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



## Nerd1000 (Aug 19, 2022)

Hi everyone,

After Minh-Than built his diesel, I got interested in making my own. I've since gone full Nerd and gone down a very deep rabbit hole reading about diesel combustion systems, in particular the work Sir Harry Ricardo (who should be a personal hero of any model engine builder) did on getting small, high speed diesels to run well in the 1920s and 1930s. I'd like to run my engine on biodiesel or regular diesel fuel, and (potentially) use it to actually drive a load so we definitely need a good combustion system to avoid poor fuel consumption, excessive smoke, etc.

My plan to achieve this is to use a swirl chamber. This demands a high compression ratio: my design is planned to be around 23:1. I'll probably also need to preheat the induction air for cold starting (I can't fit both a glow plug and injector into the 10mm diameter swirl chamber). Fueling is planned to be by a bosch style jerk pump, which will be lubricated by the fuel itself so that we can tolerate some small leakage past the plunger.

Here's the current progress designing the guts of the engine.







Due to a goof-up on a customer job, I now possess a very large number of 20mm ID manganese bronze bearings, so they will be the mains. You can see the piston (three compression rings and one oil control) and conrod (fairly conventional) along with the heavy duty crankshaft and a stand-in for the camshaft and lifters. In the background is the injection pump housing.

Most of the work I've done so far was on the cylinder head. The counterflow intake design should help keep heat in the charge for smooth combustion and encourage swirl in the cylinder to aid the second phase of combustion outside of the swirl chamber.





The swirl chamber is formed by a hemispherical pocket in the head and a matching stainless steel insert that forms the lower part of the chamber and includes a tangential port connecting it to the cylinder. The Injector is of the inwards opening pintle type:




The barbed fitting on the top is a spill-over drain for fuel that escapes past the needle into the upper part of the injector. The needle seals to the bore via a delrin piston ring, and as rings have a gap there will always be some leakage.

I hope everyone's interested in the start of this big project! Much more design work remains to be done, along with prototyping fuel system components etc before starting the engine build itself. Wish me luck.

-Nerd


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## minh-thanh (Aug 19, 2022)

Nerd !!
A little bit of my experience
1 - When I tested my injector, it was the same as you designed it, it needed a greater amount of oil and pressure to compensate for the loss of oil and pressure through the injector body - this could also be due to I don't make injectors good enough - so consider this when you design your pump and stroke
    2 - Make sure that the force of the needle spring is greater than the pressure in the cylinder, so you must measure the maximum pressure in the cylinder and the force required to open the injector (I try and fail , so it took longer)
  Good luck !!!


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## Nerd1000 (Aug 19, 2022)

My pump has a 4mm bore and 2mm effective stroke, so it should be able to supply a lot more fuel than we need to inject at full power. Hopefully this is enough to compensate for any leakage.

I've run some simulations in Lotus Engine Sim software to estimate the cylinder pressure. It should peak at around 100 bar during the power stroke. Rather high, but the nozzle is only 1mm in diameter so the force should not be too high. The injector spring is sized to have the needle lift at 100 bar on the 4mm needle diameter. Of course I'll need to do testing to make sure my maths isn't wrong haha.


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## peterl95124 (Aug 20, 2022)

Nerd,
         its pretty easy to compute how much fuel it takes to combust all the air in the cylinder, and its a really small number, way smaller than 4mm diameter x 2mm stroke pump per cycle. Will you have a way to vary the pump.  I've read about the Bosch type and its got an adjustable overflow port, kind of like a "sleeve valve" in some obsolete engines, which seems difficult to model.

may I ask out of total ignorance, why do you think you need a swirl chamber. I'm building a "Hansen" that doesn't have one.

yes, there is interest in what you're doing, keep us up to date !!!
Peter.


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## Mechanicboy (Aug 20, 2022)

Nerd1000 said:


> My pump has a 4mm bore and 2mm effective stroke, so it should be able to supply a lot more fuel than we need to inject at full power. Hopefully this is enough to compensate for any leakage.
> 
> I've run some simulations in Lotus Engine Sim software to estimate the cylinder pressure. It should peak at around 100 bar during the power stroke. Rather high, but the nozzle is only 1mm in diameter so the force should not be too high. The injector spring is sized to have the needle lift at 100 bar on the 4mm needle diameter. Of course I'll need to do testing to make sure my maths isn't wrong haha.



Nerd1000...

Fuel pump with big bore and small stroke is much difficult to regulate the amount of fuel by regulator and operator.  Better to have small bore and the stroke is regulated by regulator depending on revolution and load. Find Hansen diesel engine has 2 mm bore and the stroke is regulated.

1 mm hole nozzle?? The hole in the injector is much smaller than 1 mm in the 2 litre direct injection diesel engine, also the hole is from 0,2 to 0,02 mm +- 0,005 mm depending on the number of holes per nozzle. More number of holes per nozzle = smaller holes. In the model engine is not practical to drill smaller than 0,02 mm hole. The practice of using electro-discharge machining (EDM) drilling of fuel injection nozzles is limited in terms of the hole size it can produce effectively and the length of time needed to drill. Fuel injector nozzles drilling with laser-based machines is attractive alternative to elder methods.

Better to use "Burmeister and Wain" injector with valve outside the injector as you find in the Find Hansen diesel engine. It's much easier to make injector than drilling such small holes in the nozzle.

 Unnecessary to have swirl camber in model diesel engines as compression is high enough to ignite the fuel. With a swirl chamber, you may have trouble getting high enough heat of compression to ignite the fuel in a small engine. With swirl camber you need glowplug to ignite the fuel under starting the engine when the engine is cold.


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## ajoeiam (Aug 21, 2022)

Mechanicboy said:


> Nerd1000...
> 
> Fuel pump with big bore and small stroke is much difficult to regulate the amount of fuel by regulator and operator.  Better to have small bore and the stroke is regulated by regulator depending on revolution and load. Find Hansen diesel engine has 2 mm bore and the stroke is regulated.
> 
> ...


Very interesting information!!!!!!!!!!!!!!

Thank you (even though I'm just following the thread!!).


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## Mechanicboy (Aug 21, 2022)

Here is how the nozzle is drilled with laser..


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## Nerd1000 (Aug 21, 2022)

Mechanicboy said:


> Nerd1000...
> 
> Fuel pump with big bore and small stroke is much difficult to regulate the amount of fuel by regulator and operator.  Better to have small bore and the stroke is regulated by regulator depending on revolution and load. Find Hansen diesel engine has 2 mm bore and the stroke is regulated.
> 
> ...


