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  #1  
Old 01-20-2006, 11:48 AM
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Fuel pump -Constant pressure or constant volume?

A fundamental disagreement has ocurred which a search of this forum did not provide the definitive answer for. Maybe someone here can settle it.

On Mercedes diesel engines is the fuel pump (not the Injection pump) a constant voume or constant pressure design?Or were there two differnt plunger type fuel pump used?

One of our (VO conversions) fourm members seems to believe that the fuel pumps on MBs use a cam to push a constant volume of fuel to the IP..the other that the cam merely provides for make up fuel and the internal spring provides a constant pressure to the IP.

Here are the actual description of the individual with the "constant volume" opinion.

Quote:
The Benz pumps I've seen are basically a siringe.... that is operated by a cam in the IP... it bolts to the side of the IP... the cam preforms the function of compressing the piston (pushing out the fuel) ... a spring returns it for another push... check valves make sure the suction pulls from the suction side ands discharge goes to the dscharge side...
And heres the link to the discussion itself.

If anyone can settle this definatively it would be of immense help.
Thanks.

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  #2  
Old 01-20-2006, 11:54 AM
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  #3  
Old 01-23-2006, 09:29 AM
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Does anyone have any evidence on this one?
Perhaps an exploded parts list view?
Perhaps the fuel pump tests from the online manual http://skinnerbox.steaky.org/Service/W123/Main.html(Section 07-145) gives the answer.

The Fuel Delivery Pressure (with the return line open) specifies a pressure of 0.6 - 0.8 Bar at idle and 0.8 Bar @ 3K RPM. This is adjusted by lengthening the pressure relief valve spring in the IP return line banjo bolt.
The Fuel pressure End Test requires that the return line is pinched (closed off), then the maximum pressure attainable should be 1.1 Bar at idle or 1.3 Bar at 3K RPM.

This indicates that, with an unloaded OM616/617 the fuel consumption would be minimal, the pressure is limited by the fuel pump, not the return line pressure relief valve (which is blocked off, along with the air bleed from the filter banjo bolt.


I would post the text, but it is a pdf file scanned from the paper document.
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Last edited by TonyFromWestOz; 01-23-2006 at 10:25 AM.
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  #4  
Old 01-23-2006, 09:41 AM
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I don't have a reference, but as evidence of the constant pressure side, I can say with some conviction, based on readings from my fuel pressure gauge I have between the lift and Injection pumps on my '84 300D, that the pressure stays at all engine speeds between 10 and 15 psi. This range appears unaltered whether I have the pedal fully depressed or am at idle. It seems logical to me that if it were a constant volume supply that the pressure would be greatest at idle when the demand from the IP was lowest.

Probably not definitive, but it's logical within my limited scope of knowledge.

Ben
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  #5  
Old 01-23-2006, 09:43 AM
Brandon314159
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I think also this has a lot to do with the spillover/pressure regulator line on the back side of the IP. (its the bajo bolt with the extra nut thing).

I think the pump puts out extra and this unit regulates a constant pressure inside the housing my letting extra fuel past at a certain pressure.

Just my thoughts
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  #6  
Old 01-23-2006, 09:57 AM
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It is not the excentric that moves the plunger that makes the pressure, it is the preload on the return spring that sets the line pressure, so that no matter how fast the cam excentric turns, the spring controls the pressure.
Where it the otherway round, then line pressure would increase as a function of speed.

FYI. Maximum volume is at zero pressure. Maximum pressure is at zero flow.

P.S. Only positive displacement pumps have rising pressure after chamber fill.
IE : The rack chambers in the IP..
.
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  #7  
Old 01-23-2006, 11:17 AM
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Quote:
Originally Posted by bhanson
I don't have a reference, but as evidence of the constant pressure side, I can say with some conviction, based on readings from my fuel pressure gauge I have between the lift and Injection pumps on my '84 300D, that the pressure stays at all engine speeds between 10 and 15 psi. This range appears unaltered whether I have the pedal fully depressed or am at idle. It seems logical to me that if it were a constant volume supply that the pressure would be greatest at idle when the demand from the IP was lowest.

Probably not definitive, but it's logical within my limited scope of knowledge.

Ben
I have observed the opposite of this to be true on mine. I have a pressure gauge right in front of the IP on my SDL, and when I hammer on the accelerator, you can see the pressure drop quite a ways. At idle I see around 10 psi (although it does fluctuate back and forth between 9 and 12 or so), then with no load on the engine I can bring the rpm up and watch the pressure gauge come up to 15 psi and level off there.
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Old 01-23-2006, 01:18 PM
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I didn't even know that our cars have a fuel pump, I thought there was just the injection pump. Where is the fuel pump located?
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  #9  
Old 01-23-2006, 01:55 PM
Brandon314159
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Quote:
Originally Posted by DieselAddict
I didn't even know that our cars have a fuel pump, I thought there was just the injection pump. Where is the fuel pump located?
Its called the lift pump...its located on the side of the IP (has the primer attached to it on the 617).

