Best Suggestions for Poor mans compound?

[cyberbadger]

This is sort of a thought experiment. You are free to tell me that this is the most stupid idea ever.

But I'm the position were trying the following wouldn't be that hard. I think I can scrounge the parts from my stores of materials on hand...

Run the exhaust from one engine to another - have two propellors (possibly of different sizes) be powered by two steam engines connected in series - sort of a poor mans compound?

I have for kicks 4 years or so ago in a stationary driveway setup run one unloaded engine with it's exhaust to the inlet steam of a second engine - also unloaded. They both ran unloaded without problem (no shaft coupling between the two engines).

I understand that in a compound you want the secondary to be a larger volume. And so larger volume on if there are a third or fourth cylinders.

1) Assuming no condensor and no stack exhaust - just to ambient after the second engine would this make any difference to vessel speed?
2) Is there a concept of volume of thrust moved by the propellors being able to add or stack to a faster overall vessel speed?
3) What would the criteria of dimensions be critical to this arrangement to work positively on overall vessel speed(e.g. the relative sizes of propellors)?
4) Would perhaps the two engine in parallel be better in some way then this poor man's compound.

I realize that there a lot variables to this question. Feel free to clarify any assumptions that I haven't typed that might be important.

-CB

[Mike Rometer]

If they ran at different speeds you would likely have steering problems. Also backpressure in the feed to the second engine would sap power from the first. It might be possible to balance one against the other but probably at only one speed or a narrow range. One thing is certain; someone has already tried it, and as it isn't common practice; they found the problems.

[DetroiTug]

Quote: "backpressure in the feed to the second engine would sap power from the first. It might be possible to balance one against the other but probably at only one speed or a narrow range. One thing is certain; someone has already tried it, and as it isn't common practice; they found the problems."

I totally agree.

It would be a very inefficient set up with a much higher than usual amount of thermal loss. As Mike points out, the second engine in series would present back pressure on the first throttling the exhaust. Only going to yield a certain amount of horsepower at the shaft in relation to the steam being generated.

I know someone that is working with a similar problem right now, two in parallel rather than series. Two identical double simples from one boiler on a car and it ain't working. I suggested he change the gearing to cut the normal RPM of one engine by one half (he changed it from 2.5:1 to 1:1), and also suggested the second engine would increase thermal loss and that is obviously exactly what is happening. He is now reworking his boiler trying to make it work. The boiler is not keeping up and when it does hes seeing excessive carry over possibly indicating very high demand to feed the two engines even at less than half normal engine RPM. The boiler as it was more than likely would have ran one engine at the proper RPM no problem at all, proven design.

As Lopez Mike pointed out once, a small steam engine out in the breeze is about as good at condensing as it is at expanding. Adding more cylinders (surface area to ambient temp) just exacerbates the problem and makes the whole plant even more inefficient than it normally would be.

Thermal loss is a big concern for plant efficiency. Steam temperature and pressure are always static, once steam leaves the boiler (heating surface) it begins to cool and keeps cooling all the way to the exhaust outlet, when it does, the pressure drops all along the way (amount of work) accordingly.

As Mike points out above and I'll elaborate, steam to useful mechanical power has been around for over 200 years and some very intelligent folks like Watt, Trevethick, Serpollet, Doble , Whitney and Besler, just to name a few have worked on the problem, 99.999% of possible experimentation has been performed, if some idea was not adopted and seeing or seen any sort of widespread use, there is a reason for it.

-Ron

[SL Ethel]

If you are looking for a poor man's compound, have you considered a 2-stage air compressor? Just make a cylinder head, sleeve one of the bores (if you need to change the cylinder ratios), and set off under steam.

From everything I've seen in reading accounts of our hobby, it seems that you can get some advantage with a small compound over a single expansion engine if your compound is in perfect running order, properly set up and timed, and if you are running with high enough steam conditions to warrant a compound. But lots of the folks who really move around quickly in their steamboats, like Consuta or Equinox, do it with well engineered single expansion machinery mated to ample boiler capacity, in a hull suited to the speed.

Cheers,
Scott

[dampfspieler]

Hi,

in my suggestion a poor mans compound is a normal or steepled one. A rich man can built such one you think about and throw his money out of the window.

Dietrich

[cyberbadger]

All interesting responses, thanks.

