Using a steam indicator

[Dick_Morris]

you can't have steam pressure at 0

Although the reasons you give are true, don't forget that many compound marine and stationary engines exhaust into a vacuum induced by the condenser and air pump.

[Steve Bratina]

The steam at the final expansion is at a temperature and pressure below 212 F and 0 psi but it is still steam and used to cushion the piston I believe. The basic question and follow ups seemed to deal with locomotives and stationary engines so that was where I was aiming.

[apm]

Steve your response had me a bit confused but upon digging a little deeper I think we must be talking different ends of the stroke. I think you are talking about the last bit of compression on the return stroke that occurs when your exhaust valve closes before your inlet port opens. This is where you get the cushioning effect and actually want some positive pressure to cushion the piston.

Here's an article that does a good job of explaining this. https://www.advanced-steam.org/5at/tech ... -diagrams/


In a single expansion engine for maximum power for maximum thermal efficiency you want your %steam pressure at release point to be as low as possible. As far as I understand while indicated horsepower is area under the curve it doesn't say anything about efficiency. In other words if you wanted to get max horsepower I supposes you could leave the cutoff open for 100% only to turn around and dump steam at full boiler pressure up the stack basically squaring up your diagram and allowing no room for expansion. In this case your steam cylinders would be functioning as a big hydraulic cylinder with full pressure through out the stroke you would get more power out of your locomotive and have a roaring fire, but you would be wasting tons of steam. Perhaps necessary starting a heavy train or working one's way up a big hill but otherwise bad practice. This is why old texts are full of stories of firemen complaining about engineers hogging steam all based on where the lever was located. Here's another diagram showing energy lost due to incomplete expansion. My understanding is for perfect efficiency (less indicated horse power) you would want to run your steam pressure all the way down to point #7 on the diagram below;


Am I off basis here? I am no expert only have a casual interest here.

I think what you are describing once again from the following location https://www.advanced-steam.org/5at/tech ... iagrams/is the following;

During the exhaust phase, the cylinder pressure – or “back pressure” – is relatively constant, being governed by the steam flow through the exhaust ports and blast pipe. It can be readily seen that the area within the diagram can be dramatically increased by reducing the back pressure during the exhaust phase. This is the reason why simple modifications such as the fitting of double chimneys or Kylpor exhaust systems, result in immediate and dramatic improvement in locomotive performance

[Steve Bratina]

At this point we need to ask a very important question. One which I think I have missed completely looking over what I have posted on the subject. That Question? Did we answer the original question brought forth by the young lad?

[k4kfh]

Here is an article published by the PLS Gazette that covers the topic of steam engine indicators:
http://www.palivesteamers.org/pubs/gazette/2013_3-4.pdf
The article mentions two British model engineers who made serious, well documented tests of miniature steam engines using electronics-based indicators and reported in Model Engineer.

One of the researchers cited above used one cylinder of a 3.5" gauge Juliet as the test subject, and the other (later) tests were conducted with a specially-constructed steam generator and engine of reasonable displacement that even had temperature sensors embedded in surffaces of the cylinder. I'm sorry to be discouraging, but In the case of your small engine, relatively large uncontrolled heat losses will completely mask whatever P/V measurements you are able to make. It's extremely doubtful that anything smaller than a Stuart Turner 10V/10H, 3/4-inch bore/stroke, would yield meaningful measurements.

B&OBob

Thanks for the article. I am curious, what do you mean by "meaningful measurements"? My engine is somewhat similar to the 10V/10H, it is 3/4 stroke but only 1/2" bore. However the cylinder walls are about 1/4" thick (maybe this is normal, but they seem a little overly thick to me). Maybe this serves to reduce heat loss?

When you say any device would not yield meaningful measurements, do you mean it would not be an accurate data set for calculating horsepower or something due to the heat/friction losses? Or do you mean that the pressure reading would be erratic and not even follow the typical curve? The reason I ask is I am not really looking to "maximize efficiency" or anything, in fact right now this engine isn't even connected to a load. I am mostly looking to learn about the use of indicators to calculate horsepower, and having an automatic horsepower reading from an electronic indicator seems like a pretty trivial thing to do with modern computing, as long as the physics of a small engine isn't going to make all the data completely erratic and useless.