I should have been more clear, the injector is a pintle type with a centerbody in the hole and a tapered valve seat, so while the hole is 1mm the actual area for fuel to flow through is much less.

I'm not planning on regulating the plunger stroke, it will always be 3mm (the first mm is used covering the inlet port, so 2mm effective). Instead I want to rotate the plunger in the barrel, combined with an angled cut or 'helix' in the end of the plunger this should cause the inlet port to be uncovered again at a controllable point partway through the injection stroke, spilling the excess fuel back to the supply. This is how Bosch style jerk pumps work on full sized diesels. Of course if I can't make it work on this scale I'll have to switch to something else.


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## Nerd1000 (Aug 21, 2022)

peterl95124 said:


> Nerd,
> its pretty easy to compute how much fuel it takes to combust all the air in the cylinder, and its a really small number, way smaller than 4mm diameter x 2mm stroke pump per cycle. Will you have a way to vary the pump.  I've read about the Bosch type and its got an adjustable overflow port, kind of like a "sleeve valve" in some obsolete engines, which seems difficult to model.
> 
> may I ask out of total ignorance, why do you think you need a swirl chamber. I'm building a "Hansen" that doesn't have one.
> ...


Yes for my engine it is something like 2 microliters, far less than the pump should theoretically provide.

However, I have low expectations for the clearances in my pump, injector, etc. I expect them to leak, a lot! So I will need extra to compensate. This is also why I was planning on using the fuel to lubricate the pump.


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## Mechanicboy (Aug 21, 2022)

Pintle injector is made for rocket engine,.. never in the diesel engine. After the fuel is injected, never fuel is dripping out of injector. The injector must be closed when the fuel is not injected into the cylinder.


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## Nerd1000 (Aug 21, 2022)

Mechanicboy said:


> Pintle injector is made for rocket engine,.. never in the diesel engine. After the fuel is injected, never fuel is dripping out of injector. The injector must be closed when the fuel is not injected into the cylinder.


The Wiki page is incorrect. Pintle nozzles are common in IDI engines. Just google pintle type diesel injector and look around.


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## Mechanicboy (Aug 21, 2022)

Nerd1000 said:


> The Wiki page is incorrect. Pintle nozzles are common in IDI engines. Just google pintle type diesel injector and look around.


Now, I remember, it's a very old construction to spread the fuel around inside the swirl camber. Allready gone out of production since the common rail diesel engine is most used in all diesel cars in Europe since 1995 as standard. (I'm car mechanic in 30 years work.) The other name on pintle injector is Pintaux injector with a extra hole in the other side of the hole.

The older engine has still pintle nozzles, but spare parts is difficult to buy today.


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## minh-thanh (Aug 21, 2022)

Nerd !! 
1mm size worked with my engine so it should be fine with yours .


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## Mechanicboy (Aug 21, 2022)

minh-thanh said:


> Nerd !!
> 1mm size worked with my engine so it should be fine with yours .


This is your injector with outside valve, true..?


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## minh-thanh (Aug 21, 2022)

I tried a lot, these are just the parts I keep, almost like Ithat  put them in the trash





*  Remember one thing: I never say what I haven't done or tried,.
And I have never heard other people speak and repeat or rely on other people's experiences*
*Remember that*


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## minh-thanh (Aug 21, 2022)




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## Lloyd-ss (Sep 7, 2022)

minh-thanh said:


> I tried a lot, these are just the parts I keep, almost like Ithat  put them in the trash
> View attachment 139450
> 
> 
> ...



Minh-Thanh,
My golly, I have so many collections just like that. Sad but true. It is soooo hard to throw that stuff out. Lots of failures before success. But something new is learned each time. Good work.
Lloyd


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## minh-thanh (Sep 8, 2022)

Hi Lloyd !


Lloyd-ss said:


> Minh-Thanh,
> Sad but true.
> Lloyd


 
There's nothing to be sad about
     You and I make engines with what we have in hand, with machines, tools ... unprofessional (or rather too bad) compared to diesel companies.


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## Nerd1000 (Sep 18, 2022)

so today I tried making a test-version of my injection pump barrel and plunger. The plunger helix came out well, as did the barrel with initial lapping. Sadly, I rushed ahead too far and began final lapping of the two parts together before getting the size close enough- the consequence was galling and seizure of the plunger in the barrel, ruining both. Lesson learned, and next time I may try hardening the barrel to minimise the potential for galling.


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## Lloyd-ss (Sep 18, 2022)

Nerd1000 said:


> so today I tried making a test-version of my injection pump barrel and plunger. The plunger helix came out well, as did the barrel with initial lapping. Sadly, I rushed ahead too far and began final lapping of the two parts together before getting the size close enough- the consequence was galling and seizure of the plunger in the barrel, ruining both. Lesson learned, and next time I may try hardening the barrel to minimise the potential for seizing.


Bummer. Its all a learning experience and sometimes the journey is the real prize of the project. How big is your reject box? Each item in there will have its own story.
Keep at it!


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## minh-thanh (Sep 18, 2022)

Nerd1000 said:


> so today I tried making a test-version of my injection pump barrel and plunger. The plunger helix came out well, as did the barrel with initial lapping. Sadly, I rushed ahead too far and began final lapping of the two parts together before getting the size close enough- the consequence was galling and seizure of the plunger in the barrel, ruining both. Lesson learned, and next time I may try hardening the barrel to minimise the potential for galling.


That's why I said in my thread about the doubt: "lapping a plunger  and cylinder  with tight tolerances"
   With small  diameter cylinders and plunger  : I Never lapping them together with tight tolerances.
  One mistake and it will destroy your hours of work.
  Before you put the plunger in the cylinder: clean them with kerosene - so clean , and then put some w20/40 oil in both before you put the plunger in the cylinder.
  Your cylinder is 4mm in diameter, so use the shank of a new  4mm drill as the standard gauge. lapping the cylinder to 4mm and then making the piston (on the shank of the drill bit there is usually a  a parameter engraving- sharpen it)
 Make the plunger larger than 4mm (maybe 4.1mm or 4.05mm and then lapping the plunger until it slides into the cylinder only with w20/40 oil)
 Or you can also use a  cheap drill shank to make a plunger


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## peterl95124 (Sep 18, 2022)

I've often wondered what is the best way to do this, because the way I see it, the size of the abrasive will determine the gap between the piston and the cylinder, so unless the abrasive is micron size the gap will be too large for a diesel injection pump, and if it is micron size it won't do any useful abrading.

my best guess is that you have to lap the piston with an adjustable dummy cylinder (an external lap), and you have to lap the cylinder with an adjustable dummy piston (an internal lap), and keep monitoring the sizes until they "just" fit. and since this is a laboriously manual and skill intensive job be prepared to have lots of throw-aways, or a few workable pairs from a large set of candidate parts.

