set of reciprocating parts. Tandem engines were typically the
compound type.

31. Double expansion steam engine: A double-expansion steam engine expands steam twice across
two cylinders prior to the steam being exhausted.

32. Triple expansion steam engine: A triple-expansion steam engine expands steam three times
across three or more cylinders.

The Titanic engines were triple-expansion, 4-cylinder units, with two low pressure cylinders being
used to prevent the use of an excessively sized low pressure cylinder. The low pressure cylinders
were exhausted into a 16,000 horsepower steam turbine, thus allowing the energy from the exhaust
of the two 16,000 horsepower vertical reciprocating engines to be captured and converted into useful
work. The designers of Titanic did not trust the new technology of the steam turbine enough to use
it instead of reciprocating engines.

33. Quadruple (quad) expansion steam engine: A quad-expansion steam engine expands steam
four times across four or more cylinders.

34. Compound steam engine cylinder ratios: The cylinder ratios define the proportioning of the sizes
of the cylinders of multi-expansion engines. The cylinder ratio should be such that nearly equal power
is developed in each cylinder.
The crank throws for multi-cylinder engines were often designed to space the forces from each cylinder
equally around the 360 degrees of the crankshaft.

A common rule for the size of cylinders in compound engines is to make the ratio of the cylinders equal
to the square of the total ratio of expansion.
A two-cylinder compound engine with an expansion ratio of 9 will have a cylinder volume ratio equal to
the square root of 9 (equal to 3), and the low pressure cylinder volume will be three times the volume
of the high pressure cylinder.

.
 

delalio

Well-Known Member
HMEM Supporting Member
Joined
Sep 23, 2019
Messages
77
Reaction score
52
Location
IE
As we generally measure pressure in pounds per square inch you will have a lot less square inches of piston area for the part expanded steam to act on with a 1" LP piston compared to a 1.75" LP piston. So will not get as much force pushing against the smaller LP piston.

Area of 1" piston is 0.78sq inch, area of 1.75" piston is 2.4 sq inch. So your small piston will only be adding 1/3rd the force that the larger piston would into rotating the engine.

Hi all, Just picking up on the point by JasonB.

I think he is correct in stating that the force on the LP Piston would be around 1/3, as the pressure drops by 1/3, and the piston size is the same.
P = F/A or F = PxA

Therefore, all being equal, the piston would accelerate at 1/3 the speed, making it a much slower rotating engine.
F = Mass x Accel or Accel = Force / Mass.

I'm not sure about the duration the force lasts on the piston etc, so not sure what speed the piston would get up to, but lets assume the pressure change over time is the same for both HP and LP pistons.

At the other end of the piston, yes the crank would have 3x the torque, as it is 3x the distance from the fulcrum, but I assume friction would be the majority of the force acting on the piston, (unless it was acting on a very heavy load). Therefore the two shafts would not spin in time.


If the two drive shafts were connected, I think the result would be as Ian says, the HP would do all the work ad the LP would not be able to keep up. I think it is highly likely the LP would end up acting as drag on the HP cylinder setup.

I seriously doubt that this premise would work in practice. While mechanically the LP piston would move up and down, in appropriate time with the HP piston, it would have to do so at (roughly!) three times the speed (ok, it's not that simple, but I'm trying to illustrate a principle). The speed of the LP piston would likely draw a degree of vacuum in the cylinder, thus negating any useful compound effect, as the incoming steam is being sucked in, having work done on the steam by the piston, rather than arriving forcefully, and doing work on the piston, as required.
But do try, and let us know!!
Regards, Ian.

Someone also mentioned about increased wall surface area etc, leading to increased cooling of the steam, and increased condensation. This would also play a role at reducing the cylinder effectiveness, until the cylinder was up to temperature.


I believe the optimal setup is to have the LP piston to experience the same force as the HP piston.


As it turns out, I've done some spreadsheet work, and a 1" x 1" HP has ~= piston force as a 1" x 1.75" LP piston at 1/3 pressure.

