# Most efficient steam engine design?



## cfellows

I know this subject has been covered some already, but I'm interested in peoples thoughts on which engine will provide the most power for a given steam source: A 2 cylinder, double acting, reciprocating engine of the vertical marine type, a 2 cylinder, double acting oscillator, or a V-4 single acting engine like the saito V4PR?

The 2 cylinder, double acting, reciprocating engine used in marine designs comes to mind first. Four power strokes per revolution, on 2 pistons with relatively small contact area, D valves which can be made pretty leak proof. But, on the downside, you have a smaller piston area on the back side because of the piston rod, potential problems with the piston rod seal, piston rod and crosshead friction.

A 2 cylinder, double acting, oscillator doesn't have problems with the crosshead friction, but shares the piston rod friction and seal issues. And it has the added problems of potentially leaks around the cylinder ports. I'm assumming the added friction of the cylinder against the standard would be similar to the added friction of the D-valves in the reciprocating engine.

A 4 cylinder single acting engine like the Saito V4 seems like it has a lot of advantages and not many disadvantages. It's true you have the added friction of 2 more pistons, 2 more connecting rod journals, and 2 more valves, so I don't know how this design would compare to the other two.

Anybody have some insight into this subject? 

Chuck


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## Captain Jerry

Chuck

I think you are right on all counts but don't forget piston valves with inside admission which may be more efficient and does not need a gland on the valve rod.

Jerry


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## Jasonb

If you want to use the steam efficiently you need to start looking at compounding to make full use of the expansion of the steam. Also think about using superheated stem.

J


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## picclock

Hi Chuck

Can you just clarify the meaning of 'piston rod and crosshead friction' (OK got it now, wikipedia lists it as slidebar for us UK based people). I would think the friction could be kept to a minimum by using two ball races, or just some rollers for the piston rod to bare against. 

Oscillator's lose a large amount of their energy moving the cylindrical mass. 

Are you going to use this with steam or compressed air. If steam and bulk/weight no object compound engine is likely best reciprocating kind.

If you want best efficiency have you considered turbines ? At small sizes they do not scale well due to the blade/housing gap remaining constant.

Don't know if this helps - perhaps more details such as power output/size weight restrictions/application would generate more ideas.

Have done various drawings for a two cylinder double acting engine which I may get round to building using the boxer crankshaft idea below. Because of the configuration the engine is perfectly balanced so no vibration (well less vibration ;D). See picture below. Although this shows single acting its the crankshaft/conrod idea which makes it interesting.

Best Regards

picclock


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## Maryak

Jasonb  said:
			
		

> If you want to use the steam efficiently you need to start looking at compounding to make full use of the expansion of the steam. Also think about using superheated stem.
> J



Plus a vacuum, some 25% of the work done by steam is below NTP IIRC the most efficient steam engine is a turbine with reaction blading supplied by a super critical boiler, ( because there is no latent heat), + superheat + condensing and this mass of bits is around 33% efficient. Your average recip engine of whatever type is between very poor to around 8% efficient. 

IMHO just pick the one that has the most eye candy for you and enjoy ;D

Best Regards
Bob


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## Ken I

Agree with Maryak - if history and thermodynamics teach us anything - a compound action triple expansion is the most efficient use of steam on a reciprocating engine. Almost all marine applictions went this way before the switch to turbines.

Why the interest in efficiency ?

Ken


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## steamer

Compounding splits the expansions over two different and smaller temperature ranges. That minimizes condensation losses......there's a problem though

Fully agree with Bob et al regarding compounding, turbines and such......however.....

In small sizes....such as small steamboat sizes on down... the surface area of the engine completely overwhelms this benefit, and in general in small scale stuff the biggest loss is through condensation loss regardless.

So IMHO....for small sized stuff.....

Double acting twins with appropriately sized piston rods and some superheat, say 100F worth, to avoid the condensation losses as much as possible are much more efficient for these applications because you can raise your steam pressure above 40 psi which you can't do with a wobbler.  Additionally, a wobbler has quite a bit of friction area.