IIRC, Cox Thimble Drone type model airplane engines' pistons and cylinders are interchangeable and accurately made to within a few millionths of an inch, they never lap a piston with a cylinder, rather they do it as described above with internal and external laps.

and the reason I'm following this thread is because I'm in the process of making a "Find Hansen Diesel" engine myself. I'm working on the big parts currently, waiting on some helical gears from hpcgears.com, and still thinking about how to make the pump and injector.

HTH, YMMV, etc...


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## Lloyd-ss (Sep 18, 2022)

For the fitting (just a generic fitting, not necessarily lapping) of the plunger, Minh-Thanh has been succesful, as have a few others.  I have worked at it, but so far, have been unsuccessful, although each attempt is a little better. So take what I say with a grain of salt.

I tend to agree with Peter that lapping the two parts together doesn't work easily.

I question the need for hardened parts. This doesn't have to work for half a million miles.

My closest attempt was using a brass barrel and a hardened dowel pin as the plunger. I reamed the brass out to slightly undersize, then repeatedly drove a hardened dowel pin, coated in oil, back and forth thru the brass. i thought it would eventually fit properly, but I think the walls were too thick and the pin never would slid thru. Thinner walled brass might work.
I tried lapping just the dowel pin spun in a lathe, using a very fine diamond lap lap. This actually worked quite well but because I had to remove maybe .0003" from the dia, it started to get a bit tapered. This method has potential.

On find Hansen's website (not in the videos) he describes a method of filling the pump body with molten solder and then inserting the dowel pin into the solder such that the solder solidifies around the pin without the tiniest of clearance.  I am sure there are some secret tricks to making that work.

I am going to re-try the brass barrel and hard dowel pin with a lap again. I think the thinner wall might do the trick.

Here the laps are. The business end is 3/4 x 2-1/2.


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## ajoeiam (Sep 18, 2022)

minh-thanh said:


> I tried a lot, these are just the parts I keep, almost like Ithat  put them in the trash
> 
> 
> *  Remember one thing: I never say what I haven't done or tried,.
> ...


Respectfully - - - I would disagree. 
I have reached a stage in life where I have realized that I just do not have enough time left to make all the mistakes to teach myself everything that I might want or need to know. 
That's one of the reasons that I'm on this forum - - - - I can share my experiences (some very hard one!!!) and then I get to learn from what doesn't work for someone else - - - sorta like I'm peering over his (or her as the case may be) shoulder - - - if that saves me effort (and especially time) - - - I am all for it!


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## Nerd1000 (Sep 18, 2022)

Lloyd-ss said:


> Bummer. Its all a learning experience and sometimes the journey is the real prize of the project. How big is your reject box? Each item in there will have its own story.
> Keep at it!


Haha, I have a 'shelf of shame' full of assorted parts, mostly created by my own absent-mindedness.

I was contemplating hardening one part not for wear resistance but because hardened steel is much less prone to galling. That being said, as Minh-Thanh says the 'lapping together' approach may not be the best option. It's worth considering other ideas. I've also put a little thought into a design that would need no precision fitting of the plunger at all (instead the seal would be provided by a stuffing box) but haven't drawn it up yet.


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## peterl95124 (Sep 18, 2022)

Lloyd-ss said:


> I tend to agree with Peter that lapping the two parts together doesn't work easily.
> 
> I question the need for hardened parts. This doesn't have to work for half a million miles.
> 
> ...


the reason for using hardened steel is that it is easier to lap just as hardened steel is easier to grind than mild steel on a grinding wheel, and oversize abrasive grains will get embedded in the lap rather than gouge out the steel so you'll get a better finish.

the purpose of the lead solder in Finn's method is to create a lap made of soft metal so that abrasive grains embed in it rather than the metal you're trying to polish so you can get a finer surface finish.  But I'd recommend looking at commercially made laps for very small diameters holes, they look like a rod with a section near one end where the rod has been split lengthwise and widened a bit, its that widened part that does the lapping.  I'd get (or make) some of them before I tried lead solder cast into the cylinder and onto a rod - in that case there's no place for the abrasive to go,  compared with the split-rod type where there's lots of space since the lap doesn't contact the cylinder all the way around, so this style can't jam and can't cause galling.

the secret to getting a piston/plunger that isn't tapered is to constantly measure with a "tenth" reading micrometer at the ends and in the middle, they should all be identical, if not apply more pressure on the part that's oversize compared to the rest. also make sure you're always moving the lap around in a circular motion while the piston/plunger is rotating in the lathe to get a perfectly flat/level result rather than one that follows any irregularity in the lap itself.

HTH, YMMV, etc...


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## Nerd1000 (Sep 18, 2022)

To be clear, I did the majority of my lapping with an aluminium split-lap. I think the main issue is that I tried to final lap with the two finished parts, rather than fitting the barrel and plunger by lapping them separately.

Minh-Thanh, a question: what grit did you use for lapping? I tried initially with 600 grit silicon carbide powder, then for the final pass I changed to 1500 grit aluminium oxide. Lapping was very slow and I wonder if I could use a coarser grit for roughing.


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## minh-thanh (Sep 19, 2022)

ajoeiam said:


> Respectfully - - - I would disagree.
> I have reached a stage in life where I have realized that I just do not have enough time left to make all the mistakes to teach myself everything that I might want or need to know.
> That's one of the reasons that I'm on this forum - - - - I can share my experiences (some very hard one!!!) and then I get to learn from what doesn't work for someone else - - - sorta like I'm peering over his (or her as the case may be) shoulder - - - if that saves me effort (and especially time) - - - I am all for it!


Because I'm so tired of arguments and informations like this :
  " lapping plunger and cylinders with tight tolerance  " , yes , it fits large diameter cylinders and pistons , but not for  2.3 , 2.4 .... to 4 mm diameter cylinders . Advice that can destroy many hours of work and a lot of effort
  ......
   I'm trying to put information that's fine with my engine
    And when I said 1mm diameter is fine and he tried to find a picture and video of me - That's just a small part of my many tests - several dozen times?? I do not remember , instead of asking me: "do you have a picture of your injectors?" or what similar sentence.


