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Limits of p/T for steam engines

Posted by Steam Captain 
Limits of p/T for steam engines
February 11, 2021 08:58AM

After having seen Jay Leno's Garage, I was actually a little surprised to learn, that some steam cars run on 1000psi and 700°F or more.
I occasionally run pV-graphs on the Desmos online graphing calculator for the design of my own (not car-related) steam engine and, well, of course the efficiency grows towards infinite the less the intake is open. Of course it rapidly reaches a point where it just isn't feasible, because with the grow of efficiency, the power produced decreases significantly.

But the overall subject made me wonder how far you can push it in a reciprocating steam engine. I guess creep is a limiting factor (although I guess much less so than in turbines, as the centrifugal forces are several orders of magnitudes smaller) And of course there must be a lubricant, that doesn't coke up in those temperatures.

Maybe the easiest way to ask would be to ask what the highest working pressure in a reciprocating steam engine ever was. But I am curious about the theory behind the limitations.
Re: Limits of p/T for steam engines
February 11, 2021 10:31AM
But the overall subject made me wonder how far you can push it in a reciprocating steam engine. I guess creep is a limiting factor (although I guess much less so than in turbines, as the centrifugal forces are several orders of magnitudes smaller) and of course there must be a lubricant, that doesn't coke up in those temperatures.???

It’s not the pressure it’s the temperature. Steam cylinder oil is only good for 700F ISO 460
I don’t know of any one with a thermal probe on the walls of the engine cylinders. You can have 1000F at the boiler and not know what it is in the cylinders. I have seen a few fried piston rings, a sure sign of over temperature.

I built my Derr boiler to run at 1000 psi. the pumps and running gear would not handle the pressure. I had to cut it back to 600 PSI. But the temperature at the output of the supper heater remained at 750F

I have been using Mobil Synthetic gear & bearing oil SHC 634 ISO 460 same as ISO 460 steam cylinder oil except that it separates from the condenser water almost immediately.

Most of the power plants I’ve worked on run at 1200F and 3600 PSI one ran at 4000. But they’re all turbines.

Re: Limits of p/T for steam engines
February 11, 2021 12:38PM
Perhaps the most critical factor in establishing high piston expander efficiency is "Friction MEP". We all know what Mean Effective Pressure is, but folks don't talk as much about friction MEP. Basically, it is the MEP the engine would have to generate simply to overcome internal friction, though we could add similar auxiliary loads such as those needed to operate pumps and blowers.

We can think of our engine's power output as being calculated by PLAN with P equalling MEP - FMEP.

Thus, your efficiency never even begins to head towards infinity, when the MEP approaches Friction MEP, the efficiency goes to ZERO. So, to determine the most efficient operating point, you need to determine Friction MEP plus auxiliary loads. Art Gardiner gave a talk about this at a Berrien Springs meet and his optimum cutoff points were higher than normally discussed.

Then, of course, you need to run a second set of calculations to ensure that you aren't overexpanding the steam and robbing output work by generating a vacuum or producing condensate that cools off the cylinder walls.



Edited 1 time(s). Last edit at 02/11/2021 12:40PM by frustrated.
Re: Limits of p/T for steam engines
February 11, 2021 01:10PM
Almost forgot. I think Jay Carter was pushing pressures near 2,200 psi and temperatures around 1,000 F.
Re: Limits of p/T for steam engines
February 11, 2021 01:23PM
Jay Leno has recently been having piston ring problems in his (Merl Norman) Doble Roadster. I know of two times now that Jay has had to put new rings in his Doble roadster because his previous rings had lost their wall tension. This last time he was dealing directly with the piston ring maker to find a correct solution. I don't know what he has found out yet. I suspect that his ring problems stem from having too high of super heat in his engine. It is costing him a lot of money to finally fix it. I listened to an observer while on our 800 mile long Colorado steam tour. They asked Stan Lucas "what would do you do for parts if you break your Doble?" Stan replied: "If you cannot afford to fix your broken Doble then you should not own one." I do not own a Doble for that main reason. The second reason is: "If your Doble's battery is dead, You Doble isn't going anywhere.
Re: Limits of p/T for steam engines
February 11, 2021 03:26PM
Rolly wrote:"I have seen a few fried piston rings, a sure sign of over temperature."
Can you explain how fried piston rings manifest? My best guess would be they become either brittle like glass or lose hardness?