I wasn't thinking of this is something practical.

I was thinking that if the second engine stalled - or couldn't start it self - it could have enough back pressure to stall the other one completely. A one of the reasons that true compounds have a common shaft to have the pistons help each other along.

I do have a lot of deck space on Nyitra to put things or try things - and I'm not afraid to cut holes in the deck for an interesting experiment. Enough room to tinker for years. But probably this particular experiment won't make the cut.

I do think my (heavy) worthington duplex(3x2x3) is going to get on board. Not really as a very practical feed water source - but mainly because I just want to run it sometimes because I love the movement. And on a really bad day - even though it is a steam hog - it is pretty robust in my book for pumping water and could pinch hit.

99.999% of possible experimentation has been performed, if some idea was not adopted and seeing or seen any sort of widespread use, there is a reason for it.

I'm not saying that isn't 100% correct - but I don't like that sentiment. If I followed that to the logical conclusion I wouldn't have built a steam launch. Just because it has been tried doesn't mean it's not worth it for the journey of seeing it yourself.

-CB

[fredrosse]

I guess I'll chime in here and disagree with many of the comments.

A compound engine is really two simple expansion engines with steam flow in series. The first engine represents the HP cylinder, and the second engine represents the LP cylinder. If you had the LP engine in proper ratio to the HP, meaning that its volumetric flow rate would be about double the volumetric flow rate of the HP engine, things could work with better efficiency than a simple engine, at least theoretically.

The proper volumetric flow rate for the LP engine could be attained with a larger cylinder, both engines running at the same RPM, or with the LP engine having the same cylinder size as the HP engine, but running twice as fast. Many other possibilities to arrive at the desired volumetric flow ratios.

As far as practical considerations go, there are serious difficulties, as pointed out by others on this thread. The setup would have the steam running thru the HP engines valving, would probably require a rather large receiver between the engines, then pass thru yet another valving in the LP engine. That would probably cause throttling losses that would kill any advantage of this concept for compounding, unless the LP engine had unusually large steam passages. Excellent insulation throughout would also be needed, to cancel out the disadvantages of all that extra piping and receiver, etc. Extra friction to run two independent valve trains, etc.

[RGSP]

The answer to whether the system COULD work is undoubtedly yes, and Webb's "Experiment" class of locomotives from the end of the 19th Century did almost exactly the same thing, but with independent driving axles rather than independent propellers.

Having said that, in steam-launch sizes (especially the smaller end) the design of compound engines with better efficiency than a larger single with the same power output is not as easy as usually assumed. All the details matter a lot, and re-heaters on the transfer pipe/receiver between the cylinders in 2-cylinder compounds are rare, but a number of independent trials have found remarkable improvements by playing a blowlamp onto an un-insulated transfer pipe. The well-known Leake Small Compound has design provision for a steam re-heater between the cylinders, and someone somewhere has probably installed one - but nobody I know.

[DetroiTug]

Quote:
I wrote: "99.999% of possible experimentation has been performed, if some idea was not adopted and seeing or seen any sort of widespread use, there is a reason for it."

You wrote: "I'm not saying that isn't 100% correct - but I don't like that sentiment. If I followed that to the logical conclusion I wouldn't have built a steam launch."

Don't like that sentiment? I have no idea how it was extrapolated that an explanation of why some exotic steam engine/boiler configurations never seen widespread use in to you wouldn't have built a steam boat had you known that.

[cyberbadger]

Quote:
I wrote: "99.999% of possible experimentation has been performed, if some idea was not adopted and seeing or seen any sort of widespread use, there is a reason for it."

You wrote: "I'm not saying that isn't 100% correct - but I don't like that sentiment. If I followed that to the logical conclusion I wouldn't have built a steam launch."

Don't like that sentiment? I have no idea how it was extrapolated that an explanation of why some exotic steam engine/boiler configurations never seen widespread use in to you wouldn't have built a steam boat had you known that.

If "99.999% of possible experimentation has been performed" means that it's not worth trying because "there is a reason for it." then you will end up trying nothing. If you try nothing then you will for 100% sure find nothing. There is a reason that few steam boats are built any nowdays - they are obsolete and not particularly known for their efficiency. If you follow that reason - why build a steamboat?

-CB