Plantoil/diesel conversion basics
Sign up Latest Topics Chat
 
 
 


Reply
  Author   Comment   Page 1 of 2      1   2   Next
danalinscott

Moderator
Registered:
Posts: 934
Reply with quote  #1 

Diesel fuel systems normally send more fuel to the injector pump and injectors than is consumed by the engine. The unused fuel is then returned to the fuel tank via what is referred to as the "return" fuel circuit.

Most stock diesel fuel systems use a "full return to tank" circuit meaning that all fuel not consumed is returned to the fuel tank.

This circuit is provided for several reasons which include:

1. Preventing the accumulation of air bubbles in the fuel supply circuit which can starve the engine of fuel.

2. Providing enough flow of diesel fuel through the Injector Pump to prevent it from becoming too hot to provide adequate lubrication of the close tolerance internals of the IP.

Since the main goal of most conversion components is to raise the temperature of the VO to fuel it is common to divert or "loop" the unconsumed VO fuel back to the fuel supply rather than return it to the VO fuel tank. This is referred to as "looping" the VO fuel return and has become standard practice in most modern VO fuel conversions. A circuit that returns no fuel to the fuel tank is referred to as a "fully looped return".


Example #2 shows the addition of an air trap and "limited return" to VO tank.



Trouble free fully looped VO fuel returns require that all connections be perfectly sealed so no air can enter or must be free of vacuum at any point. In fact the majority of initial "bugs" in most VO conversions is due to air being drawn into the fuel supply circuit and accumulating in the "loop" until the engine is starved for fuel and appears to lose power.

To counter this a third type of return circuit was developed which allows a small amount of VO fuel to return to the tank and so provides a place for any air bubbles to "escape" from the loop. This is referred to as a "limited return" circuit.

A limited return is essentially an air trap installed in a fully looped VO return circuit which which allows any air bubbles to accumulate and an return circuit to the VO fuel tank which is "limited" by either a valve or fixed orifice so while air can easily pass back to the fuel tank freely only very small amounts of VO fuel are able to if no accumulated air is present. This air trap can be as simple as a "tee" fitting in a high point of the return fuel line with a needle valve installed in the line back to the fuel tank which allows the amount of flow to the tank to be adjusted.

Below is a rough  example of this simplest of air traps. Fuel flow is from left to right.






__________________
Dana danalinscott@yahoo.com
dsofio

Registered:
Posts: 5
Reply with quote  #2 
Hi Dana,

Thanks for the diagrams.  I think I am getting lost in trying to visualize the two systems interlinked/overlapping as one, with two pumps, running in two modes...  Do you have a flow diagram (no heat need be shown) for a two-electric-pump system that shows the partial-looping you're describing, along with solenoid valve?

In the case of a "fully-looped" or a "limited return" systems you describe, it seems to me that the liquid (whether diesel or WVO) throughput at the IP is, inevitably, greatly constricted in comparison with the full-return config.  That was a given?  Anyway, when you pull the return-line off of the stock-setup tank with the lift-pump running, it's a veritable gusher...right?  (I never tried this with my OEM pump, but my similarly-rated electric pump certainly pushes a lot of excess fuel through the IP, and I assume there is not a huge difference between mechanical and electric pump curves).  Maybe my observation that this limited- or full-constriction config you're describing will stop virtually all return to the tank was intended to be self-evident.  But I'm still confused, since it seems that on the one hand, you're acknowledging that it's part of the IP design that excess fuel should flow through the IP (and out the pressure-relief valve, or "PRV") for the sake of cooling and/or lubrication of the IP...but then, on the other hand, you're saying that it's common practice to reduce this flow to very little or even nothing by looping, (partially or fully) in order to cut down on the heat-loss from hot WVO that would otherwise be sent back to the tank.  Are you saying (other than WRT trapped air concerns) that it's OK to loop fully/partially, long-term, with no ill effect to the IP, despite the choking-off of the normal return-to-tank flow via the IP PRV?  (at least on the P7100/12V IP?)

2)  This may or may not be a totally different question, but I'm still trying to figure out how to avoid substantial cross-contamination from the purge-fuel returning to the WVO tank during purging.  So as I'm addressing above, if it's "OK" to simply stop up the return-to-tank flow and force the system to recycle and burn whatever has passed the filters (except maybe for what little goes through the WVO air-bleedoff line) then this has an easy answer:  it's only a tablespoon or so that transfers each cycle, so no worries.  But if I've misunderstood you, and it IS indeed still important for the IP to have a continuous flow of cooling- and/or lubricating-fuel...well, what's the schematic then?  The way I see it, with each minute or two of purge-time, the bypass/return tube would carry on the order of quarts or even gallons of purge fuel into the WVO tank, which would not work for long before overflowing and/or contaminating the VO to the point where a "WVO" tank would have no real meaning.