"I built my Derr boiler to run at 1000 psi."
That's what I am designing my test-bed-boiler with and similar to you experience, the pump working against such a large pressure seems to be the hardest design issue. The drive train is exposed to tremendous bearing loads at 1000psi.
Good lawd! I just read the Carter engine uses a bash valve! At 2200psi! And 3000rpm! Unbelievable. But it seems to work. Does he state somewhere what type of oil he's using?

Frustrated, you mention friction losses. I was wondering how high pressure determines piston ring design. At 1000psi or even more, the forces pressing the ring out against the cylinder walls must be extreme. Those pressures aren't even found in IC engines. The piston rings might even be designed to be much smaller. They wouldn't work so well in the lower pressure region, but work well in a certain design pressure range (enough thickness for the pressure to act against, but small enough not to create excessive friction and wear)

"2,200 psi and temperatures around 1,000 F. "
That's insane. That pressure means the crank bearings are statically loaded with tons of force from the pressure - TONS. I have high respect for people, who design around these challenging conditions.

SSsssteamer, these experiences sound like Jay needs high temperature steels. Actually there ARE alloys suitable to do the trick. It's just tedious work to figure it out as a private hobby developer, because it can dig deep in the wallet. But technically, the alloys exist.

Edited 2 time(s). Last edit at 02/11/2021 11:12PM by Steam Captain.
Re: Limits of p/T for steam engines
February 12, 2021 09:44AM
Tether hydros run temperatures and pressures even higher those mentioned. Of course their time between overhaul is measured in minutes, not hours. They run cast iron pistons with Dykes rings in a steel liner. See below.

Lohring Miller

Re: Limits of p/T for steam engines
February 12, 2021 11:25AM
Hi Steam Captain.

For solid numbers, you would have to test a given engine. On the other hand, there are references which give approximate friction for various mechanical devices and these numbers can be used to give an initial estimate for engine design.


Re: Limits of p/T for steam engines
February 12, 2021 01:04PM
How would someone learn how to build these hydro engines? I'd like to see what I can take from it for my own projects. The website flysteam is nice to scratch the subject, but there are no extensive information about hydro engines out there.

Btw thanks for the tip, frustrated.

Edited 1 time(s). Last edit at 02/12/2021 01:05PM by Steam Captain.
Re: Limits of p/T for steam engines
February 13, 2021 08:30AM
There is a great series of articles on the flash steam hydros. Unfortunately, the copies I have are too large for this forum.

Lohring Miller
Re: Limits of p/T for steam engines
February 13, 2021 03:03PM
Hi Captain,
Interesting tool this Desmos graphing calculator. Also very cool that you summarized that you need maximum pressure with a short cut-off to achieve any power. Note that power would be in Horsepower.

A quick aside and pertaining to Jay Carter. Jay is the one famous for the Carter Copter. Jay is, god rest his soul, a very smart man. I met him at my first Steam Automobile Club of America (SACA) meet that I attended in September of 2007. He gave a presentation and asked the participants "what factor increases the RPM of a steam engine?" I immediately answered, "pressure". Jay is also noted for his Volkswagen steam engine conversion where he modified an IC combustion engine to what we call a bash valve, uni-flow steam engine. And yes, he ran over 2,000 PSI as he told me during common talk around and about.

Note that the picture is myself at the tiller with Tony Grzyb giving me lessons on how his buggy works during that 1st meet of mine.

Another interesting feature of a steam engine is that it has maximum torque from a stall. This is very much like an electric motor. My understanding of a steam engine is to use it to provide torque and direct drive like a locomotive, Stanley Automobile and my steam powered scooter. If you go to pictures on this steam automobile website, you can view pictures of Rick's Steam Scooter.

It took me since then-to-now to realized the reason for Jay to use a bash valve, uni-flow steam engine. I admit at first brush I didn't like the idea of this high revving engine that required a transmission. He went through a very extensive process to develop the bash valve to make the engine work at that pressure and RPM. This was the key element he needed to be successful with this steam condition, working range.

Another noteworthy device that Jay used is something called a De-Super Heater. This is where water is squirted into the mono-tube steam generator right before it enters the engine. This device tempers the heat to allow lubrication to be effective.

