A SOLID FUEL SYSTEM IS IMPORTANT FOR ANY AIRCRAFT ENGINE.  THESE ARE THE SYSTEMS USED BY THE THREE MOST POPULAR LIGHT AIRCRAFT ENGINES IN USE

TYPE OF FUEL SYSTEMS USED

ROTAX

• Dual external fuel pumps
• External single fuel regulator
• Fuel return commonly back to primary fuel tanks
• 6 port duplex fuel selector valve

VIKING

• Dual in-tank pumps
• In-tank dual fuel regulators
• Header tank based system
• No fuel valve needed

UL

• Dual external fuel pumps
• External single fuel regulator
• Fuel return commonly back to primary fuel tanks
• 6 port duplex fuel selector valve

EXTERNAL FUEL PUMPS

• UL and Rotax use external fuel pumps for their injected engines.  They are a copy of what was used in cars prior to 1998.
• When 2 pumps are used together, for takeoff and landing, the amperage draw can be 10-14.   This is a lot since these engines are not equipped with good alternator systems.   Even with a single pump running, the draw is scary high if either the generator or external voltage regulator were to fail.  The dual fuel injectors and sparkplugs cannot work without electricity.  
• External pumps produce close to 2 x the required fuel pressure for the injectors.  The excess pressure is regulated back down, using an engine mounted fuel regulator.  Excess fuel is returned to the fuel tank.  
• All this additional fuel, when returned, brings heated fuel to the tank.  The total fuel circulating between the main fuel tank and the engine, then returned, is about 30-35 GPH.  
• Due to the excessive fuel circulating, the amount of fuel gravity fed to the firewall mounted pumps is also 30-35 GPH.  Hence the large hoses specified.
• Rotax and UL each use a single fuel regulator at the end of the fuel rail, mounted to the hot, air cooled cylinders.  The Rotax brand is likely to be Bosh but the UL is a no-name copy.  Each is filled with rubber parts and will fail, sooner or later.
• This is now an antiquated way to feed fuel to a fuel injected engine.  

ISSUES RELATED TO SYSTEMS DRAWING / RETURNING FUEL FROM / TO A MAIN FUEL TANK

• A fuel injected engine MUST have a solid fuel pressure at all times, or the engine will stop
• Having reliable gravity feed, all the way from the wing tanks to the firewall mounted pumps, of 30-35 gallons / hour is much harder than the 2-12 GPH that the engine is actually using.  When this system was used by cars, the pump was mounted right below the fuel tank.
• There is an illusion that these systems work because fuel usually come out of the pump when turned on.  What is not so obvious is how small the margin is for the system to stop working. 
• It is likely that even though the pump was never designed to pull fuel on the suction side, that in the airplane application it actually does this to keep up with demand.  Since there is suction, any air leak prior to the pump has the potential for pump cavitation.   An O-ring leak in a fuel drain, gascolator or selector valve would not be good. 
• Since the fuel is at low pressure (suction) as it enters the hot engine compartment, any exposed metal fuel pump body or fuel system component could cause the fuel to boil and cavitate the pump, dropping the pressure from 43.5 psi to less than 10 psi.  Once the pressure has been dropped, the pump will not be able to re-gain pressure due to the 43 psi fuel regulator at the other end of the engine fuel rail.  (Here is where a small bleed bypass around the pressure regulator would be handy but this is not implemented by either Rotax or UL.  The bleed would allow some fuel to flow, re-priming the pump with fresh fuel and again be able to make pressure) 
• A larger amount of unusable fuel.  Because more fuel is being returned to the tank, than is used by the engine, fuel is on the move in the tank and not always available at the pickup location.  To guard against this, more fuel must be kept in the tank, shortening the available range.  Also, as the fuel level decrease in the tank, the warm return fuel becomes more and more pronounced and pump cavitation is even more likely.
• Serious concern of un-porting the fuel pickup location.  As mentioned above, one second without fuel pressure is one second the engine will not operate.  Carbureted engines have a fuel bowl from which the engine can draw fuel.   Not so with an injected engine.  There has to be fuel available to the pump at all times.  With low fuel, and in an extended descent, this is usually not the case.  Some installations add more complexity to the system by the installations of door-post mounted “sumps” in order to have some protection from this.  However, keep in mind that these fuel pockets only last for a few seconds when the system wants to pump 35 GPH in a circle.  The pump will draw the fuel from the pocket in no time flat, bringing back the un-porting issue.

THE FUEL SELECTOR VALVE

• The UL and Rotax systems usually use a duplex / 6 port fuel selector valve.  These are complex, costly and have 6 fuel line connections right inside the cabin.  Another style is from a pickup truck with dual fuel tanks.  These are of terrible quality and are electrically operated.  Cut one open and inspect the construction if considering using one.  Again, there is no backup so be sure it works. 
• Viking does not use a fuel selector valve.  Fuel simply drains from the two wing tanks into a single header tank.  From there, no selection is required.  The popular C-150 also has no fuel selector. 

SO, WHY IS THE VIKING SYSTEM BETTER AND HOW DOES IT WORK

• The Viking system consists of a header tank and two fuel pumps.  That’s it.  Fuel fills the header tank by gravity.  If the engine uses 6 GPH, the system only has to flow 3 gallons from each wing tank.  (only about 1 qt every 5 minutes)
• The pumps are inside the tank and submerged in fuel, just like every modern fuel injected car.  The fuel regulators are right on the pump bodies, pre-filer screens are part of the system, etc. 
• A precise fuel sender unit and gauge are available, allowing the main tanks to be used to a lower level with complete confidence.  The 30 min VFR daytime reserve is in the header. 

CAN I USE A VIKING HEADER TANK WITH A UL OR ROTAX ENGINE

• The short answer is – yes you can.  Viking has such a system and it is being tested on a UL engine.  The long answer is much more complex and you need to understand it if you are contemplating such a conversion.  Here are the details.
• First it is important to understand how the original system works and the reason for why things are the way they are. 
• Fuel pumps:  The fuel pumps are big, heavy and draw a lot of current for a reason.  In order to reliably produce 43 psi of pressure, additional capacity is used, and then regulated down.  Because the pumps are external to the fuel, they are only cooled by the fuel running through them and by convection.  If the pump slowly got hotter and hotter during operation, a vapor situation would surely occur.   
• In order to use fuel for pump cooling, excess fuel needs to flow through the pump.  Most systems run 3-5 times more fuel through the pump than what is used by the engine.  The main reason the fuel is returned back to the fuel tank is to cool it.  The fuel also cool the fuel rail and purges air from the rail when first primed for starting.  
• If we eliminate the large pumps, we also eliminate some of the heat put into the fuel.  Some fuel must still be returned to the header tank since these fuel rails are not designed to purge air without a return at the last injector.  A small amount of return also helps cooling the fuel injectors. 

THE VIKING HEADER TANK SYSTEM USED ON THE UL OR ROTAX ENGINES

• The system operates exactly as if used on a Viking engine, with one exception.  There is a small amount of calculated fuel return from the fuel rail back to the header tank.  The existing fuel regulator is replaced with a bleed large enough to purge air from the system but small enough for the in-tank pumps / regulators to easily maintain 43 psi of rail pressure.  

Test setup with pump and fuel return

UL pressure regulator replaced with machined part to provide steady return along with pressure control from in tank dual fuel regulator.

Measuring return fuel amount.

keeping an eye on fuel pressure

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Viking Fuel System 1