Lobo Pup Twin 1.6 cc diesel

[GailInNM]

Well, it's not going to run today.

First off my ride to Thanksgiving dinner will be here shortly and I don't expect to be back until late.

But more importantly, I have something screwed up on timing. It's too early to tell for sure if it is design or I screwed up machining. But it is at least consistent in that it is the same on both cylinders. At bottom dead center, I have a small over lap where the intake, transfer and exhaust are all connected. So, no suction on the intake. Determined by blowing into the venturi while turning the engine over. I just make up a connector to replace the carb with a tube connected to it so I can play. I will put a disc on the crankshaft so I can measure what is opening where so I can try to deduce what is happening before I tear it down.

I will also go over all the timing calculations for the ports so see what I can find out there. If nothing shows up on the design, then it will be time to tear it down to see what is happening. That should keep me busy tomorrow, and then a day or two (optiminst) to make new parts to correct the error.

So, although I did not plan it that way, we will all get a lesson in trouble shooting a new design.

I still have faith that it will run soon.

Gail in NM

 

[arnoldb]

Thank you very much for the update Gail.

Sorry to see about the timing issue - but I guess that's all part of our hobby!

A silly question - if you don't mind. Was the solder you used on the barrel just thin "resin core" solder, like normally used for electronics, or something more specific? - I cannot see any additional flux applied to the parts in your photos, so I presume you used a form of flux core solder with very clean parts.

Once again, thank you very much for sharing this build

Kind regards, Arnold

 

[GailInNM]

Thanks Arnold.
First off, to answer your question. Yes, the solder is regular electronics grade solder with a flux core. The flux core is not necessary however. I used a small amount of regular non-corrosive paste flux and worked it into the joint. Then I wiped off all the excess so only a small film remained. That is why none shows in the photo. Since both parts were mechanically clean that is all that is necessary. If one does not have thin solder, you can clamp a length of thick solder in the mill vice to flatten it and then cut small strips from that.

Got in too late last evening to do anything on the engine, but got started a couple of hours ago. First off, the primary problem is that I can't follow my own drawings. I had the pistons and cylinders installed with the intake and transfer ports reversed (post #100) Not only did it reverse them on installation, I took photos and described how to do it! OK I said. It must have been late. No, the time of the post was 7:30 in the evening. So I guess I just had my head some where that it should not have been. That is the problem when you design something. You KNOW what it should be, so you don't check the drawings.

So I reassembled it (correctly) and it was more like it should have been. I had made up a fitting to replace the carb and had a length of tubing attached to it. I put the tube in a cup of water and turned the engine over by hand. Suction was there but was not very strong and it bleed off too quickly. More disassembly and reassembly wit some tests during and in between. Too much leakage between the two cylinder crankcase halves is the conclusion. Even though I had tested the bearings for leakage around the seals before I started the project, there is just too much leakage after they have been "exercised" a little bit. I think what happened is that I just tested with pressure on one side of the bearing, but in operation there is pressure and suction alternating each revolution and the seals do not get a chance to seat.

So, back to the drawing board for a little bit. I am replacing the ball bearings with sleeve bearings, which means making a new center bearing housing and a sleeve bearing. No big deal, but as long as I am doing that, I am going to change the crankshafts to make them easier to make. I could get by with out doing that, but I think that by doing so it will take less time overall as I can then ream the sleeve bearings with imperial reamers that I have and get rid of the metric dimensions that I was using to match the metric bearings. Nothing against metric. I just don't like to mix imperial and metric on the same project. I am comfortable with either, but most of my tools are imperial. Also, with the ball bearings gone, I have room to fit an o-ring seal on the center bearing retainer to further reduce leakage betweenthe two crankcase. I wanted to do so on the original design, but there was not enough room. This is probably not necessary, but the o-rings are only a few cents each plus the time to put two grooves in the housing.

Current status is that the drawings are about half way done with the updates. I should be ready to start on the replacement parts later today. They should only take a day or two to make.

It's all part of bringing a new design to life. That's part of the fun in it. Of course it would be nice to have things work perfect the first time, but.........

Gail in NM

 

[kustomkb]

Great looking engine Gail!

Im sure it will run great too.

Thanks for the great thread and all the best with her maiden voyage!

:bow: :bow:

 

[GailInNM]

DIY,
The ball bearing internal seals leaked too much under the alternating pressure/vacuum. There was no room to put additional seals in without making a new crankcase. The bronze bush will get rid of the sealing problem because of the long annular passage. They will have lots of lubrication with the alternating pressure/vacuum and the fuel will have 33 percent castor oil.

