Fuel pump -Constant pressure or constant volume? - Page 2 - PeachParts Mercedes-Benz Forum

coachgeo · #16 01-26-2006, 10:22 AM

ok...... now that definitions are settled how bout we get back to constant pressure or constant volume etc.

TonyFromWestOz · #17 01-27-2006, 09:16 AM

The question came from a position that "deadheading" of a MB fuel pump would cause excessive fuel pressures, not relating to the use of it to pump diesel Vs other fuels.

There is no problem using this fuel pump to pump viscous fuels like unheated Vegetable oils. It works well in this application.
This is not related to the use of the priming pump mounted on the exterior of the fuel pump, which I believe pumps when the plunger is pushed down.

So, does anyone have something of substance to add to the question of whether the fuel pump (lift pump) on the Mercedes OM617 diesel engine pumps when the cam in the IP pushes the push rod (actuating pin), or when the push rod returns under spring pressure?

dkveuro · #18 01-27-2006, 10:39 AM

Quote:

Originally Posted by TonyFromWestOz

......................
So, does anyone have something of substance to add to the question of whether the fuel pump (lift pump) on the Mercedes OM617 diesel engine pumps when the cam in the IP pushes the push rod (actuating pin), or when the push rod returns under spring pressure?

How many more times..!

I replied about this days ago !
My post read......

"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."

.

TonyFromWestOz · #19 02-12-2006, 10:27 AM

The opponent to my post, on the other forum, that the fuel pump on the OM617 is a constant pressure fuel pump, has reversed his stand and apologised for the argument.

Thank you all for your comments, whether related to the question or otherwise.

BTW, the pump is only pumping biodiesel (processed from Used Cooking OIl (UCO)). I am using a different pump for the straight UCO (not processed into biodiesel)

miner · #20 07-10-2006, 02:22 AM

This discussion seems to have ended, but not because a definitive answer was found. Just because some guy on some other forum decided to change his mind.

Reading through the postings, though, both sides seem to be argued. Some say it is the IP return that regulates the pressure and others say it is the spring in the fuel pump plunger that does it. I am not intimatly familiar with the fuel system so I can't give an answer. But I would think that it is most likely that there is a pressure relief in the lift pump AND that the IP regulates pressure at the input via the cigar return line. But it actually doesn't matter as long as you don't deadhead the fuel pump. I mean, that was the whole point of the question right? To avoid damaging the lift pump. And I don't see why you couldn't just put a little bypass after the lift pump but before the fuel selector valve that ties into the tank return line. That would just cycle diesel fuel continuously to the tank when using vegoil. This would only be if you are putting the fuel valve between the lift pump and the stock diesel filter. If you are putting it after the stock filter then you are not deadheading it as there is return to the tank. Right?

I mean, it actually could be the case that it is a constant volume pump (relative to rpm?) where the pressure to the IP is regulated by the IP return line and all the excess fuel volume is returned to the tank. That probably isn't the case, but if it is then deadheading the lift pump could really mess something up. So just don't deadhead it and you'll be safe.

But it seems very likely to me that the IP would regulate pressure at it's intake via the return line. And I have seen where a lot of WVOers take the IP return line out. Is that a good thing to do? I don't see why it is neccessary. Surely you could rig your plumbing somehow so that you kept the stock setup on the diesel side. No?

TonyFromWestOz · #21 07-10-2006, 09:48 AM

Jackson,
The other party agreed with me after looking at his fuel pump again. I have the fuel pump dead headed into a Pollak valve and it works fine. Max 20 PSI.

WD8CDH · #22 09-07-2006, 02:17 PM

Hi Tony,

It seems to me that a better description of the MB lift pump is "Constant Volume/Pressure Limited". It is constant volume (per stroke) as long as it's output pressure is below it's pressure limit and Constant Pressure when the IP use or other restriction allows the output pressure to rise to the lift pump's set limit.

It also seems to me that since the pressure regulator in the IP return Banjo Bolt is lower than the Lift Pump pressure limit, the pump is normally operating as a Constant Volume pump with any excess flow from the lift pump not needed by the IP returned to the tank once the internal IP pressure reaches the regulators set pressure.

This makes me believe that there would be no return flow (looped or not) if an external lift pump was used that has a pressure output of less than the .8 bar setting of the return relief valve (like a 4-6psi electric pump).

Does this concure with your findings?

Also, does it look like there would be any damage to the MB lift pump if it's chamber was filled with cold solidified WVO?

The reason that I am asking this is I am thinking of changing my 2 tank system to use the stock MB lift pump for WVO and the electric lift pump for just the start-up diesel.