Compound steam engine.

[Richard1]

Just an idle thought about compounds. If I built an engine having 1" and 1.75" cylinders with a 1" stroke I would have a pretty normal compound engine expanding steam about 3 times in each cylinder.
What would happen if I built an engine with HP 1" bore x 1" stroke, LP 1" bore x 3" stroke? Clearly the lower pressure steam in the LP cylinder only has the same piston area as the HP cylinder but does have a much longer crank throw to apply this lower pressure.

Would it work? I have never seen such a thing or even any mention of it. Too tired, hot and bored right now and my mind is running in strange directions.

 

[ianjkirby]

I seriously doubt that this premise would work in practice. While mechanically the LP piston would move up and down, in appropriate time with the HP piston, it would have to do so at (roughly!) three times the speed (ok, it's not that simple, but I'm trying to illustrate a principle). The speed of the LP piston would likely draw a degree of vacuum in the cylinder, thus negating any useful compound effect, as the incoming steam is being sucked in, having work done on the steam by the piston, rather than arriving forcefully, and doing work on the piston, as required.
But do try, and let us know!!
Regards, Ian.

 

[Richard1]

Piston speed by itself could certainly be a problem and could certainly not be extended to a triple or more expansion engine.

I don't think the LP piston would draw a vacuum as the cylinder volume is pretty much exactly the same as in a normal compound. So the volume and rate of steam flow should also be pretty much the same.

cylinders 1" x 1" and 1.75" x 1" have volumes of .78 and 2.4 cu inches
cylinders 1" x 1" and 1" x 3" have volumes of .78 and 2.35 cu inches

Unless the LP cylinder was steam jacketed to keep it hot there would be more condensation as the surface area of the cylinder walls is almost double.

Maybe one day I will build one and see if it works.

 

[Richard Hed]

Piston speed by itself could certainly be a problem and could certainly not be extended to a triple or more expansion engine.

I don't think the LP piston would draw a vacuum as the cylinder volume is pretty much exactly the same as in a normal compound. So the volume and rate of steam flow should also be pretty much the same.

cylinders 1" x 1" and 1.75" x 1" have volumes of .78 and 2.4 cu inches
cylinders 1" x 1" and 1" x 3" have volumes of .78 and 2.35 cu inches

Unless the LP cylinder was steam jacketed to keep it hot there would be more condensation as the surface area of the cylinder walls is almost double.

Maybe one day I will build one and see if it works.

I thimpfk the LP piston would have to travel too fast, however, I like the idea. I thimpfk maybe if you shorten the LP to say 1-1/2" you might be able to do it, that is with an appropriate sized piston for that crank size.

 

[Richard1]

Maybe it would be possible to build a multi expansion engine with both the bore and stroke maintaining the same ratio as the pressure drops. The example shows the cylinders volume increasing approximately 3 times at each stage. Such an engine is certainly going to look odd.

HP 1" x 1"
IP 1 7/16" x 1 7/16"
LP 2 1/16" x 2 1/16"

 

[Richard Hed]

Maybe it would be possible to build a multi expansion engine with both the bore and stroke maintaining the same ratio as the pressure drops. The example shows the cylinders volume increasing approximately 3 times at each stage. Such an engine is certainly going to look odd.

HP 1" x 1"
IP 1 7/16" x 1 7/16"
LP 2 1/16" x 2 1/16"

The cube root of three. Might work.

 

[Steamchick]

Hi, Richard. Yes, you are right, in that thermodynamically, the volume increase will extract power from the steam in the LP cylinder. Considering full-sized engines kept the same stroke but used progressively larger pistons, I think is based on logistics of achieving the progressive volume increase with the most efficient arrangement.
Points to note.

1. The crank design is "consistent and optimised" - apart from dynamic balance and forces from the extra mass of L pistons... - if the stroke is the same for all cylinders.
2. The cylinder wall area is naturally larger with larger diameter pistons, but the optimum chamber for heat loss to the walls is always an approximation of a sphere.... I.E. bore and stroke equal.
3. As these engines were often in ships or locos, where space is a consideration, longer stroke engines or cylinders were not so suitable for their application.
4. As the pressure difference between intake and end of stroke (exhaust valve opening) is usually higher in HP cylinders than IP and LP, it does make mathematical sense to utilise relatively large LP pistons to get good forces from the low pressure on the piston to drive the crank to increase efficiency there.
5. Try some simple sums to try and predict the cylinder pressures - hence forces - on each piston and con-rod. When you change diameter of LP piston you'll get a better appreciation of the optimum.

