I agree Glenn..some cool air over the pumps can't help..
Geoffrey - you may be right with some of the other cases in that the fuel just started to vapourise and the pumps could still just get enough flow to generate the required pressure...not sure.
But to be 100% (in my mind anyway) - the solution is to 'push' cool fuel from nearer the tanks (inside the cabin somewhere) to the Rotax pumps, no matter how high they are on the firewall. This means there is positive pressure in the fuel lines firewall forward - this makes a big difference to the boiling point of any fuel, but it also means that fuel is being delivered to the pumps if there ever is any issue..this would force any vapour rapidly through the system (and at worst allow a quick restart).
With Lycomings (and other engines - perhaps such as the 912ULS) they do not require 45psi - this is quite unusual (and more like an fuel injected car setup from what I am told). Auto fuel pumps are often in the fuel tanks and are 'pushers' - this is why fuel injected cars these days don't suffer vapourisation anywehere near as much as older cars.
A lot of people on hot days (even with Avgas) with have engines that cough and splutter a bit on the ground - usually though, as soon as they hit their boost pumps, the situation resolves itself (all the RV builders I know have their boost pumps in the channel inside the cockpit).
One way to see how quickly your engine will stop if you have vapourisation in the pumps... run your engine on the ground, turn one fuel pump off. Then switch the other one off - it is instant.
One thing I have come to realise with this engine (and the new 915iS which I am considering for my next plane) is that fuel system design is CRITICAL. Really great engine this, but has some differences to 'traditional' or other fuel injected engines.
I have also spoken at length with Mark Paskevich (President of Rotech in Canada) - he designs fuel systems for aircraft and has a great youtube video on a 912iS installation (albeit with a high-wing). Interesting to see the header tanks etc.
https://youtu.be/TzTieV0EvSY
I'm not an expert in any of this, but with the help of some very experienced builders and tech guys here in Perth we have analysed this in a lot of detail, done testing etc. and have tried to come up with as reliable a system as we can. I also like using mogas because it's half the price of avgas so I had to make it workl!
Any small amount of fuel vapors (do not think of it as air, it is mostly gaseous Butane) that gets pumped past the fuel pumps to the high-pressure side will be recompressed back into the liquid phase due to the 45 psi of pressure. This is not a problem.
Under normal fuel flow, the majority of fuel that the pumps deliver to the engine is returned to the tanks.
The pumps move about 30 gallons an hour even if the engine is OFF.
At Idle 99% is returned by the fuel pressure regulator.
At WOT the engine uses about 5 gallons (~15%) and returns the other 25 to tanks.
As the fuel flow through the fuel line, it tends to cool the line as best as it can.
If at some point, the temperature and pressure reach the proper conditions, the fuel in the line forms a vapor bubble, it can become irreversible.
The bubble no longer cools that section of line and the line gets even hotter quickly, causing the bubble to get even larger.
The pumps are able to pumps vapors to some extent, but that only moves more fuel into the hot spot in the lines and that fuel also vaporizes.
It becomes a losing situation. This is why it is often referred to as a Vapor Lock. There is no way out!
Once the bubble starts and the fuel pumps stop moving fuel, the bubble just keeps getting bigger and doesn't move.
If you have a lot of altitude and the option to glide for 5 minutes or more, you might want to wait for the ambient pressure to increase in the descent and the engine to cool from the airflow before depleting whatever battery power remains before attempting a restart.
It might not start at altitude but you might just get lucky if you can muster the patience to try it later at a lower altitude with a cooler engine.
You have two ways to collapse the bubble, remember it is Not Air, it is fuel vapor, and can be changed back into a liquid under the proper condition.
Either lower the Temperature or raise the Pressure.
Temperature...
If you do not already have Fire Sleeving on the Fuel Lines, Install it, and keep it away from the exhaust.
It is not there just for fire retarding. It is also a great heat insulator.
Heat-shields that keep the fuel lines from "Seeing" the radiant heat can also help. (Think of a Campfire!)
Pressure...
The common fuel pumps used in the carburated engine world can produce up to ~5 psi.
This has the same effect as descending 10,000 ft or reducing the temperature by ~40°F.
An upstream fuel pump that has liquid fuel available has the capability of collapsing that Vapor bubble instantly.
A low-pressure pump close to the fuel tanks is never a bad idea, especially in a low wing aircraft where it is uphill all the way.
Bill Hertzel Rotax 912is North Ridgeville, OH, USA Clicking the "Thank You" is Always Appreciated by Everyone.
Adding cooling air flow can't hurt but in high ambient temperatures it may not be able to overcome the heat absorbed from the engine. Each installation is different.
Thanks, it sounds like you and Ian confirmed my original thought to install a third fuel pump in a cooler location to pressurize the fuel going to the Rotax fuel pumps.
Ian, did you use by-pass fuel lines and check valves in your facet pumps? Can you please describe how you plumbed the pumps? Did you plumb them in series or parallel?
If you used check valves, what type and specification? Most advertise a 1.0 PSI or 1.5 PSI opening pressure, which I think is a little too much.
Again a quick hand drawn schematic would be very informative.
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