This becomes a problem if you run condensing as superheat REQUIRES internal lubrication which is hard to get back out of the condensate which can result in oil in the boiler feed water.  Boilers don't like that much!

So for small scale stuff......blow the exhaust up the stack.....

Just my opinion ....worth exactly what you paid for it.....

and for the record...my boat engine.....all 2 HP ....is a compound......cause I like them... ;D

Dave


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## Harold Lee

Ken I  said:
			
		

> Agree with Maryak - if history and thermodynamics teach us anything - a compound action triple expansion is the most efficient use of steam on a reciprocating engine. Almost all marine applications went this way before the switch to turbines.
> 
> Why the interest in efficiency ?
> 
> Ken



I know this thread is about efficiency, but one of the drawbacks to a compound engine is since the cylinders are in series, they are not necessarily self starting and have the same "dead spots" as a single cylinder engine. On a model that is static run on a display this is not a problem but if one wants to put an engine in a radio controlled model boat or a model train, this can create problems. The real world solution was to put a "simpling valve" in the low pressure line which would allow boiler steam to directly enter the low pressure cylinder and effectively start as a simple two cylinder engine. After the engine was running the valve would be closed and the engine would continue to run as a compound. I built this compound a few years back based on Rudy Kouhoupt's design. I played with building a simpling valve for it and finally moved on since I did not have any plans on putting it in a boat.






Just my $.02

Harold


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## Wrist Pin

Since we are on the subject of efficiency...
Years ago, I got into a discussion on steam mill engines. A lot of steam /energy loss was in the slide valve(s). The prevailing solution was to incorporate solenoid valves instead of mechanical. The theory being that having sharp opening and closing valves would capture more of the expansion of the steam, increasing efficiency and horsepower.
Has anyone gone down this road or would like to discuss the concept?


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## Dan Rowe

Corliss valve gear is close to electric solenoid valves only actuated by a mechanical system. This makes the expansion much more efficient.
http://en.wikipedia.org/wiki/Corliss_steam_engine

Dan


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## Wrist Pin

Fascinating stuff, Dan.
But one still has the drag of the associated mechanical parts to contend with. In the discussion years ago we thought of using electric solenoid valves fired by relays that were triggered by a Hall Effect sensor or some such.


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## Dan Rowe

There is no free lunch. You still have to take in account the power required to operate the valves in the overall efficiency calculations. see:
http://en.wikipedia.org/wiki/Variable_valve_timing#Electro-mechanical

Dan


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## Wrist Pin

More fascinating stuff!
Just goes to prove nothing is new under the sun! It also proves one can find anything on line if one knows where to look. Thanks Dan, for enlightening me!


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## lescad

Hi Chuck
Another loss of efficiency with conventional valve arrangements is the relatively cool exhaust steam taking heat from the cylinder and valve.

Maybe a single acting V4 uniflow with piston or poppet inlet valves ??

Les


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## JorgensenSteam

Chuck-

My slant on the answer to your question is as follows, and is just my hunch only, not necessarily based on hard evidence.

The oscillators used after the big steam boat paddlewheels were generally small units, and were known to be steam hogs, but were simple and reliable since they had so few parts.
The oscillators were generally made for intermittant loads, and efficiency was not a concern, just simplicity, reliability and low cost. The steam ports were probably designed for no cutoff, so the steam would not see any serious expansion, but again efficiency was not necessarily a concern with an oscillator.

For other type engines, the critical factor will be the cutoff, and how much of the power of the steam can be extracted during the expansion phase. Any steam engine will be highly inefficient if it does not cut off early in the stroke.
I am assuming that the ports, valve and valve travel have beeen correctly designed, but more often than not, in modern model engines these items are anything but correctly designed.

So the bottom line is that any engine with a good valve/port/valve travel design will be more efficient than a poor design, regardless of whether the engine is an oscillator, a compound, single, double acting, etc.

The effect of the rod is minor. You can even compensate for that in valve timing.

A single acting engine will be more effective if it has to operate at higher rpms.
For low rpms, the double acting engine probably will be more efficient than a single acting engine and an oscillator just because an oscillator cannot control cutoff, and the single acting will have more pistons and friction.
A well designed single acting steam engine can be very efficient though.

