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°--I would expect to see it somewhat before 90°. 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° 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° 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° 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°, lp1 at 270°, hp2 at 90° and lp2 at 180°....
> 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° at upward ways and 180° downward way. Also if the steam comes in to early, the slam upwards at 180° and downwards at 0° 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°, the lp cylinder was at 120° and both (!!) lp cylinder were at 240°, 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° crank angle.
But this is only a guess, not the truth.
So I am still confused about these strange crank angles.