Steam port sizes

[addiator]

First let me introduce myself and my problem. I am a Mechanical Engineering student, who has endeavored to build a Uniflow engine as my final project. I am trying to look at the matter thermodynamically and scientifically in general, however a lack of literature available is making it all a tad difficult (I do have Stumpfs book BTW).

One thing that I have been trying to figure out is how to determine the port area for the inlet. I actually decided to go with piston valves, as it is to be a small engine - but aerodynamically designed ones to prevent excessive loss. I imagine that I could figure out the engine speed from the ports - slow enough for the friction loss not to be excessive, with the losses being possible to be determined for any assumed port area by some basic equations of fluid dynamics. Yet I wonder what would be a better way to go about it.

Another thing I wonder, is how to determine how much steam will escape by the exhaust ports and at what speed. Now, if they were to be perfect, it is easy - enough for the pressure in the cylinder and condenser to equalize. But how do I find out if there is enough time for that to happen? And finally, how can one predict the shape of the 'dip' in the end of the indicatior diagram for when the ports are uncovered?

I know that there are some people here who dealt with similar issues, so I am looking forward to your input. I have actually been told that I am over-thinking this, but I want a perfect engine

[Lopez Mike]

Bart and others will chime in here as they have more recent experience and knowledge in this area.

That said, forget the idea of your engine design being perfect. I.C. engines are very well developed but the vast, vast majority of that sorting out was done on test stands and flow benches over years and years of work. They are not designed from first principles as a structure might be.

You would do well to examine carefully the work already done on I.C. poppet valve engines (almost all of them) and the exhaust ports on two stroke cycle engines (most of them small engines).

On the two stroke cycle engines I would stay away from high output designs as they are very dependent on exhaust system resonances for high engine performance.

As an additional thought, you will probably find that at the revolution rates and flow regime of a practical uniflow steam engine, the port designs are not a critical design element. Thermal losses are likely more important. Unlike most I.C. engines, steam plants are a very integrated design. Everything works together. Boiler, engine and all.

[DetroiTug]

Basically echoing what Mike has already written.

To design and build a steam engine that will run is easy, to design a "perfect" engine, one could invest a lifetime, and many have and perfection has eluded them. A simple device to turn a pressurized gas in to work - there are hundreds (probably thousands) of designs, when one attempts to design one with very high efficiency, durability and practicality - competitive in today's power market, the task becomes vastly more daunting.

There is the SACA Steam Automobile Club of America forum I would suggest as a source that typically deals with high steam engine efficiency theory, mostly engineers posting there. Lots of spreadsheets and entertaining debates as well Some very knowledgeable folks there on Unaflow design, and high output steam generators.

For a class project with a deadline, I would go with an relatively inexpensive fast-building existing design. And for simple exhibition, the converted 2 cycle bash valve unaflow is pretty easy to build.

-Ron

[barts]

Some random ideas on the subject....

Like almost all interesting engineering projects, there's a whole bunch of interacting choices. The uniflow can be more efficient than counterflow engines because the decreased heat leakage allows for much greater expansion in a single cylinder. In order to realize this efficiency, the uniflow typically uses significantly larger cylinders, and admits steam over a much shorter period. This means the amount of steam to be admitted is actually fairly small, and the intake valve size is not particularly critical. Poppet valves are a natural in this application; Lentz valve gear made them work well w/ eccentrics, but a rotating cam can work as well. Bash valves are certainly simple, but lack a certain design 'elegance' . By conventional engine practice, the clearance should be held as small as practical - a few percent.

I'd reason about the required valve sizes based on gas engine practice, since you can get a pretty good idea of effective sizes of a poppet valve by looking at cylinder fill rates at rpm of maximum torque on a naturally aspirated engine. I'd choose a small industrial engine for this; they're known for pretty conservative cam timing and good slow speed behavior. Remember that max flow of an automotive poppet valve is reached at 1/4 valve diameter opening - no more is needed.

