I have been reading this thread with interest...I can tell you of my experience and findings regarding vapourisation.
I have a low wing, kit built with 912iS Sport - fuel system and engine installation exactly as per the manual. I had around 60 hours when I experienced vapourisation.
Was a warm day in West Australia (around 32c), had just had taken a friend for a flight from a strip up the coast, landed then headed home (about a 1 hour flight) - so under the cowl was already hot.
I got to around 8000 feet (running mogas) and the engine just stopped. No warning. EFIS yelling at me. Luckily for me, I was by this time over another airstrip and managed to glide into that strip. I remember feeling a power surge around 2000 feet (as I occasionally tried restarting and advancing the throttle etc), but I had committed to the landing and it was the best thing to do.
Anyway - thanks to the ability to analyse the Garmin G3x data and some ground testing, we now know it was vapourisation. We even replicated the problem on the ground by running the fuel system and generating some heat with heat guns - after about 3 minutes, the pumps just screamed and cavitated - the fuel was boiling and the pumps were sucking air. And without 45psi, this engine is not happy.
At the time, I had a gascolator which was really just acting as a heat sink - we just shone the heat guns onto the gascolator for a minute or 2 on the ground and vapouristaion occurs. My fuel pumps are mounted high on the firewall - and sucking (creating lower pressure in the lines).
To save this getting too long, this is what I have now done - and not a single issue anymore. I regularly fly on 35-42 degree C days, and up high (10000 feet), with mogas. And I don't worry anymore.
- installed a third fuel pump assembly (made of 2 facet pumps in the cool part of the channel of the cockpit) - this pushes fuel through to the Rotax suction pumps (so there is always positive pressure there) - positive pressure increases the boiling point of fuel. Also, the fuel is 'pushed' from a cool area (down lower than the bottom of the fuel tanks) where there is no chance for it to have air/vapour in it.
- I always run my engine at all times with both Rotax fuel pumps on - why not? I test them both every single flight on the ground and 2 of them failing in flight independently is a very slim chance. They are rated to operate for thousands of hours - so I figure, just run them both! Just like every other dual thing on this engine.
- Removed the gascolator and installed 2 inline fuel filters instead (the gascolator wa mainly being used for filtration, rather than water draining) - I have 2 wing drains and by the time you have run the engine for 2 minutes so much fuel is pumped in this circular system anyway, so any residual water will have moved through the system
In my view - if you use mogas and are sucking fuel through a hot part of the firewall, there is a chance of vapourisation - especially up high. Fuel can boil at very low temperatures at altitude, especially under suction. And this engine needs continuous high pressure (around 45psi) to operate at all times - if this drops (as I experienced) the engine doesn't cough and splutter and keep running. It stops.
If I build again with this engine (or the 915is) I would be installing the fuel pumps so they PUSH fuel to the engine, so any lines in heat-soaked areas are under positive pressure. I would also probably install a third pump again, just to make sure if the Rotax pumps start cavitating, there is fuel being pushed into them. And heat shield all lines on the suction side.
Service Bulletins (from my Kit Manufacturer) have recently been quite a bit with the fuel system with 912iS engines - because of experiences with vapour lock.
An interesting test for you to try - take your top cowl off, get 2 heat guns and blow some hot air onto your fuel pumps for a few minutes (fuel pumps on, engine off). You will immediately hear the point at which the vapour starts and the fuel stops sucking. It is sudden and a bit scary. I actually did this, but from memory we were aiming the heat at the gascolator, but I am sure it would happen with the fuel pumps as well.
I really do believe that future installations should have either
a) a third fuel pump pushing fuel from the cool part of the cabin (I treat this as my boost pump - on in climb etc).
b) the fuel pumps should be mounted down low, inside the cabin as close to the wing tanks as possible. So gravity will feed fuel under positive pressure to the pumps, then the lines up forward will always be under positive pressure - never sucking in a hot area (as this will decrease the boil;ing temperature of the fuel).
I am sorry I have bashed this reply out so quickly and haven't gone back to check (as I am pretty busy) but this was just my experience.