Also, with the increased room, I am putting o-rings in to the housing to seal against the crankcase wall.

At the 10,000 RPM max that this engine should run at, wear will not be a problem. The piston/cylinders will wear out long before the center bearing becomes a problem.

UPDATE: The new center bearing assembly is complete. Photos later. Still deciding what to do about front bearings.

Gail in NM

 

[Deanofid]

Glad to hear you're getting it sorted out, Gail!

The plain bronze bearings ought to last nearly forever with that fuel mix, for sure.
I've had glow engines that used only one plain bearing (aluminum!) that ran weekend
after weekend for years.

Thanks again for sharing an excellent build. Looking forward to it's maiden run!

Dean

 

[Twmaster]

Sweet. I'm glad to see you have that sorted out Gail.

I'm watching this thread as I fly RC planes and would love to someday build my own diesel.

 

[arnoldb]

Thank you very much for the answer on the soldering Gail

And it definitely sounds like you're well on your way to sort out the problems!

Kind regards, Arnold

 

[GailInNM]

The Lobo Pup Twin whimpers into life.
It's been cold here so operation on the test stand outside has been limited.

Last weekend I put the Pup on the test stand for the first time. No video yet as it was too cold to both set up the camera and play with the Pup. I had changed the center bearing out to a sleeve bearig, but left the front bearings as ball bearings. With the new center bearing I had good suction on the intake.

The engine started up fairly quickly and was running it rich and at low RPM for the first run. Compression setting was about where I anticipated it would be. But, as the run continued, I had to open up the needle valve and increase the compression some. As expected there was some black oil in both exhausts, indicating that some wearing in was happening. But the black exhaust continued and did not clear up. Soon the RPM started to fall off. After running , I brought the Pup inside for a tear down to check.

Compression was way down. Pulling a cylinder, the piston to cylinder fit was way too loose. So now I knew the source of the black exhaust grunge. A couple of measurements and test fitting of the piston to cylinder produced and unexpected result. BOTH the cylinder and piston were worn. Why???? I don't know why. I suspect, but don't know, that they are approaching similar hardness.

The cylinder was 12L14 and the piston was cast iron. The same as I had used on the three PMC IMPs a few month ago, and the same as I have used in the past. Same lapping technique followed by careful cleaning. The only differences I can think of is that I as using a different source of cast iron for the piston, and that I has colored the cylinder by heating to 550 degrees F. The heating should not have affected the structure of the 12L14. According to my numbers, it would not have been affected any until about 800 degrees. The cast iron machined the same as what I had been using as near as I could tell. So, it seems that for some still to be determined reason the piston and cylinder materials are not happy with each other. It acts like they are similar in hardness.

Plan of attack. To make it easy to experiment, I have fitted the front cylinder position with a dummy plug to replace the front cylinder and removed the piston and connecting rod. The dummy plug is necessary to block off the intake passage for the front cylinder. So now I have a single cylinder engine to play with. I plan to make up several piston/cylinder pairs. They can be switched out easily in just a few minutes. I will try some different material pairs for them to see what happens. The progression is planned as follows.

1. 12L14 cylinder that has not been heated. Cast iron piston.
2. 1144 cylinder. Cast iron piston.
3. 41L40 cylinder. Cast iron piston.
4. 4142 HT cylinder. Assorted pistons.

The new 12L14 cylinder is almost done. I have found some 1144 steel in the box. It is enough harder that if it is a hardness problem the difference should show up. I did not have and 41L40 or 4142 on hand, but it is ordered and should arrive tomorrow.

If nothing else, I should learn something. Not really looking forward to machining the 41L40 or 4142 steels. Nothing too difficult, but a lot slower than the 12L14 and 1144 steels. The 4142 is prehardened to about 300 Brinell. This is still machinable, but not by much for a duffer like me.

I will let you know how it goes.

Gail in NM

 

[GailInNM]

That's a possibility Diy. Thanks. I will add that to my list. I have enough cast iron on hand to do it so I might give it a try early on. Depends on when the shipment of steel arrives tomorrow. I will probably make us several sets of piston-cylinders to try as this cold spell looks like it will hang on for a few days. Then all I will have to do is switch them out when I get so weather warm enough for playing.
Gail in NM

 

[Maryak]

Gail,

Commiserations on the problem after your first run.

I can't remember what lapping process you used but your description of the massive wear over such a short period of time suggests to me that a residue of the lapping process was still present.

I hope this helps.

Best Regards
Bob

 

[Twmaster]

I hope this is not a question born of ignorance...

Why not aluminum pistons?