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## minh-thanh (Sep 19, 2022)

Lloyd-ss said:


> This actually worked quite well but because I had to remove maybe .0003" from the dia,* it started to get a bit tapered*. This method has potential.


I can tell you : " my cylinder is 100% straight ., no taper "
  But that is the info: Impossible for me.


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## minh-thanh (Sep 19, 2022)

Nerd1000 said:


> Minh-Thanh, a question: what grit did you use for lapping? I tried initially with 600 grit silicon carbide powder, then for the final pass I changed to 1500 grit aluminium oxide. Lapping was very slow and I wonder if I could use a coarser grit for roughing.


 Silicon carbide powder .
I have 2 options: 400 or 600 grit to start with.
    Because in my opinion the first step is just to remove scratches caused by drilling or reaming, if I want to be fast, I use 400, 600 is also ok but longer.
    If I use 400, then 800 and 1800
If I use 600 then just 1800 is enough


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## Nerd1000 (Sep 19, 2022)

minh-thanh said:


> Silicon carbide powder .
> I have 2 options: 400 or 600 grit to start with.
> Because in my opinion the first step is just to remove scratches caused by drilling or reaming, if I want to be fast, I use 400, 600 is also ok but longer.
> If I use 400, then 800 and 1800
> If I use 600 then just 1800 is enough


Ok, so my choice of abrasives was in the ballpark. Just need to refine my technique


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## Lloyd-ss (Sep 19, 2022)

Nerd,
I am enjoying following your progress. Your candid observations are refreshing.
At the beginning of the thread you mentioned using Delrin for some seals and gaskets. Watch out for high temps (over 180F) with Delrin. PEEK is more expensive but MUCH tougher and is good for 480F.

I have made lots of small parts with both Delrin and PEEK. Both are excellent, but I always use the PEEK when I need the strength or heat resistance.

Also, please consider using copper for seal-washers and gaskets, and crush-rings. I have several sizes  of soft copper tubing on hand and with a little practice, they can be cut nicely with a good tubing cutter, with only minimal clean-up needed. If you want them super annealed, just heat to red hot and let air cool or quench in water. If the heat-tarnish is a problem, just dip in a little muriatic acid (brick acid) for several seconds and they will brighten right up. I really like functionality of the diy copper seal-washers.
Lloyd


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## peterl95124 (Sep 19, 2022)

Nerd1000 said:


> To be clear, I did the majority of my lapping with an aluminium split-lap. I think the main issue is that I tried to final lap with the two finished parts, rather than fitting the barrel and plunger by lapping them separately.


and also to be clear, I'm being wordy because I too will be embarking on this challenge when I finish the frame and cylinder for my Find Hansen Diesel, and the more I learn from your endeavors the less trial-and-error I'll have to go through, so
many thanks for sharing.


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## Lloyd-ss (Sep 19, 2022)

Here is a barrel lap from the junk box. The lap is drilled and tapped all the way thru. The gashes were cut thru with a Dremel thin cut-off wheel. The screw in the picture threads in to align and expand. The drive rod threads in from the other end. I have tried many ways to expand the lap, including a rubber plug in the middle that gets compresses by the two screws. Many methods work.
Nerd, you are correct about "technique."  Anyone can get it to "work." But getting it to work WELL requires technique, or what I like to call, knowing the "tricks".


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## Lloyd-ss (Sep 20, 2022)

I have a retired diesel fuel injection mechanic kind of mentoring me, and he is a walking encyclopedia of hands-on knowledge. The method that he uses, is to do the barrel first and the plunger last. He uses a piece of wood with a V-notch in and loads it with compound and then spins the plunger in the lathe and monitors it closely. That is his "trick."


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## Steamchick (Sep 20, 2022)

For fine abrasives - much finer than some of the grades mentioned here, try jewellers' polishing pastes?








						Diamond Polishing Paste 5g 1 Micron
					

Ideal for high precision polishing operations 1 micron  14000 Grit Final stages of polishing and cleaning Perfect for glass stone cer.. - Product Code : 998 3002



					www.cooksongold.com
				



 - Or the polishing solutions used when making sections for microscopic study? - You really need to try to achieve a mirror finish and clearances smaller than 1000 grade grit for diesel injectors to work. 
Lapping your parts together will make the gaps bigger than the grit size. 
In industry, working to such fine tolerances, (as happens on millions of engines made every week!) the way is by making a lot of parts and selecting the best fit from the distribution of manufacturing sizes. 
So make 1 of the complex parts, and 5 times more of the "simple" parts - then select "best fits". Measuring by using the complex part as the gauge is the only way. You need to measure just a micron or so variation between parts - greater accuracy than "tenths of a thou". (0.0001" = 2.54mu). A single tenth of a thou variation is the difference between good and bad.
Does this help?


			https://www.buehler.com/uk/blog/grinding-and-polishing-guide/
		

K2


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## Steamchick (Sep 20, 2022)

I would not use WD40 as any form of lubricant. It is a water dispersant. But use watch maker's lube oil, or sewing machine oil. - Even the very light olive oil you would use for salad dressing is good, light (small molecules!). But the BEST is simply diesel fuel (the most obvious and best for this application) ... as the final job will run in diesel fuel anyway! - DERV contains special lubricant additives to hugely increase the LUBRICITY and AVOID SCUFFING... and detergents to aid washing the grit away from surfaces. (Central heating oil is specifically lacking these detergents and lubricity additives! - so don't use non-"road-use" oils).
I have been using DERV for washing dirty engine parts etc. for 50 years or more.... much healthier and safer than petrol! - and leaves parts with a very light oiling on the surface that resists overnight corrosion from condensation when the workshop cools... (Paraffin and petrol can both permit overnight condensation corrosion!).
Hope that is useful?
K2


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## Nerd1000 (Sep 20, 2022)

Lloyd-ss said:


> Nerd,
> I am enjoying following your progress. Your candid observations are refreshing.
> At the beginning of the thread you mentioned using Delrin for some seals and gaskets. Watch out for high temps (over 180F) with Delrin. PEEK is more expensive but MUCH tougher and is good for 480F.
> 
> ...


I'd use PEEK if I could afford it! it's extraordinarily pricey stuff. I think if I need more heat resistance than delrin can provide I might try glass-fiber filled PTFE.