Please set the record straight, or set me straight, or whatever...

--Dave


danalinscott

Moderator
Registered:
Posts: 934
Reply with quote  #3 

That is a lot of fairly confusing questions DAVE...not sure I can answer all of them here. And I DO try to stay on topic as much as possible.

 

Quote:
Do you have a flow diagram (no heat need be shown) for a two-electric-pump system that shows the partial-looping you're describing, along with solenoid valve? 

 

No...I don't.

 

Quote:
In the case of a "fully-looped" or a "limited return" systems you describe, it seems to me that the liquid (whether diesel or WVO) throughput at the IP is, inevitably, greatly constricted in comparison with the full-return config. 

 

Constricted?   I don't think so.

In a "looped" return the fuel is simply returned directly to the IP feed line instead of being returned to the tank. The I will have the same amount of fuel available to it. Fuel that is used (by the injectors) is simply "made up" for by fuel from the tank.

 

Quote:
when you pull the return-line off of the stock-setup tank with the lift-pump running, it's a veritable gusher...right?  
That depends a great deal on the make/model of the engine. Some have barely any flow from the return some are a gusher.

 

Quote:
you're acknowledging that it's part of the IP design that excess fuel should flow through the IP (and out the pressure-relief valve, or "PRV") for the sake of cooling and/or lubrication of the IP...but then, on the other hand, you're saying that it's common practice to reduce this flow to very little or even nothing by looping 

 

The flow through the IP remains unchanged when a "looped" return is used so lubrication is not effected.  Cooling of most IPs is accomplished as long as VO entering it is under 150°F (as a rule of thumb). Looping merely conserves heat it does not create it...with a few exceptions.

 

Quote:
Are you saying (other than WRT trapped air concerns) that it's OK to loop fully/partially, long-term, with no ill effect to the IP 

 

Yes..as long as you do not overheat the VO being provided to the IP.

 

Quote:
I'm still trying to figure out how to avoid substantial cross-contamination from the purge-fuel returning to the WVO tank during purging.  So as I'm addressing above, if it's "OK" to simply stop up the return-to-tank flow and force the system to recycle and burn whatever has passed the filters (except maybe for what little goes through the WVO air-bleedoff line) then this has an easy answer:  it's only a tablespoon or so that transfers each cycle, so no worries.  

 

Returning a bit of diesel fuel to the VO tank during the purge cycle is a non issue AFAIK.  You can also loop the return during purge so all the diesel/VO  mixture is "burned. This is slower of course.

 

Quote:
But if I've misunderstood you, and it IS indeed still important for the IP to have a continuous flow of cooling- and/or lubricating-fuel 

 

Again..the flow through the IP doe snot diminish even when  the return is fully looped and NO fuel returns to either tank.

 

Quote:
The way I see it, with each minute or two of purge-time, the bypass/return tube would carry on the order of quarts or even gallons of purge fuel into the WVO tank, which would not work for long before overflowing and/or contaminating the VO to the point where a "WVO" tank would have no real meaning. 

 

This might be true for an unrestricted return on a few vehicles. But with a fast flow the purge is quickly accomplished possibly as quickly as 15 - 30 seconds. Once the purge is complete the return can be switched to full return to diesel tank.

 

I hope I have understood your questions and provided useful answers.  If not..try again.


__________________
Dana danalinscott@yahoo.com
dsofio

Registered:
Posts: 5
Reply with quote  #4 
Yeah, getting to the nub now.

I recall from some previous e-mail exchange that a reasonable target WVO temp is about 165F at injectors, no?  So if the looped WVO is up to that temp, then... that's also the "cooling" temp of the WVO being recycled into the IP, isn't it?  Still OK with that temp? 

If not, the way I see it, the WVO would need to somehow cool to the 150F you mentioned before being recycled to the IP.  This suggests the following thermal 'cycle' of temps needs to occur within that loop:
a)  IP inlet temp (max):  150F
b)  IP PRV (outlet) temp:  150F plus conductive heat gained heat from IP (varies...I'd be guessing)
c)  Injector temp target 165F (presumably reached from IP conduction, plus electrical heating of IP lines and/or further heat conduction from injector bodies)
d)  Injector bypass flow, combined with IP PRV outlet:  somehow cool to max 150 before recycling, i.e. remove all heat that had been added in the rest of the loop.