I guess what I am asking is...would an indicator for this engine be a "cool toy" that is just not an extremely accurate instrument, or is this such a small model that any indicator would fail to provide any good data at all?

[k4kfh]

At this point we need to ask a very important question. One which I think I have missed completely looking over what I have posted on the subject. That Question? Did we answer the original question brought forth by the young lad?

It would appear that you guys have answered the first two questions, I understand now that I'd have to drill a hole into the cylinder, and I know if I wanted to take an indicator card off both ends I'd need two separate holes.

The one thing I am still a little confused on is how exactly you'd tune the cutoff of an engine with a piston valve. I totally understand why an indicator is useful in improving efficiency of a Corliss engine, because you can change how far and thus for how long the valves open. But I don't see how to do that on a piston valve engine (or a slide valve engine, but I say piston valve because the engine sitting on my desk is of the piston valve variety). How would you change the cutoff without having to physically alter the valve itself?

[Dick_Morris]

A good illustration of how the cut-off is change is the suite of valve gear simulations created by the late Charlie Dokstader. Your engine probably only allows it to be changed by mechanical modifications, but many engines allow it to be adjusted on the fly.

His software allows simulations of a number of types of valve gear and includes simulations of the indicator cards. Most of the parameters of his example designs can be adjusted and the impact on the valve events shown on the indicator card can be observed. By selecting the correct example you can adjust the parameters and see what your valve gear should look like. I don't see a simple valve gear, but slip eccentric or outside admission Stephenson should be pretty close. If you have a piston valve, the Stephenson inside admission may be the closest.

It's free for download at http://www.billp.org/Dockstader/ValveGear.html

[James Powell]

At this point we need to ask a very important question. One which I think I have missed completely looking over what I have posted on the subject. That Question? Did we answer the original question brought forth by the young lad?

It would appear that you guys have answered the first two questions, I understand now that I'd have to drill a hole into the cylinder, and I know if I wanted to take an indicator card off both ends I'd need two separate holes.

The one thing I am still a little confused on is how exactly you'd tune the cutoff of an engine with a piston valve. I totally understand why an indicator is useful in improving efficiency of a Corliss engine, because you can change how far and thus for how long the valves open. But I don't see how to do that on a piston valve engine (or a slide valve engine, but I say piston valve because the engine sitting on my desk is of the piston valve variety). How would you change the cutoff without having to physically alter the valve itself?

Some parts of the motion can be adjusted, depending on what style of valve gear is fitted. If the valve gear is simple (like a single excentric), then there will be less ground to adjust before you run out of area to make changes that don't involve mechanical changes. That being said, a good card can tell you where you can make changes, and roughly how much of a change to make. So, they can be useful...

If you have either Stephenson's or Walsharts (or other...) valve gear, then you will have more adjustments possible. And more room that the original parts are not really the best design possible...

Now, as mentioned further ^, you need to have a port into the cylinder to be able to measure what is going on. Then you need to have a way of figuring out crank position. Neither of them are hard with electronics, there are examples out there of electronic indicator. (one with software is available from Station Road Steam).

The link shared upthread is interesting. Further reading that I know of is Leggett's "October Experiments" in Live Steam in the late 80's/early 90's.

James

[apm]

Here is an article published by the PLS Gazette that covers the topic of steam engine indicators:
http://www.palivesteamers.org/pubs/gazette/2013_3-4.pdf
The article mentions two British model engineers who made serious, well documented tests of miniature steam engines using electronics-based indicators and reported in Model Engineer.

One of the researchers cited above used one cylinder of a 3.5" gauge Juliet as the test subject, and the other (later) tests were conducted with a specially-constructed steam generator and engine of reasonable displacement that even had temperature sensors embedded in surffaces of the cylinder. I'm sorry to be discouraging, but In the case of your small engine, relatively large uncontrolled heat losses will completely mask whatever P/V measurements you are able to make. It's extremely doubtful that anything smaller than a Stuart Turner 10V/10H, 3/4-inch bore/stroke, would yield meaningful measurements.

