Triple expansion engines


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Mark Chirnside reports in his last article about the propulsion system of the Olympic, that the engins run without any vibration.
Thats a nice point of discussion.
Because most triple expansion engines have no vibrations. The pistons are working in an angle of 120°, so at each point of a full revolution allways 1 and 2/3 of all cylinders generate power, not more, not less. So compared to other multi cylinder engines, like four cylinder Tandem engines of two cylinder compound engines, only the three cylinder engine is able so generate such a smooth and soft power output to the main shaft. A two cylinder engine has an piston angle of 90°. so if the first piston is in a death end positon, the other piston is exactly middle of the piston stroke, so having the highest point of power output. This produces vibrations, which someone could feel. Four cylinder engines have this up and down, or forward-backward shake also. because again the angle is 90°, so the two pistons are in death end postion and two are in full power output. Tandem engines act like two cylinder engines, so having the same shake of the engine.
So all triple expansion engins should not produce vibrations, and if, this vibration might come from the propellers and not as suggested from the engines itself.
 
Jan 5, 2001
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<FONT COLOR="ff0000">Mark Chirnside reports in his last article about the propulsion system of the Olympic, that the engines run without any vibration.
Thats a nice point of discussion.


Quote:

(...Olympic docked, passengers) mentioning that there was ‘very little vibration.’ In fact, as the reciprocating engines turned in opposite directions, this assisted to prevent vibration; although it must be mentioned that the propellers can be the prime cause of vibration, rather than the engines themselves.





I am saying that the engines themselves produced little vibration.

One of the factors was the fact that they were balanced on the Yarrow, Schlick & Tweedy system:


Quote:

'To reduce vibration to a minimum, both engines are balanced on the Yarrow, Schlick & Tweedy system.'

- Shipbuilder, Autumn 1911, referring to Cunard's Laconia.





However, the Laconia's engines were not quite the same; they were quadruple-expansion engines.

I believe that of the little vibration that did occur, it was mostly the propellers' turning; this was suggested after Olympic's maiden voyage, and the fact that her propeller design was slightly modified during her 1913 refitting would seem to support this. However, Olympic was never a ship that vibrated much; it was only slightly noticeable, and you get vibration of a kind in all ships.

During the 1932/33 ovehaul of the ship's engines, there was a proposal to reduce vibration even more and improve further the engines' balance, which I will post later.
 
Dec 29, 2000
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Hello Mark,
you're right, but most people think if they hear of a revolvering steam angine of an shaking, vibrating action, compareable to modern diesel or gasoline engines. But triple expansions engines do not have. The run in complete silence, only a slight hissing noise if the sealings or valves of the linkage may have produces some noise, as the shaft itself, by the rolling in its beds.
But the engine will never shake. A Quad-engine, which is again foru cylinders will have, but never a 3 cylinder type.
so I guess all the vibration must have come from the turbine and the propellers. the turbine may have slight more vibration than the recipoking engines, so I guess the balance system was not to reduce the engine vibration, more to keep the engine in complete balance, to avoid torsion power of the powerfull cylinders to the shaft, to keep also the shaft allways in straight level.
You can see a triple expansion engine at my homepage. I was build with components from ships manufactures, but the design is for inmobile locations, so the linkage is missing, as the large wheel will never been used in ships.
http://members.tripod.de/Reichel/HE/IronLady.html

The page is german, but you can watch the pictures...
 
Jan 5, 2001
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Steffen, I had written the following post before seeing yours, so I will add it here.

I completely agree with you that the engines run very smoothly and don't shake like many people think, but from your initial post I took you to be saying that there was no vibration whatsoever. There's bound to be some, that is virtually unavoidable, but not necessarily a lot of vibration; we agree that there was very little.

Written Post:


<FONT COLOR="ff0000">Another vessel, Orsova, had quadruple-expansion, rather than triple-expansion, engines, but they were balanced on the same principle.


Quote:

…Special attention had been paid to this last feature in the design (Yarrow, Schlick & Tweedy), the position of the cylinders and crank angles being arranged so as to ensure that vibration will be practically non-existent.





To talk of the Olympic’s engines’ 1932/33 overhaul:

It had been planned to fit additional ties for the engine columns to ensure that the balance of the engines were maintained, but upon the fitting of the new crankshafts the engines were performing so well that there was simply no need to add them to the engines as the balance was fine. (I am quoting this from memory, though, because I could not find the specific information and won’t be able to for another week.)
 