It could be interesting making the LP cylinder shorter and fatter to account for pressure losses between the cylinders.
E.g. a 0.5"L x 2.475"Diameter LP cylinder. (for a 1" x 1" HP)
This would have ~2x Piston Force ignoring losses.

Not sure if I explained this clearly, but hopefully it made some sense!

Kindest Regards, Del
 
Joined
Apr 2, 2020
Messages
2,678
Reaction score
810
Location
Sunderland , UK.
I think I follow your reasoning, Del, on the last bit. But I have to think again about the first part. I am a bit puzzled by the pressure - speed relationship you mension. Also you suggest separate cranks for the cylinders ("2 drive shafts"??), does this mean you are not thinking of a single crankshaft? - confused by that bit...
K2
 

Richard Hed

Well-Known Member
Joined
Nov 23, 2018
Messages
1,861
Reaction score
450
Location
Moses Lake in the Great Soviet of Washington
Aha! Sometimes I am wrong, but just for a change I got it partly right.
Error: There are 5 main bearings on the compound twin in the garage, not 3 as stated above, but otherwise I was right.
Correct: It is a 180 degree crank, with eccentrics as described. See pictures attached. Hope this helps?
Kz.
SO! You are holding out on us, Ken, the little sneak has an engine he has never shown us! A spanking for you, Ken! Tell us more about that beautiful little two cyl. compound. What's more, it has a use! it's driving a generator.

I have been thimpfking for a long time that any compounds might benefit by having a small 'reservoir' or 'capacitor' from the HP to just at the entrance to the LP which would more quickly evacuate the HP and have quick boost for the LP. What does you thimpfk?
 

Richard1

Well-Known Member
Joined
Feb 13, 2009
Messages
140
Reaction score
11
In my original idle speculation I hadn't even thought about single or double acting or how the crank throws would be set up. It seems building it as a cross compound would be the way to go to simplify what is already a crazy idea. There should in this case be very little need for a receiver as the steam would flow direct from HP to LP so the pipe should be enough. I think receivers are mostly a source of heat and pressure loss unless they are somehow set up as superheated reheaters. Something else to speculate about but too complicated for this. I would envisage both cylinders either being on a common crankshaft or on two crankshafts geared together but in either case turning at the same speed.
 
Joined
Apr 2, 2020
Messages
2,678
Reaction score
810
Location
Sunderland , UK.
RH. I have not been holding out on anyone. You have probably read more of my history than my wife knows!
This little engine was made by some Sunderland apprentices, otherwise unknown, sometime in the 1920s or 1930s.... or so I was told when asked to care for it. (Something like 15 years ago). It belongs to the Sunderland Model Engineers. So it runs regularly at shows. I'll be running it on steam on 20th Aug. In Roker Park, Sunderland (UK), if you want to pop over.....
I am not a thermodynamics expert, but sure that an intermediate reservoir for steam is a loss of energy... so direct transfer from HP cylinder to LP cylinder is the better option, with minimum volume of valve space and connection between. R1 explained it anyway.
The generator is a dummy. It is was a simple 3-pole armature rotor and 2-pole field coils, just worked as a motor, in that it could drive itself at 6V. 1A. But only at a couple of hundred rpm. Connected as a dynamo, it didn't generate a milivolt with the engine fairly chuffin' along at a few hundred rpm. So I installed a 4V PM DC motor to generate, but it still runs so slowly that it won't light my smallest LED. Next is to make a different version of internals, maybe a multi-pole PM rotor, with multi-pole stator... and light an LED? But that is a future project in a long list...
Cheers!
K2
 

