Engines > Your Own Design
My new project ...Diesel engine .
Minh Thanh:
--- Quote from: PaulR on August 01, 2026, 09:08:45 pm ---I haven't had time to follow this thread so I'm not at all sure what I'm looking at... but I do know it looks fantastic!
--- End quote ---
Thank you !
Hi All !
I don't know if this helps, but this is how I machine the needle shaft.
The old live center wasn't very stable, so I made a new one.
Another method is that after machining a section, I open the clamp on the rotating center support and move it in - clamp it again, and then open the clamp on the chuck and pull the workpiece out a bit - clamp it again - and continue machining.
Hopefully this gives you some ideas.
Roger B:
Hi Minh,
Are you using TIG welding rods for your needles?
Minh Thanh:
--- Quote from: Roger B on August 14, 2026, 08:16:53 am ---Hi Minh,
Are you using TIG welding rods for your needles?
--- End quote ---
Hi Roger !
I've been a little busy, and I often have trouble accessing the forum, so I'm replying to you a bit slowly. Sorry !
Yes, I used stainless steel TIG welding rods to make the needles. (I usually use what is readily available around me.)
I want to know: "Why..." so I made a few needles with a diameter of 0.5 mm for testing, but they were mostly useless - there was a way to make it spray better, but it started to alter the texture of the whole size I wanted, so I went back to the familiar size and with few adjustments, the results were good.
http://www.youtube.com/watch?v=6ph2MciY5WE
Minh Thanh:
An update: As you saw in the video from the previous post, the fuel spray pattern was excellent. I fabricated five injectors based on those specifications, ensuring every component was as precise as possible; however, after assembly and adjustment, the results fell short of expectations. Upon disassembly and a thorough inspection under a magnifying glass, I found absolutely no differences. Reassembly and adjustment: no change.
After examining the injector in the video, I concluded that the fuel is sprayed through an extremely tiny gap.
So, I conducted a few experiments with the nozzle by making the outlet port as small as possible: I drilled a 0.5 mm hole (resulting in an actual diameter of about 0.55 mm) and inserted a short section of a 0.5 mm drill bit shank into it—effectively reducing the injector's orifice to 0.05 mm. However, this still didn't solve the issue regarding the 1 mm³ fuel volume (I had limited the pump stroke to yield an output of maximum 1.5 mm³).
To date, I have built over 20 injectors and adjusted each one more than 10 times—meaning over 200 adjustments in total—yet I’ve only had two successful outcomes.
...Blah, blah...
I tried some other crazy experiments to get the nozzle down to 0.0... mm.
I decided: I would not pursue the injector angle approach.
Nor would I try to make the injector as small as possible—unless I could manufacture one with a 0.1 mm diameter.
Based on previous designs and Try to adjust it so that it sprays as well as possible.
All engines I have designed and built, and all the components I have worked on, the diesel fuel injector is the one where I was forced to compromise—due to the maximum fuel volume of 1 mm³.
I hope this saves someone some time and effort. Of course, you can give it a try—but I certainly won't.
Roger B:
Hello Minh,
That is my experience as well, the fuel is atomised in the very small gap between the cone on the end of the needle and the body. This is the same for needle and poppet style injectors. I have tried plain holes down to 0.2mm but there was no atomisation with the fuel volumes we are using. Full size multi hole injectors have holes around 0.2mm but with much greater fuel volumes.
I previously tried to measure the needle lift with a 1 micron resolution dial gauge but got no sensible results.
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