Fuel tank pressure

[aussiemick]

Hi.

I am having fuel tank foaming issues with my Srimok.

I have opened the tank opening up by 2.5mm, the tank is pretty loose in the frames.

I have a OS91HZR engine and I am think the fuel tank pressure maybe too high, the tank gets pretty tight in the frames once the engine has fired up and run.

Is it possible to regulate the fuel tank pressure by varying the length of the pressure line that goes between the crank case and the One Way Valve?

The OS manual for the HZR recommends 100mm for the pressure line, I cut the line exactly 100mm.

Am I correct in thinking that a longer line between Crankcase and OWV would reduce the tank pressure?
Would the regulator still work as designed if the tank pressure was less?

Thanks in advance

Mick

[TmMugen]

Actually I think that is the problem with pumped engine..We got people flying trex 700 with YS91SR and still the foaming is seen in the rex..My humble conclusion is that as long as there is no lean runs, I personally do not care too much. Just enjoy srimoking..

[dale-pabajo]

the trex 700 has a fuel tank plug witch I consider would work as a relief valve at some point where the tank has enough pressure to be released by that plug so the tank does not exploed or braek.
the Srimok fuel tank only has one large plug with four holes with no type of release plug.
in the manual Kasama recommends a OMI fuel magnet witch I imagine is for reduceing the foaming action caused by vibration.
the Srimok fuel tank does have a tight fit on the frame and just mabe the space where the fuel tank is located need's to be enlarged enough so the tank could swell with the pressure.
but I have yet to consider this opption cause I'm still on the building phase of my Srimok.

[Mercuriell]

Originally Posted by aussiemick

Is it possible to regulate the fuel tank pressure by varying the length of the pressure line that goes between the crank case and the One Way Valve?

The OS manual for the HZR recommends 100mm for the pressure line, I cut the line exactly 100mm.

Am I correct in thinking that a longer line between Crankcase and OWV would reduce the tank pressure?

Would the regulator still work as designed if the tank pressure was less?

Thanks in advance

Mick

I don't think the length of the tubing between OW and crank will alter pressure much - the pumped engines do pressurise very high and if the frames cutouts are widened still further the tank will likely expand to fit. The pressure generated is really a function of the cracnkase design and rpm. I don't see why the regulator should not work with a reduced pressure - may be worth trying exhaust pressure but start off rich to avoid leaning it out too much. Does your engine lean out as the fuel level goes down ??

[aussiemick]

The reason I think that a longer fuel tube between OWV and crankcase would reduce pressure is, the line would be a greater volume (in theory if the line was a large tank, the pressure increase would be tiny for every stroke of the engine) and as it has to pressurise to greater than in the tank before pressure can get through to the tank, therefore pumping less air into the tank, also line friction losses will all stand to help reduce pressure......?? Just guessing a little bit here armed with a fair bit of materials handling experience and practical experience as a fitter

I will just give it a go this weekend, lengthen it by 50mm (increase of 50%) and see what happens. As long as the regulator still lets fuel through then it should all at least fly.

I will set everything pretty rich to try it out.

Mick

[Mercuriell]

Interesting thoughts - I guess it depends on the ratio of the volume of the tubing to the stroke volume produced by the engine and also the elasticity of the tubing - maybe thinner walled tuning might help !

PS Where you flying your Srimok - I fancy flying my nitro this w/e and might bring the Faifa out for an airing too though waiting for a canopy !!

[aussiemick]

John

Am planning on flying at Dakabin on Saturday, probably later on in the day.
Then going down to Canungra on Sunday.

Will see if I have any thinner tubing in my box of tricks.

Mick

[husafreak]

Does the foam affect your engine run? I see foaming in my tank but it runs great and at the very end of the flight I don't see foaming anymore the small amount of fuel at that point sits quietly. There are some videos of spectacular hard 3d flying of the Srimok that clearly show foaming on takeoff... so I'm not bothered by it.

[Mercuriell]

I found when I had engine foaming the engine did tend to lean out a bit more on the last 1/2 tank - since I Dremelled out the cutouts there is NO foaming or boilng at all

[aussiemick]

I have had pretty good success with lengthening the fuel line between Crankcase and OneWayValve.

The foaming is now at a level where the engine will hold a tune, it is still a bit rich if anything. Before the pressure in the tank was insanely high, now it is possible to push the tank in a bit once it is pressurised. I reckon I will try out a 25cm length of tube, it is now 20cm, and see if the engine still holds a tune with the reduced pressure.

I had a hard time getting the pinion into the clutch bell during the original build and I think the vibrations are coming from there, Kasama was kind enough to send a replacement (unfortunatey he packaged it a bit light and it got damaged on its way to me) I will try it out next outing if I can panel beat the dent out of the clutch bell.

Mick