Cal Haines

Member
Nov 20, 2000
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Tucson, AZ USA
Steffen wrote{
... Because most triple expansion engines have no vibrations. The pistons are working in an angle of 120&deg;, so at each point of a full revolution allways 1 and 2/3 of all cylinders generate power, not more, not less. So compared to other multi cylinder engines, like four cylinder Tandem engines of two cylinder compound engines, only the three cylinder engine is able so generate such a smooth and soft power output to the main shaft. ... So all triple expansion engins should not produce vibrations, and if, this vibration might come from the propellers and not as suggested from the engines itself.

Hi Steffen,

It may just be our language difference, but it sounds as if you are saying that a three cylinder, triple-expansion engine is producing power only on one half of the stroke. You say, "... at each point of a full revolution allways 1 and 2/3 of all cylinders generate power, not more, not less." Actually, most of the time all three pistons are producing power. Only when a piston is near the end of its stroke, either top or bottom, is it not producing power. For the benefit of the non-technical folks here, the pistons of most steam engines do work on both the up and down stroke (unlike the internal combustion engines in our automotobiles). Steam is alternately admitted to both the top and bottom of the cylinder, causing the pistion to alternately push and pull.

While it is true that Titanic's engines were triple-expansion, they were not three cylinder engines. They had two low pressure cylinders, for a total of four cylinders. Titanic's engines can be describes as:
[ul][*]Triple Expansion - three stages of steam expansion, carried out in three different sized cylinders [*]Four Cylinder - consisting of one high pressure (H.P.), one intermediate pressure (I.P.), and two low pressure (L.P.) [*]Double Acting - producing power on both the up and down stroke [*]Inverted, Vertical - the pistons were above crankshaft[/li][/list]
That Titanic's engines ran smoothly is explained by the fact that they used the "Yarrow, Schlick & Tweedy" system of balancing. This article explains it better that I can:


Quote:

Balanced Engines.


The constantly varying pressures on the crankpin result in corresponding variations in the twisting stresses exerted by the engine, the range of torsional stresses varying with the type of engine, number and position of cylinders, and the steam distribution in each cylinder.

These unequal stresses continued for long periods often result in the development of flaws on the shaft, and may finally lead to total breakage.

It is therefore desirable to so balance up the moving parts that an even turning movement on the shafting may be obtained, and vibration damped down to a minimum.

Regarding this subject Professor W. E. Dalby, M.A., B.Sc., in a paper read at a meeting of the Institution of Naval Architects, says


"The only way of balancing a 3-crank marine engine of the usual type is by the addition of balance weights, or bob weights, to the moving parts. In this case, therefore, balancing necessarily means the actual addition of considerable masses of material to the machinery which have no other duty but that of producing forces equal and opposite to the unbalanced forces caused by the motion of the moving parts which are concerned in doing the proper work of the engine. It is well known that Messrs Yarrow, Schlick, & Tweedy made a departure from existing practice when they began to build engines in which the moving parts concerned in doing the proper work of the engine were so arranged that they were in balance amongst themselves. In engines of this kind no part of the machinery merely turns round or reciprocates for the sake of the forces its motion causes on the frame. No such arrangement is possible, however, unless the engine has at least four cranks. This condition and the progressive increase in the power of marine engines have together determined the gradual introduction during the last ten years of the 4-crank engine into the Navy and the Mercantile Marine. Yet that the possibilities of balancing the 4-Crank engine have not been generally recognised is shown by the fact that many engines of that type have been and are still being built with their cranks at right angles, even when absence of vibration is imperative. Four cranks at right angles is just the one particular arrangement of a 4-Crank engine which makes it impossible to effect balance without the addition of balance weights. A change in the crank angles, however, and a small change in the mass of the moving parts is all that is necessary to obtain an engine in which the moving parts are balanced amongst themselves; to change, in fact, a 4-crank unbalanced engine into a 4-crank balanced engine of the Yarrow, Schlick, & Tweedy type. These changes cannot be made in any arbitrary manner. The masses, crank angles, and centres of cylinders must be mutually adjusted to satisfy certain conditions."​


The necessary calculations required in accurately determining the above arrangement of cranks, balance weights, etc., are worked out from the indicator diagrams, crank effort diagrams, and the carefully calculated weights of the various moving parts, and involve a considerable amount of labour.

Balance weights are sometimes fitted to the crank webs of the H.P. and I.P. engines, which are lighter, while the crankpins of the two L.P. or heavy engines are bored out hollow, so that the weights of the parts may be correctly adjusted.

In the Yarrow-Schlick-Tweedy system of engine balancing, the calculations are usually so carefully determined that the addition of balance weights is not always required, the necessary balance being found by the relative crank angles and crank sequence, or order of rotation.

In an ordinary 3-cylinder triple-expansion engine the sequence is either H.P., I.P., and L.P., or L.P., I.P., and H.P., but when four cylinders are fitted two L.P.) the sequence is usually as shown in the sketches on p. A4.