Keep in mind that a uniflow engine w/o aux. exhaust valves or efficiency-robbing extra clearance will require at least 20-25 inches Hg vacuum on the exhaust side to prevent excessive compression, so if you're actually going to run this engine, a condenser of some sort is required. If you can leverage the university steam plant as a source of steam, a steam ejector can be used along w/ some copper tube and a large container of water to create sufficient vacuum for tests. Plumbing the cylinder drains with significant piping away from the cylinder before the valve will let you use the volume of the pipe as aux compression space -this is handy for starting the engine If the drain valves are below the cylinder, that space will soon fill w condensate and remove the effect of the extra volume.

Remaining steam in the cylinder is compressed adiabatically; w/o sufficient vacuum the pressure can exceeed the inlet pressure. Unbalanced poppet valves are thus a natural in this application, as they function as automatic safety valves if vacuum is lost. Due to this compression, single cylinder uniflow engines require healthy flywheels to run smoothly, particularly at low RPM. The requisite flywheel size may be calculated using energy methods; don't forget to include piston and connecting rod kinetic energy as it all helps. If you use a piston valve, you will need to include a poppet-style safety to prevent breaking things if you either lose vacuum or a slug of water is ingested.

Some interesting data points from the literature: typical uniflow full load BMEP w/ 200-250 psig inlet is 50-60 psi. Uniflows would typically be efficient from perhaps 1/4 full load to 50% or more over; the large cylinder size makes large overloads possible. Remember there are high peak loads due to the large cylinder; uniflow engines should be ruggedly built for their power output w/ ample bearings and a stiff connecting rod.

As you may gather, I'm designing a uniflow engine for my next steam boat .

Good luck w/ your project... it will take some time.

- Bart

[fredrosse]

I have built and operated an atmospheric exhaust pressure Uniflow Engine which generated electricity for my house, with the exhaust heating the house. It is a single cylinder single acting engine (23 cubic inches displacement) which produced 4.64 brake horsepower (BHP) at 1040 RPM, using 21.1 Pounds per Hour (PPH) per BHP. Inlet is 150 PSIA, saturated steam. The engine needed to have high clearance volume to keep compression pressure within reason, but, as illustrated above, still had very good economy, rarely achieved with most steam engines.

There is an ASME paper about this steam plant, I will send you a copy if you would like.

The analyses of valve pressure drop for both the admission valve and exhaust ports are accomplished as follows;
Flow and Pressure drop thru the ports will be via the general compressible gas flow equations, often including choked flow for some portion of the valve events.
The analysis is performed using a time-step method, which eliminates all kinds of fancy mathematics, thanks to the computer that can do a lifetime of manual calculations in the time it takes to blink your eye.
The mass flow into and out of the cylinder is calculated with time steps that are small enough that the conditions between time steps do not vary significantly. The parameters relevant here are 1. Valve opening area, 2. Valve geometry discharge coefficient, 3. Upstream Steam Conditions, generally Pressure & Quality, 4. Downstream Pressure.
For example, say you have a Uniflow engine at 600 RPM, a connecting rod that is 4x the engine stroke, and exhaust ports that begin to uncover at 90% stroke. This engine will have the exhaust ports open for 39 degrees of crank rotation at the end of the power stroke, followed by 39 degrees rotation on the compression stroke. That is 22% of one engine revolution, and occurs over the time interval of 0.022 seconds. Use EXCEL and break this interval into, say 100 time steps, or 0.00022 seconds per time step.
Beginning with the cylinder steam mass, quality, and pressure known at 90% stroke (just before exhaust), at each time step, calculate the piston position and cylinder volume, plus mass flow out of the cylinder space, then the resulting remaining mass in the cylinder space, and its expanded pressure (based on the then existing cylinder volume), which will be decreasing. Then on to the next time step, you have a new set of values for iteration. As long as relevant parameters do not change more than 10% of the previous time step, reasonably accurate results will be obtained. If they do change significantly, then you need smaller time steps.

