Fuel return line design obstacle

Roger Lee wrote:

On all the planes I work on I see many different return line installation points. Some are in the gascolator, some in the supply fuel hose out of the engine compartment, into a header tank and then some into either a wing fuel tank or another fuel tank location. So they all seem to work.Most of these are all aircraft Mfg placed locations. 

Roger are you still happy about returnto the gascolator or into the supply hose? Either of those is a definite no with the LAA here in UK. I know CTs started with that arrangement (into gascolator) but they changed it later to go to the tank didn't they?

All

From the viewpoint of purpose the concept is to reduce air in the line from vapor with hot fuel.  To return to the gascolator you have a very small volume capacity so are also mixing air bubbles and hot fuel back into your pickup to the engine.  This therefore is not ideal.  If an aircraft has a header tank and it is able to keep the hot fuel and any bubbles separate from the fuel pickup then it is a reasonable compromise.  Return to the fuel tank on a carb engine in some cases is not always feasible due to the lift required in the case of a high wing tank system and the issues of a suitable return line location from the pickup.  (the fuel pump only has from .4 to .5 bar pressure at maximum) In any injected engines this is easly solved with a duplex fuel valve that returns all fuel to the tank one is drawing from.  

As Roger has pointed out this is normally designed by the OEM of the aircraft.  As in the UK there may be rules that disallow return to a gascolator and i agree they are based on best practice of that regulatory body.   The gascolator is too small and circulates too much of the hot fuel and bubbles to be a good place in my opinion. 

Cheers

(Once again, just lost  my response.)

I agree with all the RW has said but would like to take the discussion a little further;

From my limited understanding/experince, it would seem that those designing/installing the Fuel Return Line (FRL) in a carburettor engine, may not appreciate its primary function and how the system should work.

The first thing to understand is that Fuel Vaporisation (FV) sometimes known as vapour lock, is primarily a problem of hot ambient air (summer) combined with a recently flown (hot engine) & parked aircraft. Aircraft operated in cooler climates/conditions may never experience FV, irrespective of how the FRL is plumbed or even if one is fitted. (Note: I am not suggesting that in some engine installations/conditions, in flight FVP may not occur)

The combination of;

  # a hot engine, generating copious amounts of convective & radiant heat,

  # the location of Rotax carburettors high in the cowling, directly above the rear exhaust pipes, 

  # fuel supply lines/distribution manifold above the crankcase,

  # automotive fuel - vaporises more readily than AvGas

  # Minimal cooling air flow and nil cool fuel flow,

                                                                         results in hot fuel, which may vaporise (form bubbles) within the fuel delivery system. Your fuel metering system (carb) is not designed to use low pressure FV. The engine will not receive an appropriate mixture of fuel:air,  to acheive/ sustain ignition. 

The second point is the role the Boost Pump (BP) plays in minimising the effects of FV.

The BP should always be turned on before engine start. This gives the pilot the opportunity, to check its function, normal pressure AND for the BP to purge hot fuel/vapour out through the FRL & replace it with cooler liquid fuel (better to start/run your engine).

FRL Plumbed Into Gascolator 

  #BP located before Gascolator.

This can not work satisfactorily. The BP will pressurise, what is in effect, a closed system (a loop). There will be nil fuel flow,  hot fuel/vapour will not be purged.  Near normal fuel pressure may be seen by the pilot. The FRL can not work before and will be less effective, after engine start.

   #BP located after Gascolator.

The effect will be to circulate the hot fuel/vapour around a loop. Minimal cooler fuel will be introduced but there may be some cooling of the FV (if your lucky) depending on the length of the circuit. The BP may "rattle" as it processes FV. This is a slightly better concept but nowhere near the optimum. Once engine start has been achieved (possibly with difficulty) the situation improves a little, as fresh fuel enters due to engine consumption.

The optimum location for the FRL exit/delivery:

 # is into a relativly low pressure break tank or top of fuel tank. The hot fuel & vapour has a chance to cool before reentering the fuel supply system.

 #The BP will, from start, be delivering cool fuel, from the tank, to the system/carburettors, pushing the hot fuel/vapours out through the FRL.

If a FV problem is suspected, ("rattling" BP/low or ni/jumping fuel pressure) extend your BP operation until consistent normal fuel pressure is seen - then attempt an engine start. It may be beneficial to keep your BP on during hot starts of this type.

I hadn't thought of this before - High wing aircraft (fuel in wings) may, if fuel valve left open, FRL plumbed into wing tank, benefit from a degree of convective fuel cooling, as hot fuel/vapour rises up the FRL, drawing cooler fuel into the system ?????