B&OBob

Thanks for the article. I am curious, what do you mean by "meaningful measurements"? My engine is somewhat similar to the 10V/10H, it is 3/4 stroke but only 1/2" bore. However the cylinder walls are about 1/4" thick (maybe this is normal, but they seem a little overly thick to me). Maybe this serves to reduce heat loss?

When you say any device would not yield meaningful measurements, do you mean it would not be an accurate data set for calculating horsepower or something due to the heat/friction losses? Or do you mean that the pressure reading would be erratic and not even follow the typical curve? The reason I ask is I am not really looking to "maximize efficiency" or anything, in fact right now this engine isn't even connected to a load. I am mostly looking to learn about the use of indicators to calculate horsepower, and having an automatic horsepower reading from an electronic indicator seems like a pretty trivial thing to do with modern computing, as long as the physics of a small engine isn't going to make all the data completely erratic and useless.

I guess what I am asking is...would an indicator for this engine be a "cool toy" that is just not an extremely accurate instrument, or is this such a small model that any indicator would fail to provide any good data at all?

I have to disagree with Bob here, at the end of the day all you are doing is plotting pressure vs cylinder position. Since a steam engine has a piston that moves there is your position, and you will have pressure in there as the thing is moving on one end of the stoke and it has to exhaust as it goes back there is your changing pressure. You have written this is more about the fun of trying than it is about getting meaningful cards which you can tune your engine.

Where I suspect you could run into the most issues is how to connect your pressure transducer to your cylinder cocks or taps. If the hole or the line you are using is too small or too long it could function as a big orifice giving you a delay for the signal reaching your transducer which would skewing your plots. This could be especially true if you used some sort of tiny tube to go to a remote transducer. If on the other hand you use a big line you could wind up having a lot of area in your measurement circuit relative to your cylinder which could also be problematic too as it could potentially function as somewhat of an accumulator giving you a false diagram. I see no reason why this won't work on a tiny engine, but you will need to be mindful of the physics at play and may need to do some troubleshooting, if at first the whole thing doesn't work on the first go around.

My thoughts are your best bet is probably to drill and tap out your cylinder head and install the transducer right in the head, or better yet make up a dummy head out of aluminum with a 1/8NPT in it for the transducer to screw right into the head proper.

Also most of my pressure measurement experience is using $500 industrial quality transducers at work, I don't know how good a $15cheapo sensor works so that could too cause issues.

Lastly not knowing how fast you are running your small engine or how cheap your DAQ will be, keep in mind sample rate vs RPM. If you have a tiny engine going 200rpm than you will need a faster sample rate than 1Hz to see what is going on in there.

[k4kfh]

I have to disagree with Bob here, at the end of the day all you are doing is plotting pressure vs cylinder position. Since a steam engine has a piston that moves there is your position, and you will have pressure in there as the thing is moving on one end of the stoke and it has to exhaust as it goes back there is your changing pressure. You have written this is more about the fun of trying than it is about getting meaningful cards which you can tune your engine.

Where I suspect you could run into the most issues is how to connect your pressure transducer to your cylinder cocks or taps. If the hole or the line you are using is too small or too long it could function as a big orifice giving you a delay for the signal reaching your transducer which would skewing your plots. This could be especially true if you used some sort of tiny tube to go to a remote transducer. If on the other hand you use a big line you could wind up having a lot of area in your measurement circuit relative to your cylinder which could also be problematic too as it could potentially function as somewhat of an accumulator giving you a false diagram. I see no reason why this won't work on a tiny engine, but you will need to be mindful of the physics at play and may need to do some troubleshooting, if at first the whole thing doesn't work on the first go around.

My thoughts are your best bet is probably to drill and tap out your cylinder head and install the transducer right in the head, or better yet make up a dummy head out of aluminum with a 1/8NPT in it for the transducer to screw right into the head proper.

