Moly Diesel Purge

[skyship007]

The following is a reply to a question I sent to LM R&D in Germany cos the UK web site has been changed.
I just asked if it was still OK to use Purge to fill the fuel filter housing when changing the element:

Marc Polzer <marc.polzer@liqui-moly.de>
To
X@yahoo.com
Today at 10:36
Dear Mr X,
It can be still used in this way.
Best regards
i. A. Marc Polzer
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F & E / Anwendungstechnik
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Phone: * * * *+49 731 1420-421
Mobil: * * * *+49 173 3420728
Fax: * * * *+49 731 1420-44895
marc.polzer@liqui-moly.de
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LIQUI MOLY GmbH
Jerg-Wieland-Straße 4 | 89081 Ulm | GERMANY
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www.liqui-moly.de

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Court of registration Ulm, HRB 1383 | Managing Director: Ernst Prost

LIQUI MOLY ist Deutschlands beliebteste Motorenöl-Marke!
LIQUI MOLY is Germany´s most popular motor oil brand!

[Tannaton]

I’m not suggesting that you can’t use liqui moly in that way, what I am saying is that it will not be as effective a use of the product as rerouting the fuel lines to run the engine off the can.

The reason I believe this to be the case is this: as the HP pump is displacement based it will pump a fixed volume of fuel for a given number of engine revolutions. The fuel pressure regulator will bleed of back to the tank via the return line a quantity of fuel to control the fuel pressure in the common rail. Given the quantity of fuel being pumped has to be adequate for full throttle, full boost and on a tuned engine, I would suggest at tick over less than 10% of the fuel pumped actually goes through the injectors.

I don’t have any hard facts to back this up, but I’m confident it makes sense?

[skyship007]

Originally Posted by Tannaton

I’m not suggesting that you can’t use liqui moly in that way, what I am saying is that it will not be as effective a use of the product as rerouting the fuel lines to run the engine off the can.

The reason I believe this to be the case is this: as the HP pump is displacement based it will pump a fixed volume of fuel for a given number of engine revolutions. The fuel pressure regulator will bleed of back to the tank via the return line a quantity of fuel to control the fuel pressure in the common rail. Given the quantity of fuel being pumped has to be adequate for full throttle, full boost and on a tuned engine, I would suggest at tick over less than 10% of the fuel pumped actually goes through the injectors.

I don’t have any hard facts to back this up, but I’m confident it makes sense?

I never suggested that using the fuel filter housing was the most efficient way to use Purge, it's just a safe easy way that avoids the need to prime the engine and is nearly as good as playing around with the fuel lines whilst holding a can of flammable fuel.

You are way off with a 10% figure. If the injectors are gummed up or the tips blown, nearly all of the fuel will be used directly. That's probably why LM changed their PDS to say there is no need to use the return line.
If the injectors are blocked inside (Rare) or there is an electronic fault, about half of the fuel will finish up going down the return line when the engine is warm.
The average figure for leak back is about one third.

The fuel flow to a common rail diesel depends mostly on RPM and temperature.

[Waynedance]

Not done mine yet.

Friend at work has added 1lt to his fuel tank which was very low on fuel and he says it made a world of difference, diesel knock gone and engine much smoother and quieter.

He tried to do it via fuel line but the engine would not start, guess the low pressure pump is needed like I guess mine would need.

[Tannaton]

Originally Posted by skyship007

You are way off with a 10% figure. If the injectors are gummed up or the tips blown, nearly all of the fuel will be used directly. That's probably why LM changed their PDS to say there is no need to use the return line.
If the injectors are blocked inside (Rare) or there is an electronic fault, about half of the fuel will finish up going down the return line when the engine is warm.
The average figure for leak back is about one third.

The fuel flow to a common rail diesel depends mostly on RPM and temperature.

Please have a think about it like this:

The HP pump on a D5 is a camshaft driven displacement unit, so the volume of fuel it pumps is fixed for each engine revolution. Engine speed is the only factor that can very the fuel pumping rate, not temperature (a volume of 1 litre is the same at 10 degrees or 90 degrees). Remember we are talking volume, not mass.

So at tickover speed (750 rpm) the volume of fuel pumped in 1 minute will be x. At 3,250 rpm the volume of fuel pumped in 1 minute will be 5x. But the injectors will be firing at 5 times the rate of tickover so the percentage of fuel pumped that is returned to the tank for a given throttle rate is the same regardless of engine speed. i.e. at 20% throttle the system may return say 80% of the pumped fuel - whether the engine is running at 750 rpm or 4,000 rpm.

Hence to keep a D5 engine ticking over at 750 rpm and run all of the ancillaries (alternator etc.) I guess would require around 7-15 BHP. The pump is capable of delivering enough fuel for 225-250 BHP, hence I suggest at tickover 90-95% of the volume is returned to the tank and this is done by the fuel pressure regulator which is an integral part of the pump - so the fuel returned never gets to the common rail or injectors.

Keep in mind also that fuel returned to the take is the excess fuel that the fuel pressure regulator bleeds off from the output of the HP pump to maintain the desired fuel pressure on the rail plus the leakback from the injectors themselves.