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914: fuel over airbox pressure

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josok-e(at)ukolo.fi
Guest

Posted: Sun Mar 18, 2007 10:29 am Post subject: 914: fuel over airbox pressure

Hi team,

Does the UMA transducer connect between the airbox and the carburettor feed line or between the airbox and the fuel feed on the pressure regulator? In other words: before or after the pressure regulator? 



In the rotax doc an after pressure regulator setup is shown as a desired, not supplied option, however i assume that it is more interesting to see a warning about fuel pressure drop before the regulator runs out of pressure to regulate  



Kind Regards,



Jos Okhuijsen

Visit -  www.EuropaOwners.org

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europa(at)pstewart.f2s.co
Guest

Posted: Sun Mar 18, 2007 10:48 am Post subject: 914: fuel over airbox pressure

Jos



I've taken mine from pre regulator - not flying yet though.



Paul

G-GIDY

On 18 Mar 2007, at 18:25, josok wrote:



 	  Quote: 	
		   



 Hi team,

 Does the UMA transducer connect between the airbox and the  

 carburettor feed line or between the airbox and the fuel feed on  

 the pressure regulator? In other words: before or after the  

 pressure regulator?



 In the rotax doc an after pressure regulator setup is shown as a  

 desired, not supplied option, however i assume that it is more  

 interesting to see a warning about fuel pressure drop before the  

 regulator runs out of pressure to regulate 



 Kind Regards,



 Jos Okhuijsen

 Visit -  www.EuropaOwners.org

- The Matronics Europa-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

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jodel(at)nildram.co.uk
Guest

Posted: Sun Mar 18, 2007 11:13 am Post subject: 914: fuel over airbox pressure

Jos,



The only measure of fuel pressure on the 914 that Rotax mentions is that the

px to the carbs should be 2-5psi over airbox px.  That sort of dictates that

you should measure the airbox px and the carb inlet px (ie between the

regulator and the carb).

I have seen somewhere in the Rotax manuals a picture of a special banjo bolt

and hose end to pick off the regulator output px (but I can't remember

where!). I also have a very vague memory that this should only be taken from

one particular side of the regulator?????



Simon



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jodel(at)nildram.co.uk
Guest

Posted: Sun Mar 18, 2007 11:16 am Post subject: 914: fuel over airbox pressure

Just to add, see page 69 of the current installation manual on the Rotax

website.



Simon



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Gilles.Thesee(at)ac-greno
Guest

Posted: Sun Mar 18, 2007 11:26 am Post subject: 914: fuel over airbox pressure

Simon Smith a écrit :

 	  Quote: 	
		    That sort of dictates that

 you should measure the airbox px and the carb inlet px (ie between the

 regulator and the carb).... 

   

Hi all,
	




The pressure on the upper side of the regulator (IN and OUT connections) 

is the fuel-to-carb pressure.

The pressure on the lower side of the regulator is the airbox pressure.

See installation manual or

http://contrails.free.fr/engine_regul_en.php



Hope this helps,

Regards,

Gilles

http://contrails.free.fr

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grahamsingleton(at)btinte
Guest

Posted: Sun Mar 18, 2007 1:21 pm Post subject: 914: fuel over airbox pressure

Gilles & all

I'm not familiar with 914 but I do remember that the return flow from 

the regulator must be unrestricted otherwise the back pressure will 

cause rich mixture? Because of the high flow rate required with all 

pumps running the return pipe needs to be at least -6 size.

Hope I'm not confusing the issue

Graham



Gilles Thesee wrote:

 	  Quote: 	
		   

 <Gilles.Thesee(at)ac-grenoble.fr>

 

 Simon Smith a écrit :

 

>  That sort of dictates that

> you should measure the airbox px and the carb inlet px (ie between the

> regulator and the carb)....   

 

 Hi all,

 

 The pressure on the upper side of the regulator (IN and OUT connections) 

 is the fuel-to-carb pressure.

 The pressure on the lower side of the regulator is the airbox pressure.