I'm planning to use a copper crush washer as the injector tip seal, IMO there's few alternatives for handling the heat and pressure. Same for the head gasket. As for the HP fuel line to the injector, this will be copper 'capillary' tube sold for use in refrigerators, I have some with a 1mm ID and 3mm OD. It can be brazed to the injector body at one end, but the other will need a fitting for assembly, was thinking of making miniature versions of 37 degree flare fittings for this.


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## minh-thanh (Sep 20, 2022)

Lloyd-ss said:


> I have a retired diesel fuel injection mechanic kind of mentoring me, and he is a walking encyclopedia of hands-on knowledge. The method that he uses, is to do the barrel first and the plunger last. He uses a piece of wood with a V-notch in and loads it with compound and then spins the plunger in the lathe and monitors it closely. That is his "trick."


That's exactly what I did
   Make the cylinder first, make the plunger later
  But I use aluminum 50. and drill holes


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## minh-thanh (Sep 20, 2022)

Hi Peter !


peterl95124 said:


> my best guess is that you have to lap the piston with an adjustable dummy cylinder (an external lap),* and you have to lap the cylinder with an adjustable dummy piston (an internal lap)*, and keep monitoring the sizes until they "just" fit. and since this is a laboriously manual and skill intensive job be prepared to have lots of throw-aways, or a few workable pairs from a large set of candidate parts.


I've been thinking a lot about this method - it's the best method, but with the small diameter it seems impossible - it's like an impossible task, and I'm also trying to find one  solution to do it but it seems 1% is successful


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## petertha (Sep 20, 2022)

I don't have a lot of lapping experience but I found the ones offered by Acro hard to beat. Some of my home brew inventions that were influenced by others I've seen in model engineering & gunsmith articles. They worked & probably viable for custom sizes. But for more routine sizes, it was pretty hard to beat  Acro considering cost & effort. One thing I'll mention, the smaller ones have proportionately smaller slits & (IMO) if they are not cut somewhat precisely & consistently (meaning radial spacing) then they don't barrel expand quite optimally. It tok me a while to figure this out. 

FWIW I did my radial engine CI liners which were about 25mm nominal bore but, the target was within 0.0001" (tenth) down the length because I was sizing to existing rings. Slow, laborious process but I think ultimately achieved. I've used smaller sized Acrolaps for other parts too, they work just as well. And you can buy the brass barrels (lap) separately which is another consideration when switching grit medium on the same hole or a series of parts.



			Acro laps Barrel  Laps and Blind Hole Laps


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## Lloyd-ss (Sep 20, 2022)

This might be old news to many of you, but I like to use Craytex sticks for final polishing. The center punch and aluminum rod were both polished on a lathe for about 30 seconds. The stuff is great for final polishing. It removes an unmeasurable amount of metal, but will make the part shine.
Abrasive is already embedded in rubber. I have been using the stick in the pic for over 5 years.


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## Lloyd-ss (Sep 20, 2022)

Sorry, forgot the pic.


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## Steamchick (Oct 2, 2022)

Lloyd-ss said:


> Sorry, forgot the pic.


Hi, Never heard of Craytex before... sounds interesting?
But a single 1/2" sq stick costs £18.50 + £12.50 postage... - From USA - Blimey! Postage costs...!
I think I'll stick to my "old fashioned" methods for now.
K2


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## petertha (Oct 2, 2022)

For a lower cost solution, I use these rubber abrasives in my Dremel all the time. They are very inexpensive & come in a range of grits. I just hold the disc wheel, without the arbor of course. Or use the cylinder/point style, but use one of those jeweler collet handles normally used for needle files.


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## Nerd1000 (Oct 3, 2022)

Ok, it took me a few tries but I think I can declare success in the lapping of a plunger and barrel!




Here's the ID and OD laps I used.



And finally, the plunger and barrel assembled with the delivery valve for my yet to be built injector tester. The black rings are delrin gaskets and the conical nose is intended to connect to a HP fuel line via a flare and nut.





The plunger seals well enough that it springs back into the bore under air pressure if withdrawn while the spill port is blocked, and I cannot move it beyond the spill port when the barrel is filled with oil. Further testing will be needed to be sure it works at higher pressures.

A few thoughts on the process: 
It's a good idea to leave yourself a healthy amount of stock to lap away, 0.02-0.03mm. This gives you room to clean up toolmarks and taper in the part. The lap will tend to climb up the part (due to spiral toolmarks) during roughing and you need to be prepared for this as it can easily become stuck if it climbs a taper.

 Abrasives wise, I used 600 grit SiC for the initial phase, followed by 1500 grit alumina for finishing. In both cases I used WD40 as lubricant.


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## Steamchick (Oct 3, 2022)

As WD40 40 is a water dispersant not a particularly good lubricant it may have been a good choice? The paraffin content would have helped clear debris and swarf from the job, as it works by lowering surface tension - I think? -  to clean surfaces.
Maybe for the wrong reason you did a good thing?
It reminds me by feel between finger and thumb, of the Delapena honing fluid I used in the workshop...  Dries the skin, because it lacks oils.
WD40 contains a lot of white spirit and clever additives. But is NOT a lubricant. They do not specify lubricity on their data sheet.
Any experts add to that?
K2


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## minh-thanh (Oct 4, 2022)

Nerd1000 !​That is good news .
  Try to do the best you can and the engine will run!
  Tool  lapping for nozzle - that's how I do it, you can do it however you want as long as the injector is sealed









Your browser is not able to display this video.


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## petertha (Oct 4, 2022)

My own experience is that WD-40 is good at removing / washing away lapping compound from parts/laps, at least when using the commercial oil/grease based compounds. Some commercial paint thinners work & others don't so I wont volunteer trade names. I think a machine oil might be a better choice to lubricate the lapping process.


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## Steamchick (Oct 4, 2022)

Be simple, get it right, use the professional stuff. £10 for a bottle of proprietary lapping fluid. And no bodging or risking destroying good work.
You want your high precision fits to be just that.
K2


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## Nerd1000 (Oct 5, 2022)

petertha said:


> My own experience is that WD-40 is good at removing / washing away lapping compound from parts/laps, at least when using the commercial oil/grease based compounds. Some commercial paint thinners work & others don't so I wont volunteer trade names. I think a machine oil might be a better choice to lubricate the lapping process.


I found ISO 68 hydraulic oil to be too thick for what I was doing. Kerosene or diesel fuel would probably be good options.


