Fuel pressure differential on my 914

[rhtrudder]

Trying to figure out whether i can plumb the differential gauge into the carby cross over tube with a T fitting or should it be getting it’s reading straight from the air box ,and then T the wet side of the senser into the fuel pressure gauge tube 

[skippydiesel]

Fuel pressure ? or air pressure/vacuum ?

[rhtrudder]

The fuel pressure has to be .25 of a bar over the air box pressure which is feed by the turbo , I bought the gauge years ago but never fitted it , it’s got the sender one side goes to fuel pressure, wet, the other to air pressure in the intake and the read out which goes to 7psi , yankee gauge, tells the fuel pressure above the air box pressure , not sure if I can t it into the balance pipe or fit it into the air box 

[skippydiesel]

I am trying (probably unsuccessfully) to read between the lines/understand:

 

Turbo pressurises air into carburetors (box) 

The fuel pressure to carburetors must be higher than the pressurised air from the turbo, otherwise the air would force fuel in the wrong direction (?)

You feel that it is important/interesting to know if the above situation is as desired/ expected.

If the gauge, you describe, is fitted to your panel you will have a "condition" monitor ie All is well OR not.

 

If the above is (sort of) correct: I surmise that you will need to connect to the up steam side of the carburetors (air box) where max turbo pressure will be found and to the fuel delivery line, somewhere between the main pump outlet and where it divides to each carburettor.

 

Plumbing into the down stream side of the carburettors (balance tube) will give a reading of sorts (might even be the same) but may no be as accurate/diagnostic as the upstream (air box) reading as its the air pressure entering and going through the carburettors which is of interest, not what is coming out/down stream of the carburettors.

[rhtrudder]

That’s how it all works , the carby bowls are sealed and the breathers are vented to the air box which pressurises the fuel in the bowls , gives it more fuel when running in the boost range

[skippydiesel]

4 hours ago, rhtrudder said:

That’s how it all works , the carby bowls are sealed and the breathers are vented to the air box which pressurises the fuel in the bowls , gives it more fuel when running in the boost range

So, was my rambling analysis/suggestions for locating sensor points (tubes) "on the ball" ?

 

Your "the carby bowls are sealed and the breathers are vented to the air box which pressurises the fuel in the bowls , gives it more fuel when running in the boost range" doesnt sound quit right to me.

 

The carby bowls are sealed is correct, in so far as being fuel tight and in the case of a turbo system the breathers (for the bowls) are vented to the air box  which normalises the air pressure between float bowl & carby inlet.

The function of the turbo charger is to deliver more air/oxygen to the carby/engine, so that it may burn more fuel for a given cylinder swept volume, thereby generate more power OR the same power, as a naturally aspirated engine, at altitude, as at sea level (known as turbo normalising) .

It does this by pressurising the air (over atmospheric) using an exhaust driven  turbine, hence the need to normalise/balance the pressure impacting on the float bowl to the same as in the pressurised air box (otherwise the high pressure would blow the fuel out of the bowl breather).

The carby, in tern, will be optimised (jet settings) to deliver more fuel (than the naturally aspirated version) to take advantage of the increased  air/oxygen - the resultant burn will liberate more power.

The method of fuel delivery is the same as for a naturally aspirated carburettor, that is by venturi (narrowing) in the carby throat, creating a pressure variation (partial vacuum) that draws the fuel up from the float bowl/chamber.

Gives it more fuel when running in the boost range - having no experience of the Rotax 914, so I dont know what this means. All internal combustion engines have a small rpm range over which they deliver their best performance - this is not exclusive to turbo charged engines.

 

My understanding of the Rotax 914 is that it is essentially a turbo normalised engine that has been configured so as to have some increase in power (115) over its  naturally aspirated 80 hp origins - this is done by using an automatic exhaust waste gate to moderate the amount of pressure being delivered by the turbine. The waste gate will be fully or almost fully open, at sea level and progressively close as the aircraft climbs and the atmosphere this. This will maintain sea level pressure in the air box/carburetors , so as to keep the engine running more efficiently at higher altitude than its naturally aspirated origins.

