Help Understanding how this Setup solves air in the FI system of a 2-stroke engine

[vonsteimel]

Greetings,
We've been experiencing some problems with Fuel Injected 2-Stroke (3-cylinder) engines. Mainly starting the engines after the fuel lines have been purged or emptied (i.e. disassembly, shipping, fuel exhaustion.etc) The engines must be setup as shown in my sketch... They use a non-priming fuel pump.

After having the engine designers/mfg investigate the problems they confirmed our suspicion that air is being trapped in the FI system in the fuel rail. They said the air will not go out of the injector and is very difficult to get out. Their solution was shown in the top portion of the attached sketch and it does seem to be working.

I'm trying to understand what exactly this setup is doing and how it works in getting the air out of the lines. I'm thinking that on the return side of the jet there will be low pressure (lower pressure than the trapped air) and the air will find the path of least resistance --- and since the injectors are high pressure (higher than the air that is trapped) it will not expel through the injector.

Does this make sense? Once we understand it better we're thinking of buying or designing something a little simpler, more practical & aesthetically pleasing to do the same thing.

We are not the biggest fish in the sea with our engine vendor, who has more problems than time to solve them, so there was not a great effort to find a 100% fix. More of a "quick fix" if you will.

Any input understanding how this Setup solves air in the FI system of a 2-stroke engine will be greatly appreciated.
Thanks,

VS

 

[dgallup]

I think that mod is simply letting some flow through the system when the pump is trying to prime. Without the bypass orifice, the pump has to be able to build up enough pressure to crack the pressure regulator. With the bypass orifice there can be a small amount of flow at low pressure, letting the pump prime and getting the flow started. Once the pump gets primed and the rail is full of fuel the bypass is just a leak around the pressure regulator.

I would think it is going to hurt normal starting/restarting as the rail is always going to bleed down in pressure as soon as the engine stops.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[ivymike]

air flows easily through the orifice and back to the tank, whereas fuel flows through the orifice with difficulty - it gives a place for the air to escape back to the tank at startup w/o dropping the fuel outlet pressure in normal operation by too much. In other words, you can purge air from the system without having to pressurize it first to open the regulator.

 

[vonsteimel]

The engines will start with air in the system but run rough as guts and will one get up to about 3500 grand, sputtering the whole way.

So once the engine is running and the Pressure Regulator Diaphragm is working, allowing fuel to Bypass, would this not "purge" the system as you describe? Or is there something that will prevent the air from moving through the Pressure Regulator? -- Obviously air will not move through the Pressure Regulator or otherwise we wouldn't need this extra bypass.

 

[ivymike]

Why run so long w/air in the lines when something relatively inexpensive will fix it? Fuel injectors can be sensitive to supply pressure, having cavitation problems when pressure is low. I can't say what in the reg. would trap air without knowing more about its design and installation.

 

[MikeHalloran]

It would work even better if the tee branch actually faced 'up'. Maybe it really does in the installation, as opposed to the illustration.

Even better than that would be an air bleed from the very top of the fuel rail.


Mike Halloran
Pembroke Pines, FL, USA

 

[Tuckabag]

I imagine that if the rail is full of air and the pump wont self prime that it is stuggling to get sufficient fuel into the pump to generate enough pressure to crack the BP reg and purge the rail of air.
Instead of having the orifice, why not substitute it for an isolation valve?
You could then open the valve, fire up the fuel pump and purge the air from the rail back into the tank, then close the valve, prime the system and fire it up?

 

[vonsteimel]

I'm a Mechanical Engineer so I'm trying to understand this system before putting it into "production"...

If this is just a simple purge, I can find better ways to do it, such as Tuckabag's suggestion. Is everyones conclusion that this is just a crude purging system? I had the impression that there was something a bit more complex going on. Otherwise, I can understand why the air wouldn't purge through the pressure reg. We know there isn't a ton of air in the system otherwise it would not run.

No, the tee is facing down as shown in the sketch. All orientations are relatively accurate.

We've never had any injector problems because of this, they seem to be rather robust.
Thanks,

VS

 

[ivymike]

is tuckabag's solution really a better one? It sounds like it would be more expensive and would require manual input, whereas the orifice method is likely effective, inexpensive and works passively with virtually no possibility for error nor requirement for maintenance?

 

[dgallup]

I would attach it to the high point in the rail.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Tuckabag]

I agree that the orifice is a more reliable solution, however the disadvantage I can see is that it will let the rail bleed down pressure once the engine stops (as mentioned above) possibly affecting restarting. Secondly if you are selling this piece of equipment do you really want it running rough as it bleeds the air from the system? Surely you want to provide a method or mechanism that will allow you to bleed the air before you run the motor so it runs smooth from the start?

 

[ivymike]

Could add a check valve in the bleed line I guess, but it doesnt sound like running rough on initial startup after a fuel system service is a huge deal...

 

[Tuckabag]

Fair enough. Guess its up to the OP to decide whats most important with regards to how these motors are operated.
He got the answer he was looking for re what function the orifice performed.

 

[turbocohen]

I used to work for a mfg/supplier of rails and injectors. Modern rails for some brands are returnless because it helps to reduce heating of stored fuel and helps keep the light ends in solution. It woulds like you are dealing with very low fuel pressure.. What is the rail pressure your pumps deliver?

 

[vonsteimel]

It is a 3 bar system. Gasoline/oil 30:1 premix.

The fuel pump is switched on first for about 5 seconds before the engine is started. It must warm up before it really starts running good anyway. And air will only get in the system following disassembly, shipping, fuel exhaustion or pro-longed storage.etc So it is not a major issue all the time.

I still do not understand how this system works. Why will air not purge through the regulator when the pump is turned on before starting? It produces the 3+ bar required to operate the regulator, as fuel comes out the return.

 

[gt6racer2]

I suspect it is due to the compressibility of the air. Normally there is no flow until full pressure is reached and the regulator cracks, by which time the bubbles have become roughly 1/4 of their original size. ( 3 bar gauge pressure system, neglecting T ). By allowing the bypass, the bleed occurs at low pressure when the bubble will be large, and more will bleed out. By simple math, the amount of entrapped air will be reduced by about 75%.