Model Diesel: 32mm bore, 38mm stroke, indirect injection

minh-thanh · Jul 24, 2024

My mistake, : near the hole
Lapping the cylinder is really good and you can try it with drill shank
If they can create good enough pressure and then you make a piston that fits the cylinder

Lloyd-ss · Jul 24, 2024

This might be a totally useless suggestion, but I have had some success with, instead of using a hardened steel barrel, using a brass barrel, reamed slightly undersize and then repeatedly forcing a correct sized dowel pin in and out of the brass to expand the hole and burnish the finish. It works best if the plunger is also made from one of the dowel pins from the same box of pins. The brass should have enough durability for a model engine, yes?

Nerd1000 · Jul 24, 2024

Lloyd-ss said:
This might be a totally useless suggestion, but I have had some success with, instead of using a hardened steel barrel, using a brass barrel, reamed slightly undersize and then repeatedly forcing a correct sized dowel pin in and out of the brass to expand the hole and burnish the finish. It works best if the plunger is also made from one of the dowel pins from the same box of pins. The brass should have enough durability for a model engine, yes?

Interesting idea. I think the main challenge with it will be the quality of the dowel pins, I know a lot of them are somewhat 'tri-lobed' due to flaws in the centerless grinding.

ruben · Oct 2, 2024

What is the bore and stroke of the fuel pump

Nerd1000 · Oct 2, 2024

ruben said:
What is the bore and stroke of the fuel pump

No pump now. Due to moving overseas I can no longer work on the engine, I did make a new cylinder head for running on petrol with a spark but I doubt I'll have time to fit a carb and ignition system before leaving, and I cannot take the project with me. So it will be shelved for 2 years until I return.

Steamchick · Oct 3, 2024

Bon Voyage Nerd,
You will be missed, unless you can continue to contribute with the brain you cannot leave at home.
Guten Tag!
K2

Nerd1000 · Oct 3, 2024

Steamchick said:
Bon Voyage Nerd,
You will be missed, unless you can continue to contribute with the brain you cannot leave at home.
Guten Tag!
K2

Hah, you won't get rid of me that easily.

Larry G. · Oct 3, 2024

If you can source or fabricate an accurately round pin for the piston, ball broaching, ballizing, or ballsizing will produce an accurately sized, accurately round, exquisitely finished cylinder. Forcing a hardened ball through the slightly undersized hole will do all those things.
Credible information is provided in the article linked here;

https://www.precisionballs.com/Ball_Sizing.php

Don't wait 'til you get a round tuit, do it NOW..

Best regards,
Larry

Nerd1000 · Oct 3, 2024

Larry G. said:
If you can source or fabricate an accurately round pin for the piston, ball broaching, ballizing, or ballsizing will produce an accurately sized, accurately round, exquisitely finished cylinder. Forcing a hardened ball through the slightly undersized hole will do all those things.
Credible information is provided in the article linked here;

https://www.precisionballs.com/Ball_Sizing.php

Don't wait 'til you get a round tuit, do it NOW..

Best regards,
Larry

Only problem is you need a press to do it!

By the way, this method works very well indeed on oilite bushes because their porous structure is naturally a little compressible.

Steamchick · Oct 4, 2024

Have you got a decent engineers' vice? - It's a good screw press for me!
K2

ruben · Nov 5, 2024

Roger B said:
This is one of the good questions, how far does the fuel spray travel in air at 30+ Bar as against in open air, will it reach the piston? I haven’t had the head off for a while but last time there was no obvious washing of the piston bowl, just signs of contact between the piston and the valves (which is why I took the head off to insert a 0.5mm shim gasket).

What is the difference between the diameter of the piston and the inner diameter of the cylinder?

Roger B · Nov 5, 2024

I don't actually know, I don't have sufficiently accurate measuring equipment. I turn a plug gauge to the planned diameter and use this to check the bore whilst lapping. The bore is lapped using an Acrolap, https://acrolaps.com/index.htm , and 40 micron diamond paste. The piston is then turned to size using a micrometer to transfer the diameter of the plug gauge. I am aiming for the closest possible running fit. The piston and cylinder are both cast iron to minimise the effect of thermal expansion. This fit is too close for an aluminium piston. I also use the plug gauge when setting up the piston ring fixture.

Steamchick · Nov 6, 2024

Sounds like the gap is "Greater than 40 micron" (grinding particle size), but less than 0.001" (smallest regular size that most modellers can measure)... at a crude guess! (which is ridiculous as 40 micron is 0.0016inch - to those that understand).
Away from such nonsense, is the Acrolap a carborundum stone holder? - or a simple "ground steel precision bar" (or whatever) holder for using with diamond lapping compound? The picture looks nice... Probably more precise than my home-made lapping bar with a carborundum stone slip.
Can you insert a human hair in the gap between piston and bore? - I doubt it.
Or the finest copper wire you can find as a single strand in and bit of scrap wire? - maybe screening off an old HiFi wire or something? My Dad uses the thinnest foil that was suck to tissue paper in the end of his cigarette packet back in the 60s, but he packed-in smoking in the 1970s, and packaging had changed by then anyway. That foil was only 0.0015in thick.
Whatever you can insert, can be measured with your micrometer, as far is its smallest reading.
Cheers,

K2

Roger B · Nov 6, 2024

The Acrolap is a split brass lap with a tapered expander which is designed to reduce the chance of bell mouthing the bore. I have used them successfully on a number of engines. I use the smaller needle laps for my fuel injection components.