A compound willl be more efficient than a non-compound, but if simplicity and easy starting are important, thena multi-cylinder non-compound is the way to go.
If you are going to steam across the ocean, then a compound will very quickly pay for itself.

A compound will also allow a much smaller boiler for the same horsepower, so that may be a good reason to go compound.

Pat J


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## steamer

Again...be careful

I small sizes ALL steam engines are very steam hungry due to condensation.

Assuming a well designed a constructed engine....superheat goes a long way, but as Dan said...there is no free lunch.

Dave


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## kf2qd

For a small engine you are going to have rather large thermal losses. To make the engine more efficient you should look at ways of thermally isolating the cylinder liner and the piston from other metal parts. Keep the steam hot until it comes out the exhaust and you will be more efficient. You want the heat to move the piston, not heat the air.


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## Ken I

Steamer is correct and my earlier comments about triple expansion should have carried the caveat that these principals do not scale well.

As an aside, about 20 years ago I spent a pleasant day in the engine room of South Africa's last working steam tug the _Alwyn Vincent _ - this had a tripple expansion engine.

This had a "big switch" simpling valve arrangement that induced live steam into all three cylinders - for starting or for emergency power if the helm rang for emergency full ahead or astern - literally quadrupling tha output in short bursts.

Sadly I took no picture but the vessel is being restored.

http://alwynvincent.wetpaint.com/page/Triple+Expansion+Engines

Ken


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## Maryak

Just for you Ken :





Best Regards
Bob


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## Captain Jerry

I know you said it was just for Ken, but I watched anyway. Great stuff.

Jerry


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## cfellows

Thanks for all the comments and info, although maybe more info than I was looking for. My fault for not being a little more specific... :-[

What I was really groping for was, does anyone see any significant difference in performance between these three engine designs, assuming the same bore and stroke for all.

Double acting twin oscillator:







Single acting V4 valve in head:






Double acting twin with D valves:






Chuck


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## JorgensenSteam

For engines of the same bore and stroke, it is going to boil down to how well the steam can flow in and out of the engine.
The engine with the least restrictive ports and passages, and one that has a valve with no lap will produce the most power.

I have a steam engine with a 1.5" bore that produces probably twice the power of another with a 2" bore, but again, it is attention to valves, ports, and passages.

For some reason, many modelers drill very tiny holes (or sometimes a single hole) for ports and passages, and use a non-domed valve, so sure, you can barely force anything through that, and while the engine runs fine with no load, it will not produce anywhere near the power it could under load.


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## Jasonb

Chuck is this engine the intended power source for your steam vehicle? That would give us an idea of the size and type of use you are after.

This may give you some inspiration, 1/2 scale model of an actual engine 

J


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## cfellows

Jasonb  said:
			
		

> Chuck is this engine the intended power source for your steam vehicle? That would give us an idea of the size and type of use you are after.
> 
> This may give you some inspiration, 1/2 scale model of an actual engine
> 
> J



Yes, this is for my steam powered vehicle. I want to maximize torque, which means lots of gearing since speed isn't important (or even desireable)



			
				BigOnSteam  said:
			
		

> For engines of the same bore and stroke, it is going to boil down to how well the steam can flow in and out of the engine.
> The engine with the least restrictive ports and passages, and one that has a valve with no lap will produce the most power.
> 
> I have a steam engine with a 1.5" bore that produces probably twice the power of another with a 2" bore, but again, it is attention to valves, ports, and passages.
> 
> For some reason, many modelers drill very tiny holes (or sometimes a single hole) for ports and passages, and use a non-domed valve, so sure, you can barely force anything through that, and while the engine runs fine with no load, it will not produce anywhere near the power it could under load.



Pat, I'm assuming, within certain constraints, port size, and volume of steam flow, is more important for higher RPM's and power than just torque alone. The ports on my oscillator engine are 3/32. I couldn't go any larger without risking interference between crossing holes.  The bore is 7/16" and the stroke is 7/8" and the maximum RPM I measured at 20PSI is about 1,700. My drive wheels are about 4 3/4" diameter I'm thinking the top RPM will be about 40. That means I'll need a gear reduction of about 40 : 1 or there abouts. 

