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Old 09-27-2011, 03:44 AM   #1
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Default Most efficient steam engine design?

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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Old 09-27-2011, 04:40 AM   #2
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Default Re: Most efficient steam engine design?

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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Old 09-27-2011, 07:17 AM   #3
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Default Re: Most efficient steam engine design?

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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Old 09-27-2011, 08:12 AM   #4
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Default Re: Most efficient steam engine design?

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 ). See picture below. Although this shows single acting its the crankshaft/conrod idea which makes it interesting.

Best Regards

picclock

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Old 09-27-2011, 08:17 AM   #5
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Default Re: Most efficient steam engine design?

Quote:
Originally Posted by 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
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

Best Regards
Bob
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Old 09-27-2011, 09:18 AM   #6
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Default Re: Most efficient steam engine design?

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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Old 09-27-2011, 09:26 AM   #7
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Default Re: Most efficient steam engine design?

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...

Dave

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Old 09-27-2011, 01:11 PM   #8
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Default Re: Most efficient steam engine design?

Quote:
Originally Posted by 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 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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Old 09-27-2011, 02:46 PM   #9
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Default Re: Most efficient steam engine design?

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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Old 09-27-2011, 03:32 PM   #10
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Default Re: Most efficient steam engine design?

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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