At the end of the day getting sensible measurements in a amateur’s workshop is unlikely. Temperature control and something like a pneumatic bore gauge would be required. The piston is simply made to fit the bore as closely as possible.

https://www.marposs.com/eng/product/pneumatic-gauges-for-internal-diameters

For an aluminium piston in a cast iron bore I would try for a clearance of around 0.05 mm. My 25 cc petrol engine seized several times in the early days due to a lack of clearance.

I think that a hair would certainly not fit, one was enough to jam the piston of my SU style carburetor.

Nerd1000 · Nov 6, 2024

For an aluminium piston I would follow the rough rule of thumb of using about 1 thou clearance per inch of bore. Or about 0.025 mm in a 25 mm bore.

But I think Reuben might be asking about the injection pump piston/plunger. If that's the case, the gap needs to be truely miniscule. Maybe 0.001mm or so?

Steamchick · Nov 7, 2024

Roger B - You do beautiful work with your models. - Loved all the photos!
But "do tell" ... what are the things that look like High Voltage chambers for something?
And "Emergency Beam Off" sounds very technical? An electron beam perhaps?
K2

Roger B · Nov 7, 2024

Nerd1000 said:
For an aluminium piston I would follow the rough rule of thumb of using about 1 thou clearance per inch of bore. Or about 0.025 mm in a 25 mm bore.

But I think Reuben might be asking about the injection pump piston/plunger. If that's the case, the gap needs to be truely miniscule. Maybe 0.001mm or so?

I was not sure which he meant but as he quoted a section refering to the piston and combustion chamber I took it to mean that.

For the injection pump and plunger I like to think I have a clearance of a few microns however once again I have no means of measuring this.

Roger B · Nov 7, 2024

Steamchick said:
Roger B - You do beautiful work with your models. - Loved all the photos!
But "do tell" ... what are the things that look like High Voltage chambers for something?
And "Emergency Beam Off" sounds very technical? An electron beam perhaps?
K2

Thank you

There are two sets of none engine photos. One is a visit to the Diamond light source:

https://www.diamond.ac.uk/Home/About/How-Diamond-Works.html

The other is various industrial electron accelerators used for crosslinking polymers, which is my day job. I am responsible for several systems from 550kV to 3 MV. Going totally off topic I have attached an old paper on the subject.

Nerd1000 · Nov 7, 2024

Roger B said:
Thank you

There are two sets of none engine photos. One is a visit to the Diamond light source:

https://www.diamond.ac.uk/Home/About/How-Diamond-Works.html

The other is various industrial electron accelerators used for crosslinking polymers, which is my day job. I am responsible for several systems from 550kV to 3 MV. Going totally off topic I have attached an old paper on the subject.

That's very cool. Working as a structural biologist I've sent samples to the Australian Synchrontron but never had a chance to visit in person.

And those are some monster electron beams you work with! I've done a lot of electron microscopy, but the instruments I've used were 200 or 300 kV. 3MV electrons would be something else... I guess you are going for deep penetration into the material when crosslinking? Just to nerd out a bit, what do you use for the electron source? Instruments I worked with used tungsten or LaB6 hot cathodes but the fancy cryo-EM microscopes at my uni use cold FEG sources, makes the beam more focused.

Steamchick · Nov 8, 2024

Wow!
That is some technology.
In 1976 I worked for a cable maker, and we obtained the early samples of Cross-linked polyethylene insulated wires. It was my job to test them in the lab to all the tests normally used to confirm PVC, elastomer and polymer cables, so we (in the factory) could appreciate the advantages of this "new" material. All I knew then was a simple comment. "It is PE electrically, but should be almost as tough as an elastomer, unlike PVC insulation."
In one test - the 30 second flammability test - it certainly was not like other materials, as the PE naturally had NO resistance to flames. Just like applying a Bunsen flame to a wax candle, it melted and burned.
But compared to "normal" PE and PVC it was considerably more resistant to abrasion and cutting with a sharp blade. But melting it, as with in-factory welding of thermoplastic insulation, just made it into normal PE... and lost all the advantageous characteristics of cross-linked PE.
I don't remember much about cross-linked PE otherwise...
I never found out how PE was cross-linked either!
K2

Model Diesel: 32mm bore, 38mm stroke, indirect injection

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