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

[Lloyd-ss]

Leak detection and repair, especially in a high pressure dynamic condition can be extremely frustrating to trouble shoot and repair. I really, really, know.
Your use of the manilla folder gasket for detection is excellent!

Also, putting a few drops of food coloring in the cylinder before doing the compression test can help reveal a leakage path.
Lloyd

 

[HMEL]

I don't know much about spray droplets, size, pressures, etc. But I'm sure someone does?
The curious thing I think I remember is the surface tension - is constant - except has a radius part of the function, so a smaller radius is a higher tension...? Then there is Surface energy: Never got my head around that... except I think there is more surface energy "per surface area" with a small radius than a larger one....
So those things (combined?) suggest to me that for smaller droplets, we need higher "pump energy = pressure - to get some shear conditions to create Aerosols instead of Droplets.
Then there is heat: A smaller droplet needs less heat to vaporise than a larger one, and has a higher ratio of surface area to mass, so smaller seems better than larger in fuel droplets, so the droplets vaporise quicker and the gas can then burn as more heat ionises it.
I guess that droplets are formed when the liquid is expelled from the nozzle/jet and the pressure gradient shears the jet of fluid? - I.E. It doesn't take any distance from the jet to convert the liquid at pressure at the jet into droplets without pressure but with velocity? - In which case, I can appreciate how the "spray" does not wash the piston, even though distances are very small.
There must be some science in it that I don't understand, so any advice is of interest. - I just "know nowt about owt" on this subject!
Thanks,
K2

I once looked very closely at nozzle and droplet size. Specifically for fuels you need a liquid mass of some amount because vapor alone can not give enough mass for the energy that is delivered. The smaller the sphere the more surface area is available for the chemical reaction and it goes faster. Injection into high pressure systems poses a theoretical problem as the engines get smaller say on model systems. It is more difficult to get the fuel to shear properly because it is difficult to machine the geometry required at very small sizes. The injector systems become a design process in itself. I have often thought that maybe at a certain size vapor will be the only way to deliver the fuel. I have seen where some model engines the fuel is mixed with a highly volatile fuel such as ether to get them to run.

 

[Nerd1000]

Do you have a link to Milo tins solution? I believe is a quite soft iron. Can it be found in something else? Is enough for purpose?

I think it's some sort of very soft aluminium, sort of like a very thick foil or maybe shim stock. Easy to bend into shape, I was able to cut out a circle and draw the ID of the hole downwards, then sort of wrap it around the inside of the paper gasket. Result wasn't too great but I feel with some practice and maybe turning up a wooden former I can probably get a neat result.

 

[NapierDeltic]

I see now, it is the sealing foil, like in older soluble coffee tins. Thank you for ideea!

 

[Nerd1000]

A brief update. Ive got my head gasket issues sorted well enough. Remilling the head didn't allow the copper gasket to work, I think I simply used up all the available compression of the copper (it measures thinner now). A composite gasket with fire ring was made as planned and is working so far with no leaks.

On to the next problem, fuel. I made a new, heavier spring for the injector, which prevents compression blowing back into the fuel lines quite nicely. However, with the stronger spring tension the injector no longer sprays fuel. Cue half a day spent chasing leaks... The initial culprit was the injector needle seal, which seems to have been damaged. Replacing that led to finding leaks in crush washers, the cone connector to the injector, etc. all easily fixed.

Finally, I got to the plunger and barrel themselves. Bad news there, the fit clearly isn't good enough. I noticed fuel was leaking below the pump, and actually ended up submerging it in kerosene and applying compressed air (using a syringe and tube connected to the fuel inlet). Bubbles leaked from the back of the barrel with only modest pressure. Seems I need a new plunger and or barrel

Now normally I'd just say oh well and get on with it. However, my partner has just recently got a job offer in Germany, and so it's likely we will be away from my shop for two years. I'd like to see the engine run under its own power before we go, so I'm weighing up whether I should convert it to a petrol engine and get some running done to validate the other parts of the design, then revive the diesel side of the project when I get back.

 

[Roger B]

I first ran my two stroke diesel on petrol to confirm that the ‘two stroke’ induction worked. It was designed to allow this so there was not too much work involved.

You may have rather more work to reduce the compression ratio and get a sensible gas flow to a spark plug, probably a new cylinder head.

Whereabouts in Germany will you be?

 

[Nerd1000]

I first ran my two stroke diesel on petrol to confirm that the ‘two stroke’ induction worked. It was designed to allow this so there was not too much work involved.

You may have rather more work to reduce the compression ratio and get a sensible gas flow to a spark plug, probably a new cylinder head.

Whereabouts in Germany will you be?

Munich.

The 'dumb' method would be to make an extra chamber that bolts in place of the injector and contains the spark plug, and use a very thick head gasket to reduce the compression ratio. The gas flow to the plug would be awful but maybe that's ok for just getting it running?

 

[Roger B]

Difficult, as you already have a pre-chamber purging the burnt gasses around the sparkplug well enough to get some fresh mixture in to ignite may be a problem.

 

[Nerd1000]

That would be the concern. It's probably worth one more try at making a workable pump plunger (at least I have a good test protocol so I don't waste my time now). Otherwise it might be time for a different cylinder head.