Observe that the H.P. and I.P. cranks are directly opposite, also that the F.L.P. and A.L.P. are opposite each other, but at right angles to the other two.

It will be thus seen that the crank angles and crank sequence are quite different when the Yarrow-Schlick-Tweedy system is adopted as in the example illustrated on p. A5, the H.P. and I.P. cylinders being inside, and the two L.P. placed one forward and one aft. Observe that the heavy engines are placed at the ends to balance up the weight of the moving parts.

The crank sequence is then (1) H.P., (2) F.L.P., (3) I.P., and (4) A.L.P. F his arrangement has the effect of reducing the vibration, and also allows of quick and easy handling of the engines.

It should be understood that the relative crank angles vary with the size of engine, power, and weight of moving parts.

source:
J.W.M. Sothern, "Verbal" Notes and Sketches for Marine Engineer Officers, 18th Edition, James Munro & Co. Ltd., Glasgow, (no copyright date given), pages A6-A7.





The problem with balancing a three-cylinder, triple-expansion steam engine is that the pistons and valve gear are all of different sizes and thus have different masses, keeping the engine from being in balance without the external weights.

As the article indicates, the crank pins of Titanic's engines were not at right angles. The actual angles were as follows:

<TABLE>[tr][td]HP[/td][td]®[/td][td]IP[/td][td]106&deg; [/td][/tr][tr][td]IP[/td][td]®[/td][td]LP (for'd)[/td][td]100&deg; [/td][/tr][tr][td]LP (for'd)[/td][td]®[/td][td]LP (aft)[/td][td]54&deg; [/td][/tr][tr][td]LP (aft)[/td][td]®[/td][td]HP[/td][td]100&deg;[/td][/tr][/table]
(Note that this is a different crank sequence than the article mentions.)​
Cal
 
Dec 29, 2000
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Dear Mark and dear Cal,

I agree but, I guess you often did not understand. Please excuse, I am german, so I often do not know the right english words for technical parts or things to describe. Please excuse.

Cal, I agree with you, but consider following:

