Leakoff pipes/ spill lines

As you know I'm a noob when it comes to diesesed engines, can someone explain what these are?
Is it just the diesel equivalent of a petrol return pipe (to the tank)? Or something else?
I've read they can leak, which lets air into the system, so they don't simply return unused fuel or it wouldn't matter how much air is in there.
I've also read that the fuel pressure in the line is important.
I'm confused, what are they for?

I just found this on LinkedIn.. some people spout utter tripe, even i know this is wrong!!

"The Diesel Leak Off Pipe is a fuel system hose that is equipped on vehicles with carburetors. Its main function is to return extra fuel from the carburetor to the fuel pump, it also has other important purposes."

Know the more knowledgeable guys will help more but I always thought they were to relieve any excess pressure

also links them to a spill rail which takes any excess fuel back to the injection pump and fuel tank

kermit_rr said:

As you know I'm a noob when it comes to diesesed engines, can someone explain what these are?
Is it just the diesel equivalent of a petrol return pipe (to the tank)? Or something else?
I've read they can leak, which lets air into the system, so they don't simply return unused fuel or it wouldn't matter how much air is in there.
I've also read that the fuel pressure in the line is important.
I'm confused, what are they for?

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Leak off pipes return excess fuel from the injectors to the tank, not always directly.

Leak off/spill/return pipes connect each injector to each other and in some cases return fuel to filter...just done mine on the van very small bore and just a push fit but if they leak lets air in and fuel drains back and starting takes a lot of cranking...

I lost count of how many times I replaced mine with cheap leakoff pipe. After a while starting would take longer and a smell of diesel would come from under the bonnet. Changed the pipe for genuine BMW and touch wood no leaks for a few years now.

kermit_rr said:

As you know I'm a noob when it comes to diesesed engines, can someone explain what these are?
Is it just the diesel equivalent of a petrol return pipe (to the tank)? Or something else?
I've read they can leak, which lets air into the system, so they don't simply return unused fuel or it wouldn't matter how much air is in there.
I've also read that the fuel pressure in the line is important.
I'm confused, what are they for?

Click to expand...

kermit_rr said:

I just found this on LinkedIn.. some people spout utter tripe, even i know this is wrong!!

"The Diesel Leak Off Pipe is a fuel system hose that is equipped on vehicles with carburetors. Its main function is to return extra fuel from the carburetor to the fuel pump, it also has other important purposes."

Click to expand...

After reading that description, you'll be even more confused.

Basically a traditional diesel injector is designed to open at a set pressure, which is how the injection pump can be precisely time the injection of the fuel. Once the injector has fired its fuel charge, the injector pintel in the injector closes under spring pressure, and so the fuel behind the pintel has to go somewhere, so it is spilled out the spill line.
Common rail injection is very similar, although instead of having a spring that released by pump pressure, the injector pintel if lifted by an electronic actuator (normally a stack of piezoelectric elements), which allows fuel from the high pressure rail to be injected out the nozzle. Once the injection period has finished, the pintel closes, and the fuel sits there waiting for the next injection pulse to arrive. There are spill lines on common rail injectors too, as some fuel always gets past the internal seals. As the internals of the injector begin to wear, there is space to allow for a greater amount of spill through the injector internal seals and down past the spill lines. It's possible to get a very rough guide on how worn common rail injectors are, simply by measuring how much fuel is spilled out the spill lines, and comparing fuel volumes of all cylinders.

That all sounds very complicated. Petrol injectors are so much simpler!! Common rail, open and close, job done.
Perhaps i just don't understand what diesel ones can't be the same, they do the same job and inject fuel into the engine?

The diesel fuel needs more pressure to atomise it finely. The more pressure the finer the fuel is and the better it burns.
Our 2.5 straight six will top 30 ish mpg in comparison to a volvo d5 engine that'll touch 50+mpg. Higher pressure. Don't forget some systems fire more than once to reduce noise and other reasons.
Complicated it certainly is.
Looking at small 1.0l petrol turbos that are now capable of late 40's mpg even at 70mph!!

1litre turbo engine are silly, they work too hard all the time.. great for efficiency but poor for mpg

kermit_rr said:

1litre turbo engine are silly, they work too hard all the time.. great for efficiency but poor for mpg

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Overstressed yes but poor mpg not at all. Depends on your needs I suppose. Quality and materials once again

Mark Piercy said:

Overstressed yes but poor mpg not at all. Depends on your needs I suppose. Quality and materials once again

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'poor' is subjective. an engine is most efficient at wide open throttle, but that is generally when MPG is worst. smaller engines will work harder than larger engines for a given load requirement, therefore have a larger throttle opening

Mark Piercy said:

The diesel fuel needs more pressure to atomise it finely. The more pressure the finer the fuel is and the better it burns.
Our 2.5 straight six will top 30 ish mpg in comparison to a volvo d5 engine that'll touch 50+mpg. Higher pressure. Don't forget some systems fire more than once to reduce noise and other reasons.
Complicated it certainly is.
Looking at small 1.0l petrol turbos that are now capable of late 40's mpg even at 70mph!!

Click to expand...

Modern diesels have much finer spray patterns so need the higher pressure to force the fuel through. They also pulse multiple times rather than one spray. Very clever but very finicky.

kermit_rr said:

1litre turbo engine are silly, they work too hard all the time.. great for efficiency but poor for mpg

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Don't forget they have a pitifully short life too.

kermit_rr said:

That all sounds very complicated. Petrol injectors are so much simpler!! Common rail, open and close, job done.
Perhaps i just don't understand what diesel ones can't be the same, they do the same job and inject fuel into the engine?

Click to expand...

Diesel common rail works on the same principles as common rail petrol engines. Although because diesel fuel is more gloopy, and doesn't readily vaporise, it's injection pressure is up over 20,000 PSI, compared to a petrol's 60 PSI or so.

Nodge68 said:

Diesel common rail works on the same principles as common rail petrol engines. Although because diesel fuel is more gloopy, and doesn't readily vaporise, it's injection pressure is up over 20,000 PSI, compared to a petrol's 60 PSI or so.

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I get that, just looking at the 2 fools side by side, you can see the difference.. apart from the nasty diesesed smell!!
Still don't get why there needs to be complicated 'spill'.. am i having a mental block? Is it simply because of the high pressure that it can't be simple like petrol and also needs more accurate injection timing?

kermit_rr said:

I get that, just looking at the 2 fools side by side, you can see the difference.. apart from the nasty diesesed smell!!
Still don't get why there needs to be complicated 'spill'.. am i having a mental block? Is it simply because of the high pressure that it can't be simple like petrol and also needs more accurate injection timing?

Click to expand...

Same on all diesels, the injection pump delivers more fuel than the injectors can use, so it has to go somewhere,

kermit_rr said:

I just found this on LinkedIn.. some people spout utter tripe, even i know this is wrong!!

"The Diesel Leak Off Pipe is a fuel system hose that is equipped on vehicles with carburetors. Its main function is to return extra fuel from the carburetor to the fuel pump, it also has other important purposes."

Click to expand...

How old is the car they were talking about?

kermit_rr said:

I get that, just looking at the 2 fools side by side, you can see the difference.. apart from the nasty diesesed smell!!
Still don't get why there needs to be complicated 'spill'.. am i having a mental block? Is it simply because of the high pressure that it can't be simple like petrol and also needs more accurate injection timing?

Click to expand...

The spill line is needed because of the high pressure needed for injection of diesel. Petrol is simpler, diesel is more economical, both have the pitfalls and benefits.

WhiskyLassie said:

How old is the car they were talking about?

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It doesn't matter, diesel doesn't use a carburettor