Checking to see if anyone has had similar problems and/or solutions.
First flight Sonex N1146R on June 24th. No problems with engine or aircraft. Fuel is 91 octane auto gas from Exxon.
Second flight July 4th, hot day, OAT = 32 to 35 degC. Normal takeoff and climb to 5,000 ft in about 7 minutes using fuel pump #1. Level cruise another 7 minutes at 120 knts when fuel pressure drops slowly (~15 seconds) to near zero and engine goes to idle (may be windmilling) with no throttle response. Confirmed fuel valve on, turned second fuel pump on and cycled both ignitions off/on but no change. About 5 minutes later while gliding back to airport the engine comes back to life, just in time for landing. Happy ending but exciting to say the least.
Some time later with engine cowls off, everything looks normal, fuel in gas tank is not hot and vent line is not restricted.
We believe fuel vaporized in the fuel pump which is mounted on firewall. Working to add heat shield and/or cooling air. Pictures of the installation can be found at: https://eaabuilderslog.org/?s=Sonex0035
A shielded line and good airflow is a start. Try changing the fuel to 100ll see if that helps any too. Did you test both pumps on the ground and make sure they are putting out enough?
Good to hear everything worked out. I’m using a Rotax fuel injected engine but learned a lot about fuel vaporization since my fuel system is very similar to a Vans RV-12is and they’ve done a good job documenting vapor lock concerns. With their fuel system anything over an OAT of 80F (26C) or fuel temp above 68F (20C) requires 100LL for any operations above a density altitude of 6000ft. A 50/50 mixture of mogas and 100LL may also lower vaporization risk. Although your operating limitations would be different, with a DA potentially over 8000ft, I’d suspect there’s a good chance the fuel type is the primary issue.
My understanding is that the highest risk area on a fuel injected engine is on the inlet side of the pump since the fuel pressure becomes the lowest in the system. A course filter before the pump intake that is dirty or that is not designed for enough flow rate could also be an issue. Fuel heating or tight bends in this area is another thing to consider.
As a Rotax 912 ULS driver, in Australia, I have some familiarisation with Fuel Vaporisation/Vapour Lock (FV).
I have no experience of winter fuel (used in warmer temperatures), which I understand from this & other Forums can be a significant contributor to FV.
Nor do I have any UL engine experince.
Reference: Ampzappers RV Service Letter -
The ambient temperature of 26C seems far to low. In my experince the temp needs to be well north of 35C . I suspect FV at around 40C
Leaving your(912ULS) boost/auxiliary pump on for the entire flight - while this may assist in overcoming a possible FV occurrences it does not address the other factors that may be contributing to the problem ie its not a good solution.
Th use of AvGas, is a temporary fix, should only be considered after addressing the various contributing factors. I guess if winter fuel is all you can purchase, then AvGas may be the way to go.
Despite the above, the SB is giving good information.
I think Timritter44’s problem is unlikly to be FV.
Ambient temperature not high enough
Occured in flight - FV is primarily a ground based phenomena, after flight/engine shut down, due to convection /radiant heat from engine, combined with high ambient temperatures and no fuel flow, boiling the inline fuel to vapour.
NOTE:
I understand that fuel injected engines may be more susceptible, than carburettor, to FV in flight. This is likly due to the more complex fuel delivery system and the reliance on high pressure pumps, for constant supply of fuel to the injectors.
Timritter44’s fuel reticulation system, mounted on the engine (hot) side of the firewall with no heat mitigation in evidence, features multiple sharp right angle bends and at least one too many filters, all of which may contribute to fuel system heating/FV
I’m curious whether anyone else has been successful with that fuel system design. In my opinion, significantly reducing the amount of fuel line prior to the pumps on the engine side of the firewall would help, also mounting the pumps as low as possible would be better. This, of course, assumes that the vapor lock is happening on the inlet side of the pumps.
If I recall correctly, most fuel system installations that I’ve seen for the UL Power engines have the fuel pumps mounted vertically on the firewall. It looks like you have yours mounted horizontally. Not sure if there are any recommendations in the installation manual but you could check with UL power.
I doubt that there is any requirement to have the pumps in the engine bay. Much cooler on cockpit side of firewall & closer to fuel tank(s).
Pumps do not need to be as “low as possible”. Sure its best to have them below the minimum in tank fuel level & this, for ease of mounting, may mean on the bottom of firewall/fuselage floor.
Pump location, relative to the tank, should when ever possible, facilitate pump priming & scavenging of last dregs of fuel (unusable fuel is just a waste.)
I know nothing of UL Power engines, however in line fuel pumps, rarely have a required orientation.
Position, yes! Relative to lowest point in fuel tank.
I think you’re probably right. I was thinking an extra 0.1 or 0.2 psi of pressure head might make a difference, but if there indeed is a vapor problem at the inlet of the pumps, that’s probably not the solution.
What’s the chance the problem is electrical? Is there any telemetry (current draw, etc) that the pumps were definitely running?
I find it interesting that the Walbro pump I am using is only rated for fluid temperature up to 150 Deg. F. I don’t know whether that is due to fuel vaporization or a physical limit of the pump. If I had installed the pumps on the engine side of the firewall, I would have been tempted to enclose and blast cool them.
Hi Wes,
65C (150F) is a significant engine compartment temperature. I would be trying to limit this to about 30-40C when engine running on the ground and even less in flight.
With my installation, I have managed to achieve no more than 10C above ambient, in the air.
On the ground, engine running, I point the aircraft into wind and on shut down, open oil inspection hatch, to mitigate engine bay temperatures.
I have one of those temperature strips, on my vulnerable electrics , which monitors max temperature achieved.
True, a heat soaked engine may deliver well in excess of 65C on the ground , shortly after engine shutdown but then pumps etc are not running and heat exposure at this sort of temperature should be relativly short duration.
I suspect your Walbro pump temperature limitations are to do with component limits (plastics) rather than fuel temperature or FV.
I’ve never measured the engine compartment temperature. I have taken the turbo AeroVee as my extreme example. The data below was collected while Sonex was trying to solve the coking <sp?> problem. The under-cowl temperature is shown as ambient. Outside temperature is shown as OAT. In cruise the temperature rise was as much as 72 Fahrenheit Deg (40 Centigrade Deg). Where I live that would push past the limit in Summer.
My Legacy has the Dynon Skyview Classic EMS ,so I have OAT & engine compartment temperatures.
On my last aircraft, with steam gauges, I used an indoor/outdoor digital thermometer, for under cowl temperatures. The long outdoor thermocouple /sensor lead can be located wherever you want, in the engine compartment. The digital screen somewhere easy to see in the cockpit.
The Sonex chart has OAT 47F (8C) & Under-cowl as 119F (39.5c) to 119 F (48c). Way too hot. I have no knowledge of this test but it screams inadequate air flow.
Black conical device is air/oil separator that comes with UL Power engines. Three lines are breather from engine, air overboard an oil return to engine.
Did a quick search on this devise - Reveiws are not good.
Efficacy of the unit and the desirability (or lack) of returning contaminated oil, to the engine, seem to be the primary concernse.
Most auto fuel has ethanol in it. Here in Canada all auto fuel has ethanol in it. Only Mogas at some airports or boat gas at marinas do not have ethanol. It is illegal to fly with fuel containing ethanol in Canada. The vapor pressure of ethanol fuel causes vapor lock.
