Fuel return Line Restrictor

[Skippydiesel]

I have a Rotax 35, PN 963820, restriction jet in my fuel return system. It looks suspiciously like a Mikuni VM22/210 type, slow idle jet #35.

My fuel return flow rate is running at 7+L/hr - far to high. I would like to reduce this to 3-4L/hr. To do this I need to fit a smaller/lower flow rate restriction jet.

I would like to fit the appropriate Mikuni VM22/210 type but do not have the experience to make the selection (my Mikuni person either doesn't know or wont advise)

The Mikuni VM22/210 comes in a range of flow rates 10-140. So fare I have not been able to find out what the flow rates are.

As I understand it, all have the same external dimensions/features;

Anyone care to suggest which Mikuni VM22/210 jet might give me a 3 - 4L/hr flow rate?

[GordonTurner]

Hi Skippy

7 liters/hour doesn’t sound like so much return, maybe enough to keep the fuel supply somewhat cool. I have planned my system with a much higher rate of return, more on the order of 10 GALLONS per hour…so I would like to hear more about your calculations and reasoning since “one of us must be wrong…”

:)

Best, Gordon

[Skippydiesel]

Hi Gordon,

Calculation - simple person simple calc. I am both lucky & cursed, my Sonex has 3 fuel tanks. Two wing and a large (40L) header tank, where the traditional single Sonex tank is usually located. In searching for a "mystery" unbidden fuel transfer situation, I disconnected the header tank from the main fuel supply, EURIKA! the fuel return was filling the header tank at the the rate of 7+L/hr (Hobbs). This was calculated by draining the header tank & dividing the hrs flown into the amount of fuel collected ( x 3 replicates)

As you rightly point out, the 7+L/hr flow rate, keeps the fuel circulating and thus relatively cool (reducing vaporisation) and assists with removing any vaporised fuel from the system.

BUT! when calculating the capacity of the main/mechanical fuel pump, to keep up with engine demand/consumption, the fuel designer must take into account the fuel exiting the system via the return line.

Rotax have a nominal Max fuel delivery by the main pump of 35L/hr.
Rotax have a nominal Max engine fuel consumption of 28L/hr at full power (usually TO/Climb) add this to the 7L/hr bypass fuel and you have a demand of 35L/hr.
35L/hr is allowing no margin for pump efficiency with age or manufacturing variation.
As pilots we usually judge the quality of fuel supply by the related pressure (psi/kpa/bar) developed in the system - Rotax requires between 2 & 7 psi.

My Sonex/engine/ fuel system are all brand new. Fuel pressure during pre-flight checks 4.5-5psi main pump 5.5psi with boost pump.
During TO/Climb pressure drops to below 2 psi and recovers to 4.5psi once cruise altitude/condition achieved.
Diagnosis - main+boost pump unable to maintain fuel pressure during peak engine demand.
Possible causes - restriction in fuel supply and too much fuel being syphoned off, in the return line.
(Note: My engine has never shown any fuel starvation symptoms)

In my opinion the engine must be able to TO/Climb on the main/mechanical pump alone, with the boost pump only required as a safety back up.

Rotax engines will operate quite happily (unless you get a vapour lock on a hot day) without a fuel return line. This is not something I would wish to try during the Australian summer months. I will go 1/2 way and put in a smaller restrictor jet and see if this brings up my TO/Climb fuel pressure above 2 psi.

It has been suggested that I just put in a larger (volume/pressure) boost pump - see may rational above against this.

Given the in-line design of my fuel system, it may be that the boost pump itself is restricting fuel flow - I may have to investigate this and redesign the system.

End of lecture!:)

PS - Are you able to help with the restrictor jet selection?

[GordonTurner]

Sorry, no knowledge about the restrictor orifices.

Our systems must be set up somewhat differently. I too have three tanks, and the return line functions to transfer fuel from the aux tanks to the main header tank. However, the return line can only return the fuel that is excess from what the carburetor takes.

In the situation where the aux tank return line flow is going to an already full main header tank, the overflow returns to the aux tanks via the vent line. The main header is vented to the aux tanks. The aux tanks are vented to the atmosphere.

[Skippydiesel]

"However, the return line can only return the fuel that is excess from what the carburetor takes."

Sorry Gordon - cant see how you arrive at this statement.

The pump supplies the distributer/fuel manifold.

The manifold feeds the two carby's, fuel pressure (nil flow) if fitted and the fuel return line.

The fuel return (on my system) has a fixed restrictor jet. Fuel will enter the return line as long as there is flow/pressure. If there is insufficient flow/pressure in the system, both the carby's and the return line will have reduced supply.

IE there is no mechanism to advantage(shut off the return line) the carby's in a low flow/pressure situation.

The key point is low flow/pressure.

[Skippydiesel]

Gordon - FYI & my apologies.

My last Rotax 912 ULS was a1999/2000 production - before fixed fixed jet restrictors became the Rotax recommended return fuel system (& before Rotax even recommended a fuel return). My aircraft was fitted with an adjustable fuel pressure system that incorporated a return to tank line. It was quit large & heavy. By adjusting a nob on the device, the spring tensioned valve could be made "heavier/lighter" thus controlling the amount of fuel returning to the tank.

I now see that it had one advantage over the current system. In this system the valve will close, below the adjusted pressure, directing all flow to the carburettors.

Your comment "...the return line can only return the fuel that is excess from what the carburetor takes." would be correct for a spring loaded return line valve.

[Skippydiesel]

Just ordered a Mikuni VM22/210 type, slow idle jet #15.

Will let you know what impact it has on my fuel system

[Skippydiesel]

FYI-

The attached chart purports to give jet ID specifications:

https://www.allensperformance.co.uk/car ... ize-chart/

The chart #35 (0.511mm) is as I have measured my Rotax 35 jet (0.53mm)
Rotax also supply a fuel distribution manifold with 0.35 mm apertures.
On the basis of one very similar measurement and the Rotax manifold, I have order a Mikuni VM22/210 type, slow idle jet #15, (0.335mm) in the hope that it will deliver about 3L/hr return fuel.

[Skippydiesel]

First flight, after fitting new smaller return flow line restrictor jet.

New jet measured at approximate 0.075mm orifice, compared with 0.52mm for Rotax supplied jet.

Unscrewed old, screwed in new - exact fit.

Ground run pressures 5.3 psi , 2500 rpm - basically the same with boost pump on.

TO/Climb Out, with boost, went down to 3 psi (improved from around 1.5 psi).

High speed cruse at 5000ft - 3 psi without boost.

So improvement all round but not as much as I would have expected. May be those that theorised a restriction (in line boost pump?) in supply may be correct.



No figures yet on new jet return line fuel flow.

[Skippydiesel]

New fuel return restrictor jet (Mikuni No 15) is returning about 3.75 L/hr - a big improvement on the 7+ L/hr that the standard Rotax Mikuni No 35 was delivering.

Fuel pressures now remain above Rotax minimums.