I may have a play with some numbers if it rains this afternoon, otherwise I'll be gardening.
K2

 

[Jasonb]

As we generally measure pressure in pounds per square inch you will have a lot less square inches of piston area for the part expanded steam to act on with a 1" LP piston compared to a 1.75" LP piston. So will not get as much force pushing against the smaller LP piston.

Area of 1" piston is 0.78sq inch, area of 1.75" piston is 2.4 sq inch. So your small piston will only be adding 1/3rd the force that the larger piston would into rotating the engine.

 

[Richard Hed]

As we generally measure pressure in pounds per square inch you will have a lot less square inches of piston area for the part expanded steam to act on with a 1" LP piston compared to a 1.75" LP piston. So will not get as much force pushing against the smaller LP piston.

Area of 1" piston is 0.78sq inch, area of 1.75" piston is 2.4 sq inch. So your small piston will only be adding 1/3rd the force that the larger piston would into rotating the engine.

Wait a minute, that is true if the pressure is the same, but these engines have progressively lower pressures as the steam goes "down hill".

 

[Richard1]

Area of 1" piston is 0.78sq inch, area of 1.75" piston is 2.4 sq inch. So your small piston will only be adding 1/3rd the force that the larger piston would into rotating the engine.

Yes this is true but the stroke in this case is 3 times longer hence the torque on the crankshaft is 3 times greater which SHOULD result in the same force as bigger piston with shorter stroke.

No idea if it actually would work like that.

 

[Jasonb]

Wait a minute, that is true if the pressure is the same, but these engines have progressively lower pressures as the steam goes "down hill".

He is talking of having a 1" Low Pressure Cylinder as opposed to a 1" low pressure cylinder. As both are the Low pressure cylinder they will get the same pressure all be it lower than the High Pressure cylinder.

What the Torque and Power figures would be will be different but without a prony brake or similar to test it's hard to say. It will be an "interesting" crankshaft to make as it will be of quite light construction to suit the 1" HP bore yet have long webs due to the longer throw that would normally be found on a lager and therefor more heavily constructed engine.

 

[Richard Hed]

He is talking of having a 1" Low Pressure Cylinder as opposed to a 1" low pressure cylinder. As both are the Low pressure cylinder they will get the same pressure all be it lower than the High Pressure cylinder. ??

 

[Jasonb]

He is talking of having a 1" dia Low Pressure Cylinder as opposed to a 1.75" dia low pressure cylinder. As both are the Low pressure cylinder they will get the same pressure all be it lower than the High Pressure cylinder.

Diameter altered but still all to do with the Low pressure cylinder

 

[Richard1]

What I am idly speculating about is having a HP cylinder of 1" bore with a stroke of 1" taking steam at 100 psi and expanding about 3 times giving an exhaust of about 30 psi. I am thinking of having a LP cylinder also having a bore of 1" but a stroke of 3" taking the exhaust steam at about 30 psi and expanding it about 3 times down to 10 psi.

The force applied to the HP piston would be about 78 lb and would act via a 0.5" crank throw.
The force applied to the LP piston would be about 23 lb and would act via a 1.5" crank throw.

This should apply approximately equal torque.

Building a crankshaft for such a crazy layout would be very interesting!

 

[Steamchick]

As you say, a crazy layout. But it will function, maybe not quite so effeciently, but it will run. I have seen many entertaining engines that are crazy, so do it. It's for fun so you have my support.
I regularly run a twin compound that belongs to the local club. I think something like a 3/4" HP cyinder, 1"stroke, and 1 1/4" LP cylinder with same stroke. While early running, but warming up, it runs on the HP cylinder at 20 psi, but when after 10 minutes or so it has warmed the LP properly, it speeds up so I can reduce steam to 15psi....
Proof of the extra power and efficiency from the LP cylinder. Hope you can achieve the same!
K2

 

[Jasonb]

Actually crankshaft shape would depend on the layout. If it were your typical vertical then a double throw crank to suit bearings at each end and one in the middle would be the most complex.