The fuel has to get to the IP somehow..all the IP does is meter and a very tiny amount of pumping.
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  #10  
Old 01-23-2006, 04:11 PM
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danalinscott, it depends on the age/generation.

The old stuff is gravity fed injection pump, meaning a low pressure fuel pump whose function is only to lift the fuel out of the tank and into the secondary filter (cartridge style). If the secondary filter-to-injection pump line comes out of the bottom of the secondary filter housing then it's absolutely a gravity fed injection pump.

The early spin-on secondary filter engines also had a gravity fed injection pump (Mercedes liked to do things incrementally - the glow system of the 1970s decade is nightmare to diagnose). Didn't work too well, so they jacked up the fuel pump pressure (then changed fuel pump?) so the fuel can go uphill through the secondary filter.

Then the injection pumps became pressurized on the inlet side via a relief valve on the injection pump. Different fuel pump from the very early example.

So you have to look at the injection pump/secondary filter to figure out the answer for a specific engine generation.

Biodieselers like you would be better off with a gravity system, in my opinion. The injection pump relief valve can cause problems unless flushed out well before shut down.
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Last edited by dabenz; 01-23-2006 at 04:21 PM.
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Old 01-24-2006, 02:14 PM
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Having been watching the thread on the WVO side I think I can pretty much say Dana, Tony etc. intends This thread is pretty specific to US and Euro spech. 61x engines lift pump found in 240D, 300D and the likes.
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Old 01-25-2006, 10:33 AM
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Wow. Just read the forum that created the question, and it's like asking "is the sky light blue or bright blue?"

Some day those guys are going to figure out their world (biodiesel) revolves around two issues: 1) constant temperature pre-heated fuel through the injection pump and 2) two lift pumps need two relief valves in a pressurized injection pump. Nothing less, nothing more.

I'm not saying it's easy. In fact, heavy fuels hasn't been done at the commercial/industrial scale in an internal combustion engine. The trains ran bunker C oil before diesel, but the bunker C engines were external combustion and diesel-electrics are internal combustion. Same drill in the marine world.
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Old 01-25-2006, 01:17 PM
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Quote:
Originally Posted by dabenz
...Some day those guys are going to figure out their world (biodiesel) revolves around two issues: 1) constant temperature pre-heated fuel through the injection pump and 2) two lift pumps need two relief valves in a pressurized injection pump. Nothing less, nothing more....
As one of "those guys" I feel compelled to inform you that you are confusing biodiesel with SVO. Biodiesel does not need to be pre-heated, while SVO does. Please do some more reading at that site to educate yourself on this matter before you trivialize our concerns and questions.
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  #14  
Old 01-26-2006, 08:35 AM
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Biodiesel is a plant-based "synthetic diesel" fuel. Straight vegetable oil (SVO), used as a fuel, is biodiesel. Waste vegetable oil (WVO), used as a fuel, is biodiesel. Vegetable oil that is processed to change it's molecular structure (I don't know if there's an abbreviation) is biodiesel. That's the way it will be unless the standards folks create completely different standards for SVO and WVO. Yes, standards are needed if this new industry is going to succeed. Straight biodiesel in any form won't work up here in the winter without preheating.

I applaud the efforts of the biodieselers. A hundred years ago the neighborhood blacksmiths built automobiles and engines. It's too bad the original question (the other forum) went from "how do I regulate injection pump pressure with two lift pumps" to the inner workings of the MB lift pump - when the MB lift pump wasn't designed for heated heavy fuels.
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Last edited by dabenz; 01-26-2006 at 08:40 AM.
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Old 01-26-2006, 08:40 AM
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Quote:
Originally Posted by dabenz
Biodiesel is a plant-based "synthetic diesel" fuel. Straight vegetable oil (SVO), used as a fuel, is biodiesel. Waste vegetable oil (WVO), used as a fuel, is biodiesel. Vegetable oil that is processed to change it's molecular structure (I don't know if there's an abbreviation) is biodiesel. That's the way it will be unless the standards folks create completely different standards for SVO and WVO. Yes, standards are needed if this new industry is going to suceed. Straight biodiesel in any form won't work up here in the winter without preheating...
Taken from the National Biodiesel Board (my bold added for emphasis):

General Definition of Biodiesel:
Biodiesel is a domestic, renewable fuel for diesel engines derived from natural oils like soybean oil, and which meets the specifications of ASTM D 6751.

Clarifying language to general definition:
Biodiesel can be used in any concentration with petroleum based diesel fuel in existing diesel engines with little or no modification. Biodiesel is not the same thing as raw vegetable oil. It is produced by a chemical process which removes the glycerin from the oil.

Technical Definition for Biodiesel (ASTM D 6751) and Biodiesel Blend:
Biodiesel, na fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats, designated B100, and meeting the requirements of ASTM D 6751.

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