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## theo van der linden (Oct 24, 2022)

minh-thanh said:


> Nerd !!
> A little bit of my experience
> 1 - When I tested my injector, it was the same as you designed it, it needed a greater amount of oil and pressure to compensate for the loss of oil and pressure through the injector body - this could also be due to I don't make injectors good enough - so consider this when you design your pump and stroke
> 2 - Make sure that the force of the needle spring is greater than the pressure in the cylinder, so you must measure the maximum pressure in the cylinder and the force required to open the injector (I try and fail , so it took longer)
> Good luck !!!


Hi 1!

very nice work !!!!
is it possible to buy the drawings form you??
i am from holland were the Kromhout engines were build


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## minh-thanh (Oct 24, 2022)

theo van der linden said:


> Hi 1!
> 
> very nice work !!!!
> is it possible to buy the drawings form you??
> i am from holland were the Kromhout engines were build


 Hi !

Ask in my thread.
   Regarding the Kromhout engine plan, which is only a provisional plan, there are many incomplete parts such as the engine camshaft and the fuel pump camshaft. I change a lot when machining ( I often ask the question : why not make this better , easier .... and change it while machining )
   If you want a plan, give me time. when i finish the engine i will redraw and edit the plan
 PS : 
  I am a very lazy person to draw
 ( About my 1-cylinder diesel plan, if I don't redraw and edit it, is like selfishness - selling the plan gives me the incentive to redraw and edit the plan  )


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## peterl95124 (Oct 24, 2022)

Steamchick said:


> As WD40 40 is a water dispersant not a particularly good lubricant it may have been a good choice? The paraffin content would have helped clear debris and swarf from the job, as it works by lowering surface tension - I think? -  to clean surfaces.
> Maybe for the wrong reason you did a good thing?
> It reminds me by feel between finger and thumb, of the Delapena honing fluid I used in the workshop...  Dries the skin, because it lacks oils.
> WD40 contains a lot of white spirit and clever additives. But is NOT a lubricant. They do not specify lubricity on their data sheet.
> ...


Steamguy,  the problem with WD-40 is not that it doesn't lubricate, its that nearly all the ingredients are volatile and once it evaporates there's practically nothing left.  Peter.


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## Nerd1000 (Nov 29, 2022)

More progress: I've got a (sort of) working injector!




Your browser is not able to display this video.




The spray pattern could do with some work, I suspect that this might benefit from lapping the needle tip together with the injector nozzle. The other issue I'm having is that the spring in the injector is not strong enough to make the 'pop' pressure of the injector reach my target of 10 MPa. I think this can be addressed by simply doubling up the springs.

Below are the main parts of the injector, prior to a few milling ops they needed for finishing. For reference the socket head cap screw with them is size M3. The injector body components are stainless steel, while the needle is tool steel.


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## minh-thanh (Nov 30, 2022)

Test your injector with compressed air, if it is sealed with compressed air it will be more sealed with oil.
  Notice every small change as you improve and perfect it.
 Keep going and you will reach your goal


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## Lloyd-ss (Dec 7, 2022)

Regarding the spring, have you considered winding your own? A very slight increase in the spring wire diameter will raise the spring force significantly. Find a spring calculator online and enter the parameters of your current spring to see what the force is. Then work from there. If you can't find any spring wire, consider straightening a spring that is the correct wire diameter, and then re-winding it. Guitar strings come in lots of different diameters, too.
It takes some practice but you can hand-wind around small nail or drill bit. I will usually wind a long length and then use only the best looking section.


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## Nerd1000 (Dec 8, 2022)

Haha yes, I often make my own springs including the example in the injector


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## minh-thanh (Dec 8, 2022)

Hi Nerd1000 !

Please consider the comment below just a suggestion
   Even though I've run my engine with injectors like yours, I really hate it and it won't be an option when I build a diesel.
  injector style like a valve (I don't remember its name) would be a better choice
  - It seems a little easier to seal it
  - Don't worry too much about the spring - because the injector only injects when the oil pressure in the injector is greater than the pressure in the engine cylinder - the force of the spring just makes it spray more "beautiful"
  Building a diesel engine has a lot of problems to deal with and don't make it any harder
    Don't worry too much about what type of needle it is.
  Focus on getting the engine running and once it's running you'll have more experience with it and you'll know what to do.


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## Nerd1000 (Dec 8, 2022)

minh-thanh said:


> Hi Nerd1000 !
> 
> Please consider the comment below just a suggestion
> Even though I've run my engine with injectors like yours, I really hate it and it won't be an option when I build a diesel.
> ...


Hi Minh,

I think there are probably pros and cons to different injector types. With mine, the valve opening is aided by cylinder pressure rather than resisted- I don't really know if this is good or bad from the perspective of the engine running.

What is clear from my research is that my style of injector is totally unsuitable for a direct injected engine like yours. It's usable only for indirect injection designs, or possibly also engines using the MAN 'M-system' of a small swirl bowl in the piston crown. I'm using indirect injection because I want to achieve smoke-free running on regular diesel fuel or even biodiesel, this would be very hard to do with a direct injected engine at our sizes. So I copied full-sized diesel practice when it comes to the injector design for this sort of engine.

Of course this is all a big experiment and I can't be sure it will work! If I have trouble I'll probably make another cylinder head mimicking your style of combustion system and injector.


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## minh-thanh (Dec 9, 2022)

Hi Nerd1000 .
My way is : make the engine run and then change it the way I want
  When the engine is running it will tell me : fuel pump : ok , ball valve : ok , injector : ok , fuel quantity : ok , injection time , compression ratio ..... ok , And then I change the type of injector  , the way it injects, change  the fuel pump, change springs....and change ..... ,
    I didn't change the type of combustion chamber so I don't know
  Everyone has a different way of doing things and a different way of thinking, I'm sure that type injector is fine with my engine so go ahead with your project - I just hate it.
  Make sure the injector spring doesn't change with the high temperature of the injector - with valve injectors it doesn't matter much.


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## Nerd1000 (Dec 9, 2022)

minh-thanh said:


> Hi Nerd1000 .
> My way is : make the engine run and then change it the way I want
> When the engine is running it will tell me : fuel pump : ok , ball valve : ok , injector : ok , fuel quantity : ok , injection time , compression ratio ..... ok , And then I change the type of injector  , the way it injects, change  the fuel pump, change springs....and change ..... ,
> I didn't change the type of combustion chamber so I don't know
> ...