Unless there is reason to think my little oscillator won't be powerful enough, I'm not inclined to build another engine just now.

Chuck


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## steamer

Chuck,

Of the three forms

Oscillator

Limited by available cut off.  

Single acting.

IMHO  Too much friction area compared to swept volume....

This leaves the double acting simple with cast iron pistons and block with piston valves...IMHO as your best choice.

Dave

...and PS....Don't forget a little superheat!...it goes a long way.


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## 11thHour

kf2qd  said:
			
		

> For a small engine you are going to have rather large thermal losses. To make the engine more efficient you should look at ways of thermally isolating the cylinder liner and the piston from other metal parts. Keep the steam hot until it comes out the exhaust and you will be more efficient. You want the heat to move the piston, not heat the air.


Hi Kf,
What is appropriate in small spaces as an insulator? for instance, will a 1mm aluminium plate between my brass cylinder and mild steel frames be worth the effort?
Tim


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## JorgensenSteam

I would not underestimate the power of a small oscillator, especially at that rpm.


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## metal

11thHour  said:
			
		

> Hi Kf,
> What is appropriate in small spaces as an insulator? for instance, will a 1mm aluminium plate between my brass cylinder and mild steel frames be worth the effort?
> Tim



I wouldn't use aluminium (which conducts heat pretty good). Better use something which does not conduct heat, such as heat resistant plastics (e.g. the stuff they use to make electric circuit boards).



			
				kf2qd  said:
			
		

> For a small engine you are going to have rather large thermal losses. To make the engine more efficient you should look at ways of thermally isolating the cylinder liner and the piston from other metal parts. Keep the steam hot until it comes out the exhaust and you will be more efficient. You want the heat to move the piston, not heat the air.



In my design of small scale (gauge 1) steam locomotives I try to "increase " thermal efficiency by:

--> using a silver soldered cylinder block assembly (air inside assembly acts as insulator) instead of a cylinder block machined out of solid

-->insulating *every* hot part (steam lines, cylinders, valves, nuts and bolts etc.) from the frame

--> using an appropriate size for supply and exhaust steamlines and make them as short and flowing as possible


Besides thermal efficiency theres also mechanical efficiency, which can be increased by using low friction bearings (e.g. roller bearings or Teflon bearings) and replacing the brass / O-ring piston by a Teflon one.

Cheers,


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## Aydelott

im making the rudy k model marine engine how are the valves attached to the rod to me by the print i looks like its just in a slot can u send pictures of  the valves? thanks


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## cbwho

Interesting read. I am surprised no one mentioned a double tandem compound. Self starting with the benefits of compounding. Simple mechanically as well.


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## Charles Lamont

Max power for a given steam supply is a matter of efficiency, as has been said. Thermodynamic considerations are at least as important as mechanical layout. Personally in a smallish model I would optimise both by using a well insulated single-cylinder to minimise both surface area and friction losses, poppet-valves to allow short steam admission with sharp cut-off and low leakage, uniflow ports to get most of the cool exhaust steam out quickly, probably single acting, and with as short a cut-off as the steam quality would allow, and running at a fair lick. I would only consider a condenser if talking about more than 1HP or so.


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## Charles Lamont

Considering the question Chuck says he meant to ask, the oscillator has no means of using the steam expansively, whereas the others can. If the Saito or the slide valve engine do have cut-off part stroke, this means that for the same bore and stoke, the oscillator could well produce a higher power output if fed with steam at the same pressure. BUT, for a given quantity of steam, ie if fed by the same boiler, it would produce less power because it is less efficient - the boiler would not have the capacity to turn the oscillator as fast as an expansive engine. If we add the condition that we are to run the engines at the same rpm as well, then an expansive engine with a somewhat larger swept volume will produce more power with the same steam consumption.


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## cbwho

A follow up question: will a single acting oscillator expansion engine work?


