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Safety of the fuel injected light sport aircraft engine fuel system
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.
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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
Related VIDEO
Comment
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Comment by Jan Eggenfellner on November 28, 2017 at 6:04am
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Actually, the fuel pumps are the shutoff valves and are in the header tank. So no valves at all, anywhere. The fuel can run in a continuous aluminum line, inside a thin PVC conduit, all the way from the aft service bay through the firewall. I suggest finding a place for such a conduit in order to run other essential stuff as well. One on each side of the fuselage is even better. In any case, no high fuel pressure connections anywhere inside the airplane. Gascolater is not used. The header tank itself has provisions for the installation of a fuel drain. Also, because the pumps regulate / circulate fuel internal to the tank, no fuel is ever stationary for water to collect into a pocket, during operation.
Be careful about your definition of a Rotax "Engine Failure" A fuel system failure is not an engine failure. It can however cause the engine to shut down :)
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Comment by Geoff Klestadt on November 27, 2017 at 10:01pm
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The only downside I can see with Jans design is that you have high pressure fuel lines in the cockpit (at least on a Ch750) and where to put the gascolater and fuel shutoff which will both be I think, working under pressure. Jans design also i think, fixes an issue with the rotax pumps getting vapour locked in some installations on hot days - instant engine failure.
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Yes, the latest fuel system has only been available for one year
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Comment by Mark Charles Kuba on November 20, 2017 at 2:24pm
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I have a 110 Viking it is different it returns fuel to the tanks.
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There is no need to change the fuel pressure based on MAP. For a specific MAP / temperature, there is a specific amount of fuel required to achieve a desired air fuel ratio.
The Rotax system is 20 years old, being pushed as the latest in aviation.
It is not even a direct injected engine.
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The Rotax system regulates fuel rail pressure with respect to manifold pressure. How does your system compensate for MAP?
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Comment by David J. Beaulieu on November 15, 2017 at 2:48pm
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Jan, I think I understand what you are trying to help me to understand. And in a way, we are saying or thinking along the same lines for the most part. I have no knowledge of how the ECU controls the injectors to provide proper amount of fuel for correct air/fuel ratio. I assume it is simply the duration of injector opening based on the injector flow properties. Knowing the flow rate of the injector, the ECU is programmed along with other variables (amount of air via other sensors) to operate the injector opening and closing as appropriate. That's kind of what I was getting at in my recap - the ECU mapping/programming calculation are based on the fact that it does indeed have a known fuel pressure to work with in determining correct amount of fuel (open time of injectors) to meet fuel/air ratio. Like you said, less pressure for same time period of injector being open would likely mean less fuel delivery and leaner mixture. Thanks for listening to me try to beat a dead horse, not my intent, I enjoy this type of discussion! Dave
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I actually, if the UL ECU is anything like I am used to, rpm has very little to do with the amount of fuel and the throttle position is there for acceleration enrichment only. so :) the two things you thought that control fuel, are not very important.
Fuel delivery is based on manifold pressure and intake temperature in order to find the density of the air in the intake manifold. Whatever position the throttle is or how fast / slow the engine is turning is not relevant. The RPM only add more fuel because thing happen more frequently, not because it controls anything.
A fuel regulator is far more precise than a restrictor. The most modern systems, and one that the Viking ECU can use, is direct pulse modulation of the fuel pump, removing further pars and reducing the draw even more. We are not doing this yet.
Since a feedback loop from an O2 sensor is not practical in aviation with the leaded fuel, there is no compensation or even knowledge, by the ECU, of incorrect fuel pressure. Low pressure will inject less fuel during the time the injector is held open. High pressure will do the opposite.
The engine can be tuned with a vacuum compensated fuel regulator, or without one. No real vacuum compensation take place in flight since the throttle is wide open and manifold pressure is close to outside pressure. At idle, the amount of fuel could theoretically be elevated if the vacuum compensation is removed but there is a wide range of acceptable idle mixtures for air cooled engines. Just don’t want it to lean.
Your Re-Cap is opposite. The regulator WILL directly affect mixture and the ECU has no way of knowing if it is operating correctly.
In the end we are talking about the same, using different words. However, I think this is more correct
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Jan, Here is what I think I know and what I know i don't know ;O
I know the injectors are controlled by the ECU which takes RPM and throttle position as primary input. The pre-programmed fuel flow and advance mapping within the ECU determine/control fuel amount thru the injectors in addition to the ignition timing and firing. The injection system is pressure and temp compensated as well. What I do not know is the injectors response to other than "normal" design operating pressure within the fuel circuit. Per the UL manual the fuel pressure is regulated to a 3 Bar pressure differential between the fuel lines and the varying pressure in the inlet manifold. So, let's say I have a failing pressure regulator that cannot hold 3 Bar differential. At what point do I start losing engine performance, and taken to the extreme, lose the engine? Maybe it runs fine with just a 2 Bar differential... I can't answer that! If the pressure regulator somehow failed closed, I assume pressure would build until I stalled the fuel pump, or ruptured a fitting. How does a fuel pressure regulator typically fail? What are the indications other than fuel pressure readings which would give me a clue? I will assume that proper operation of the engine is dependent on the ECU (with all it's inputs from the sensors and mapping) having a constant known fuel pressure relative to inlet as a reference for the mapping of flow. Hence the reason for a regulator providing a source of fuel to the injectors at a set differential to the inlet. If the ECU does not need constant 3 Bar fuel pressure differential to inlet pressure, why not have a calibrated bleed with no moving parts to allow for return fuel and pressure within the fuel circuit? Open for discussion at this point...
Sidebar... The newer ECU's from UL do indeed have a static port on the exterior of the ECU to measure ambient pressure. I "think" it may be a way to more accurately assess and provide ambient conditions to the ECU with improved mapping for power or efficiency?
To recap, I do not see the fuel pressure regulator as a component that "controls" mixture, but more as a device to provide a known variable for the ECU to control fuel flow, ignition, etc. associated with engine operation. I do not know to what extent, or range of, a lower or higher than "3 Bar differential relative to inlet" effects the ability of the ECU to manage the engine.
Great discussion, appreciate the dialog...help me fill my (and maybe others) clue bag!
Dave
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I did see a reference to altitude in your post. The ECU has no idea of what altitude the airplane is flying at. All it knows is what info it is being given.
It knows MAP and intake temperature. That is enough info to calculate the ‘amount’ of air entering the engine and therefore the amount of fuel to introduce
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