We have a triple cylinder engine. So we have a hp, ip and lp cylinder, acting to a singlre crank shaft. We also consider that engine as a typical marine engine, standing upright wit hthe cylinders pointing straight upwards to the celling and the sharf in its bearings mountet to the deck. We consider now the engine at position zero, which means hp piston at death end position up. This means that the hp crank is also pointing upwards, and we consider this as angle 0&deg;.
Most engines with three cylinders I know having now the hp crank at 120&deg; and the lp cylinder at 240&deg;.
Okay, if we know open steam valves, the hp cylinder won't move, because in death end position, but ip cylindr will start to move down, heading to death down end and the lp cylinder will move up, heading up death position. both cylinders together act as if 1 and 2/3 cylinders were giving power to the shaft.
Now the turn beginns, and at 5&deg; the hp cylinder beginns to generate power, but less than any of both other cylinders, first at 10&deg; the hp cylinder get into a position were the angle is right to send the incredible to the crank. The ip cylinder is at 130&deg; and the lp cylinder is at 250&deg;. The lp cylinder is close to the maximum effort point, which is allways at 90&deg; or 270&deg; of the crank, so it is only 20&deg; before, so we can consider that lp piston start to bring is most and best power and effort to the crank, with the best point of lever and the highest possible torsion power. So still only 1 and 2/3 cylinders are acting, if we add all the power together: hp cylinder just has started to bring effort to the shaft, ip cylinder is closer to death end position, so making less effort to the shaft, but lp cylinder is close to maximum effort point. Now the lp cylinder reaches crank point 270&deg;, sending maximum effort, while hp crank is at 30&deg;, one-third away from maximum effort point, and ip cylinder crank is at 150&deg;, closer to the death end position. So hp and ip cylinder are away from producing the same effort than the lp cylinder that moment. Now the ip cylinder reaches death end down at 180&deg;. The hp cylinder is at 60&deg;, close to the maximum effort point, still increasing its power output. Lp cylinder is at 300&deg;, and thus has leaft its maximum power output point, starting to loose power output to the shaft, because the steam does not expand much more. So we still have only 1 and 2/3 of all available cylinder power acting to the shaft. Now the hp cylinder reaches 90&deg; crank angle, being now in the highest possible effort and power output point, because again the crank is in best lvereage to the shaft, giving the most torsion power to the shaft. The ip crank is at 210&deg;, still only 30&deg; after death end position, so far from its maximum power pojt at 270&deg;, to which it has start its journey now on that turn. Lp cylinder is at 330&deg;, very close to the iupper death end position, now generation much less power than before, because being only 30&deg; before death end. So we have again only 1 and 2/3 clinder power at the shaft, not less not more.
Short after hp cylinder has left is maximum effort point at 90&deg;, its crank is now at 110&deg;, lossong somewhat of its power output. The ip cylinder crank is then at 230&deg;, increasing power output as much the crank angle gets closer to 270&deg;. The lp cylinder is now at the death end zone, were actually not realy power is generated, thus why the point 10&deg; before and 10&deg; after the death ponts is called 'black zone'. So lp cylinder generates still some power, but very, very less. So we still only have 1 and 2/3 cylinder power at the shaft... and the turn continues....
You understand what I mean? This is how three cylinder, triple expansion angines work. the move very smooth and very soft, and without any vibration. Vibration at a steam engine comes, alike the shake, because if a cylinders moves throuh the black zone, were only one cylinder must generate at its maximum effort point all the power, but its less power as is both cylinders were bringing power to the shaft. so the engine will have a rattle, a shake and as more revolutions, as more it becomes to a vibration, than a shake. Such a shake can damage the bearings and has very high torsion power effects to the shaft, and thats why two cylinder engins with cranks at 90&deg; angles were not often used in ships. Also four cylinder compund engines shale, a little, much more less than a 2 cylinder engine, but little more than a 3 cylinder type.
Most four cylinder engines act as paired engine, so we have two hp cylinders and two lp cylinders. Each hp cylinder has its own lp cylinder. So the cranks were set as following: hp1 at 0&deg;, lp1 at 270&deg;, hp2 at 90&deg; and lp2 at 180&deg;. this priciple was found be De-Glen, who also build the two axle drive for railroad steam engines with four cylinder engines, to reduce the incredible torsion power. In inmobile engines this is not of regard, because the main crank shaft can be put in special bearings and the engine can be balanced that way, that the torsion power will have no critical effects on the shaft, which is neccessry, because steam engines have a realy incredible torque.
As we can se, the 4 cylinder compound engine has still two cylinder in death end position, and other two cylinders are in most effort point. this is why this engine still vibrates, and a triple cylinder engine will not, because two cylinders bring the thrid cylinder through the black zone, so it will not shake. Steam engines were commonly build in parts, so each cylinder is mounted to its own carrier, and the carrieres at the bottom together form the bearing beds for the crank shaft. Often we found in marine engins, that the carriers were in larges distances to give space for thelarge and heavy block type flywheels, which replace the large in large in diameter found flywheels of the inmobile steam engines, maybe in company manchinery halls, as example. Also Titanic had flywheels, but they were not inside the carriers, but outside mounted and smaler than commonly found.
so the carriers were mounted together, bringing the cylinders to the desired set and row, and no the carries must be balanced, and here again I agree with you Mark and Cal. Balancing is a must, because I realy know no engine which had a single carriers for all cylinders, if it had more than two. So the carriers must be at th same level, to avoid the crank shaft bearings to be out of the same level and to absorb the incredible power of torque generated by the engine to the shaft. Without balancing the shaft will not rotate free, it will stick and take damage, or in worst case bend or break. So an engine must be balanced to avoid this damage, and in a complete overhaul, the balancing of an multiple cylinder engine is checked and if possible a reblanacing is done or the balancing system is replaced. This is must. Each overhaul the balacing is checked.
But triple expansion engines should have not a real critical effect to the balancing system, so a rebalancing should not be needed, but sometimes it was done in mobile used engines, because the decks or ground an which the engine was mounted has changed, making balancing need.
So if we have the engine as a single triple expansion, three cylinder engine, there should be no vibration, and if we have, the engineer may have realy a problem. Vibrations at such engines point to shaft damages or bearing problems!
Thanks to Cal, we all know that Titanic, as Olympic had four cylinder engines with triple expansion. Because the lp cylinder was so large in diameter, so it was divided into two lp cylinders, which together have the needed piston area to work proplerly.
so I personally expected, that if titanics hp crank was at 0&deg;, the lp cylinder was at 120&deg; and both (!!) lp cylinder were at 240&deg;, but Cal mentioned someting different.
The engles Cal wrote down here seem, for me as steam engineer appearance, confusing. Because if the were true, the engine must have had a shake, realy. Because the crank angles were not in a harmonical turn setting, thus a little shake of the engine must have been present. Because if hp cylinder is death end up, the ip cylinder is short after most tractive effort and lp2 crank is much closer to most effort point, thus having nearly two cylinders at the most effort point giving an higher torque to the shaft than in any other sharft position, so here we have a shake point.
Thats why I wondered...
 
Jan 5, 2001
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Steffen,

Your English is fine. I'd certainly not be able to write that well in German.

(Mein Deutsch ist nicht sehr gut.)

smile.gif


Best regards,

Mark.
 