 See installation manual or

 http://contrails.free.fr/engine_regul_en.php

 

 Hope this helps,

 Regards,

 Gilles

 http://contrails.free.fr

 

 

 

 

 

 

 

 

 

 

 

 


	


-- 

Graham Singleton



Tel: +441629820187

Mob: +447739582005

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Gilles.Thesee(at)ac-greno
Guest

Posted: Sun Mar 18, 2007 1:36 pm Post subject: 914: fuel over airbox pressure

Graham Singleton a écrit :

 	  Quote: 	
		   I'm not familiar with 914 but I do remember that the return flow from 

 the regulator must be unrestricted otherwise the back pressure will 

 cause rich mixture? Because of the high flow rate required with all 

 pumps running the return pipe needs to be at least -6 size.


	


Graham,



You're right.

I recently conducted some tests to verify this point. I pressurized the 

regulator to the max 1,3 bar absolute, while running the pump. All the 

flow was by-passed by the regulator, with no problem to maintain the 

specified ambient + 0.25 bar.



While I was perched on a ladder, doing the pressurizing with a U-tube, 

my buddy sitting on the wing forgot to switch the different pumps, so 

some additional tests are necessary.



Best regards,

Gilles

http://contrails.free.fr

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rehn(at)rockisland.com
Guest

Posted: Sun Mar 18, 2007 3:47 pm Post subject: 914: fuel over airbox pressure

UMA gauges makes a gauge specifically for measuring the difference between 

the fuel pressure and air box pressure. The difference in pressure ( fuel 

pressure above air pressure) must be between 2.18 and 5.08 PSI. This gauge 

measures the difference which is much easier than using two gauges and doing 

the math. The fuel pressure is measured at the starboard outlet of the fuel 

pressure regulator. A banjo bolt fitting is connected at that point to form 

a T. The air pressure is connected in the line that goes to the airbox 

pressure sender via another T. Both of these lines are connected to the UMA 

sender which in turn electrically sends the info to the gauge on your panel. 

The gauge has range markings on it for the 914. I have had mine for two + 

years and it works great and was easy to install. Check out spruce for more 

details   http://aircraftspruce.com/catalog/inpages/umadiff1.php



Jerry

 	  Quote:

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rparigor(at)suffolk.lib.n
Guest

Posted: Sun Mar 18, 2007 7:08 pm Post subject: 914: fuel over airbox pressure

Hello Jos



Sorry I did understand exact wording of your question, but will describe

what you need to know and do.



The 914 uses a turbocharger to squeeze air molecules together at a greater

than ambient pressure. Unlike a normal aspirated engine, if you were to

vent the float bowl to ambient pressure on a turbo charged engine and

began to boost above ambient pressure, air would in fact flow the wrong

way through the carb jets and starve the motor for fuel.



Thus on a 914 the float bowl is vented instead of to ambient pressure, to

airbox pressure. When not in boost airbox will be close to ambient, when

boosting it will go above ambient pressure. 



OK so now we have at least bowl pressure following turbo output.



Now you need greater fuel pressure than float bowl pressure (which is the

same as airbox pressure) to be able to fill float bowl with fuel. This is

the job of the fuel pressure regulator. You want between approx 2 and 5

PSI fuel pressure (absolute) over airbox pressure (absolute).



In absolute terms, you need your fuel pumps to work harder in thinner air

than in thicker air. It is very possible that filters and pumps can

provide at 2K, but not at 8K. If pressure can not be kept up, you are in

danger of having your motor playing silent night. Turning on second pump

can help, going down in altitude can help, and dropping manifold pressure,

which will drop airbox pressure will help. Remember a stuck wastegate can

overboost a 914. The fuel pressure regulator can not keep up if you go too

much over redline boost pressure. Thus you can run out of fuel on take off

in a very bad position. If you are not between 2 and 5 when rolling, abort

take off. If you get airborne, and have engine quit, see if reducing

manifold pressure helps get back differential of at least 2 PSI, if it

does,you can probably fly pretty safe at a reduced power till you can

land.



