Fitting an electric fuel pump

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pcatterall

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My mechanical pump on my Perkins 4108 works fine and I am not really tempted to change it.
My question is therefore really a matter of interest.
Would there be any advantage in 'going electric' or are we just making something simple more complicated?

My real interest that has been exercising my little brain is how does an electric pump supply the right amount of fuel?
My mechanical pump will vary the supply to match engine revs with an amount of oversupply which will be returned? ( as I see it!)
An electric pump would give a fixed ( unless there was a complicated means of adjusting to match revs).
Could the flow from an electric pump vary with the amount of fuel in the tank due to the different head height?

Perhaps a simple explanation from one of our experts would help me and others
 
I'm not an expert as you know but my understanding is that the lift pump supplies an excess of fuel at all times to the metering pump. The surplus is usually returned to the tank or, in your case, to a spare tapping on the fuel filter. It's not necessary to vary the flow as the metering pump will take what it wants depending on throttle setting and return the rest.
 
ghostlymoron said:
I'm not an expert as you know but my understanding is that the lift pump supplies an excess of fuel at all times to the metering pump. The surplus is usually returned to the tank or, in your case, to a spare tapping on the fuel filter. It's not necessary to vary the flow as the metering pump will take what it wants depending on throttle setting and return the rest.
Click to expand...

+1

Our pump starts as soon as the key is turned and the engine just sups what it needs, the rest being returned to the tank clean after the filters. Brilliant for filter changes, no manual bleeding necessary.
 
pcatterall said:
My mechanical pump on my Perkins 4108 works fine and I am not really tempted to change it.
My question is therefore really a matter of interest.
Would there be any advantage in 'going electric' or are we just making something simple more complicated?

My real interest that has been exercising my little brain is how does an electric pump supply the right amount of fuel?
My mechanical pump will vary the supply to match engine revs with an amount of oversupply which will be returned? ( as I see it!)
An electric pump would give a fixed ( unless there was a complicated means of adjusting to match revs).
Could the flow from an electric pump vary with the amount of fuel in the tank due to the different head height?

Perhaps a simple explanation from one of our experts would help me and others
Click to expand...

Whether mechanical or electric pump, as long as it keeps up with demand, delivering too much will not be a problem. Constant speed electric fuel pumps are the norm on diesel cars.

Colin. Www.solocoastalsailing.co.uk
 
don't most mech lift pumps provide a pressure, not a fixed volume of fuel?
The return to the tank is from the injectors, not the lift pump?

A typical petrol injection system has a pump with excess capacity, plus a pressure regulator which dumps the excess back to the tank.
A yacht diesel with mechanical HP pump could do the same on the LP side.
Not sure about modern diesels!
 
lw395 said:
don't most mech lift pumps provide a pressure, not a fixed volume of fuel?
The return to the tank is from the injectors, not the lift pump?

A typical petrol injection system has a pump with excess capacity, plus a pressure regulator which dumps the excess back to the tank.
A yacht diesel with mechanical HP pump could do the same on the LP side.
Not sure about modern diesels!
Click to expand...

I'm still struggling to understand just what does happen. No problem with a common rail engine, or one in which there is a return to the tank directly from the injection pump or the secondary filter bowl. But in engines like my Yanmar 3GM30 there is no return pipe from anywhere other than the injectors. As I understand their function the injector pump delivers a metered amount of fuel to each injector at the precise moment it is needed. A very small amount of fuel is not injected, as there is a delay while the pressure builds to the 2000 bar or so that is needed. This very small amount comes back to the tank. I measured the flow on mine when doing 2000 rpm and collected a tiny amount, something like 1-2 ml per minute.

The lift pump is a positive displacement type that cannot be throttled down. So on my engine presumably the flow must be going somewhere, or the lift pump design somehow allows recycling. I would be very pleased to hear from someone who knows.

Thinking more about it, in the days when I had various petrol vehicles with electric fuel pumps, there was no weir system or return to the tank, the pump stopped when there was no demand. Do mechanical pumps work the same way?
 
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There's the potential for a big bonus if the pump is fitted between the tank and the primary filter.

A pair of filters can be set up in parallel, with both primed & ready to go. Run on one, with a spare in reserve. Should on filter block, it's seconds to switch to the other and you could change the duff one while the engine's still running - as long as the pump has sufficient capacity, you could even prime the new one on the go. I have a similar setup, though gravity fed, so I doubt I'd get away with priming if the engine's running at much more than idle.
 
I have found the answer to my question at http://www.cdxetextbook.com/fuelSys/diesel/comp/dieselliftpump.html The pump does not work in the way I had assumed, it is not the force applied by the cam-driven lever arm that pumps the fuel, it is the return spring.

'Atmospheric pressure at the fuel tank forces fuel along the fuel line, to open the inlet valve. Fuel flows into the pumping chamber. The eccentric keeps rotating, and the rocker arm is released. The spring exerts force on the diaphragm, to pressurize the fuel in the chamber. This pressure closes the inlet valve, and opens the outlet valve, letting fuel be delivered to the injection system.

If the system doesn’t need all of the fuel delivered, the pressure in the outlet fuel line rises to the same level as in the pumping chamber. That holds down the diaphragm, and keeps the diaphragm return spring compressed. When this occurs, the split-linkage in the rocker arm, allows the lever to maintain contact with the eccentric, without acting on the diaphragm pull-rod.'
 
