Fuel solenoids causing troubles

[Jens]

I ran into an interesting situation today that I thought I would share.
I do not know if this is in fact true but I am at least 75% certain ....

I had starting issues with Thumper (more than usual) recently.
Thumper had completed a nice long run last night, I flushed all the veggy oil out of the system as usual and, I flipped off the primary load circuit from the generator. This is a habit I started very recently. I like the idea that Thumper comes to rest very gradually.
My setup is such that if I switch off Thumper it closes the fuel rack and shuts off the fuel flow solenoid. Those two happen immediately but Thumper of course keeps running for a while. In the past, I would not bother to switch off the loads on Thumper (this may be a bad thing if you run an ST head ... but I don't) and she would come to a stop fairly quickly. Well, with the loads mostly switched off it might take a minute or so to come to a complete stop.
Fast forward to this morning ..... Thumper started as usual but it took a bit longer to get up to speed and there was hardly any power available. I quickly traced that to air in IP#2 which is not unusual. Since she ran I disconnected the injector hoping that as usual the fuel would eventually start flowing (remember she is running at this point). That didn't happen and I had to take the delivery valve off the IP to get things flowing. Once flow was re-established full power was available and all was well (I ran into an intermittent circuit on the fuel pump so things died but that is another issue).
After thinking about this some time I decided that this is what is happening:
Thumper is running, everything is peachy, I switch to Diesel to flush the veggy oil, I turn off the main power circuit to the generator and then I switch off Thumper. The fuel solenoid switches off allowing zero fuel into the IP pump system. Thumper gradually winds down while merrily injecting fuel into the cylinders - a very small amount but it is not zero, possibly due to the rack not closing completely. Since no more fuel is allowed to flow but the IP's are still pumping, air is sucked into the IP fuel lines somehow (the system was completely replaced but is still doing this). At this time IP#2, possibly because the longer low pressure supply line, has a big air bubble in it and looses prime. The engine is now stopped completely.
The next startup proceeds as usual because for some reason IP#1 can recover from the situation quickly. IP #2 never catches again and Thumper limps along on one cylinder.

There are a number of ways that this can be addressed and I will look into this during the summer but I thought I would post this here in case other people run into weird and bizarre problems with the fuel system which they have trouble explaining.

[Ronmar]

I have run across a few instances of valves that were pressure rated, but had very low tollerance to vacume.  What is the fitting size on your solenoid valves?  I have also seen issues with solenoid valves used that were not specifically intended for fuel/oil.  I would say that any loss of fuel pressure/flow to the IP while it is still stroking would be a bad thing.  It is way easier to contain liquid oil than it is air.  You could have all kinds of air capable leaks, just waiting to admit air when the pressure drops below atmospheric, yet only weep the slightest trace of fuel.  I have seen this many times troubleshooting hydraulic systems.

[sailawayrb]

Agreed, running out of SVO on a cold night was a major concern of mine.  I built some fuel level switches from SS tube, SS floats (that have internal magnet), and reed switches.  If the SVO tank runs low, my fuel solenoid valve toggles over to the diesel tank.  If the diesel tank runs lows, my emergency shutdown system shuts down the engine.

[sailawayrb]

The switchover from veg to diesel is a tad more complicated but also easily done.

Not too complicated.  My fuel solenoid valves feeds from the SVO tank when it is energized and it feeds from the diesel tank when it is not energized.  When SVO gets low, the reed switch in SVO tank opens, fuel solenoid valve is no longer energized, and valve then feeds from the diesel tank.  

I was able to build the level switches by buying the raw parts from a level switch company in lieu of the assembled level switches.  I believe they wanted close to $100 each for these assembled SS level switches.  I believe I built them for less than $10 each.

[BruceM]

I also use homebrew (mini magnet inside) floats and reed switches for my oil sight tube/level sensor.  Piece of cake to make, and very cheap, as sailawayrb says.  I'll add the same to my diesel fuel tank sight tube one of these days.

12V delay relays shouldn't be too hard to find.  Or just let your micro controller handle it.   

[Ian]

Remote diagnosis is full of wrong assumptions and misunderstandings... However, with that in mind, may I suggest there could be a slight twist to your air in IP issue ?

As the IP has no seals other than fine tolerance mating parts, it relies upon the film of fuel between them to form the seal. You run the engine in a hot enclosure and the fuel supply lines and IP are going to be pretty hot within that enclosure.  This is not any kind of issue when the solenoids are open.

However, when you shut the engine down and close the fuel solenoids, the whole thing starts to cool. The fuel in the fuel system will also cool and contract and forming a partial vacuum. Air can be drawn into the IP (between plunger and barrel) to relieve the vacuum.

When you start up again (from cold) there will be an air bubble in the IP barrel. IP 1 may be able to compress this enough to actually move some fuel and allow cylinder 1 to fire, but IP 2 could have a bigger bubble because the fuel supply line is longer and pulls in more air because it contracts more. Maybe IP 2 cannot compress this bigger air bubble enough to move fuel and therefore cylinder 2 will not fire without bleeding.

I have learned the hard way that you should never shut down the fuel supply to a diesel engine when it is hot. Always keep a fuel supply available to provide for contraction of the previously heated fuel. The answer (if my remote diagnosis is correct) is to ensure that at least one of the solenoids allows fuel to the IP even when the engine is not running – or arrange for a bypass.

You may have a problem with an IP rack not fully closing but I doubt it. The fuel solenoids are probably doing exactly as they were designed – shutting off the fuel completely. A simple test to allow a fuel supply to the IPs when the engine is stopped could verify this.

I hope this helps in some way.

Regards,
Ian

[BruceM]

Jens, I have the same problem with my gravity fuel fed 6/1 IP.  If I turn off the fuel at the tank, and come back to start a week later, I've got air in the IP and have to remove the delivery line to clear it.  If I leave the fuel tank valve open, then there is no air in the IP and no problem starting.

Shutting off the valve after the engine is cool has the same result.

There is no noticeable weeping of fuel at the IP.

So until I can find and fix the air leak, I leave the fuel on, even when I'm going to be absent for a couple weeks.

[rl71459]

I think that Ian has a valid point.... I can see how air could be pulled by a sealess piston. this could
also explain how only 1 cylinder could have the problem (differences in the fit of the piston or quality
of the bore/piston)

It may also allow small amounts of fuel to seap into the crankcase oil.

Are the IP's Indian made?

Rob

[Ronmar]

You said you have seen air in the lines while running.  Are you using a feed pump of some sort?  If so, look to the suction side of that pump.  If it is restricted in any way such as by cold thick oil, it could be lowering the pressure enough to induce air.  This is very common on hydraulic systems and causes pump cavitation/additional noise as well as erattic cylinder operation as the air works it's way thru the system.  Common description is that it "gets better as the system warms up"

Good point about contraction in a sealed system.  This would certainly test the vacume tollerance of all your components...  I don't ever turn the fuel off on my 6/1.  It dosn't leak(if it did I would fix it) and it fires like a champ first compression stroke, pretty much every time...

[listeroil]

You do not switch off a lister by switching off the fuel.

The only reason to switch off the fuel is to change the fuel filter or work on the fuel system.

The IP carries on pumping when you switch off the fuel and sucks air into the IP.

Heres a page from a genuine 1971 Lister cs manual that tells you how to switch off the engine.



Mick

[veggie]

Listeroil,

What's the chances of posting that manual ? (PDF)
Looks like a good (modern) copy written in real english !

veggie