912 ULS: Partial power loss on takeoff - vapor lock?

Hi Oliver,

I would like to see some more photographs -

 # Showing fuel delivery system  (manifold)

 # Both sides of engine - showing fuel delivery to each carburettor

"The only thing I find somewhat suspicious is that the fuel return line only goes to the gascolator, what however also appears common."

I understand that this fuel return method (to gascolator) is not uncommon HOWEVER it will compromise the purpose of the fuel return system.

The fuel return system is supposed to:

 # Remove vapour/bubbles from the fuel system. 

 # Remove hot fuel, to a tank, where it can be cooled by the larger volume of tank fuel.

 # Allow cool liquid fuel to enter, from the tank, and cool the fuel system & enter the float bowls.

 # Provide some fuel line pressure control (not a big factor).

The system you have, just reintroduces vapour/bubbles and hot fuel, at the gascolator, effectively compromising the above functions.

I recomend plumbing the return line back to a tank.

Boost Pump Location/Configuration

 # Your Boost Pump (BP) should be before your gascolator - not after. This configuration is further compromised by your fuel return system - your pump may be trying to pump vapour/bubbles.

 # A partial blockage of the gascolator filter, may cause the BP to create a low pressure area, making fuel vaporisation more likly, further compromising the BP's efficiency.

 # The BP should have a bypass circuit fitted, in case of pump failure causing fuel blockage.

Fuel Return Restrictor Jet

 # Does your fuel return line have a restrictor jet fitted?

😈

Hi Sean,

Thank you for your fast response, I really appreciate it.

I'm not quite sure what you mean with manifold. I attached an image, from before we insulated the fuel lines, showing the location of the gascolator as well as the auxiliary fuel pump.

I share your concern that we might just be circulating hot gasoline / vapor if the return line only goes to the gascolator.
I also attached two thermal camera images. Right after shutdown, the bowl of the gascolator was at 41 degC / 106 degF. The top was even at 50.7 degC / 123.26 degF. I don't see how at such temperatures the returned gasoline is supposed to sufficiently cool down.

I however think that it makes sense to have the boost pump installed behind the gascolator, so that the fuel is getting circulated even when the engine is not running.

I also attached images from other perspectives.

I agree that running the return line back to the tanks would probably be best. This would however be a massive undertaking.
If needed, I could however envision to shield the gascolator and to direct cold air to it. installing a small header tank inside of the cockpit might be an option as well.
It also doesn't help that the gascolator is located directly next to the tailpipe.

The fuel restrictor jet is installed and clean. 
The Facet boost pump does not have a bypass valve installed, I however understand that it is also not needed for this model.
The screen in the gascolator is perfectly clean.

What puzzles me is that nobody else seems to have these problems, even though there a tons of other Zeniths with the exact same configuration out there.

"I'm not quite sure what you mean with manifold."

The fuel manifold/distributor usually has a 5 way split:

1 Fuel from mechanical pump IN

2 & 3 Fuel to carburettors

4 To fuel pressure sensor

5 To Fuel return line

"I also attached two thermal camera images. Right after shutdown, the bowl of the gascolator was at 41 degC / 106 degF. The top was even at 50.7 degC / 123.26 degF. I don't see how at such temperatures the returned gasoline is supposed to sufficiently cool down."

I think you have convincingly proven, that the way your return line is plumbed, can not be very effective, particularly in hot climates/seasons.

It may be possible to fit a fuel radiator - fuel injected engines (especially diesel) often have a fuel radiator fitted in the return line.

"I however think that it makes sense to have the boost pump installed behind the gascolator, so that the fuel is getting circulated even when the engine is not running."

I don't know what you mean by "boost pump installed behind the gascolator". Rotax recomend the BP be installed between the fuel supply tank(s) & the filter (gascolator) not as you have it, after the filter.

The BP cooling & purging any vaporised fuel,  is severely compromised by the way you return line is plumbed. In the event of a significant fuel vaporisation event, the BP  itself, may not circulate the fuel.

"The Facet boost pump does not have a bypass valve installed, I however understand that it is also not needed for this model."

So in the event of a BP failure, will your fuel supply to the engine be compromised?

Your high wing/fuel tanks should deliver sufficient gravity assisted fuel, even without a BP - assuming the BP isn't blocking the only supply line.

You may be able to install an alternate (gravity) fuel supply line to the mechanical pump.

Rotax recomend a BP fuel bypass be installed.😈

In the UK, taking the return line to the gascolator is not accepted by the LAA, it has to go to the tank.

I thought Rotax also prohibited or at least disapproved of this.

Most likely vapor lock. Recommend fuel return to tank instead of gascolater that is located in a high temperature area.