An awful lot of words. Simple way to follow is to have a differential

gauge and keep it between 2 and 5.



I hope the following will answer your question:

You need to plumb 2 lines to your differential sender, 1) fuel pressure

and 2) airbox pressure.



1) Fuel pressure. You need to connect to one of your carburetor fuel

supply lines that run between the fuel pressure regulator and the

carburetors. You can purchase a double length banjo bolt from Rotax and

extra crush washer and stack another banjo (run to your sender) on the

output of the fuel pressure regulator. Don't reuse old crush washers.

(that said when you you are stuck somewhere you can heat real hot with

blow torch or cigarette lighter and quench crush washers in oil to soften

them, then do figure 8s on a piece of wet dry sandpaper on a flat surface

to make them flat, get to 600 grit should do it). I am using Earls -3

teflon hose and will fill lines with 100LL. Don't want 20 years old mogas

in there after a while



2) Now for the other side of the sender. Airbox pressure. You can do this

2 ways. You can purchase an extra nipple for a spare plug in your airbox

and run that to sender. If you don't have an extra hole, you can T to the

fuel pressure regulator line or the actual airbox pressure side of the

enrichment solenoid. 



Or you could do as I am going to do. There are 2 pressures in the airbox.

Actual pressure (Port most side of airbox) and Impact air pressure ( Think

starboard most side of airbox). Impact air pressure is a bit more than

Actual airbox pressure. It is used to increase the float bowl pressure a

bit more to in fact allow the engine to run a bit richer. If TCU

determines that boost is OK to apply, above 108% power the wastegate will

close, and the enrichment solenoid will in fact plumb impact air to the

carburetor float bowls, instead of plumbing actual airbox air. This will

give a richer mixture that will greatly aid in cooling engine that is

huffing and puffing quite a bit harder than if run at 100% power. 



I will use a injected molded 316 Stainless steel "T" from www.mcmaster.com

to take instead of actual airbox air, the pressure going to the

floatbowls. This pressure will be actual airbox air up to ~108% throttle,

and impact air if the solenoid is in fact working. I don't think there

will be a tremendous difference between the 2, but I am hoping I can see a

flicker of the differential needle when the solenoid switches from actual

to impact. This "T" needs to go somewhere between output of the enrichment

solenoid and the carb float bowls. Be very careful not to break the nipple

on the enrichment solenoid, it is plastic. If there was a leak going to

carb, this T should indicate higher than normal differential pressure that

you probably would not see if you were plumbed to the airbox. Running lean

in during war Emergency Power (115%)is not very conducive for long engine

life. Just a note, I don't like the white plastic Ts Rotax uses and

changing to the stainless ones as described, and since I need to re wrap

the hoses, will not put back the cheap stuff Rotax provides but Teflon

spiral wrap also obtainable from www.mcmaster.com 



Try burning a piece of the Rotax supplied spiral wrap, then try teflon.



Bob as far as your question, you want as little restriction on return flow

flow of the fuel pressure regulator. Do not consider a restrictor. If

restrict a bit too much you could easily exceed the max 5PSI especial in

thick air. Parallel pump design best check that when both pumps are

running in thick air you do not exceed 5PSI. Rotax in their manual tells

you how to measure restriction of return.



Differential pressure is what you are interested in on a 914. A restricted

filter or gasculator, or plugged pre pump filter will restrict flow. If

you put your thumb over output of pump in one of the mentioned conditions

you probably have great pressure. The way the fuel pressure regulator

works, is it needs a specific flow for its internal designed restrictor to

 restrict enough for "Crack" pressure to be achieved and begin flowing

fuel to carbs. Differential gauge can give you heads up to fuel

starvation. Thinner air, pumps worked harder, failure will most like be

not enough pressure. Thick air and 2 pumps running in parallel, or

restricting return line will most like fail in that more than 5PSI going

to carbs. 



Ron P.

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