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

[petertha]

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.

http://www.acrolaps.com/index_003.htm

 

[Lloyd-ss]

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.

 

[Lloyd-ss]

Sorry, forgot the pic.

 

[Steamchick]

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

 

[petertha]

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.

 

[Nerd1000]

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.

 

[Steamchick]

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

 

[minh-thanh]

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

 

[petertha]

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.

 

[Steamchick]

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

 

[Nerd1000]

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.

 

[theo van der linden]

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

 

[minh-thanh]

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 )

 

[peterl95124]

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

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.

 

[Nerd1000]

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

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.

 

[minh-thanh]

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

 

[Lloyd-ss]

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.

 

[Nerd1000]

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

 

[minh-thanh]

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.

 

[Nerd1000]

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.

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.