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## Charles Lamont

cbwho - I think there is a way to vary the valve events on an oscillating engine, using an eccentric driven disc valve equivalent to a slide valve. There is a drawing of one in the book 'Model Stationary and Marine Steam Engines' by KN Harris, but I have never taken the time to fathom out how it works.


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## Rocket Man

I took engineering in college I can do math and draw models on paper but I need to build engines and test them to know for sure how well they really work.

You need to design each engine for the work it needs to do.   Sometimes you need low RPM engines, sometimes high RPM engines, some times medium speed engines.

Engines with short crank shaft throws will turn high RPMs.  Low RPM engines are best built with long crank shaft throw and long rod.

If you build 2 identical engines with 1" bore so one engine has 1" stroke and the other engine has 2" stroke you will learn when testing it 1" stroke turns about 7000 RPMs while the 2" stroke only turns about 3500 rpms.   The short stroke engines has 2 times more RPMs and the long stroke engine has 2 times for Torque.   They both produce the same amount of HP.   Both engines use almost the exact same amount of steam when producing the same amount of HP.   These engine are both single acting.

When I build the same 2 engines again 1" bore with 1" & 2" stroke this time they are double acting I get same results as before, a high rpm engine & low rpm engines, both use the same amount of steak at the same HP but this time double acting produces about 2 times more HP than single acting.   Rod diameter reduces piston are on bottom side of the engine.

Read history of stream, lots of people have tried lots of things.   I have not built all those designs to see how well they work.  1 of the most efficient steam engine every built has the engine cylinder inside the boiler fire box surrounded in hot boiling water.   It was a long stroke engine with rod & crank on one side of fire box and cylinder head on the other side of fire box.    Cylinder stayed hot all the time there was no cold starting the engine it started up at full power.

Build a better high efficient boiler.  I did a lot of boiler testing experiments I could upload pictures but don't know how?  It appears to me I have to put pictures some where like photo bucket then link them here.   Photo bucket has turned into jerks it cost $$$$$ for me to look at my own pictures so I don't go there anymore.


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## cbwho

Rocket man, in regards to single acting and double acting, same dimensions, the double acting will double the power but also double the steam consumption. That is what I found with a Mamod TE oscillators.


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## Peter Twissell

The simple Mamod traction engine has variable cut off, using the eccentric 'throttle' changes the position of the cylinder oscillating axis, which in turn changes the period of the inlet.
Maximum efficiency of a boiler is achieved using maximum superheating of the steam. The energy required to boil water (latent heat of evaporation) is required whatever the boiler pressure. Additional heat used to increase steam temperature and pressure is realised during the expansion phase and is delivered as output power.
The most efficient steam engines are supercritical, where the steam is superheated beyond the point at which the pressurised stem is as dense as water, allowing for efficient heat transfer. This happens at pressures on excess of 3000psi and temperatures in excess of 400 degrees Celsius. These figures are only realistic in flash steam boilers, where the pressure vessel consists of small diameter tubes which have high strength relative to their inside surface area. An interesting topic, which I intend to pursue at some point.


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## dazz

Hi
There are two parts to thermal efficiency in the cylinders.  There is the net flow of heat out of the cylinders.  There is also the alternating heat loss as hot steam enters the cylinder, expands and cools and energy is extracted, then leaves through the same ports.  In both cases, efficiency can be improved using a poor thermal conductor, like stainless steel.  

Stainless steel has its problems (like galling) but it is worth considering given your design goals.

Dazz


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## Peter Twissell

For the engine itself, have you looked at an opposed piston uniflow design?
The uniflow concept gives an advantage with thermal efficiency, keeping the inlet end hot and the exhaust end cooler, reducing the transfer of heat between the working fluid and the mechanical parts. With ports at each end opened and closed by the pistons, cutoff can be controlled by moving the cylinder tube itself and there is no energy absorbed in moving separate valves.


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## Rocket Man

Some steam locomotives in the late 1940s & early 1950s were equipped with a steam exhaust cooling system to turn exhaust cylinder steam to water so it can be used again.   Has anyone every built a model engine that cooled exhaust steam to reused water.


