Compound Condensing Engine and Thrust Box - ME1923

[Maryak]

As a retired marine engineer..............if the engine stopped on a dead centre then it will not start without moving off the dead centre. If the vessel has way on when this happens it would take an almighty effort to bar it off the centre against the load on the propeller. In this configuration, impulse valves are no help because the other cylinder is also dead centre. Baring is normally only used when the engine is cold and for setting up or in multi prop vessels for locking a shaft.

Perhaps the vessel was not intended to be maneouvered and relied on tugs for harbour work; or the engineer was relied upon to ensure the engine stopped off centre, (unlikely); or the drawings are in error for a reversing propulsion engine.

Some 2 cylinder compound marine propulsion engines were "balanced' by placing the cranks at angles other than 90 or 180 thus starting from a HP dead centre was by use of an impulse valve to the LP.

Best Regards
Bob

[Jasonb]

Just out of interest would it have been possible to use that same load on the prop to help turn the engine over if the bar were used to turn it in the same direction as the movement of the ship is trying to turn the prop rather than against it? To my mind you would be using the load to help you rather than work against it.

J

[Dan Rowe]

The answer to why the crank pins are 180 not 90 is the type of cross compound engine. There are two basic types the Woolf compound and the receiver compound.

This is a Woolf compound and the steam leaving the HP cylinder goes directly into the LP cylinder.

The Woolf compound was invented before the receiver type of compound engine.

http://www.thermodynamicsheatengines.com/HeatEnginesVol%202%20Chapter%202%20RS.pdf

Dan

[Jasonb]

Actually the original 1923 design chose a 180deg crankshaft to give a shaft that was easier to make and a more balanced engine as it was felt that self starting was not an issue on a model that was unlikely to be put to use. Nothing more complicated than that

Also I thought that on a lot of these engines that the external pipe linking the two valve chests acted as a reciever together with the volume of the passages rather than a direct passage in the block found on a Woolf.

J

[Maryak]

Just out of interest would it have been possible to use that same load on the prop to help turn the engine over if the bar were used to turn it in the same direction as the movement of the ship is trying to turn the prop rather than against it? To my mind you would be using the load to help you rather than work against it.

J

Quite right Jason, the down side of this is................. if it goes with a woosh so will you and the bar.

Thanks to you we now know that this is a model compromise and not a model replica of the actual engine.

Best Regards
Bob

[steamer]

And actually the model's receiver pipe is not large enough for a "cross" compound.    A cross compound's receiver should have a volume of at least the swept volume of the LP cylinder.....with a cylinder ratio of 4  (D²/d²) then the receiver should have a volume equal to 4 times the HP ...or equal to the LP .

Dave


Wot Bob and Jason Said!

[Dan Rowe]

Guys, before this morning I was not really sure what a Woolf compound was. That is why I went looking for a definition. If you scroll down just a bit on the link in my last post to section '2.3 Types' you will see the authors definition of a Woolf compound.

I think that no matter how this engine got this way it fits the definition.

The original Woolf compound was a Cornwall beam engine which had 2 cylinders on the same end of the beam. Here is a nice model version with cut away sections. Notice there is not a receiver as the Hp and Lp are in phase.
<a href="https://www.youtube.com/watch?v=6xWKqneqFu4" target="_blank">http://www.youtube.com/watch?v=6xWKqneqFu4</a>

Dan
Edit: From my library,
 Steam-Engine Theory and Practice By William Ripper 1905 pg 126-127
"Double-expansion compound engines may be divided into two main classes: (1) the Woolf type in which the pistons of each cylinder commence the stroke simultaneously, as in tandem engines (Fig 140), or those with cranks at 0⁰ or 180⁰ apart; (2) the Receiver type, in which the cranks are set at any other angle other than 0⁰ or 180⁰ with each other, and in which the steam exhausted from the first cylinder is passed into a chamber called the receiver,"

[GWRdriver]

Very impressive work Dave.

[Rivergypsy]

Hi,

Sorry for the delay, but many thanks to everyone for taking the time on the explanations, and to Dan for those links - fascinating stuff!

Jason - would a 90deg crank be any harder to machine? Once the centres are in I'd have thought it was pretty much the same.

Harry - Many thanks too 

Given a couple more days work there should be a crank sat in that bedplate too (barring any noisy and expensive mishaps!)

[Jasonb]

No not much more effort to do a 90deg crank now, but back in '23 it would have been a lot more work on the average treddle lathe

[Rivergypsy]

And a little more progress - the crank is done (finally!  ), and loosely laid in place ready for final fitting, plus the prototypes of the bedplates are off of the laser. They'll need some more work I think, but in general I'm happy with the look, plus its a start to the building stand.

I had hoped to get the connecting rods on shortly, but just had a big end bearing come out of the lathe chuck, so there's one more item in the black museum, and another couple of chunks of material awaiting soldering together to make a new one...

[Rivergypsy]

After accumulating a desk drawer full of roughed or part machined components I decided it was time for a major crackdown here, so the latest to be evicted are the piston rods. The lower crosshead areas were first finish machined, then a turning fixture made in order to turn the shafts relative the the crossheads, before getting them up in the lathe and skimmed back to 3/8" dia, radiussed, and then threaded M8x0.5. The first finished yesterday, so it seemed rude not to take it home for a trial fitting
The second is now done too, so just the front faces to chamfer, oil grooves to add, and the holes to open up from the M5 thread they are now to 5.1dia...

Next off I think the guide bars, wear plates, and then pistons...

[ChooChooMike]

WOW  Great work so far. Didn't realize how large that engine is going to be !!

[Rivergypsy]

And a nice concerted effort on bearings and caps sees the piston rods in place for good. Next stop the wear plates and guide bars...

[ozzie46]

WOW! That is really looking good. 

  Ron