Bore and stroke ratio in steam engine.

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Bore and stroke ratio in steam engine.

This topic has 26 replies, 11 voices, and was last updated 7 November 2011 at 12:31 by Alf Jones.

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22 October 2011 at 11:40 #4069
Jens Eirik Skogstad 1
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@jenseirikskogstad1
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22 October 2011 at 11:40 #76639
Jens Eirik Skogstad 1
Participant
@jenseirikskogstad1
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
22 October 2011 at 15:11 #76648
David Littlewood
Participant
@davidlittlewood51847
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
22 October 2011 at 16:04 #76650
michael howarth 1
Participant
@michaelhowarth1
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.
22 October 2011 at 16:10 #76652
michael howarth 1
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@michaelhowarth1
Edited By mick H on 22/10/2011 16:18:33
22 October 2011 at 16:59 #76655
Sub Mandrel
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@submandrel
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
22 October 2011 at 17:45 #76658
colin hawes
Participant
@colinhawes85982
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
22 October 2011 at 19:00 #76666
Jens Eirik Skogstad 1
Participant
@jenseirikskogstad1
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?
22 October 2011 at 23:50 #76685
ady
Participant
@ady
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.
23 October 2011 at 03:17 #76686
Chris Trice
Participant
@christrice43267
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.
23 October 2011 at 14:31 #76707
John Allan Watson Brown
Participant
@johnallanwatsonbrown
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.
23 October 2011 at 23:30 #76747
Jens Eirik Skogstad 1
Participant
@jenseirikskogstad1
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
24 October 2011 at 20:38 #76812
Sub Mandrel
Participant
@submandrel
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
25 October 2011 at 00:55 #76826
Jens Eirik Skogstad 1
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@jenseirikskogstad1
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.
25 October 2011 at 10:44 #76833
Ian S C
Participant
@iansc
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
25 October 2011 at 17:34 #76851
David Littlewood
Participant
@davidlittlewood51847
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
25 October 2011 at 18:00 #76852
Chris Trice
Participant
@christrice43267
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?
25 October 2011 at 20:37 #76862
Jens Eirik Skogstad 1
Participant
@jenseirikskogstad1
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
25 October 2011 at 21:32 #76871
Sub Mandrel
Participant
@submandrel
I wonder if you have more (controllable) variables with a steam engine?

Neil
25 October 2011 at 23:41 #76880
Chris Trice
Participant
@christrice43267
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
26 October 2011 at 02:47 #76885
Chris Trice
Participant
@christrice43267
Cough, er… that was obviously meant to say ‘functioning’. Oops.
30 October 2011 at 10:48 #77053
Sub Mandrel
Participant
@submandrel
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
30 October 2011 at 10:58 #77055
Anonymous
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
30 October 2011 at 21:59 #77093
Sub Mandrel
Participant
@submandrel
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
31 October 2011 at 10:49 #77106
Ian S C
Participant
@iansc
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
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