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## cbwho

Peter Twissell said:


> The simple Mamod traction engine has variable cut off, using the eccentric 'throttle' changes the position of the cylinder oscillating axis, which in turn changes the period of the inlet.
> Maximum efficiency of a boiler is achieved using maximum superheating of the steam. The energy required to boil water (latent heat of evaporation) is required whatever the boiler pressure. Additional heat used to increase steam temperature and pressure is realised during the expansion phase and is delivered as output power.
> The most efficient steam engines are supercritical, where the steam is superheated beyond the point at which the pressurised stem is as dense as water, allowing for efficient heat transfer. This happens at pressures on excess of 3000psi and temperatures in excess of 400 degrees Celsius. These figures are only realistic in flash steam boilers, where the pressure vessel consists of small diameter tubes which have high strength relative to their inside surface area. An interesting topic, which I intend to pursue at some point.


That is definitely true of the later Mamod TEs but not true on the early 60s TE. My experiment involved a non-adjustable TE frame mounted with a 1980s Mamod "locomotive" double acting oscillator cylinder. The double cylinder clearly had double the power but it also clearly consumed twice the steam. Admittedly my sample size is small.


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## dazz

Hi
Condensing steam does more than just recycle water.  If done right, it creates a reduced pressure on the engine exhaust to below atmospheric.  This improves engine power and efficiency.
Condensed steam would need filtering before it could be put back in the boiler.  

There is a steamboat called the TSS Earnslaw on a lake in the South Island of New Zealand.  This is fitted with two locomotive boilers and uses lake water to condense the exhaust steam. 

Dazz


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## popnrattle

I made a "steam" engine with cylinders. 1/2" stroke and 3 inch bore is short on torque but can be geared down to meet your needs.


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## Rocket Man

I remember reading once how exhaust steam from #1 cylinder can be reused to power another #2 cylinder.   I don't remember how well that works but I understand the physics, it appears to me back pressure on #1 cylinder will reduce its power and power to #2 cylinder should not be very high.   This engine design might have a special name?


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## Shopgeezer

The triple expansion engine is a common marine engine. This is three cylinders. Exhaust from the first cylinder enters the second, and exhaust from the second enters the third. Each cylinder is larger than the previous one. It requires lots of pressure to the first cylinder. Very efficient. Triple expansion engines ran a lot of the commercial shipping in the world right up until the 1950’s. I know an older engineer who worked on a steamer in the Caribbean bouncing around the islands. That’s the way I would like to spend my winters! Here is a description of this engine. 

https://en.m.wikipedia.org/wiki/Compound_steam_engine

Here is a model kit for one of these engines:

https://www.ministeam.com/acatalog/Martin-Triple-Marine-Steam-Engine-Martin_Triple.html#SID=289


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## Charles Lamont

Rocket Man said:


> This engine design might have a special name?



An engine with two-stage expansion is called a 'compound'. The physics is about minimising the temperature variation in each cylinder. In engineering practice it also reduces the effect of valve and piston leakage.


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## Jules

The Titanic ran the exhaust from its triple expansion engines into a steam turbine. 
Now that’s really efficient. 
The output power from the turbine was close to that of the two triple piston engines. 
The turbine drove the central propeller.


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## k2steve

Jules said:


> The Titanic ran the exhaust from its triple expansion engines into a steam turbine.
> Now that’s really efficient.
> The output power from the turbine was close to that of the two triple piston engines.
> The turbine drove the central propeller.


I did not know that - Thanks


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## Shopgeezer

Now you have me thinking about running my own steam engine exhaust into a turbine. What a great idea. Interesting historical note. I understand that the Titanic was the fastest liner of its day.


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## TimTaylor

Keep in mind that the Titanic boilers (29 of them) generated steam at 215 psig working pressure - much higher than is safe for any hobby application - so they had lots of pressure drop to work with. It's triple expansion marine engines were specifically designed to provide the low pressure inlet steam to the Parsons low pressure turbine that drove the center propeller.

Steam turbines work off of isentropic enthalpy drop. At the typical pressures we use for hobby boilers, to get any useful work out of the steam engine exhaust, you would have to use a rateau type turbine designed specifically for the pressure and flow - there simply isn't enough pressure drop to use a simple curtis stage design. You'd also likely need a condenser to get enough pressure drop.

