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Old 25-12-2018, 15:27   #61
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Re: Injector HP Fuel Line - How to Make?

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Originally Posted by Compass790 View Post
I don't know but suspect they are so heavy-walled because the pump pulse delivers a hammer blow to them which causes vibration. Always you have more trouble with pipelines that have pulse loads and it's also what causes hydraulic hoses to fail. But you would need a hydraulics engineer to chime in for a definitive answer.
I'm curious as well as to why i.d. is critical as assume the diesel is incompressible as mentioned by A64 earlier
Liquids are not incompressible, rather relatively incompressible in comparison to vapors and gasses.

Oil has a compressibility of 1.36 E-06 per psi of pressure increase. In comparison water is 3.4 according to the reference I use. If I have the decimal point in the right place I calculate about a 3.5% decrease in volume at 2,500 psi.

There is a certain level of stress below which metals will not fatigue so from a structural viewpoint provided one does not cycle the hoop or circumferential stress above this level fatigue is not relevant to either the retention of pressure integrity or ultimate working life of the tube. However, even though one may be working below this critical stress level the tube will still expand under pressure thereby requiring a greater volume of liquid to be pumped into it to attain a specific pressure.

Mechanical, engine driven diesel fuel pumps do not have seals in either the pump plungers or the injector plungers therefore there is a small leakage of fuel bypassing them when they are under pressure.

I suspect that if you sum the volumes related to fuel compressibility, fuel line expansion and leakage the total would come to a significant portion of the fuel being pumped and that the designers try to minimize these effects by minimizing the volume of fluid to be compressed and the ID of the conduits to minimize tube expansion.

The tubes are also subjected to considerable vibration in service and for handling purposes require a certain degree of robustness which would tend to favor thick walled tubing.

I doubt that any single factor dictates tube dimensions but they all add up to what we see in service as the most practicable.
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Old 25-12-2018, 16:59   #62
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Re: Injector HP Fuel Line - How to Make?

Thanks very much for your comprehensive explanation. I was thinking of making up a spare injector line out of brake line just to see if it worked. Actually just found a possible flaw in your explanation. Whilst a mechanical injector pump has no seals at least ours has a delivery v/v that is a non-rtn v/v in effect ASFAIK. Therefore the injector line should hold pressure just below the pop pressure of the injector ( assuming all in good order) It only has to hold that pressure for a very short time. So if the volume decrease is only 3.5% I still think it's maybe worth a try to make up a spare injector line outa brake line ( assuming wall thickness sufficient) just as an experiment.
I wont be rushing into it as that sounds like a winter job to me but will post result if & when I get around to it.
I will wear safety goggles when testing. lol
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Old 25-12-2018, 17:30   #63
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Re: Injector HP Fuel Line - How to Make?

There are commercially available spare injector lines.
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Old 25-12-2018, 19:24   #64
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Re: Injector HP Fuel Line - How to Make?

Yes I'm well aware of that thanks. I often like to do things just because some say it can't be done & I like experimenting. E.G. everybody ( who didn't know ) said I couldn't rebuild my injection pump but I did & it wasn't that hard. Have plenty of stuffups too as when I made a big-end bearing outa bearing bronze when I was told it was too hard by a machinist mate & he was right.
Madness maybe but it's my madness
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Old 25-12-2018, 19:35   #65
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Re: Injector HP Fuel Line - How to Make?

I know what you mean Compass790. I'm the same way sometimes. The internet makes it possible to do things that couldn't be done even 5 years ago. You can track down parts that used to be impossible to find.
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Old 26-12-2018, 03:40   #66
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Re: Injector HP Fuel Line - How to Make?

This whole thread has been helpful to me. I didn't really know much about injector lines and maybe I still don't enough to be successful at a DIY project but I do know that if I can source the stock, there is a odds on chance that they will work OK.

As to their design, I believe this was worked out decades and decades and decades ago - at least for the small basic marine diesels I play with. Nothing has changed so I just need to reproduce what they did 50 or 80 or more years ago.

Stay tuned!
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Old 26-12-2018, 14:15   #67
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Re: Injector HP Fuel Line - How to Make?

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I know what you mean Compass790. I'm the same way sometimes. The internet makes it possible to do things that couldn't be done even 5 years ago. You can track down parts that used to be impossible to find.
Yes, it's an exciting time to live in if you have a DIY bent. Google & you tube have saved my bacon many a time
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Old 26-12-2018, 14:23   #68
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Re: Injector HP Fuel Line - How to Make?

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This whole thread has been helpful to me. I didn't really know much about injector lines and maybe I still don't enough to be successful at a DIY project but I do know that if I can source the stock, there is a odds on chance that they will work OK.

As to their design, I believe this was worked out decades and decades and decades ago - at least for the small basic marine diesels I play with. Nothing has changed so I just need to reproduce what they did 50 or 80 or more years ago.

Stay tuned!
A cobber, when I was floating the idea of machining a big-end bearing outa LG2 bronze, said "you are trying to overturn 300 years of bearing design". Unsurprisingly he proved to be right so your approach is definitely the one most likely to succeed with the added bonus of saving time
Still wont stop experimenting tho
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Old 01-01-2019, 04:34   #69
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Re: Injector HP Fuel Line - How to Make?

Believe I've got it figured out, or at least figured out enough for me to use the explanation if questioned.

Would like to say that it was a reasoned calculation based solely on the ideas given above and powers of reflection, but, sadly, this is not the case.