S

Stephen Stanger

Guest
Hey CAL!
Great definitions of the engine specifics (thank you) questions though.
I've always seen the engine described as DIRECT acting not double acting, same difference?
Also you missed out the "surface condensing" part of the engine description. Would you have a definition for that and what surface are they referring to?
 

Cal Haines

Member
Nov 20, 2000
308
1
263
Tucson, AZ USA
Steffen wrote:
> ... I am german, so I often do not know the right english words for technical parts or things to describe. ...

Hi Steffen,

Actually, your English is pretty good, much better that my Deutsche! The only term you use that sounds odd is "death end" of stroke. I would use "top dead center" (TDC) or "bottom dead center" (BDC).

>Cal, I agree with you, but consider following:
>(Steffen's description of rotation and forces in three-cylinder engine)​
>You understand what I mean? This is how three cylinder, triple expansion angines work. the move very smooth and very soft, and without any vibration. ...

I understand what you are saying, but I'm not sure I agree. If all three cylinders were the same I suspect that your description would be pretty close, at least in theory. I would have to sit down with a set of indicator diagrams and work through the crank angles, etc., before I agree. My immediate concern is that the pressure in the cylinder, and thus the force is exerts, drops as a piston move up or down. It's not a given that the point of greatest torque will be when the crank is at 90&deg;--I would expect to see it somewhat before 90&deg;. My second concern is that a three-cylinder, triple-expansion engine has three different sized cylinders, all operating at different pressures--they may not all produce exactly the same amount of force over their entire stroke.

>... Most four cylinder engines act as paired engine, so we have two hp cylinders and two lp cylinders. Each hp cylinder has its own lp cylinder. So the cranks were set as following: hp1 at 0&deg;, lp1 at 270&deg;, hp2 at 90&deg; and lp2 at 180&deg;....

> As we can se, the 4 cylinder compound engine has still two cylinder in death end position, and other two cylinders are in most effort point. this is why this engine still vibrates, and a triple cylinder engine will not, because two cylinders bring the thrid cylinder through the black zone, so it will not shake. ... Often we found in marine engins, that the carriers were in larges distances to give space for thelarge and heavy block type flywheels, which replace the large in large in diameter found flywheels of the inmobile steam engines, maybe in company manchinery halls, as example. Also Titanic had flywheels, but they were not inside the carriers, but outside mounted and smaler than commonly found.


I've never seen a marine compound engine with a flywheel. Titanic's reciprocating engines certainly did not have flywheels. Maybe you are thinking of the thrust bearings?

6725.gif


The thrust bearings were necessary to transfer the thrust of the propellers to the hull of the ship. Otherwise, all the power of the engine would be at work trying to push the engine of it's mounts.

> ... Thanks to Cal, we all know that Titanic, as Olympic had four cylinder engines with triple expansion. Because the lp cylinder was so large in diameter, so it was divided into two lp cylinders, which together have the needed piston area to work proplerly.
so I personally expected, that if titanics hp crank was at 0&deg;, the lp cylinder was at 120&deg; and both (!!) lp cylinder were at 240&deg;, but Cal mentioned someting different.
The engles Cal wrote down here seem, for me as steam engineer appearance, confusing. Because if the were true, the engine must have had a shake, realy. Because the crank angles were not in a harmonical turn setting, thus a little shake of the engine must have been present. Because if hp cylinder is death end up, the ip cylinder is short after most tractive effort and lp2 crank is much closer to most effort point, thus having nearly two cylinders at the most effort point giving an higher torque to the shaft than in any other sharft position, so here we have a shake point. ...


I refer you to the excerpt from Sothern that I quoted. The designer of an engine can control the power of each cylinder by adjusting it's dimensions and working pressure. It sounds as if you assume that both of the LP cylinders together had about the same power as the HP or IP cylinders. I would expect that the LP cylinders would each be about as powerful as the others. I've never seen a marine engine with both LP cylinders at the same angle--it would be almost guaranteed to be out of balance due to the large amount of mass in the two pistons and cranks.

Cal
 

Cal Haines

Member
Nov 20, 2000
308
1
263
Tucson, AZ USA
Stephen wrote:
> Great definitions of the engine specifics (thank you) questions though.
> I've always seen the engine described as DIRECT acting not double acting, same difference?


The "direct acting" part refers to the fact that the pistons are directly connected to crank shaft, as opposed to the type of mechanism common in paddle-wheel steam engines. In the latter case the piston drives a large lever or "walking beam" (sort of like a see-saw) which in turn works the crank.

Here's a photo of a model of the engine of the paddle wheel ferry Eureka, on display at the San Francisco Maritime Museum, showing the walking beam.

