The Return of No. 83, a Hot Air Engine

[vtsteam]

steadying hand pressure by increasing revs.

Have you tried clamping it down to the bench?

Russell

Heh, nope, Russell! 

[vtsteam]

I think I mentioned that the original intention was to run this little engine on wood. I wish I'd had the time when I first built it to make that possible.

Today I started making a little latch for the fire door on the firebox. I also began insulating the firebox. It will be insulated just like my melting furnace, but on a tiny model scale.

Here I've fitted a layer of fiber insulation to start with:

 

[vtsteam]

Working on my foundry is slowing me down on No. 83. I'm thinking about doing an iron casting to replace the brass power piston cylinder and displacer guide block. Foundry was in a sad condition, but have made big strides over the last few days

Back at it shortly.......

[jiihoo]

I am following with high interest.
I'd been ignoring MadModder for some time and when I started visiting again I found many interesting projects. This is one of them  .

[awemawson]

I’m watching with great interest but my get up and go to a large extent has got and gone! I do still occasionally power up the gen set for the induction furnace to blow out the cob webs and run the pump to raise water from the bore hole through the heat exchanger as this keeps the field drain return to the stream clear, but apart from sometimes raising a bar to red heat in the pot for a bit of fun it remains idle sadly.

[vtsteam]

Glad of the company, guys! 

Today I made an ashpan out of some scrap galvanized tin:

 

And put some "small logs" in it -- ya know, just to see what it looked like......


 

 

[vtsteam]

Well, then I kinda figured might as well light them and see what happens........

What happened was Number 83 took off at 1500 RPM and the base of the smokestack oxidized to a beautiful shade of purple:! Finally Number 83 was running on wood, after 18 years, doing what it was intended to do! 
 

[BillTodd]

Excellent 

[vtsteam]

Thanks, Bill! 

[shipto]

Brilliant. 

[Country Bubba]

Ah the sweet smell of success! Happy for you.
 

[vtsteam]

Thanks Dwayne, Country Bubba! 

[vtsteam]

I've made up a Prony brake to measure power output, in conjunction with a tachometer ( https://www.madmodder.net/index.php/topic,13897.0.html ) and now that I'm ready for more tests, it's time to tear down and clean the engine. Inspection shows a few issues:

1.) Flywheel grub screw is loose.

2.) A wear pattern of the outer rim of the main bearings is visible on the flywheel boss. This means the flywheel was contacting there, rather than bearing against the inner journal of the ball bearing. I need a shim washer to eliminate that source of friction.

3.) The steel wrist pin is lightly marking the power cylinder. I need to substitute a brass wrist pin, or shorten the pin and plug the ends with some graphite plugs.

4.) I suspected that the displacer pushrod was loose in the displacer. It screws into the far end, and was originally sealed there with a high temp adhesive. I probably broke that bond when adjusting the pushrod length awhile back.

To check for leaks, I lowered the displacer into a glass of hot water. This expands the air inside the displacer, and any leaks immediately show up as bubbles. Sure enough, they popped right out of the screwed end joint, so I have to fix that. Otherwise the internal volume of the displacer adds markedly to the dead space of the entire engine, and with the internal volume expanding and contracting a little out of phase, absorbs power that should be going to the power cylinder.

5.) The tiny amount of oil from lubricating the external parts has worked into the power and displacer cylinders, congealing, and adding friction. It was washed out with soap and water.

6.) The finish inside the displacer cylinder is slightly rough to the touch from boring. I didn't think much about it before because it's a non-contact surface, but I now imagine that it must produce air resistance at speed as it shuttles back and forth. So I think I'll hone all of the test displacer cylinders to a smoother (and similar) finish for testing. I don't think lapping is necessary, but maybe some day I'll try that as well to see if it makes a difference.