One complication is cylinder initial condensation (during admission) and re-evaporation during expansion and exhaust portion of the stroke. This is what makes reciprocating steam engines have generally low efficiency. I would suggest you initially evaluate just using an ideal gas with no phase change, then after you get the program running OK, I will provide methods of addressing the cylinder condensation and evaporation issues.

[fredrosse]

Although not addressing your specific analysis task, some review of the Whitecliffs Solar Steam Plant would provide some insight for a very efficient small steam plant. This plant used a 3 cylinder uniflow steam engine, based on an industrial Diessel engine lower end, and GM 53 Series cylinder liners for the uniflow steam cylinders. The plant had very good efficiency, and used simple "bash" valves for admission.

The simple arrangement was possible as the plant ran with 600 PSI / 600F inlet steam, and vacuum exhaust. The relatively high inlet conditions allows the "bash valve" concept to work OK, as admission is only occurring during a few percent of the stroke near TDC, so the "bashing" is not too punishing to the machine. The engine was almost a direct copy of typical CO2 engines used for model airplanes, only much much larger. There is technical data presented on the web for this project, which you will find interesting.

[Lopez Mike]

I knew Fred and Bart and Ron would be founts of information.

I had forgotten those airplane engines. Always frosting up. And they would run either direction just fine. I busted at least one model airplane that way. It kicked back on flipping it to start and I didn't notice. I was amazed to have the plane reverse itself into the ground.

That brings up the thought that there is no favored direction of operation with one of these bash valve engines. All sorts of issues with inadvertently not reversing when approaching a dock. Hmm.

Some very low performance I.C. motorcycle engines, specifically trials bikes with not much spark lead, are prone to running backwards. I had one cough and start running backwards just as I crested a small hill. I was deposited in a heap at the bottom of the hill to the intense amusement of my friends.

[Kelly Anderson]

One thing that I have been trying to figure out is how to determine the port area for the inlet.

A rule of thumb that if often repeated (sometimes with variation*) in engineering texts for the area of steam pipes and ports is to size them so the steam flows at 6,000 feet per minute when the engine is at operating speed. So if the piston speed averages 600 FPM during the part of the admission stroke that the valve is open, then the port area should equal 1/10 of the piston area. If the piston speed averages 60 FPM during the part of the admission stroke that the valve is open, then the port area should equal 1/100 of the piston area.

Avoiding making the ports too large reduces clearance in conventional engines. Avoiding making steam pipes too large reduces the amount of time the steam spends in those pipes, reducing condensation losses.

* Some texts call for speeds as high as 10,000 FPM, but these are for very large engines. The higher port friction losses of our small engines require us to be more conservative.

[Lopez Mike]

Very interesting number, 6000 f.p.m. That translates to 100 f.p.s.
The reason that I find it interesting is that long experience in I.C. engines has yielded numbers not too far from that.

The most restrictive port arrangement is the flat head or valve in block arrangement and the maximum power always occurs in these engines very close to 250 f.p.s. (average velocity) and I suspect the velocity at maximum torque will usually happen a bit lower speed at around 200 f.p.s. The best F1 engines are getting the max power around 300 f.p.s. and this has been about the same for fifty years or more. The astounding high outputs are mostly from higher r.p.m. rather than any great break throughs in port flow.

These are test bench numbers, not calculated, and it is reassuring to hear that non-engine practice is not that different.

As you note, at the speeds that our steam engines turn the ports and valves can be very small and are.

[fredrosse]

Kelly, "size them so the steam flows at 6,000 feet per minute when the engine is at operating speed....." Nice to get an old, "tried and true" rule of thumb.

I notice this thumb-rule is based on the volumetric space increase created by the piston movement. That would be true if the engine was using something like a non-condensable gas. For a steam engine, the amount of steam admitted to the cylinder would actually be the steam needed to fill that volume, plus the steam that becomes initial cylinder condensation. For small engines this can be about the same amount of steam as needed to fill the cylinder, so about double ideal flow, and double the calculated ideal velocity, approximately.

A subject that should get some additional technical evaluation, for those of us who are inclined to investigate this. Hope addiator shares with us his analysis findings.