I use stainless steel springs and the injector is actively cooled by fuel flow (some leaks past the needle seal ring and will flow back to the tank via a return line). So hopefully the injector won't get too hot. Heat is also a big problem for coking of the nozzle tip so certainty something to avoid.


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## Lloyd-ss (Dec 9, 2022)

Nerd1000 said:


> edit for brevity ............
> 
> What is clear from my research is that my style of injector is totally unsuitable for a direct injected engine like yours. It's usable only for indirect injection designs, or possibly also engines using the MAN 'M-system' of a small swirl bowl in the piston crown. ......................................


Nerd 1000,
I am really enjoying your posts and appreciate your sharing, warts and all. Your progress is uplifting. You seem to do a lot of research and planning and that is a good thing, but we all certainly have our own philosophical approaches to such a project.. which is really a total learning experience that enriches our lives.

It looks like you are using a pump-line-injector system, with the injector being a differential pressure type. I am working on one also but haven't posted in a while. I downloaded the shop manual for the Yanmar "L-series" single cylinder portable engines that run at 3600 rpm and have displacements from .19 to .35 liter. The manual is 130 pages and is very complete with lots of theory, diagrams and specs. They also use the P-L-I system, with a differential injector, and inject directly into a chamber in the piston crown like you mentioned. I bought one of their injectors and pumps (pretty cheap, actually) to look at their construction. Impressive in their robustness considering the small size of the engine. They are obviously built to last. Just for reference, their firing pressure in a test set-up is 19.6MPa, and I tested the one I bought and found that to be true. Seems pretty extreme and it seems like the leakage problems of a home-built unit increase with the square of the operating pressure, LOL. I just thought I'd throw this info into the pot.

Keep up the excellent work that you are doing. I will get back to sharing the progress on my thread very soon.
Lloyd


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## Roger B (Dec 19, 2022)

Some thoughts and suggestions on small diesels. I have got as far as a runnable 20cc, 25mm bore 40mm stroke, 4 stroke diesel. This runs on standard pump diesel. The video is a warm start, opening the fuel rack as I start to crank.



The atomisation from the injector is not very good yet. At least half the fuel is ejected unburnt from the exhaust. Compression pressure is around 35 bar and the injection pressure is over 100 bar.

The cylinder head is flat with a hemispherical combustion chamber in the piston. The injector is set at an angle aiming at the centre of the chamber.












The injection pump is 2mm bore and 2mm working stroke, the first 1mm closes the inlet port. The fuel quantity is controlled via a spill helix so the timing of the start of injection is constant. The injector is a poppet or mushroom type based on the GM 5.7L design. I have experimented with needle type injectors but getting a good seating down the end of a long bore was very difficult. With a poppet style the seating is visible. Find Hansen and Don Comstock both use this type.






The fuel pump plungers are made from commercial hardened and lapped pin gauges. The metering helix is ground with a small cutting disc. The pump body is made from hardened silver steel lapped to size using an Acro needle lap and 1 micron diamond paste.











The injector needles are turned from silver steel and just the tips are hardened. The bodies are stainless steel. The seating is lapped with 1 micron diamond paste. I have tried several different angles for the tip, currently 10° included seems to work. If the angle is too shallow it will tend to jam.

I built a test pump for the injectors so I can apply a known load to the plunger with a spring balance. 50N on a 2mm plunger gives around 150 bar opening pressure. Don’t get it anywhere near your skin. The spray pattern is still not very good with a mixture of well atomized fuel and wet drops.


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## Nerd1000 (Dec 19, 2022)

Very cool Roger, you've done what I am trying to do!

Regarding your incomplete combustion issue, I have a few thoughts...

There is some good public domain info on diesel combustion systems on the web if you look hard enough. What's clear from my reading is that you need two factors to get good combustion in a diesel: adequate atomisation, and sufficient mixing of the air and fuel during the burn. The swirl chamber I'm planning to use is one solution to this; the strong swirl in the chamber ensures intimate mixing of the injected fuel with the air in the chamber (about 1/2 of total). The remainder of the mixing process happens as the hot partly burnt mixture rushes from the swirl chamber into the cylinder, creating lots of turbulence that mixes the remaining air with the still burning fuel.

In a DI engine getting the mixing to work well is much harder. You need to provide sufficient air motion in the cylinder itself to mix the fuel spray with the air. This is usually done through careful design of the inlet ports to create a strong swirl in the cylinder. Having more fuel jets aimed around the cylinder reduces the required motion, hence most full sized DI diesels using multi hole nozzles. On our scale, perhaps you would want to look at adding a 'lip' to the injector needle to try to spread the spray pattern out more.


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## Lloyd-ss (Dec 19, 2022)

Roger, your set-up with the adjustable injector test pressure is clever. Iike it.


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## Roger B (Dec 20, 2022)

Thank you both

I have a couple of sources of diesel information, some older versions of the ‘The Modern Diesel’ and T D Walshaw’s ‘Diesel Engine Design’. (Washaw published model engineering things as Tubal Cain).

Diesel Engine Design by Walshaw T D - AbeBooks

The Modern Diesel - Fifth Edition

Turbulence appears less of a problem in a small engine as there is less distance for the fuel to travel to mix with the air. I have designed moderate turbulence with the air from the squish band being forced into the combustion chamber as TDC is reached. Too much turbulence can cause excessive heat loss.

I still thank most of my problem is atomisation. One of my tests is to ignite the fuel spray with a small flame. Experience shows that if I can’t light the fuel spray this way it won’t ignite in the engine. Whenever I make this test there is always of layer of fuel on the workbench underneath where the heavier droplets have fallen. The ‘ink blot’ test shows the same.


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## Nerd1000 (Dec 20, 2022)

Roger B said:


> Thank you both
> 
> I have a couple of sources of diesel information, some older versions of the ‘The Modern Diesel’ and T D Walshaw’s ‘Diesel Engine Design’. (Washaw published model engineering things as Tubal Cain).
> 
> ...



It's curious to me that your injectors if anything seem to make a narrower spray than mine, despite my design being a pintle type that really should be expected to spread less than the poppet type you're using. My injector does seem to lack consistency, so maybe flaws in the finish of the tip are making the spray wander around the place and appear to be more spread out than it actually is.

Anyway, I've certainly noticed that I get a distribution of different droplet sizes like you do. I guess higher injection pressure would help- the only other thing I can think of for your style is to add a second conical surface to the poppet so it looks like this:






The sealing surface would still have a 10 degree included angle, but there's another cone below with a larger included angle. I'm thinking that impingement of the spray on the lower cone would help break it up into smaller drops, kind of like some garden hose nozzles.