In condensing applications, the condensate may or may not require filtering for re-use, depending on the nature of the contaminates, i.e. oil from displacement lubricators.


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## Andy Munns

Many people think they started with the High Pressure and added the IP and LP, but historically they started with LP atmospheric engines where the vacuum did most of the work and added higher pressure cylinders fed by higher pressure boilers. The reluctance to embrace high pressure steam was a bit like the reluctance to embrace DC power (pro kerosene lobby) AC power (pro DC lobby) and today's renewables and EVs. Compound, triple and quadruple expansion reduce the temperature (actually heat) loss in each cylinder and this made transatlantic steamships possible. Titanic was a bit old fashioned and slower cf Cunard ships that had already gone 100% turbine. They still expand the steam over multiple stages. Recip engines cannot sensibly build LP cylinders with big enough exhaust ports, hence the push to LP turbines with their huge exhaust ports that can get the last bit of energy from huge volumes of low pressure steam.


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## deverett

They were still building vessels with triple expansion engines and low pressure turbines in the 1950s.  I know because I sailed on one in my later Merchant Navy career.

Dave
The Emerald Isle


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## Shopgeezer

Twenty nine boilers!  Did Titanic burn bunker oil?  I can’t imagine shovelling coal for that many boilers but she was old enough that maybe so. You would need a lot of stokers.


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## Peter Twissell

She did indeed burn coal.
One of the coal bunkers already had a fire burning in it before she set sail.


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## redryder

One of the most efficient designs ever was the steam-diesel for marine use. While very efficient compared to other existing power plants, it was a commercial failure given that it's operation required tending and maintaining by both a diesel engineer and a steam engineer. The limited number of engineers who were certified for both steam and diesel stopped going to sea and became marine surveyors. Not installing it vs hiring 2 full time engineers for every shift, I believe, kept the installed fleet at just two ships.


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## Richard1

There is partly described in "Audel's" engineering encyclopedia. Exact name and publication date forgotten a single acting quintuple expansion engine as I recall the HP piston was 1" diam and the LP was 8.5" with an inlet pressure of something like 2,500 PSI. I have no idea if it was ever built or what the efficiency would have been.

Very rough back of envelope calculations suggest the cylinder bores would have been
1", 1.75", 3", 5", 8.5" and would have expanded the steam 3 times in each stage.


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## Vietti

Don't forget that the D valve can be balanced and reduce sliding friction.


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## Richard1

Vietti said:


> Don't forget that the D valve can be balanced and reduce sliding friction.



My guestimated inlet pressures are:-

HP 2500 PSI
1st IM 833 PSI
2nd IM 269
3rd IM 92 PSI
LP 31 PSI

Condenser inlet 10 PSI absolute.

With these pressures I don't think they would be using any sort of a slide valve balanced or otherwise at least on the first 3 cylinders. I would be very curios to know what sort of valve would stand up under those sort of pressures and temperatures. Without any superheat at all the inlet steam is 668°F I assume if you are going to the trouble and expense of such a complex engine you would want a lot of superheat.

Lubrication would be interesting to.


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## Andy Munns

Not sure about 'simpling valve' for a marine compound or triple... I teach that these were usually called an Impulse Valve and only gave a shot of steam into IP or LP receiver for nudging a stuck HP piston off dead centre - Rarely needed if you had some vacuum as working the links was faster. The impulse valve does introduce back pressure on the exhaust of the previous cylinder so the indicator diagram would be somewhat irrelevant.

Then, in the last days of recip steam, designers went back to double compounds with poppet valves + serious superheat + HP to LP reheat + an exhaust turbine on marine jobs. Sometimes also a exhaust turbine from the LP exhaust to the LP receiver. Search for Lentz engines.


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## Richard1

Andy Munns said:


> Not sure about 'simpling valve' for a marine compound or triple...



I did briefly work on a paddle steamer where the simpling valve simply turned full boiler pressure onto the LP valve chest and diverted the HP exhaust up the stack. I don't think this is usual though.


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