Please let me discuss what I would call the standard steam engine like a Stanley. The same concept applies with the higher the pressure, the higher the engine speed. The difference is that this steam engine uses torque as opposed to high RPM (higher horsepower). As SSsssteamer remarks about J Leno, he would agree with me that there is nothing sweeter than the sound of one of these steam engines puffing away.smoking smiley

An example of one of these engines is the Stanley 10 Hp. My engine is pictured as the day I bought it. This engine has a "D" slide valve and really can't begin to withstand pressures over say 600 - 700 PSI. By the way as SSsssteamer mentions about J Leno and his Doble Car, it uses a piston valve and allows it to handle significantly higher pressures. I believe the Doble is ~ 1,000 psi. Again, this is a direct drive engine on the rear axle.

As you can see I attached some PV diagrams. The difference is really pertaining to the RPM and use of torque compared to high RPM (higher HP). It depends on what you like and how it's applied to your application.

Enough for now and hope this provided some insight to your pressure question and the elements behind it.

Kind regards,

Re: Limits of p/T for steam engines
February 13, 2021 04:47PM
Thank you very much Rick H. for the abundant info. I love pV-diagrams. They say so much about an engine if read properly.

I never heard about a de-superheater being used on a small scale. I guess Jay Leno didn't want to alter the boiler to reduce the temperature.
Re: Limits of p/T for steam engines
February 14, 2021 07:03AM
You are Welcome...it is my pleasure to talk steam,

When working with PV diagrams, I find it useful to work with the equation PV=nRT. You can identify what is happening with temperature around the graph. Just a thought.

If you like PV diagrams then I think you might enjoy TS diagrams. One is attached to spark your interest. Let me know if any questions. I actually find using a TS diagram more useful in explaining the total picture of a steam engine including burner, boiler (proper term is steam generator) and engine. Where the PV is just kind of focused on the engine part.

Do let us know about what projects you're working on? Hope to see you at a future SACA meeting hopefully this year. Also if you are not a member, please join for the Bulletin provides interesting articles and information plus contact info for all its members. Note J Leno is a member.

Kind regards,

Re: Limits of p/T for steam engines
February 14, 2021 01:48PM

One thing you must keep in mind when referencing Jay Carter's steam plant is that the steam generator ran at a varying pressure, with a max of 2200 psi. The engine ran at a varying pressure also, depending on the loading. Jay had the throttle set up to control the entire steam plant's output.. His air flow, fuel flow, and FW flow followed the demands of the engine-maintaining a 200 psi drop across the throttle.

Of course, it might be possible to put 2200 psi to the engine if you put the throttle to the floor and held it there long enough for the generator to catch up-then let off the throttle, But I seriously doubt that Jay did that while doing the Federal Air Quality Tests! In addition to the effect that the 200 psi pressure control had on the BMEP to the engine, he was using bump valving running at a low cutoff. His set up would have kept the generator pressure only 200 psi greater than the pressure to the engine at any given moment, and that made his steam plant very efficient.

Jay also had a de-superheater set up on the generator, with 2 separate temp sensors controlling 5 different injection points within the coil stack...way beyond what Doble did with his normalizer.

Jay was far ahead of all of us with his design and accomplishments...another one of the great men we've lost from the steam group!

Re: Limits of p/T for steam engines
February 15, 2021 10:49AM
Rick H., I'd honestly love to chat with steam enthusiasts in person on meetings. Unfortunately, I'm located in Berlin at the moment and I'm not talking about the 26 Berlins in the US smiling smiley
Which is a little sad, because there is just a trace amount of steam enthusiasts here and the overwelming part is about steam locomotives only (But which I support of course.)

Hey, Chuk,
Amazement bestartles me. Amazing what people can achieve when they put all their love into something. Respect for Mr. Carter and honestly, everyone, who create something amazing because they just love to do it.

You write "only 200 psi greater". I myself consider 200 psi a tremendous pressure drop. I mean it's 200 psi of 2200 psi max. That is a whole lot of pressure loss. Do you know the reasoning behind it as to why he wanted to keep a 200 psi pressure drop through the throttle? Dry steam? more controlled super heat by pressure drop? His setup sounds very very fine tuned, as indicated by the five different injection points inside the de-superheater.
A 5% cutoff is like the nigh-impossible-plus-ultra. Everything below is something I've never heard of. And if it existed, it wouldn't put out much power due to an extremely squeezed pV-diagram, whereas a longer cutoff increases the output at the cost of a reduced efficiency. 5% is like the best you can practically wish for in a reciprocating engine if only the efficiency counts.
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