 

[Lloyd-ss]

A brief update. Ive got my head gasket issues sorted well enough. Remilling the head didn't allow the copper gasket to work, I think I simply used up all the available compression of the copper (it measures thinner now). A composite gasket with fire ring was made as planned and is working so far with no leaks.

On to the next problem, fuel. I made a new, heavier spring for the injector, which prevents compression blowing back into the fuel lines quite nicely. However, with the stronger spring tension the injector no longer sprays fuel. Cue half a day spent chasing leaks... The initial culprit was the injector needle seal, which seems to have been damaged. Replacing that led to finding leaks in crush washers, the cone connector to the injector, etc. all easily fixed.

Finally, I got to the plunger and barrel themselves. Bad news there, the fit clearly isn't good enough. I noticed fuel was leaking below the pump, and actually ended up submerging it in kerosene and applying compressed air (using a syringe and tube connected to the fuel inlet). Bubbles leaked from the back of the barrel with only modest pressure. Seems I need a new plunger and or barrel

Now normally I'd just say oh well and get on with it. However, my partner has just recently got a job offer in Germany, and so it's likely we will be away from my shop for two years. I'd like to see the engine run under its own power before we go, so I'm weighing up whether I should convert it to a petrol engine and get some running done to validate the other parts of the design, then revive the diesel side of the project when I get back.

Nerd !!!!
That is a lot of news, and I hope the move goes well, but I have really enjoyed your project and posts. They will be missed, and I mean that sincerely.

May I suggest a last ditch effort for your injector? I know how you must feel. Since the leak is at the back of the plunger, have you considered a packing gland with a Teflon washer, such that you can adjust the compression on the Teflon? The Teflon will compress very little and will probably not upset the balance of the injector. I had to use one on my injector for a minimal loss out the back of the injector and it works well. The adjustability of the gland is important because on mine, I had to tighten the gland to the point of locking up the plunger, and then back it off just the tiniest amount to allow movement. The usual o-ring materials won't allow that type of fine tuning and you end up with a stick-slip type of action.

I hope you stay active on the forum, and that the move is wonderful for you and yours.
Lloyd

 

[Lloyd-ss]

Munich.

The 'dumb' method would be to make an extra chamber that bolts in place of the injector and contains the spark plug, and use a very thick head gasket to reduce the compression ratio. The gas flow to the plug would be awful but maybe that's ok for just getting it running?

You are a smart guy. You will figure something out.
Lloyd

 

[Nerd1000]

Another idea would be to leave the injector in place as a seal and fit a carb to feed model diesel fuel straight to the intake, so still compression ignition but not a true diesel.

I'll have one more go at the pump first however.

 

[Roger B]

I agree with continuing with the fuel pump, that's working in the direction you want to go.

Munich is around 4 hrs from me by car or train if you are interested in meeting up. There is also a German model engine maker I know who lives just south of Munich.

 

[Nerd1000]

I agree with continuing with the fuel pump, that's working in the direction you want to go.

Munich is around 4 hrs from me by car or train if you are interested in meeting up. There is also a German model engine maker I know who lives just south of Munich.

That sounds like fun! I'll have to reach out once we get settled. We are planning to move in September, though it remains to be seen whether this is possible given various bureaucratic delays that are inevitable with such things.

 

[minh-thanh]

Nerd1000 !
Can you take pictures of the inside surface of the cylinder ?

 

[Nerd1000]

Nerd1000 !
Can you take pictures of the inside surface of the cylinder ?

Engine cylinder or injection pump cylinder?

I think it is quite difficult to do the injection pump one, as it is only 3.5mm in diameter and the surface smooth enough to be shiny if the photo is at an angle. I cannot see the surface finish with my eyes, let alone a camera. Perhaps with some special equipment (fiber optic camera?) one could do it.

 

[minh-thanh]

It's not too difficult
Phone camera is good enough
Or just need the light behind and use a magnifying glass to see
(What the surface looks like in the picture, it looks like I'm looking at it with light and a magnifying glass)

 

[Nerd1000]

It's not too difficult
Phone camera is good enough
Or just need the light behind and use a magnifying glass to see
(What the surface looks like in the picture, it looks like I'm looking at it with light and a magnifying glass)

View attachment 158316

 

[minh-thanh]

Personally, I would keep lapping until the surface is really good - like the area in the picture
A little more patience, a little more care: you will do it better.

 

[Nerd1000]

Personally, I would keep lapping until the surface is really good - like the area in the picture
A little more patience, a little more care: you will do it better.

View attachment 158417

The area you point to at the top of the cylinder is a screw thread. Or do you mean further in near the hole?

Anyway the main issue appears to be the plunger, I can see low spots on it where it was not burnished by the cylinder during running. I will lap the cylinder again to be sure it is as good as possible, then fit a new plunger to it.

I suspect the issue with the plunger is still heat treatment distortion. I think I will actually harden it before turning, and use a CBN insert to machine it. So the new order of operations will be drill pin hole, harden and temper, turn to a bit over final size (leaving allowance for lapping), grind helix and finally lap OD to fit the cylinder.