6729.jpg


Maybe "double acting" is the wrong term for the fact that the pistons worked on both up and down stroke. (I'm not aware of any steam engines that only work on the down stroke, in the fashion of an internal combustion engine.) After sifting through several of my books I'm not finding the term "double acting", or anything else for that matter, used to describe this aspect of the engines.

> Also you missed out the "surface condensing" part of the engine description. Would you have a definition for that and what surface are they referring to?

That refers to the type of "condenser" used to condense the exhaust steam back into water. It's not a part of the engine, per se, but an integral part of the whole system (just like the boilers). Titanic's surface condensers used tubes full of cold sea water to cool the steam. The steam was condensed by contact with the surfaces of the condenser tubes. The other type of condenser would be "direct contact", where the cooling water is sprayed directly into the steam itself. Titanic's feed water system used a direct contact heater that operated on this principle. It wouldn't work when sea water is used for cooling, since that would introduce salt into the boiler feed water.

By the way, the main condensers did more that just condense the steam for recycling. The process of condensing the steam produces a powerful vacuum that boosts the efficiency of the engine. (Shooting from the hip, the gain is around 10-20%, but I would need to check that number, so don't quote me.) The low pressure cylinders of Titanic's engines actually exhausted at a pressure below atmospheric and the turbine worked entirely in the vacuum range. By the time the steam reached the condensers it was close to room temperature, due to the high vacuum of the condenser. The fact that the total pressure drop from the boilers to the condensers is larger than it would be without the condensers allows the steam to expand to a greater volume and do more work than it could otherwise, accounting for the increase in efficiency.

Do you have the part of an engineer at the Orlando exhibit? That could be fun!

Cal
 
Dec 29, 2000
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Dear Cal,

Steffen wrote:
> ... I am german, so I often do not know the right english words for technical parts or things to describe. ...

Cal quoted: Hi Steffen,

Actually, your English is pretty good, much better that my Deutsche! The only term you use that sounds odd is "death end" of stroke. I would use
"top dead center" (TDC) or "bottom dead center" (BDC).


THX. I allways try my best, so trying that yiu might be able to understand what I am talking.


>Cal, I agree with you, but consider following:

>(Steffen's description of rotation and forces in three-cylinder engine)


>You understand what I mean? This is how three cylinder, triple expansion angines work. the move very smooth and very soft, and without any
vibration. ...

Cal: I understand what you are saying, but I'm not sure I agree. If all three cylinders were the same I suspect that your description would be pretty close,
at least in theory. I would have to sit down with a set of indicator diagrams and work through the crank angles, etc., before I agree. My immediate
concern is that the pressure in the cylinder, and thus the force is exerts, drops as a piston move up or down. It's not a given that the point of
greatest torque will be when the crank is at 90&deg;--I would expect to see it somewhat before 90&deg;. My second concern is that a three-cylinder,
triple-expansion engine has three different sized cylinders, all operating at different pressures--they may not all produce exactly the same amount of
force over their entire stroke.


No, not all three cylinders are same, they must be different in size, just to ensure that all three cylinders have the same power!!! But the most effcient force point is at 90&deg; crank angle. Well, the advantage of a steam enige is, that the torque will only vary slightly during the full piston stroke. gasoline engines have the most torque short after the ignitation of the gas-air mix in the cylinder.
So compare: gasoline engine: Slam a hammer onto the piston head, this makes it move.
steam engine: push the piston with your hands full force down, and if you can drag it with the same force up again

So the gasoline engine just got once a giant force, when the fuel is enlighted and smashes the piston down... The steam engine got this downward force over the whole stroke, up and>/B>down. Double action!
In action, the steam engine will not allways have the need to generate such a power. So the torque reaches good datas even at low crank angles, but most torque is applied to the shaft at 90&deg; crank angle, if we asume full power, full filling and full pressure. All other angles have lower torques, but not so much less, that we must consider it critical. But all cranks follow the mathmatic laws and physical laws of force and levers, so only in 90&deg; angle we have the longest lever and the best force directions, thus having maximum force at the turn point. You agree?
Compare my homepage http://members.tripod.de/Reichel/HE/IronLady.html
There you can see a triple expansion engine. The only difference betwen a marine engine is the large flywheel and the linkage, but from the least engine, both are same.
The engine has an alltogether power of approximately 700 PSi, while the hp cylinder generates 270 PSi, the ip cylinder has 250 PSi and the lp cylinder has 190 PSi. There is only slight difference, but as ideal, all three cylinders should have same power!

>... Most four cylinder engines act as paired engine, so we have two hp cylinders and two lp cylinders. Each hp cylinder has its own lp cylinder. So
the cranks were set as following: hp1 at 0&deg;, lp1 at 270&deg;, hp2 at 90&deg; and lp2 at 180&deg;....