On the other hand if you did it as a horizontal cross compound type layout then you could have the flywheel in the middle of a straight shaft with separate crank arms or a disc type crank at each end and just the two bearings inside the cranks.

 

[Richard1]

On the other hand if you did it as a horizontal cross compound type layout then you could have the flywheel in the middle of a straight shaft with separate crank arms or a disc type crank at each end and just the two bearings inside the cranks.

That would seem to be by far the easiest and simplest arrangement.

It would be possible to build two single throw crankshafts with bearings at each end like a normal vertical engine but gear them together in the middle. The LP crankshaft could be lower than the HP due to the longer cylinder and it wouldn't matter that both halves of the engine turned in opposite directions.

 

[Richard1]

As you say, a crazy layout. But it will function, maybe not quite so effeciently, but it will run. I have seen many entertaining engines that are crazy, so do it. It's for fun so you have my support.

Thanks due to circumstances I am a long way from the shop but will keep thinking about this and hope to perhaps build it one day. It was just idle speculation but it would be fun to make it real

 

[Steamchick]

Hi Richards (1 and all), I am enjoying your idle speculation. I would suggest however, that a fine improvement on the "conventional" twin (or other) compound would be to really minimise the volume of passage from exhaust valve of HP to intake of LP cylinder. To my mind (vague understanding and idle notions, that is...) most of the wasted heat is from steam expanding between cylinders, so the exhaust pressure of the higher is reduced before becoming the intake of the lower pressure cylinder. - You 30 psi HP exhaust is quite likely to become a 15 psi LP cylinder intake pressure after expanding down a "conventional" U-tube connection pipe, or other. e.g. if you have 30psig in 10% of the HP cylinder when the exhaust valve opens, it is quite conceivable that there is at least the same volume for the steam to expand into just transiting down the connection to get to the intake of the LP cylinder. So the 30psig is down to 15psig when the LP cylinder intake opens. Expanding into a "similar" volume of the LP cylinder (just before the LP piston expands the volume further) the 15psig drops further to say 8psig... which steam is expanded 10 times to 0.8psig.... when the LP exhaust valve opens. That means that the expanding LP cylinder has extracted 90% of 1/3rd of the energy that was expelled from the HP cylinder. I.E. 30% of the exhaust energy from an otherwise "single cylinder" engine. Sounds quite efficient - even more so if you re-write this in Absolute pressure and open the LP exhaust into a condenser, at just a few psi ABS. However, considering the total energy loss (simply!!): The 100psi steam input to the HP cylinder - with 50% cut-off - and assuming 10% volume at exhaust opening - will be below 55psi when the exhaust valve opens. this 55psi steam fills the exhaust valve space, transit pipe and inlet valve space of the LP cylinder so may be down to below 25psi when the LP inlet valve opens. Assuming 10% of LP cylinder volume - which you suggested should be 3 x the HP cylinder volume - then the steam pressure would be below 10psi in the LP cylinder when it is allowed to expand. (To around 1psi?). So we have 100psi x 0.5 HP cylinder volume, expanding 45psi, plus the 9psi expansion in the LP cylinder, makes 54psi (54% of the steam energy putting it crudely!) - less lost heat into the metal and then atmosphere....
It seems to me logical that compounds were successful for power generating stations pumping stations and ships.... but not locomotives (even some compounds were re-timed and fed as simple multi-cylinder engines for the extra power!). You more than double the mass of pistons and cylinders, etc. to gain an extra 10% fuel efficiency... (Probably much less with other heat losses, etc.).
An interesting topic!
K2

 

[Jasonb]

The problem with making the passages too small is that they will not hold enough steam to fill the LP cylinder once the HP exhaust is closed. The tube linking the two cylinder sis often termed the receiver and is sized to contain enough of the lower pressure steam to be able to fill the LP cylinder. Crankshaft design also comes into this as a 90deg crank will see the LP valve opening at a different time relative to the HP exhaust closing. The steam chest volume also comes into this and can be why some compounds have piston valves for the HP and slide valves for the LP