NapierDeltic
Well-Known Member
I see now, it is the sealing foil, like in older soluble coffee tins. Thank you for ideea!
Model Diesel: 32mm bore, 38mm stroke, indirect injection
- Thread starter Nerd1000
- Start date
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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.
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
Well-Known Member
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?
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
Well-Known Member
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.
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.
Nerd !!!!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.
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
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.
I'll have one more go at the pump first however.
Roger B
Well-Known Member
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.
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.
Nerd1000 !
Can you take pictures of the inside surface of the cylinder ?
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.
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)
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)
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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.
A little more patience, a little more care: you will do it better.
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.
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
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
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.
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