> As we can se, the 4 cylinder compound engine has still two cylinder in death end position, and other two cylinders are in most effort point. this is
why this engine still vibrates, and a triple cylinder engine will not, because two cylinders bring the thrid cylinder through the black zone, so it will not
shake. ... Often we found in marine engins, that the carriers were in larges distances to give space for thelarge and heavy block type flywheels,
which replace the large in large in diameter found flywheels of the inmobile steam engines, maybe in company manchinery halls, as example. Also
Titanic had flywheels, but they were not inside the carriers, but outside mounted and smaler than commonly found.


Cal: I've never seen a marine compound engine with a flywheel. Titanic's reciprocating engines certainly did not have flywheels. Maybe you are thinking
of the thrust bearings?
(Picture of engine blue paint with thrustblocks)

Not realy, I knew the difference betwen a thrust block and a flywheel, but many marine engines use the heavy and large thrust block as flywheel. Common gasoline engines still have a flywheel, not as large as we knew from pictures of stationary mounted steam engines, but effectife enough to bring the engine throught the black zones. Steamers with paddle wheels lack flywheels, becausde the large and heavy padle wheel is used as flywheel. The compact marine multi-cylinder engines do often have no need for a flywheel, so small block type flywheels are build in, to give the engine a snoothy turn and rotation, to absorbt the heavy shake of the up and downward flying masses.
The most destructive shake is a result of the pistons slamming into the bearings at crank angles of 0&deg; at upward ways and 180&deg; downward way. Also if the steam comes in to early, the slam upwards at 180&deg; and downwards at 0&deg; crank angle will damage the crank bearings, were the stroke beam is mounted to.
So all steam engines have so called 'death spaces', were some steam after exhaust is left, as a buffer for the piston to avoid this destructive power, but thsi will only slightly absorb the shake of an steam engine, as less cylinders, as more the shake.
So flywheels can help to reduce the shake, and with a good linkage adjustment, most steam engines turn well without any visible shake.
Anyway, triple expansions engines, have were balances masses, because there the piston itself if often build larger and heavier more and less, so that the three pistions have compareble weights. This is important to reduce shake. So it could be that the hp piston is made of heavier materials, then th ip cylinder, while the lp piston is made in a special profile and 'light-construction', so avoid improper masses, which cannot readily moved.
Otherway: If we cannot act like this, the sharf at the cranks get balance weights, to equal the masses of piston, crank and stroke beam. Such weights can be found at most railroad steam engines, and in some cases at stationary steam engines. Seldom I have seen them at multi cylinder engines, with more than two cylinders.


The thrust bearings were necessary to transfer the thrust of the propellers to the hull of the ship. Otherwise, all the power of the engine would be at
work trying to push the engine of it's mounts.


Yes, yes I know. Also the thrust blocks are used as point were propeller shaft and engine shaft are 'screwed-together'. But, did anyone saw a block type flywheel? The thrust blocks of many marine engines I saw were used as block type flywheels, if the engine was large enough, and Titanics engines were that large....

> ... Thanks to Cal, we all know that Titanic, as Olympic had four cylinder engines with triple expansion. Because the lp cylinder was so large in
diameter, so it was divided into two lp cylinders, which together have the needed piston area to work proplerly.
so I personally expected, that if titanics hp crank was at 0&deg;, the lp cylinder was at 120&deg; and both (!!) lp cylinder were at 240&deg;, but Cal mentioned
someting different.
The engles Cal wrote down here seem, for me as steam engineer appearance, confusing. Because if the were true, the engine must have had a
shake, realy. Because the crank angles were not in a harmonical turn setting, thus a little shake of the engine must have been present. Because if
hp cylinder is death end up, the ip cylinder is short after most tractive effort and lp2 crank is much closer to most effort point, thus having nearly
two cylinders at the most effort point giving an higher torque to the shaft than in any other sharft position, so here we have a shake point. ...

Cal: I refer you to the excerpt from Sothern that I quoted. The designer of an engine can control the power of each cylinder by adjusting it's dimensions and working pressure.