 

[mu38&Bg#]

Making lapped piston & liners work is about thermal expansion. The piston runs hotter than the sleeve so you need to have a slightly smaller coefficient of expansion in the sleeve material than the piston. Aluminum pistons work, but not with steel liners. Typically ringless aluminum (high silicon content) pistons are run in chromed or nickel plated brass or aluminum ( different than the piston itself) sleeves.

I thought cast iron and 12L14 was a common combination. Is it possible you ended up with material different than you ordered or thought you had?

 

[GailInNM]

Bob,
It sure acts like it, but they were all cleaned with my regular process. It is to first scrub with solvent, then with hot water and detergent. I use a stiff bristle test tube brushes for the cylinder, different one for each process. The only thing I can think of is that heating the cylinder reduced it hardness, although it should not have, enough that my brass lap embedded some lapping compound in the cylinder. But, this does not explain why the cylinder would also have worn. Only the piston should have worn then. But, because of the possibility, that is one reason why I am going to try a harder steel for the cylinder. But, it is obvious that it has something to do with my process or materials. This is the 7th IC engine in this size range that I have built, and the 5th with 12L14/cast iron. No great amount of running time on any of them, but certainly they were not worn out at 10 minutes.

Twmaster,
Aluminum has it's own set of problems. It is sometimes used with glow engines, but the normal practice there is to put a brass liner in the cylinder and chrome plate it. For compression ignition engines the coefficient of expansion of the piston and cylinder need to match fairly closely. Due to the high compression ratio, the match between the piston and cylinder needs to be very close. On larger engines where rings can be fitted this can be compensated for. For engines under 1/2 inch bore rings are a real pain. I don't know of any compression ignition engines in this size range where it has been done. I have seen rings down to 3/8 bore on glow engines, but there the compression ratio is only about 6 or 8 to 1. For compression ignition the normal operation point is about 18:1.

Dieselpilot.
12L14 and cast iron are what I have used before. The cast iron is a new stick, but from the same supplier. The 12L14 I know is the same as it is the same stick that I used on the last engine, but it was not heated on the previous engine. The heat was low enough that it should not have affected it by my charts, but it may have anyway. That is why I have another one to try that has not been heated. Have to make piston and contra piston to match it yet.

Making parts is the fun part, so I don't mind making a few different sets of piston & cylinder to try and learn something. It will run properly eventually and hopefully I will find out why I am having problems now.


Gail in NM

 

[Twmaster]

Thank you Gail. I appreciate the education.

 

[cobra428]

What He Said,

I was thinking of how to make an ABC piston for the Whittle.

Keep the faith

Tony

 

[mu38&Bg#]

Gail, I look forward to see what you find with the new parts. Maybe it was just the heat.

Tony, ABC pistons must have 22-30% silicon to work properly. The interference fit at TDC is like -.0001" which should change to about +.0001 when at operating temperature. Taper is important and I've not yet measured any engines to see what the taper actually is.

 

[cobra428]

Greg,
I have a few model airplane engines laying around here (in that hobby 50 years). I'll have to take a look see. It will be hard to measure
and even harder to make!

Tony

 

[zeeprogrammer]

Wow. What a great thread.
So much to learn.
I'm looking forward to see what you find out.

 

[GailInNM]

You faithful followers must have given up on me by now. It has been too cold to do much test running, at least for an old man like me. Over the last couple of weeks we have twice tied the record low temperatures gong back 70 years. The highs for the days have not been much better, mostly about 5 to 10 degrees F below my threshold of pain for playing with toy engines. Still nothing like the current snow storm you guys in the NE USA are current experiencing.

I have made several new cylinders. Tightened up the lapping tolerances a little bit. I don't know what the problem was with the original cylinders, but current ones made of 12L14 seem to be working fine. Although I cleaned the original ones quite well, it sure acted like there was still some lapping compound in them. Just as Bob said. Maybe it was just too late that night and I just thought I cleaned them well and forgot the final detergent/hot water scrub. :shrug:

Current problem is that the intake fuel draw is still low. I have to open up the needle valve about1 to 2 turns more than I should. And then the setting is more critical than it should be. I think that I am going to have to reduce the venturi ID a bit. I have checked the timing diagram and that looks OK, but I can increase it a little bit, but not much, if need be. Of course that means making new cylinders (again).

Total run time on the Pup is probably about 45 minutes total at this point.

Thanks to everyone here who voted the PUP as POM for December. It sure helps to keep the drive up to get it running right.

Anyway, here is a short video of the Pup running with only one cylinder installed. I am creeping up on things a little bit at a time.
Gail in NM

[ame]http://www.youtube.com/watch?v=ez6Si49tfls[/ame]