Relationship of steam preasure to cylinder size

[capnbob]

Years ago I was talking to a person how made steam engines.  I wanted him to make two cylinders with pistons for a sternwheeler.  The cylinders were to be 3/8" bore by 1 1/2" stroke. I would finish the rest of the engine.  He said that cylinders that size would require too much steam pressure to be safe in a model.

Is there a relationship between the size of a cylinder ( 3/8 X 1 1/2 verses 5/8 X 3/4 ) and the pressure requirements?

Bob

[derekwarner_decoy]

Hi PD's.......Bob....there are many misconceptions with steam.......& all too often from too little knowledge 

The cylinder stroke is simply a number or unit of measure that reflects the swept volume or displacement when referenced to the cylinder bore......

The engine stroke is not a function of pressure 

There are a number of good reference books on model steam engines........well worth spending a few $ gaining knowledge

K N Harris texts would be a good starting point.........we all are never to old to learn  ........Derek

[R.G.Y.]

Bob my friend Derek is very knowledgeable, he is right. The swept volume has little relevance to the pressure in this case. Although higher pressure will give more expansion. The area of the piston is the ruling factor here.
To explain in simple terms (I am only a simple carpenter). If a 1inch by 1inch square piston was working at 50psi.  A .5 inch by .5 inch square piston will need 200 psi to produce the same work. As obviously the later is only using a quarter of the pounds per square inch.
Why not have square pistons . R.G.Y.

[capnbob]

Derek and R.G.Y.  Thanks for the answers. 

BTW, R.G.Y., in "Steam & Stirling Engines Engines You Com Make" there is an engine with a square piston.

Bob

[Bierjunge]

We can't answer you question if you don't tell us:

• the diameter of the sternwheel
• the surface area of each bucket
• the average number of buckets immersed
• the scale of the model boat
• the rough size (length, front surface, displacement) of the prototype

Then I would volunteer to make some calculations. It's all interconnected somehow, you know.

Moritz

[capnbob]

Moritz,

The question was not about boats it was about engines.
Does a long stroke engine require more steam pressure than a short stroke?
What I get from the answers so far is, If the diameters are the same then pressure is the same.
Or, is there more to it than that?

Bob

[Bierjunge]

Bob, it's all about torque. And the (maximum) torque of a (two cylinder) engine is piston surface times crank radius times steam pressure. So for a given steam pressure, you could obtain the same torque either by means of a small piston with a long stroke, or a large piston with a short stroke. It is as simple as that.

But what I was trying to say:
You simply cannot design a good engine independently from the boat. That's why I was asking for the number of paddle floats and so on: Torque is what makes your wheel spin. And for a given hull, a given wheel and a desired speed, you will need a certain torque.
But if you take an engine designed independently from these needs (say 3/8 x 1 1/2), it could be totally sufficient for a small boat, but far to weak for a large one. Or in other words: It may be the case, that the (theoretically) needed steam pressure to propel a large model at prototypical speed may be too high to be handled in a model safely. But nobody will be able to answer this if you don't tell us a single thing about the boat!

Moritz

[R.G.Y.]

And of course the crank must be half the stroke. Unless a rocker is introduced. Which could help in a stern wheeler. But remember longer the stroke the slower the engine came run. A square engine (stoke near same as bore ) can rev faster.  R.G.Y.

[Bierjunge]

Theoretically, you could replace a given engine by another one with half the stroke and the double piston surface. Or vice versa.
In practice however, the piston speed was the limiting factor. So a slow engine (esp. sternwheelers..) could be built with a much longer stroke (and slimmer piston) than a high-revving propeller engine.
So classic sternwheelers usually were built with a very long stroke, usually 4 or even 5 times the piston bore.

And now to designing a steam engine for a model sternwheeler:
The stroke should be scaled correctly  (let's say 1:32), because it is visible from the cranks. If you also build your cylinder bore to scale  (1:32 as well), theroretically, 1:32 of the prototype's boiler pressure (e.g. 0.3 Bar instead of 10 Bar) should be sufficient to achieve Froude speed, or to spin the wheel SQRT(32) faster than the prototype's.
But model engines have a lower efficiency and a higher relative friction than the prototype. But even with 1.5 Bar, which is a common value for many model engines, you have enough safety margin to overcome these influences and to obtain an adequately motorized model.

PS: I just had a brief look at some original specifications: A stroke of roughly 1/30 of the hull length seems to have been a typical value for original sternwheelers. So your first engine, being a long stroke engine with a stroke-to-bore-ratio of roughly 4, could be a suitable choice for a 45" long model hull. If it can carry the weight of the boiler an engines...

[R.G.Y.]

 BOB.  My 1/2 inch bore + 1 inch stroke double acting twin. Can be seen running on 20 psi. at.
http://www.youtube.com/watch?v=-ANuLa4jRKQ        This should give you hope.   even this engine will be geared down 3 to 1 driving side paddles. R.G.Y.

[Bierjunge]

As you don't provide any steamboat specifications, I will chose one arbitrarily:

Let's take the 50-Foot River Steamer, No. 1 in the Marine Iron Works of Chicago Stern Wheeler Catalogue 1902.
Please allow me further to do all calculations in metric units, it's much easier this way.
This boat has a 2130 mm dia wheel with 1980 mm wide buckets (7' x 6 1/2') and engines with 127 mm bore and 508 mm stroke (5" x 20").
I further assume the buckets being 200 mm (8") wide, an average of two buckets being immersed, and a steam cutoff after 75% of the stroke.

Now let's look at the static bollard pull scenario as kind of worst case: The wheel churning when the boat is kept motionless. Even worse, each bucket shall independently act against stagnant water.
Then, at a boiler pressure of 11.4 bar (165 psi), we would get a medium torque of 3500 Nm, spinning the wheel at 26 rpm (which is equivalent to a circumferential speed of 2.9 m/s or 6.5 mph) and a mechanical output power of 9.5 kW.
This is a worst case, as I told. If the boat is moving, the wheel will spin faster, and the speed and the power will increase. As a hint, in the catalogue the engine is rated with 22 indicated hp (16 kW).

Now let's look at a fictional model in the odd scale of 1:14, which would perfectly match your first example engine with 9 mm bore and 37,5 mm stroke (3/8" x 1 1/2"). With a pressure of 11.4 bar/14 = 0.8 bar, the 152 mm dia wheel would spin (same bollard pull scenario) at 97 rpm which matches exactly Froude's scaling law. The output power would be 1 W. If you assume a model engine having only 50% mechanical efficiency (due to higher relative friction) compared to the prototype, the speed will go down by the factor 0.7 (divided by square root of 2). If you double the boiler pressure to 1.6 bar (which is more typical for model boilers), the speed will increase by the factor of 1.4 (square root of 2). Quite simple, isn't it?

Edit: Model output power added.

[R.G.Y.]

Bob.  Dont let that put you off. R.G.Y.

[Bierjunge]

Bob.  Dont let that put you off.

Why should this put him off???
It was meant to confirm that his original choice of engine size was a sensible one!

[derekwarner_decoy]

Guys...... ..I think we may have a misunderstanding here......

Moritz was only asking a technical question to respond or enable a more succinct reply.....over the years Moritz has provided many sound mathematical steam answers or and advice 

I am happy with his questioning questions...... ...for the sake of debate & learning......


Derek

[R.G.Y.]

SORRY I SHOULD HAVE SAID US NOT THIS R.G.Y.