Also most of my pressure measurement experience is using $500 industrial quality transducers at work, I don't know how good a $15cheapo sensor works so that could too cause issues.

Lastly not knowing how fast you are running your small engine or how cheap your DAQ will be, keep in mind sample rate vs RPM. If you have a tiny engine going 200rpm than you will need a faster sample rate than 1Hz to see what is going on in there.

This answers my question. I figured the hardest part would be attaching the actual sensors, and that assuming I could sort that out, I'd at least have a fun "toy."

The trick is the cylinder is so small that 1/8 NPT will look absolutely ridiculous on it (even on a temporary "dummy" cylinder head that can be swapped out for the original).

I was thinking I'd have to use copper tubing to a remote transducer, because the cheap sensors are not exactly rated for steam temperatures, so I believe a siphon/gauge protector is necessary. It really wouldn't be that hard to drill some very small holes and run some thin copper tubing into the wooden base, with outlets on the sides to connect a pressure sensor, like many do with regular cylinder cocks on larger Stuart and similar engines. The trick here is the valves. The smallest I can find a cylinder cock type valve is 5/32-40 thread, and my cylinder is constructed such that it would look pretty ugly to fit any valves bigger than about 3/32 thread.

Photos: https://imgur.com/a/7X4ksjY

Notice the cylinder has very little room for mounting drain cocks without destroying the cladding area. The other two pictures are just to give a sense of scale for those not familiar with this particular engine. It's 1/2" bore, 3/4" stroke.

What would be the best way to try to fit valves on this thing?

[apm]

I took a look at those pictures and my recommendation stands, take the 4 screws out and make a dummy cylinder head that the pressure transducer screws into.

Sure it will look goofy but who cares, using some very tiny tubing is going to introduce a host of new variables and problems that you don't want and could create all sorts of issues.

From reading your postings my impression of you is that you are a young engineering student who has picked an interesting learning project to test your new found skills in measurement, instrumentation and programming. I think this is pretty cool, should be commended, you you will learn a lot and get to put something neat on your resume to talk to at an interview for an internship but if you are going down this path why not go the whole way and do it right?

I get called all the time by my customers when a machine isn't working correctly to come out and do all sorts of crazy measurements. My recommendation if you want to teach yourself some engineering skills is to go the whole way and think of this as a customer's machine that you are called out to investigate and have 2 options;

1. You show up at their facility with a temporary cylinder head with a pressure transducer screwed right in that you made ahead of time in your company's machine shop. You unscrew the cylinder head set up your instrumentation run the tests and prove your ideas, and show them it will work. When you are done at the end of the day you put the machine back together the way you found it go back to the office and produce a nice report showing them the performance of their machine.

2. You show up, with all sorts of tools, perform major surgery on a customer's machine, drilling a hole or two that you can not undo that physically alters their machine to install a setup that will impart a whole host of issues and uncertainty that could taint your results. Then you put your completely unproven software and DAQ system to the test only to find out that nothing is working and you are forced to walk away (or walked out of their plant) with results that are inconclusive at best.

Which path sounds better to you?

In my experience in engineering always pick the simplest, easiest, lowest risk route first that will not impart a whole host of variables beyond your control over one that looks pretty! Even if the requirements are to eventually make it look pretty and elegant and over complicate it, almost everyone will give you the option/funds to try the the quick and dirty, reversible cheap proof of concept route first!

[Dick_Morris]

I haven't worked with electronic transducers, but my impression is that they take little volume and movement of the fluid to make them register, compared to an indicator which uses a piston which has to be displaced. If that's true, a small tube would work just fine. With minimal volume and flow, there will be minimal friction and inertia of the fluid in the tube to affect the measurement. Ensuring that the tube is full of water will avoid lost motion due to compression of any air in the line to the transducer. Because there will be no exhaust from the tube between the cylinder and the transducer, it's unlikely that you would get live steam at the transducer. If would all condense in the tube.