No, not the dimensions!!! He can only adjust the linkage, which has a time effect of the steam entry to the cylinder. So the linkage opens and closes the ventiles of the cylinder, in Tatinics engine I guess we have no ventiles, I thinks she got sliders. So the ventiles open to let steam in or open to let the steam pass to exhaust.
We can adjust the linkage in percent, this means how long the steam should enter the cylinder, and how long at same time the exhaust is opened. 100 percent means the full way of the stroke steam is let in, or the exhaust is open. 50 percent means, if the piston is at half stroke, the inlet is closed, as the exhaust is closed.
So most triple expanbsion engiens were driven betwen 35% and 80%.
This has no effect on dimension, more an amount of steam which can enter the cylinder, as how much steam is left as buffer in the cylinder to absolb the flying mass of the pistion. So you can spare much steam if you set the linkage to low percentuages, but if you 'drive' to low dosages, example 15% the buffer of exhaust steam which is left in the cylinder is not enough to absorb to power if the pistion slams into the death center, which an engineer could her, because a special sound will appear, a rumble in the bearings, as a signal of to less steam in the engine cylinders.
Also you cannot control the power of each cylinder. You can control the power of the whole engine, but not for each cylinder. You can close the throttle, to reduce the pressure to the engine, thus reducing the force of the engine, but this is very ineffective. Fist you should set the linkage to lowest percentuages, which will also reduce power output, before you start to reduce pressure by the throttles.
Thats why the throttle is commonly a single hand-wheel, not realy impressive in size, but the most impressive hand-wheel at engines with a adjustable linkage is the 'steering-wheel', were the engineer will adjust the linkage and direction.
So the desinger can construct an engine and change the dimensions of cylinders and pressures, to adjust the power, but he must do this carefully. A triple expansion engine can only work, if all three cylinders have the same piston force, pressure to piston surface! Idealy, same. Practicaly there are differences. So in the triple expansion engines I know and described at my homeapge, we will find following:
hp cylinder diameter 490 milimeters and pressure of 19 kg/square centimeter -> 35 Tons of force
ip cylinder diameter 830 milimeters and pressure of 5 kg/square centimeter -> 27 Tons of force
lp cylinder diameter 1300 milimeters and 0,9 kg/square centimeter -> 12 tons of force
So I agree, but if a designer will not pay attention to this, espcially making all three cylinders in same size, he will never making thisengine work properly. So even a desinger cannot change the dimensions and pressures as he likes. So he must carefully watch the priciples the engineers and founders of compound engines have found and published.
As you descripe, it is important to have the lp cylinder work in vacuum, because if has an positive effect to the condensor, so what if the desinger does not know and sets the lp cylindrer to 5 kg/square centimeter presure, at exhaust of ,5 kg/square centimeter? He will make the engine ineffective, thus we can consider titanics engine desinger as well known, carefully paying regard to the laws of compound engines.


It sounds as if you assume that both of the LP cylinders together had about the same power as the HP or IP cylinders.

Yes, but only theoreticaly. This is idealy. The best possible compound engine has all three cylinders, or four or five or more in the same power. But practicaly this is impossible. But we should never forget and have before our eyes: The ideal compound engine has all cylinders in same power.
But then we must talk about the ideal way and possibilty to the thing the designer had made practicaly. and here wen can see, how good the constructer realy was.

I would expect that the LP cylinders would each be about as powerful as the others. I've never seen a marine engine with both LP cylinders at the same angle--it would be almost guaranteed to be out of balance due to the large amount of mass in the two pistons and cranks.

That's what most people expect. Most triple expansion engines do not have the lp cylinder divied in two. the most triple expansion compound engines I know, have only a single lp cylinder, thus having not realy that mass problem.
In triple expansion engines the desinger will make the hp piston slightly heavier, by a massive profile, maby a different steel.
The ip piston get a common profile, because having the desired mass and weight. The lp cylinder is now made of a special profile and material, light construction, so all three pistons should idealy have same masses. But again, this is theoreticaly and only idealy. This means, you will not find this in reality.
But, triple expansion engines can deal got will slight improper masses, because once in move, the steam will carry the piston weights, so the difference will not realy count.
But even if we share the lp cylinders in two parts, two cylinders I mean, why than such disharmonical crank angles?
I guess because to get close to the ideal crank angles of the triple compound engine: 120&deg; crank angle.
But this is only a guess, not the truth.
So I am still confused about these strange crank angles.
 
M

Morgan Eric Ford

Guest
Cal Wrote: I refer you to the excerpt from Sothern that I quoted. The designer of an engine can control the power of each cylinder by adjusting it's dimensions and working pressure.


Steffen wrote: No, not the dimensions!!! He can only adjust the linkage, which has a time effect,,,

Morgan comments:

Steffen I think you may have misunderstood what Cal meant by designer. The designer is the person who creates the original plan "Blaupause" for the engine and sends them to the machine shop "mechanische Werkstatt". This is the person who decides the stroke, bore and other details of the engine.


I think the German word for designer is "Zeichner" or "Baumeister".


Regards,

Morgan
 
Dec 29, 2000
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Well, surely, I first missunderstood, but even a designer, as mentioned later in my posting, cannot vary the dimensions just by guess. He must depent onto the laws of compound steam engines, commonly developed and found by Anatoile Mallet, the first who build compounbd steam engines, so called mallets. The triple expansion engines appeared later...
 
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