[nickle]

Running on wood pellets as originally imagined is a fantastic outcome. Well done. On the cylinder roughness... It may actually help in some circumstances. A turbulent boundary layer caused by roughness can actually reduce overall drag under certain conditions... Think dimples on a golf ball type thing... Smooth golf balls are higher drag and don't go as far. I've heard some engine tuning types actually prefer rougher ports when porting performance engines for the same reason. I can't recall how to set up the math's as it's been an awful long time since I learned about it. Before and after figures would be interesting and maybe a plan to roll back to rough surface finish if that works better. Either way, well done on getting it to run so well.

Nick

[vtsteam]

Thanks Nickle for your thoughts  . I did wonder if the roughness might contribute to heat exchange, which I think is the biggest concern of all in hot air engines, especially if increasing their size (which I intend to do). We'll see. And yes, a possible experiment is to try otherwise identical cylinders with different internal finishes.

BTW, one other variable, besides smoothness, is the increased side clearance created by honing the ridges down. Does that help or hurt? Well could be both, also. More clearance reduces air resistance, but decreases heat exchange.

The number of variables in hot air engine design is huge compared to IC or steam. That's because of the gas heat exchange requirements within the engine itself. Also why they are still so subject to experimentation..... and so therefore, interesting.

[vtsteam]

I think experiments with hot air engines often lose sight of the fact that they have an advantage over IC and steam in that they can use any form of heat (unlike IC engines) and don't require the danger, operational watchfulness, and expense of boilers (unlike steam engines). Most experimenters seem to have in mind an ultimate goal of fueled transportation, and use gas fossil fuels to experiment with in hot air engines. They search for efficiencies with engine configurations that favor these fuels.

Hot air engines can run on non-fossil biological solid fuels, including waste like sawdust, nut shells, rice husks, mulch hay, leaves, pine cones, branches, chips and waste wood, and they can run on solar heat, and waste heat from industrial processes.

Stationary uses where some fuel or heat source is inexpensive and available is a niche use that they seem eminently suitable for to me, rather than transportation, which is better suited to fluid fuels and electricity.

I don't believe hot air engines will ever be able to replace high power density fossil fueled IC engines. But as stationary engines they can utilize high availability local fuels that IC types absolutely can not. And so provide a low cost power source, and a different definition for the concept of fuel efficiency. That's been my interest all along.

[nickle]

They are fascinating things... I still have not built one despite them being the main reason for buying my first lathe 20 years ago! I have the beginnings of a fizgig on the shelf in my shed. The surface area for heat exchange thing is an interesting aspect of Stirling Engine design... I wonder if some sort of ribbed displacer cylinder would help... but then you get more drag from a larger surface area and the thermal mass of the cylinder can become an issue. As you say, there are so many variables. Aside from the ability to use low energy density fuel, I think one of the most interesting aspects of these engines is the ability to use waste heat. there are endless industrial and domestic processes that have heat disposal as a problem... including a Stirling cycle in that process allows some energy extraction to happen. It wont always be optimal temperature differentials between the hot and cold sides, but its waste heat so efficiency doesn't have to be the main driver. Have you got a copy of Edgar Westbury's "A Practical Treatise on Hot Air Engines"?

[vtsteam]

Hi Nickle!  I do have Westbury's treatise, also a lot of old Model Engineers, which I've collected over the years. Coincidentally I have just been reading yesterday about Westbury's last engine, a Robinson type, published a couple months before he passed away. And a then an editor's note, about a year later:, 

 

I find this not only a little sad, but also somewhat mysterious, and it makes me want to build that engine just to see for myself whether this is true, or not.

[nickle]

Isn't that design in the book? that would be a shame as i was eying that build off for a some day project.

[vtsteam]

I'm not convinced that it doesn't work as drawn. But if it doesn't, it would be instructive to make the changes to get it running. Part of the fun actually, and a place to learn things.

I will give one interesting side note (clue). Robinson engines used a perforated displacer with regenerative material inside. The Westbury design does not, and E.T.W. seems not to have realized this, and had drawn a sealed hollow conventional displacer.

Still, I believe it should work that way, as well.