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Bore and stroke ratio in steam engine.

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Jens Eirik Skogstad22/10/2011 11:40:09
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Hi all folks!
 
Some time i am thinking what is difference properties between 3 steam engines who has bore and stroke as here:
 
1. 5/8" bore 3/4" stroke
2. 5/8" bore 5/8" stroke
3. 3/4" bore 5/8" stroke
 
I know the steam engine who has 3/4" bore, 3/4" stroke will give more power and need bigger boiler than the steam engine who has 5/8" bore and 5/8" stroke.
 
Let me listen to your answer..
 
Regards Jens Eirik
 
 
 
 
David Littlewood22/10/2011 15:11:46
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Jens,
 
The over-simplified answer would be that the power of the 3 designs you mention would be in proportion to the swept volume of the cylinders:
 
5/8" bore x 3/4" stroke - 0.23 in^3
5/8" bore x 5/8" stroke - 0.192 in^3
3/4" bore x 5/8" stroke - 0.275 in^3
 
In practice many other features of the design might affect this: the streamlining of the steam inlet/outlet passages, the dead space in the cylinders, the valve timing (cut off etc.) and the efficiency of the insulation. Probably several other factors I haven't though of.
 
David
mick H22/10/2011 16:04:39
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Jens,
 
This is a superb question that I have been trying to find a simple answer to for some time now, without success. From what I have discovered I believe David to be quite correct in his analysis, but especially where he says it is an oversimplification.
 
 
Earlier this year I bought a book called Steam Locomotive Design: Data & Formulae by Phillipson, 1936, from Camden Miniature Steam (www.camdenmin.co.uk) for the princely sum of £12.95. It is quite a technical book and it may not contain all the questions that you want answered but it did answer some of mine.
 
If you message me with your e mail address I could send you a "sample" of what the book contains.
mick H22/10/2011 16:10:58
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Edited By mick H on 22/10/2011 16:18:33

Stub Mandrel22/10/2011 16:59:28
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Don't forget the maximum torque will, all other things being equal, be greater for an engine with greater piston area.
 
To do a full analysis I imagine you would have to draw theoretical indicator diagrams, making assumptions of the steam chest/exhaust pressure and using the cut-off data for the valve... and then do lots of hard maths.
 
Neil
colin hawes22/10/2011 17:45:24
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Work done by a steam engine is (pressure) x (area of piston face) x (distance the piston travels) . An early cut off allows the expanding steam to be used more efficiently due to greater pressure drop at end of stroke,so I would expect a longer stroke to make better use of the available steam but at lower rpm.
Colin
Jens Eirik Skogstad22/10/2011 19:00:58
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Posted by colin hawes on 22/10/2011 17:45:24:
Work done by a steam engine is (pressure) x (area of piston face) x (distance the piston travels) . An early cut off allows the expanding steam to be used more efficiently due to greater pressure drop at end of stroke,so I would expect a longer stroke to make better use of the available steam but at lower rpm.
Colin
 
Think the compound and triple expansion steam engine has short stroke in low expansion cylinder (same stroke in all cylinder and difference bore sizes..) Then the cylinder with big bore will make about same torque with lower steam pressure leaved from other hi pressure/middle pressure cylinder.
 
But in case the steam engine has big bore, when the steam in same pressure is used in the steam engine with under squared or squared cylinder, then the steam engine with over squared cylinder will give more torque.
 
But the revolution is a question when we are talking about comparing between 3 steam engines witn under squared, squared and over squared cylinder when the steam pressure is same and timing/overlap is same. Which of them will give more revolution?
 
ady22/10/2011 23:50:46
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1. 5/8" bore 3/4" stroke --highest torque
2. 5/8" bore 5/8" stroke
3. 3/4" bore 5/8" stroke --highest revs...so the most power
 