For my design this isn't possible, maybe I could experiment with different pintle shapes, or go to a poppet type nozzle. Testing will be needed to see how well my style performs.


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## Nerd1000 (Dec 20, 2022)

The design of my engine is more or less done, I still have some odds and ends (governor, fuel tank, exhaust pipe) but we've reached the stage where I can start making major components.





I'm planning to make a lot of the larger parts (block, lower crankcase, etc) as castings. Here are the patterns and core box for the main block,  I did have a




go at casting this part on the weekend, but had a misrun due to inadequate venting of the mould, and couldn't re-try due to having only made one core. Next try will be after Christmas, I'm fairly confident it will work.


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## Arild (Dec 22, 2022)

Hi! I've made a lot of castings in both aluminium and bronze. Remember to have some overshoot at the surface you need to mill off, and add a 2% to it all, (in aluminium)


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## Roger B (Dec 22, 2022)

It’s an interesting problem to know what is going on when the fuel is injected into high pressure (and hence high density) air. My initial experiments with needle type injectors with various orifices from 0.2mm – 0.5 mm seemed to produce good atomisation but were not successful when installed in the 2 stroke diesel. The airflow/swirl was obviously different to the current four stroke.



My needle injector design will need significant modification to fit the four stroke cylinder head.


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## Nerd1000 (Dec 24, 2022)

Roger B said:


> It’s an interesting problem to know what is going on when the fuel is injected into high pressure (and hence high density) air. My initial experiments with needle type injectors with various orifices from 0.2mm – 0.5 mm seemed to produce good atomisation but were not successful when installed in the 2 stroke diesel. The airflow/swirl was obviously different to the current four stroke.
> 
> 
> 
> ...



There are a couple of things we can know for sure: in the cylinder the atomisation will likely be worse (less pressure drop across the nozzle, though perhaps the poppet type nozzle isn't affected by this because the pressure helps the spring hold it closed?) and the air is much denser, so the droplets will not travel so far.


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## Lloyd-ss (Dec 24, 2022)

Nerd 1000,
With your tenacity, success is the only possible outcome.

I hope you are not too tired of being bombarded with slightly relevant information (LOL, no offense intended), because here is some more. Yanmar makes a LA series portable engine in a range of 0.2 to 0.4 liter, and up to 3600 rpm. It is a direct injection. Notice that the combustion chamber is off center, but remains  in line with the injector nozzle. Whether this was strictly because of space limitations around the valves, or whether it is off center strictly by design, I do not know.
I know that you are at the point of no return on many of the design aspects of the engine, but just for reference here are a few detail drawings from the Yanmar service manual. The pump plunger is .293" in diameter, the full srtoke is approx 5mm with a quick rise and then a 180 deg dwell on the plateau. The helix ramp on the plunger has a slope of approx 0.17" and controls the end point of the injection. When I measured the parts, I was rather surprised at how large the dia and stroke were for that displacement of an engine. The opening pressure of the injector nozzle is set at 200kg/cm^2 (2,800 psi). Even if the compression pressure in the engine is as high as 1,000 psi, that still means the relative injection pressure is 1,800 psi. Again, that pressure surprised me.
I hope you find something useful in this info.
Lloyd


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## peterl95124 (Dec 25, 2022)

Lloyd, thanks for posting the drawings for the Yanmar LA,
FYI, the plunger length is about 5 X its diameter, a good rule-of-thumb,
one thing that still puzzles me is the inlet hole to the pump chamber
is blocked by the plunger most of the time, so it seems that fuel
can't get back in, yes its open when the plunger is fully retracted, and
when the cut-off groove matches up with the inlet, but between those
two extremes it seems there would be "vapor lock" so to speak


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## Lloyd-ss (Dec 25, 2022)

Peter, yes, I had already given the vapor lock situation some thought and do have a reasonable guess, but there are a couple of more oddities.

The first thing, which really isn't odd, is that the cut-off groove is connected to the tip of the plunger via a drilled hole. You can't see that drilled-hole feature in the drawings I uploaded. So when the cut-off groove passes over the inlet hole, all of the residual pressure in the pump chamber shoots back into the inlet line. The part I find a bit odd is that the supply line and the return line are the same thing. So there must be a LOT of turbulence in that fuel line. 

What may be mitigating the vapor lock phenomenon is the fact that the plunger is pumping upward, and that the fuel tank is directly over the fuel inlet line, so there is strong gravity flow into the pump body, but also with all that turbulence. Somehow, that combo seems to work.

The second odd thing is that the cam lobe for the fuel pump (you cannot see the lobe in these drawings), holds the plunger at the maximum pressure portion of its stroke for a full 180 degrees of the camshaft revolution, which would be for a full 360 degrees of crankshaft rotation. Maybe holding the plunger all the way in, after it has spilled all of its residual fuel and pressure back into the inlet line, helps any vapor lock to dissipate. 
 Just my thoughts.
Lloyd


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## minh-thanh (Dec 25, 2022)

Here are 2 pictures


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## Lloyd-ss (Dec 26, 2022)

Again, interesting that the high pressure fuel pump spends 2/3 of its time in the full compression position, and only briefly blips up to expose the inlet port. The low pressure pump is synced with the inlet timing of the high pressure pump, which certainly makes sense.


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## Nerd1000 (Dec 26, 2022)

Lloyd-ss said:


> Again, interesting that the high pressure fuel pump spends 2/3 of its time in the full compression position, and only briefly blips up to expose the inlet port. The low pressure pump is synced with the inlet timing of the high pressure pump, which certainly makes sense.


This feature is likely intended to prevent the engine running backwards (with the exhaust valve serving as the inlet valve) if it 'kicks back' while being started. This is a legitimate problem for diesels with jerk pumps whether 4-stroke or 2-stroke, many older Mercedes cars have a one way valve in the inlet so the engine chokes on its back pressure if it tries, and you can find many stories of Detroit Diesel 2-strokes doing this resulting in black smoke coming out of the air filter.

Edit: an addendum regarding the 'vapor lock' issue on the pump. My injection tester pump plunger has no helix or drilling, therefore there is no way for fuel to enter the pump on the return stroke until the spill port is uncovered by the top of the plunger. Presumably the result is the formation of a partial vacuum (and some boiling of the residual fuel in the pump) during the return stroke, but this doesn't seem to cause any problems so long as the return spring is strong enough.


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