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rowlandcarson(at)gmail.co
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 Posted: Fri Jun 21, 2013 9:20 am Post subject: fuel return line restrictor |
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While delving into the various Rotax manuals in pursuit of information about the fuel manifold, I noted the dimensions of the restrictor in the fuel return line - 0.35mm ID.
I have been considering the most elegant way to incorporate the restrictor supplied by Europa (FS02) into my home-brewed alternative to the Rotax manifold and so checked its dimensions. I find that the restrictor hole will easily pass a number 71 drill bit, but barely accepts the shank of a number 70 drill bit. So I deduce that the orifice is about 0.7mm ID, ie twice the diameter of the Rotax one.
Why should Europa have supplied a different size of restrictor from that called out by Rotax?
Looks as though the Rotax orifice will (in any otherwise identical circumstance) allow the fuel pressure to build up slightly higher, and return less fuel to the tank, than the Europa one. It might just make the difference between pass and fail in a fuel flow test.
Has anyone had any issues traceable to this difference between the Europa-supplied restrictor and the Rotax one?
I might consider machining up my own manifold (obviously I'm heading into deep water with LAA engineering here) and if so, which size of orifice should I incorporate? Should it be removable, rather than integral, to allow fine-tuning of fuel pressure?
in friendship
Rowland
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carl(at)flyers.freeserve.
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| Posted: Fri Jun 21, 2013 10:43 am Post subject: fuel return line restrictor |
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Hi Rowland,
We have the Europa restrictor and with the new pump the fuel pressure comes
in at 5psi (the higher end of what is required).
We did some fuel pressure checks with an analogue guage and the pressure was
pretty much the same at idle and full power. I think the new Rotax pumps are
self regulating (pressure wise) soi the size of the rerstrictor would seem
to be irrelavent.
IMHO the more fuel that bleeds back to the tank the better (within reason)
as this keeps the temps in the engine bay as low as possible.
Carl - G-LABS
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Remi Guerner
Joined: 14 Dec 2010
Posts: 284
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| Posted: Fri Jun 21, 2013 11:50 pm Post subject: Re: fuel return line restrictor |
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Rowland,
I have tested several restrictor diameters from .35 to .8 mm. Here are my conclusions :
The restrictor diameter affects the fuel pressure, but not as much as you would think. Even with a plugged return line you barely see a higher fuel pressure.
Of course the return flow is greatly affected. Return flow with the .7mm restrictor is four times the flow with .35. Obviously, the higher the return flow the better, for vapor suppression, provided that the pressure is within the specified range.
The .35 restrictor is easily plugged by debris. This happened to me twice.
If you build your fuel system as described in the Europa Manual, using the .7 mm restrictor as supplied with the kit works the best.
Personnally after 8 years, 700 engine hours of testing different configurations of the fuel system, I am back to the .7 mm restrictor.
Now, why Europa specifies .7mm while Rotax says .35 ? This may be because in the Europa, the return line goes to the bottom of the tank while the Rotax diameter is designed for aircraft whose return line goes to the top of the tank, which is the case in most other aircraft. I believe that returning to the bottom of the tank offers more resistance, so a bigger diameter is necessary to ease the flow ? It would be interresting to hear the Europa designers on that subject.
Regards
Remi
F-PGKL, XS Monowheel, 912ULS, 1018 airframe hours
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grahamsingleton(at)btinte
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| Posted: Sat Jun 22, 2013 12:25 am Post subject: fuel return line restrictor |
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Remi
I think it makes more sense for the return to go to the top of the tank, at the bottom warm fuel can be taken straight back to the engine.
That's how I always did it anyway and never had a problem.
Fuel gets heated while it is in the engine driven pump, when the engine is hot the pump can be as much as 75deg C.
With low fuel flow, when taxiing for example, fuel will heat up to vaporization point. Jabiru engine is problematic , isn't the pump buried at the back of the engine?
Graham
From: Remi Guerner <air.guerner(at)orange.fr>
To: europa-list(at)matronics.com
Sent: Saturday, 22 June 2013, 8:50
Subject: Re: fuel return line restrictor
--> Europa-List message posted by: "Remi Guerner" <air.guerner(at)orange.fr (air.guerner(at)orange.fr)>
Rowland,
I have tested several restrictor diameters from .35 to .8 mm. Here are my conclusions :
The restrictor diameter affects the fuel pressure, but not as much as you would think. Even with a plugged return line you barely see a higher fuel pressure.
Of course the return flow is greatly affected. Return flow with the .7mm restrictor is four times the flow with .35. Obviously, the higher the return flow the better, for vapor suppression, provided that the pressure is within the specified range.
The .35 restrictor is easily plugged by debris. This happened to me twice.
If you build your fuel system as described in the Europa Manual, using the .7 mm restrictor as supplied with the kit works the best.
Personnally after 8 years, 700 engine hours of testing different configurations of the fuel system, I am back to the .7 mm restrictor.