Thats my guess.
Chris Trice23/10/2011 03:17:11
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I know nothing about steam or whether the following applies but in internal combustion terms, the longer the stroke compared with the bore, the more torque there is (from a single stroke) at lower rpm's but also the lower the maximum rpm of the engine. Formula one cars are the opposite having short strokes compared with the bores which allows them to rev much faster and the peak power is at the top of the rev range. Average road cars as a general rule have strokes that are roughly equal to the bore of the cylinder. I have no idea if this translates into the steam world but I would imagine the physics involved would be similar.
John Allan Watson Brown23/10/2011 14:31:49
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Your question had me get out my old books. From my Sotherns verbal notes for Marine Engineers (7th Edition, 1911) steam expands in a cylinder of a reciprocating engine approximately hyperbolic or Isothermal. Turbines it is Adiabatic. Your expamples are single cylinder but as others have mentioned compound or triples (or Quadruples) which drop the temperature in stages for efficiency. I have examples in the text book of this calculation. Steam is delivered saturated or dry, which will give different indicated horse power (I.H.P.) than wet steam or superheated steam. Superheated steam gives a rise in temperature and you get increased pressure for a constant volume. So your question of work done from each example is dependant on several factors not just the physical volume of the stroke. These other factors as shown in above posts inlude cut off, heat drop and clearance volume. Power is work done in a given time so I.H.P. is equal to 33000 lb raised 12" in 60 seconds. Heat drop is usually allowed at 1 B.T.U. for each 778 Foot-Poundsof work done, where B.T.U is British Thermal Unit. Shaft horse power (S.H.P.) is the useful measure of delivered power not I.H.P. Rough ratio is S.H.P. = I.H.P * 0.9. Sorry about giving 'old school' measures but I couldn't find my metric text books yet. So what steam pressure and its condition prior to entry of the cylinders is of more importance to calculate the power output, so get a better boiler than a bigger bore. Sorry if I have bored anybody.
Jens Eirik Skogstad23/10/2011 23:30:58
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Thanks for all answers..
 
I got acess to the information about the design of steam engine as here: http://sites.google.com/site/jorgensensteamvalvegear/design
 
 
Stub Mandrel24/10/2011 20:38:44
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Steam engine is max torque when it is stopped - as you get full steam pressure on the piston. Unfortunately power output at zero revs is...zero. But that's why conventional steam engines never have gearboxes, and why diesel engines need some form of torque converter or gearbox as they don't produce any torque at all when stopped.
 
Neil
Jens Eirik Skogstad25/10/2011 00:55:58
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Posted by Stub Mandrel on 24/10/2011 20:38:44:
Steam engine is max torque when it is stopped - as you get full steam pressure on the piston. Unfortunately power output at zero revs is...zero. But that's why conventional steam engines never have gearboxes, and why diesel engines need some form of torque converter or gearbox as they don't produce any torque at all when stopped.
 
Neil
 
 
So clearly, with increased bore will give increased torque in zero revs.
Ian S C25/10/2011 10:44:51
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Torque (measured with one or other sort of brake), In my case with hot air engines, in inch oz X RPM, divided by 1352. This gives the power in Watts. To go metric, its
Watts = grms X cm X RPM X .00001026. Ian S C
David Littlewood25/10/2011 17:34:11
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Whilst the force on the piston is proportional (other things being equal) to the cylinder cross sectional area, this does not translate directly into tractive effort. Remember that the reciprocating motion of the piston rod is turned into rotational torque at the driving wheel by the action of the con rod on the driving wheel crank pin. The shorter the stroke of the piston, the nearer this needs to be to the axle centre, so the greater is the reduction factor to get the torque (torque is force times distance from point of application to axis).
 
I'm no great expert on IC engines, but I belive the above comments about short stroke IC engines are valid. However, F1 engines rev at incredible high speeds, and I suspect that the inertia of the moving masses and the ability of gases to get in and out become dominant factors there. Steam locomotives, even at speed, only have the wheels rotating at say 6-7 times/sec, i.e. 400 rpm at most.
 
And, to return to the OP's question, he asked about power and steam consumption; this is much more related to swept volume than is torque.
 
David
Chris Trice25/10/2011 18:00:02
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This is an opportunity for me to be educated because as mentioned I know zero, zilch, not a jot about steam engines so am I right in saying that unlike an internal combustion engine where the fuel air gases only expand after they are ignited in the cylinder, with steam, the system is already pressurised and the inlet valve (?) only opens to allow the pressure into the cylinder and push down on the piston?
Jens Eirik Skogstad25/10/2011 20:37:38
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Posted by Chris Trice on 25/10/2011 18:00:02:
with steam, the system is already pressurised and the inlet valve (?) only opens to allow the pressure into the cylinder and push down on the piston?
Chris, the valve is open and there is allready full pressure at piston in beginning from TDC and later about 60-70% from TDC the valve is closed then the steam is expanded to make force at piston until exhaust valve is opened (same valve), from BDC thr piston is pushing out used steam until the valve is closed early before TDC then the cylinder get compression before the piston is on way to TDC..
 
See the second picture from top in this link: http://sites.google.com/site/jorgensensteamvalvegear/diagrams

Edited By Jens Eirik Skogstad on 25/10/2011 20:39:43

Stub Mandrel25/10/2011 21:32:04
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I wonder if you have more (controllable) variables with a steam engine?
 
Neil
Chris Trice25/10/2011 23:41:31
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So presumably the fuctioning of a steam engine is more akin to a two stroke engine? 

Edited By Chris Trice on 25/10/2011 23:43:48

Chris Trice26/10/2011 02:47:33
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Cough, er... that was obviously meant to say 'functioning'. Oops.
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