 

Yeah Gods I do go on and could however I will stop there, as I belatedly recognise the my terminal VD (verbal diarrhea) is probably boring you to death.

[rhtrudder]

Spot on

[skippydiesel]

59 minutes ago, rhtrudder said:

Spot on

The VD ????

[fallowdeer]

“The 914 is a "altitude" or boosted engine; the manifold pressure goes over atmospheric pressure. As opposed to a "normalized" engine that tries to maintain sea-level manifold pressure.
-When the wastegate is closed the max amount of exhaust pressure routed into the turbo.
-When the wastegate is open the exhaust pressure is allowed to escape into the muffler.
-Even with the w/g fully open you will get some boost.
-the wastegate will be closed at idle. Strange but true: The TCU is trying to reach its target MAP and the w/g will be closed until it achieves this target.
-When the throttle is set to 115% the wastegate is fully closed until 40 InHg is reached.
If the cable is not adjusted correctly: too loose - the wastegate cannot close and you will lose manifold pressure.
If the servo/cable is not setup correctly (throttle closed/Master ON/allow servo to cycle/Master OFF/adjust w/g to fully closed position) then it is possible the servo cannot move and the warning light will flash.”

 

Rob Seaton, Rotax Owner’s Network

 

 

[skippydiesel]

Thanks Fallowdeer ; While I live I learn - I always thought the 914 was a whole mechanical turbo/wastegate engine (didn't know it had an electronic TCU). Doesn't change the fundamental points of my description .

 

I dont recognise your (Mr Seaton's)  "altitude" or boosted engine" terminology but get the point - I would suggest almost all turbocharged engines make more Hp at sea level than their naturally aspirated origins.

Aviation is probably one of the few applications, where only a small gain at sea level is not uncommon (turbo normalising). The primary objective being high altitude performance.

Overwhelmingly, turbo charged engines (even in aviation) make a lot more sea level Hp than their naturally aspirated origin. Thus better engine power:weight ratios are achieved, while also gaining high altitude performance.

For a turbo charged engine the 914 is relativly "lightly" boosted at sea level, making only an additional 35 Hp over its 912, 80 HP origin (if you believe this was the foundation engine)

[rhtrudder]

14 hours ago, fallowdeer said:

“The 914 is a "altitude" or boosted engine; the manifold pressure goes over atmospheric pressure. As opposed to a "normalized" engine that tries to maintain sea-level manifold pressure.
-When the wastegate is closed the max amount of exhaust pressure routed into the turbo.
-When the wastegate is open the exhaust pressure is allowed to escape into the muffler.
-Even with the w/g fully open you will get some boost.
-the wastegate will be closed at idle. Strange but true: The TCU is trying to reach its target MAP and the w/g will be closed until it achieves this target.
-When the throttle is set to 115% the wastegate is fully closed until 40 InHg is reached.
If the cable is not adjusted correctly: too loose - the wastegate cannot close and you will lose manifold pressure.
If the servo/cable is not setup correctly (throttle closed/Master ON/allow servo to cycle/Master OFF/adjust w/g to fully closed position) then it is possible the servo cannot move and the warning light will flash.”

 

Rob Seaton, Rotax Owner’s Network

 

 

That’s how I understood it

[skippydiesel]

2 hours ago, rhtrudder said:

That’s how I understood it

Fair enough .

 

No turbo charged system stays under atmospheric irrespective of how its configured - turbo normalising, in maintaining sea level pressure to altitude , in the inlet manifold, is by definition going over atmospheric pressure. Other than "ground effect" aircraft all fly at altitude so all go over turbo charged engines go over atmospheric - simples😁.

 

As for "boosted engine" - this  terminology would appear to cover all systems where higher than atmospheric pressure is induced/created in the inlet manifold - sounds like BS to me🙃.