While trying to determine where best to put anti-hammer stacks (or whatever they're called) in copper lines while moving a friends water heater, and realizing I didn't know why they were even needed (some systems hammer, some don't), I decided to look it up (though a minutes consideration would have answered the question---thanks [for nothing] Google). Well, turns out it's the inertia of the water in the pipe, or rather the sudden opening and closing of valves interrupting the inertia built up by the moving water, which cause transient pressure spikes, which, depending on the volume moving through the pipe, can be huge.

Neither the pressure nor the volume-induced transient pressure spikes seem relevant in the tiny volumes contained in the injection lines, nor does the leakage around the P&Bs (because the 'leakage' should be the same for each; thus they cancel out of the equation).

The 'expansion' of the pipes also seems an unlikely candidate; if the volume changed significantly enough to swell the pipe, given the (roughly) million cycles in 10 hours of running mild steel would undergo, permanent distortion should result. I've yet to see this in any engines I've worked on.

Additionally to the 'pressure question', common rail engines run at about 4-6 times the injection pressure of mechanically injected engines, yet their injection line walls are only around 2 mm thick (for 10 mm OD lines), and they're (depending on the manufacturer) subject to 3-5 times more of these pressure spikes, due to the multiple injections-per-power-stroke of common-rail engines.

So my story, and I'm sticking to it (until a better one comes along), is that the reason for having the lines the same length is precision of injection, which is kept as accurate as possible by having the same, smallest amount of fuel necessary in each line, thus avoiding the 'inertial inaccuracies' entailed in trying to move differing volumes though the line.
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Old 01-01-2019, 14:37   #70
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Re: Injector HP Fuel Line - How to Make?

Holy hell jimbunyard! That brain worm got to you.Ya got me convinced but not enough not to try the experiment of making a non-std length/i.d/o.d. line come winter. lol.
I doubt there is any significant leakage in plunger & barrel as if there was ( at least in our I.P. a Bosch PFR ) the fuel would be getting into the crankcase as the close tolerances are the only barrier btwn high pressure fuel & the crankcase.
Great post to mull over
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Old 01-01-2019, 15:07   #71
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Re: Injector HP Fuel Line - How to Make?

Firstly the water hammer. If you want to do some maths on it, it's called "the celerity of a pressure wave".

Never gave any thought to the differences between conventional and common rail from a pressure regime viewpoint but since the injection timing is governed by the opening of a solenoid valve in the injector I suspect you are correct in your analysis on both the timing aspect and the tubing wall thickness issue.

Beg to differ on the leakage issue. The leakage at the injectors is handled by the return line from injectors to tank, and in the pumps themselves there is a circular drain slot on each plunger which is also ported to an un-pressured return line and so the crank case end of the plunger never sees diesel under pressure and consequently little leakage past the plunger. There may even be an elastomer seal in the non pressured part of the plunger barrel, I've never actually looked in there although I have seen the plungers out of pumps numerous times.
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Old 01-01-2019, 15:18   #72
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Re: Injector HP Fuel Line - How to Make?

Well I beg to differ as i have had my injection pump completely apart & there is no seal,elastomer or otherwise, in the plunger & barrel & I say again the only seal is the close fit of the plunger & barrel. Look at Bosch PFR vid on youtube or an exploded parts diagram if you wish to see it's setup. Without the close fit the pump couldnt get to the 2500psi approx that it does.
But my argument only applies to the Bosch PFR as that's all I've stripped & rebuilt
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Old 01-01-2019, 16:36   #73
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Re: Injector HP Fuel Line - How to Make?

I think itís simply a matter of economics.
Yes you can build your own, your shop can build you one, likely for not a whole lot more than you can buy OEM though.
Most probably they only make one up when it canít be bought, lots of things are like that now, itís why nobody repairs shoes anymore or fixes TVís.

Iíd bet a brake line would definitely work, but not indefinitely. I bet it would fatigue and crack. 100 hours or 1000 hours, who knows.
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Old 01-01-2019, 18:49   #74
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Re: Injector HP Fuel Line - How to Make?

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Well I beg to differ as i have had my injection pump completely apart & there is no seal,elastomer or otherwise, in the plunger & barrel & I say again the only seal is the close fit of the plunger & barrel. Look at Bosch PFR vid on youtube or an exploded parts diagram if you wish to see it's setup. Without the close fit the pump couldnt get to the 2500psi approx that it does.
But my argument only applies to the Bosch PFR as that's all I've stripped & rebuilt
There are many designs of diesel fuel injector pumps some of which are lubricated by the diesel being pumped with case drains.

I think the post wherein the issue of lube oil dilution was raised was referring to those such as the Yanmar injector pumps where the plungers are actually driven by an extended camshaft and share lube oil with the enging rather than those which have the entire injector pump in it's own housing with their own crank or camshaft and being driven by a shaft or power take off from the engine.

Since I have never had the need to disassemble a Yanmar fuel pump I cannot be unequivocal re the existence of plunger wiper seals but if one wanted to ensure against engine oil dilution one may decide to design them in.
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Old 02-01-2019, 01:01   #75
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Re: Injector HP Fuel Line - How to Make?

Minor setback (ok, maybe major setback - not sure yet).

Contrary to what has been posted, these Yanmar injectors lines do not have nipples silver soldered or brazed on. There is a forged nipple and a separate slide on ferrule. I found this out while trying to un-braze the nipple - not possible with a forged nipple ()

So not only do I have to source the tube stock, I now have to source the nipples as clearly I can't DIY the forged nipple.

Maybe have to start saving for off the shelf ones .

Next stop is a visit to the local injector shop to see what they can supply in the way of tube stock and nipples but right now, they are closed for a Christmas break.

Meanwhile, keep on discussing lines and pumps and stuff

Compass790, it's not looking good for your wager but you never know...
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