Now, why Europa specifies .7mm while Rotax says .35 ? This may be because in the Europa, the return line goes to the bottom of the tank while the Rotax diameter is designed for aircraft whose return line goes to the top of the tank, which is the case in most other aircraft. I believe that returning to the bottom of the tank offers more resistance, so a bigger diameter is necessary to ease the flow ? It would be interresting to hear the Europa designers on that subject.
Regards
Remi
F-PGKL, XS Monowheel, 912ULS, 1018 airframe hours
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Duncan McFadyean
Joined: 18 Jan 2011
Posts: 222
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| Posted: Sat Jun 22, 2013 12:41 am Post subject: fuel return line restrictor |
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Rowland,
The restrictor orifice dimension you quote is the stock Europa size, i.e.
about twice what Rotax recommend, and what everyone else has!
Duncan McF
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budyerly(at)msn.com
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| Posted: Sun Jun 23, 2013 10:46 am Post subject: fuel return line restrictor |
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<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]--> Rowland,
I noticed the same thing a couple months ago installing Jeff Robert's new engine. Rotax now uses the .35 mm and our FS02 is .025 inches or .7mm.
My A&P researched the difference with Lockwood and I did it through Rotech.
We asked if we needed to change. They didn't care. The fuel pump (mechanical) has been changed slightly and as usual Rotax never bothered to explain the difference in flow and performance. We were told to expect slightly higher fuel pressures with the new restrictor mounted on the fuel split fitting. Basically if the fuel pressure is between 2.2 min and 5.8 max fine. No optimum is set by Rotax at max power at sea level that I have found.
As for flow you will see no difference since the carbs are using the fuel based on the need of the engine, not the pressure in the fuel line. The float needle cuts off the flow to the carb. The flow back through the return line will be that volume of fuel which is unused. Regardless of pressure (provided it is within limits.) Only if the float needle sticks open will you see a significant difference in flow (and of course engine performance).
Other detail:
We did a test using the supplied Rotax restrictor on the new 912S vs our old FS02. Fuel pressure dropped a bit with the FS02 but did not get anywhere near the 5.8 max of the fuel pressure with both pumps running and was above the 2.2 min with just the engine pump and when pulling the fuel line off the engine pump and putting only the electric pump into action it was still above the min. We saw a bit of an increase with the smaller fuel restrictor of .3 psi. on our old hand held pressure gauge. We were still getting 3.2 psi with the FS02 and 3.5 with the Rotax (as measured with my really old gauge which was calibrated three years ago to 3 psi or 83 in. H2O) so not much difference (Jeff's electric gauge was about 4 psi with the Rotax engine pump and a bit lower on the electric pump.
The manufacturer of the aircraft is responsible for his setting up of the fuel system. If the FS02 works within limits, why change it.
Kerry at Lockwood is ambivalent about the difference. He has found some builders who weren't using restrictors at all and the 912S ran fine on high wing aircraft (Rans S 6). No report on the low wing aircraft, they all had restrictors supplied by the kit manufacturer or Rotax. If it is within limits, at full power, it is within limits.
In the US we don't have to do fuel flow tests, but to check you just need to put a clear tube inline on the return side and look at the flow or collect it into a measuring cup. If the pressure in the line with the FS02 is only .3 psi different, then the higher pressure of the Rotax orifice will flow faster through the orifice but the flow will be the same as with the FS02. Pump volume makes a huge difference in return flow. Nice of Rotax to change the pump slightly without providing tests and results. Note that the wrong auxiliary pump (such as the automotive low pressure style, like the Facet or Purilator 4104/5) will make a difference as they don't provide the volume. Years ago Europa did SB 04 specifying the Facet 40106. Only if one installs a restrictive fuel supply system by using a small diameter fuel selector, fuel filter, gascolator, etc. will there be a problem. Keep the fittings and fuel selectors as specified and the Europa fuel system works well. The LAA requirement for excess fuel flow check is a great idea on an untried engine/fuel system, but on a tried and true design it is just scary as hell and a bit noisy and tedious. We just empty the pax side of the tank, and run the engine checks on the main and note that the fuel returns to the tank and then fiddle with the fuel flow on the Dynon or EIS until close to 7 GPH. After about 15 minutes of running, you will see a small amount of fuel in the reserve or pax side and have a fuel flow that is fairly close for initial cruise checks.
Pretty stupid and lazy and not very scientific of us, but that is all we have ever needed.
At my shop we check that the fuel pressure is in the range with only one pump and does not exceed the max with both on. We have never had a problem with the min pressure. Our US DARs don't care as long as full power runs have been made and the kit system is installed as per the manufacturer of the kit. If the kit manufacturer does not specify a fuel system, a good DAR will want evidence of a full power run only, but no excess fuel flow requirement is set. We are at sea level so the fuel flow should be at its max with the prop set to 5200-5600 RPM at full throttle. My DAR and FSDO are happy with that.
Best Regards,
Bud Yerly
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