Richard Hed

Well-Known Member
Joined
Nov 23, 2018
Messages
1,861
Reaction score
450
Location
Moses Lake in the Great Soviet of Washington
RH. I have not been holding out on anyone. You have probably read more of my history than my wife knows!
This little engine was made by some Sunderland apprentices, otherwise unknown, sometime in the 1920s or 1930s.... or so I was told when asked to care for it. (Something like 15 years ago). It belongs to the Sunderland Model Engineers. So it runs regularly at shows. I'll be running it on steam on 20th Aug. In Roker Park, Sunderland (UK), if you want to pop over.....
I am not a thermodynamics expert, but sure that an intermediate reservoir for steam is a loss of energy... so direct transfer from HP cylinder to LP cylinder is the better option, with minimum volume of valve space and connection between. R1 explained it anyway.
The generator is a dummy. It is was a simple 3-pole armature rotor and 2-pole field coils, just worked as a motor, in that it could drive itself at 6V. 1A. But only at a couple of hundred rpm. Connected as a dynamo, it didn't generate a milivolt with the engine fairly chuffin' along at a few hundred rpm. So I installed a 4V PM DC motor to generate, but it still runs so slowly that it won't light my smallest LED. Next is to make a different version of internals, maybe a multi-pole PM rotor, with multi-pole stator... and light an LED? But that is a future project in a long list...
Cheers!
K2
Why not get a real generator?
 
Joined
Apr 2, 2020
Messages
2,678
Reaction score
810
Location
Sunderland , UK.
Hi Richard. It depends what you call a "real generator". ... It is a "real generator" at a couple of thousand rpm. But the engine only does a few hundred rpm. As it was made nearly 100 years ago, I can keep it original, or simply modify the generator to light an led, or make a "proper (modern design, multi-pole) generator". No rush to do anything. Volts are generated by speed of change of magnetic field. Slow speed = very few volts! Field strength and number of windings determine the current deliverable. The original windings are few, and it was never designed to give a good magnetic field. Wound/wired as a series motor, it would never generate anyway...
The club "philosophy" is to keep the old models as originally made, but maintain and use them. We are curators of history...
Not like "Washington's axe". Is it true that the axe he used to chop down the family apple tree had a rotten handle, so it was replaced when it went into the museum, but they didn't know the original axe head was badly rusted and blunt, and had been replaced with a new (sharp steel) one when the family became wealthy...? Living just 10 miles from the original family home in the UK - that shows their wealth - they would have given the old one away and bought a decent new one anyway.... so maybe I should follow suit and buy a cheap Chinese engine and generator instead of running the antique model?
Or maybe just keep it original....
K2
 

Richard Hed

Well-Known Member
Joined
Nov 23, 2018
Messages
1,861
Reaction score
450
Location
Moses Lake in the Great Soviet of Washington
Hi Richard. It depends what you call a "real generator". ... It is a "real generator" at a couple of thousand rpm. But the engine only does a few hundred rpm. As it was made nearly 100 years ago, I can keep it original, or simply modify the generator to light an led, or make a "proper (modern design, multi-pole) generator". No rush to do anything. Volts are generated by speed of change of magnetic field. Slow speed = very few volts! Field strength and number of windings determine the current deliverable. The original windings are few, and it was never designed to give a good magnetic field. Wound/wired as a series motor, it would never generate anyway...
The club "philosophy" is to keep the old models as originally made, but maintain and use them. We are curators of history...
Not like "Washington's axe". Is it true that the axe he used to chop down the family apple tree had a rotten handle, so it was replaced when it went into the museum, but they didn't know the original axe head was badly rusted and blunt, and had been replaced with a new (sharp steel) one when the family became wealthy...? Living just 10 miles from the original family home in the UK - that shows their wealth - they would have given the old one away and bought a decent new one anyway.... so maybe I should follow suit and buy a cheap Chinese engine and generator instead of running the antique model?
Or maybe just keep it original....
K2
I thot u said that u put that "generator" there. I would simply make the generator go faster by proper pulleys or gears.
 

Jasonb

Project of the Month Winner!!!
Project of the Month Winner
Joined
Mar 30, 2008
Messages
3,094
Reaction score
783
Location
Surrey, UK
Depends what you call "proper" the in-line configuration is quite common and introducing gears or belts would completely spoil the original layout and look. No doubt one could thrash the life out of the old engine to get the revs up but I'm with Steamchick and would treat the old girl with respect, besides it's nice to see an engine's parts moving rather than look at a blur.
 