Does this bear out in practice?  Maybe there's a reason it isn't so critical?  I'm thinking there's not going to be much temperature drop in that loop, so the IP will be getting WVO at something maybe considerably higher than 150F.



danalinscott

Moderator
Registered:
Posts: 934
Reply with quote  #5 

Quote:
I recall from some previous e-mail exchange that a reasonable target WVO temp is about 165F at injectors, no? 

 

I prefer a pre-injector temp of around 250°F actually. This reduced VO viscosity to nearly the viscosity of diesel fuel for better combustion/less carbon accretion.

 

Quote:
So if the looped WVO is up to that temp, then... that's also the "cooling" temp of the WVO being recycled into the IP, isn't it?  Still OK with that temp? 

 

Actually higher IP temps tend to be the case only with IPs that have a high rate of return from the injectors. Most don't.  In nearly all cases even with a small amount of very hot VO being "re-cycled" back to the IP the temperature tends to remain fairly stable at only a few degrees higher than if no fuel is begin looped from the injector return line.

 

If you have a huge amount of return fuel from the injector lines I believe that you need to check to see if this is normal..or if the fuel is actually coming from just the injector lines.  Most injector return lines are also connected to filter return and/or IP return lines. The large amount of fuel may be coming from one of these other sources.

 

If you have replaced your OEM fuel pump make sure that the pump(s) you replaced it with are either self regulating or have a pressure regulator that limits the PSI fuel is supplied to the IP to that the OEM fuel pump supplied it. Over pressuring the IP might cause a greater than normal amount of return fuel flow.

 

I suspect that there are probably some exceptions to the rule of minor injector line return flows. In this case it might not make much sense to add injector line heaters since the flow might prevent them from significantly raising the temp of the fuel flowing through the injector lines.  I don't believe that one would need to worry too much about excessive heat being added by the IP in this case either.

 

I think that you may also be confusing my posts indicating that (as a rule of thumb) heating VO to over 150°F prior to the IP  is not efficient since  IPs will "absorb some of that heat in most cases  with my posts stating that exceeding the design temp of IPs is unwise if IP longevity is a major concern.

 

 

 


__________________
Dana danalinscott@yahoo.com
Dan

Registered:
Posts: 3
Reply with quote  #6 

Dana,

I have a Greasecar kit on my 1987 Mercedes, which uses a completely looped return.  As you know, this leads to trapped air in the VO system.  I am thinking of using your partial return loop configuration to manage the problem of air in my VO lines. 

 

My question for you is this: the needle valve is supposed to let air out and send it back to the VO tank; but what prevents air from coming in through the needle valve?  An air leak is an air leak, right?  What makes it one-way?  Wouldn't you need some sort of check valve, to make sure the air is only getting out, and not coming in? And would a plumbing check valve even work for this?  It would seem to me you would need a valve like on a car tire - but this would likely get clogged by the VO.

 

Am I going awry somewhere in my logic here?  

dsofio

Registered:
Posts: 5
Reply with quote  #7 
Ooh! Ooh! Can I answer?

I believe the loop pressure can never fall below atmospheric while the engine is operating, so there can never be backflow through the vent connection.  This is the same relative-pressure physics of our blood circulation system:  it keeps our veins from collapsing and makes us bleed out (for a while, at least) and tends to keep bad stuff out simply due to the pressure gradient. 

True, with the engine OFF, those pressures equalize, but as there is no OTHER place for air to enter the system, the air stays out--like having your finger over the end of a soda-straw keeps the liquid within, even as you raise it out of the glass.  If you DID have a place for air to get in under those conditions, it would quickly show up as a leak when the engine is running.

--ds

<afterthought edit>:  When I put my system together, rather than use a small line-sized tee, I think I will adapt a much larger tee to create ample space at the top, so that the liquid flowing through has ample dwell-time to let air bubble up towards the vent.  I'll place this as high as possible in the engine compartment, and run the air-vent hose from the high-point in order to maximize the air-removal, with a needle valve somewhere along that hose.
Dan

Registered:
Posts: 3
Reply with quote  #8 

So I guess the partial return hose, when it arrives at the VO tank, will have to remain completely submerged in fuel at all times, otherwise air would travel back through the needle valve, and into the return loop? 

dsofio

Registered:
Posts: 5
Reply with quote  #9 
Should be no need to submerge, since with engine running, the pressure in the loop is always above that in the tank/atmosphere due to added pressure from both lift pump and IP pressure-relief valve.  That's the pressure that causes the flow in the loop to  return to the tank and recycle to the IP inlet--it wouldn't move otherwise. 

