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Bore and stroke ratio in steam engine.

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Stub Mandrel30/10/2011 10:48:31
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Chris,
 
A steam engine is different to an internal combustion engine in many ways. Even the basic analogy of a gas expanding behind a piston breaks down because 'steam' (unless superheated) is a vapour and it has the property of both condensing and evaporating at different points in the cycle. the biggest differences are:
 
The pressure on the piston doesn't rise to a maximum at ignition, then fall. Instead when the valve opens it rapidly rises to maximum and (assuming the steam passages are big enough) stays more or less constant until cut-off, when the pressure starts to drop. This longer, steadier push (and the eas of making the pistons double acting) is why steam engines generally have fewer cylinders than petrol engines. A V12 steam engine would be pointless.
 
Secondly, there's the influence of a condenser, if fitted. Very few model steam engines have condensers and the only one I have come across more than basic details of was by Tom Walshaw. The condenser takes the exhaust and literally condenses the steam back into water in an airtight box, out of which it pumps the water. Now, if the condenser was perfect and the steam contained no air, you could get a perfect vacuum, adding an extra 15psi or so to the difference between steam pressure at each end of the engine.
 
Even though this can't be achieved in the real world Walshaw's condenser was still effective enough that it allowed his engine to run steam at LESS THAN ATMOSPHERIC PRESSURE! That means a partial vacuum even in the steam chest and boiler, so the boiler was boiling at less than 100 degrees!
 
Try doing something like that witha pertrol engine...
 
The theoretical maximum efficency of a heat engine is determined by the temperature difference bewteen the 'input' and the 'output'. Another way of looking at a condenser is that it is 'refrigerating' the exhaust, and hence it increases the efficiency by allowing greater power to be extracted from steam at a given input temperature (and pressure).
 
This is why Watt's condensing engines were such a revolution.
 
I won't get into superheat in detail, but you can easily see that /raising/ the input temperature is another way of increasing efficiency.
 
Neil
Andrew Johnston30/10/2011 10:58:22
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Posted by Stub Mandrel on 30/10/2011 10:48:31
 
This is why Watt's condensing engines were such a revolution.
 
 
That's a shocking pun.

I thought that the main advantage of Watt's condenser was that it was separate from the cylinder. Hence it increased efficiency as there was no need to heat and cool the whole mass of the cylinder every cycle?

Regards,

Andrew
Stub Mandrel30/10/2011 21:59:01
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You are right Andrew,
 
The very earliest steam engines condesned the steam in the cylinder, Watt's genius was to permit condensing while not cooling the cylinder as you say.
 
Neil
Ian S C31/10/2011 10:49:25
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The early steam engines worked on the partial vacuum caused by the cooling steam, I'm not sure that it was Watt, but the big change came when the engine was powered by the pressure of the steam. Ian S C
Jens Eirik Skogstad31/10/2011 20:50:02
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Posted by Ian S C on 31/10/2011 10:49:25:
The early steam engines worked on the partial vacuum caused by the cooling steam, I'm not sure that it was Watt, but the big change came when the engine was powered by the pressure of the steam. Ian S C
 
The first steam engine (Newcomen) was not a pure steam engine who maked the steam as power source. Newcomen used the steam to push the piston out and then the vacuum maked as a power source caused by watercooling in the cylinder. James watt discovered the watercooling in the cylinder to make vacuum was wasteful then he maked the condenser to make vacuum while the cylinder was still hot.
 
Later Watt maked the steam as power source instead the vacuum as power source in his reciprocating steam engine since the steam is practical to make increased pressure in cylinder, but the problem was poor material (or maybe unknown stretch properties in material caused by pressure) in steam engine/boiler to keep against increased pressure hence the maximum pressure was limited to atmospheric pressure.
Alf Jones07/11/2011 12:31:00
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There is a slightly different way to look at it, and thats the function of the boat itself.
 
If you want an all out racing boat, it will be a very different engine design, and therefore selection of criteria such as bore vs stroke, and if you were making a tug to be used in bollard pulls, where torque would be major function, or if you want a nice simple, easy to live with boat, where you would choose a simple design.
 
If you wanted a high speed racer, general thinking is that the line to follow is an over-square stroke, multiple cylinders and a uniflow exhaust. The engine is driven purely by initial input pressure, which can be 1000's of PSI, and there is essentially zero benefit taken from the expansion properties of steam, as the engine is turning too fast. This might give an engine turning at 5,000 - 10,000 RPM, which would usually be coupled directly to a small racing designed prop.
 
For say a tug, where you look for maximum torque, then long stroked compound engines would be where you would start, giant propellors, and going to lengths to eek out every scrap of energy from the steam, such as the condensers discussed above.
 
Like anything else in this lovely hobby, the "right" answer depends on the question you started with.
 
One other thing I would say is that Jens make a comment about needing a "larger" boiler in his first post.
I would say that what is actually needed is a more efficient boiler, rather than a larger one. One of the big benefits of steam powered toy boats, over steam powered toy trains ( apart from the significantly lower drain on the wallet ! ) is that you get an unlimited supply of free water, so the question of boiler size comes less down to "how long will it run for before I have to fill her up" and turns into "How can I make enough water turn to steam"
 
You can make very small boilers which produce vast amounts of steam, as long as the water is available to actually make it in the first place.
 
 
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