Steamrguy

New Member
Joined
Aug 13, 2014
Messages
4
Reaction score
11
OK , I have been digesting this a bit and her is my two cents. This is off the top of my head so it my not be exact but....

The difference between the two designs is the Torque produced between the two cylinders and the force produced between the two cylinders.
In a typical design

1" stroke
HP Piston with a area of 1 cu/in
LP piston 3x greater or 3 cu/in
pressure on HP piston 100 psi
Pressure on low pressure 1/3 or 33 psi

Force of the HP piston is 100psi x 1" = 100 lbs
Force of the LP piston = 33 psi x 3" = 99 lb
Everything balances out so the each piston is doing the same amount of work. and producing the same Torque and so the same Horse power.

In the presented design, the LP piston is the same diameter but with longer stroke so...

force of the LP piston would only be 33 psi x 1" = 33 lbs. if it has 3 times the stroke it would produce the same HP but the torque must also be 3 times that of the HP piston for that to happen.

I am not sure in the numbers presented that the horse power in the two cylinders would actually be the same but you can see the logic. The traditional design is a well balanced design. The presented design has unequal force and torque in each cylinder but should still produce the same horse power. It would be interesting to see the performance of the presented design under load ( if that were even possible)

Dean
 

Jasonb

Project of the Month Winner!!!
Project of the Month Winner
Joined
Mar 30, 2008
Messages
3,094
Reaction score
783
Location
Surrey, UK
Dean as torque is lbs.ft the crank throw possibly comes into what torque is being applied to the crankshaft

HP 100lbs force x 0.5" crank throw 4.16 lbs.ft
LP 33.3lbs force x 1.5" crank throw = 4.16 lbs.ft
 
Joined
Apr 2, 2020
Messages
2,678
Reaction score
810
Location
Sunderland , UK.
Hi Dean, Jason.
I like the simple approach you are taking. However, "Life isn't like that" is what I heard for decades every time I tried to "calculate" what should happen.
As a lover of numbers - but not the most competent - I enjoy all these number crunching exercises. And learn a lot in the process. Simply:
The pressure explanation is a bit too simple. While it expresses what the simplest theory should predict in "work preformed" by each cylinder - and Jason, I agree with your calcs. - this isn't really true of a more complex (more real?) model. You need to consider real cut-off, mean cylinder pressure, expansion into intermediate cavities... etc. to get a better numerical model of what the steam can do. In fact a lot of energy is lost to cylinder walls, and passage walls, while transiting the engine, so this "loss of efficiency" should be estimated - even if you use a fudge factor of 50% or something at each stage. As the steam expands into the HP cylinder, then from that to an intermediate chamber volume when the exhaust valve opens, followed by the LP cylinder, these expansions must be considered to guesstimate the mean pressures in both cylinders for the period when they can "accept the work from the steam" and translate this into thrust on the piston to produce motion power. I tried to estimate this in a "thought model" in post #19... In fact it isn't so clear that I find it confusing to read now... (Sorry about that!).
But the bottom line is that the 100psi supply steam - if exhausting at 33psi - would only be applying 66psi AVERAGE pressure for the length of the stroke. At exhaust, the steam is expanded into the transfer passage volume - reducing pressure further - and therefore the inlet steam to the LP cylinder is a much lower pressure than you have modelled.
So maybe a real set of sizes needs to be computed to better understand this? (I'm not clever enough to work it all out).
Conclusion: I think the simple "volume" based calculations say that the 2 confirgurations that Richard 1 postulates are effectively the same. But when further losses of transfer passages, and heat loss are considered, the larger piston LP is the way to go, because there is less cylinder wall to loose the heat in this configuration. (Maybe that is why it has always been done that way?).
K2 - Now I need to lie-down and rest the grey cells!
 
Top