I have a small electric pump that I use to prime the system in stead of hand operating the lift pump, when I had some trouble following running out of diesel I ran it continuously and It stopped the engine cutting out and ultimately cleared things out. I have two filters and the 12v pump is inline between both so it sucks and blows....
 
Thanks all I was happy with the concept of the mechanical pump providing excess fuel as, at least there was some control when the revs increased I suppose ( thinking some more) that the electric pump would only grossly oversupply at tick over and that at cruising revs there would only be modest oversupply and at full belt just small oversupply .
So if I fit one do I block off the mechanical one or just simply disconnect the in and out?
Some responses seem to suggest that they have an electric pump in line with the mechanical one and presumably switched separately. I can see the 'bleeding' benefits of this and assume that one doesn't interfere with the other?
 
pcatterall said:
Some responses seem to suggest that they have an electric pump in line with the mechanical one and presumably switched separately. I can see the 'bleeding' benefits of this and assume that one doesn't interfere with the other?
Click to expand...

Once you obtain a pump that allows flow without being energized, then the pump can sit there until needed for bleeding or supply in the event that you have a failure with the lift pump.
 
I agree, not necessary to vary the flow, and like the questioner, also have a Perkins 4108, never had a problem with the pump, but, keep a spare just in case.
I also have a dual system, let me explain; following a engine stoppage just outside a well used entrance, ferries etc, due to taking on dirty fuel, and having to spend an hour changing filters whilst ferries passed doing 15kn. and the subsequent violent motion caused, including seasickness, I was determined never to be in a similar situation ever again, To that end I built a 'fuel polisher' based on a design seen elsewhere, using a 12v petrol pump which picked up fuel from the tank, passed it through a water separating filter then a fine filter and returned to the tank, so 'polishing' the fuel. I further added a three way valve so that the polished fuel could be directed for other purposes, namely to quickly using flexible pipe feed the final engine filter, bypassing the mechanical one if it has failed. So far, other than a test run, has not been needed. The other benefit, besides ensuring clean fuel to the primary filters (two) I have access to fuel to fill the replacement filters, rather than using the manual mechanical pumpslever, which does lasting damage to the thumb!
 
Stemar said:
There's the potential for a big bonus if the pump is fitted between the tank and the primary filter.

A pair of filters can be set up in parallel, with both primed & ready to go. Run on one, with a spare in reserve. Should on filter block, it's seconds to switch to the other and you could change the duff one while the engine's still running - as long as the pump has sufficient capacity, you could even prime the new one on the go. I have a similar setup, though gravity fed, so I doubt I'd get away with priming if the engine's running at much more than idle.
Click to expand...

Good in theory but in practice it doesn't work with ours. It has a small cartridge filer about the size of half a cotton bobbin which blocks well before the primary filter - ask me how I know:)
 
vyv_cox said:
I'm still struggling to understand just what does happen. No problem with a common rail engine, or one in which there is a return to the tank directly from the injection pump or the secondary filter bowl. But in engines like my Yanmar 3GM30 there is no return pipe from anywhere other than the injectors. As I understand their function the injector pump delivers a metered amount of fuel to each injector at the precise moment it is needed. A very small amount of fuel is not injected, as there is a delay while the pressure builds to the 2000 bar or so that is needed. This very small amount comes back to the tank. I measured the flow on mine when doing 2000 rpm and collected a tiny amount, something like 1-2 ml per minute.

The lift pump is a positive displacement type that cannot be throttled down. So on my engine presumably the flow must be going somewhere, or the lift pump design somehow allows recycling. I would be very pleased to hear from someone who knows.

Thinking more about it, in the days when I had various petrol vehicles with electric fuel pumps, there was no weir system or return to the tank, the pump stopped when there was no demand. Do mechanical pumps work the same way?
Click to expand...
I think lift pumps are not fixed displacement pumps rigidly driven by a cam, they are driven by a mechanism which exerts a limited force, so when the back pressure is high enough, the pump doesn't pump any harder.
It's often a spring which provides the pressure, the cam drives the suction stroke against the spring.....

http://2.bp.blogspot.com/-HgsHQPKQ1...600/BLEED_YOUR_DIESEL_ENGINE_042910_PE-P3.jpg
 
lw395 said:
I think lift pumps are not fixed displacement pumps rigidly driven by a cam, they are driven by a mechanism which exerts a limited force, so when the back pressure is high enough, the pump doesn't pump any harder.
It's often a spring which provides the pressure, the cam drives the suction stroke against the spring.....
Click to expand...

Yes, that's my understanding too. My MD21B doesn't have a return to tank and once the lift pump has built enough pressure it 'floats'. You can feel this by pumping the priming lever: after 1 or 2 strokes it is at full pressure and the priming lever just flaps up and down without doing anything. At this point the actuating lever/roller in the pump is away from the cam lobe and only returns onto it once the pressure drops.
 
lw395 said:
I think lift pumps are not fixed displacement pumps rigidly driven by a cam, they are driven by a mechanism which exerts a limited force, so when the back pressure is high enough, the pump doesn't pump any harder.
It's often a spring which provides the pressure, the cam drives the suction stroke against the spring.....

http://2.bp.blogspot.com/-HgsHQPKQ1...600/BLEED_YOUR_DIESEL_ENGINE_042910_PE-P3.jpg
Click to expand...

Yes thanks, I found it after I posted the question, see #10.
 
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