When the engine is off, there couldn't be a place for air to get into the loop, other than a defect-- that would have presented itself as a leak while running.  So though gravity might tend to pull the liquid out of the loop, you'd first have to allow air in somewhere directly within the loop.  You should connect the air-bleed at the high-point of the loop to catch air at the place where it naturally will tend to collect.  Without another place for air to enter /within/ the loop (can't happen at the open or submerged end of the connected air bleed line) the loop will remain vacuum-locked and unable to drain.
rkpatt

Registered:
Posts: 1
Reply with quote  #10 
So what are actual examples or needle vaves or orifices to use  for the air trap / limited return and where do  i purchase them ? - Thanss
danalinscott

Moderator
Registered:
Posts: 934
Reply with quote  #11 

Quote:
Originally Posted by rkpatt
So what are actual examples or needle vaves or orifices to use  for the air trap / limited return and where do  i purchase them ? - Thanss


Needle valves are available at hardware and plumbing stores...as are tees and hose barb fittings if you want to fabricate and install a limited return "air trap". This is a fairly new (used around 2 years so far) development and so they are not off erred commercially as far as I know. One heated filter maker has incorporated an air trap/vent orifice in his filter head...but it being very slow about bringing his product to market. I will link to it in the component vendors section when he does.

if anyone is interested in fabricating a separate air trap and selling them let me know and I would be happy to work with you on this and provide a link in the components vendors section here once a good product has been developed.

__________________
Dana danalinscott@yahoo.com
zx9rn

Registered:
Posts: 1
Reply with quote  #12 
I am trying to install my Davco 234 plus filter in my 85 MB SD300. My wish is to bypass the stock filter completely. I wish to run the fuel through the aftermarket filter then into a nickel 16 flat plate heat exchanger then directly to the injection pump. This will be a single tank conversion. Does anyone have any suggestions on how to do this or where to obtain the fuel line diagram? I am also using manually controlled injector line heaters and manually glow plug switch.


__________________
Terry Pickard
danalinscott

Moderator
Registered:
Posts: 934
Reply with quote  #13 

Quote:
Originally Posted by zx9rn
I am trying to install my Davco 234 plus filter in my 85 MB SD300. My wish is to bypass the stock filter completely. I wish to run the fuel through the aftermarket filter then into a nickel 16 flat plate heat exchanger then directly to the injection pump. This will be a single tank conversion. Does anyone have any suggestions on how to do this or where to obtain the fuel line diagram? I am also using manually controlled injector line heaters and manually glow plug switch.


Have you read the info on "The differences between "single tank and "two tank" VO conversions ", "The basic concepts of two tank conversions and components," and "A good place to start .............." yet?  
 
The info on a simple MB single tank conversion may be on the Fattywagon website or the Lovecraft Biofuels sites.  The one you describe sounds similar to both of the single tank conversions promoted on these sites.
 
Please post future questions on this at "I have a question."

__________________
Dana danalinscott@yahoo.com
PabloCasa

Registered:
Posts: 2
Reply with quote  #14 

Would a standard air eliminator as is used in hydronic heating sytems work in this application?  Cheap, small, simple, lets air out of pressurized systems w/o letting any of the liquid out.


__________________
SVO Vehicles
SVO boiler
SVO Cogen
dsofio

Registered:
Posts: 5
Reply with quote  #15 
I don't know those devices per se but their cousins, as used in solar water-heating panels, are commonly known to blow off for various reasons other than their intended purpose.  If there were a seal-less model (no rubber) it might be worth a try, but then again, running a 'trickle' line back to the fuel tank where air can be purged without potential problem doesn't seem like too much trouble to me compared with the possibility of having an engine-compartment showered with a fine spray of WVO.  As far as I know we're talking about minute quantities of air here, so no need for wholesale removal devices.  I do like the notion of providing an amply-sized high-point chamber, though, so that if a big slug of air comes along it will have a place to accumulate rather than being passed along.
Previous Topic | Next Topic
Print
Reply

Quick Navigation:


Create your own forum with Website Toolbox!

This forum is brought to you by A proud sponsor of Invest in the future.