Updated 08/12/05
Proper fuel system design is very important to ensure trouble free performance when installing an EFI engine into an aircraft. Many people fail to consider certain aspects when designing or modifying their system for use with EFI. Fuel system malfunction and fuel starvation are among the leading causes of homebuilt aircraft crashes.
System Basics
It is important to familiarize yourself with the basic EFI mechanical components and function to be able to understand why certain things need to be a certain way. All EFI systems use a high pressure pump to supply fuel to the injectors. This is almost always electrically driven. Most systems run between 35 and 45 psi. Fuel is supplied to fuel rails or a fuel block which is connected to the injectors. The other end of the fuel rail or block is connected to a fuel pressure regulator. Its function is to hold the fuel pressure at a constant differential above the intake manifold pressure. It does this by returning unused fuel back to the fuel tank. The pump always puts out a constant volume of fuel and more than the engine requires at full throttle so most of the fuel is returned back to the tank under idle and low power conditions. Below is a proven fuel system used in racing cars which undergo high G forces. The system for aircraft is a variation of this and has also been flight proven in our RV6A and others.
General Concerns in Aircraft
An EFI fuel system must be designed to supply fuel to the injectors under all anticipated flight conditions. EFI engines do not tolerate getting air into their fuel systems. Unlike a carb which has a float bowl to dissipate air bubbles, if air is present on the high pressure side of the pump, air will be injected along with the fuel. This will lead to a lean condition until the air is purged. It should also be noted that most EFI pumps do not process air very well due to their design nor do they reprime well if there is much head involved. In short, a constant, air free fuel supply must be available at the inlet of the high pressure pump.
On high wing aircraft with the tanks in the wings, fuel can be gravity fed to a firewall mounted header tank or even directly to firewall mounted high pressure pumps. Be sure that your system will feed from at least one tank during high bank angles in uncoordinated turns and at steep pitch angles as well. If not, low pressure pumps with good pickup qualities should be used to feed a firewall mounted header tank as shown in the graphic below for low wing aircraft.
Low wing aircraft with wing tanks either must have in-tank high pressure pumps or a system as depicted above to ensure a constant supply of fuel. 2 Low pressure and 2 high pressure pumps are recommended for redundancy. One of each can feed the engine. Check valves should be installed on the discharge side of all pumps to prevent back flow through them when only one pump set is on. Most EFI pumps have these internally already. One of the low pressure pumps can be an engine driven type if your engine has the location or drive for it. The decision to use redundant pumps is your choice of course.
Fuel Pumps
For electrically driven low pressure pumps, the Purolator/Facet, solid state oscillating units are light, reliable and inexpensive. They are available in 3 models with different flow rates, pressures and draw head. Aircraft Spruce carries the 2 popular models rated at 30 GPH. A higher flow race model with higher draw head and flow rate is available from sources in the UK. One low pressure pump needs to be able to flow enough fuel to feed the engine at full power. As a rule of thumb, each US gallon per hour will feed about 11 hp. If you take your engine hp and divide by 11, it will give you the flow rate needed to feed it in US gallons per hour.
For high pressure pumps, it is hard to beat the Bosch designs. These have been around for 30 years and are well proven and reliable if installed correctly and fed clean fuel. They are not inexpensive though. Note that most Bosch EFI pumps use a barb (slide on) inlet fitting which does not allow the use of an AN type fitting. Some models also use a barb discharge fitting. Be aware that the standard barb connections have been well proven in billions of hours of automotive use and are probably more reliable than AN fitting connections. Auto fuel pumps are often located underneath the car near the tank. They are constantly in a wet and dirty environment and rarely see any maintenence or inspection performed for years. Proper high pressure fuel line needs to be used throughout the high pressure system. Don't even THINK about using standard auto fuel hose designed for carbureted engines here. Two popular Bosch pumps are shown below, discharge left, inlet right:
For those wishing a lighter, smaller pump with threaded type connections on both ends, we will soon be offering the popular Walbro series shown below. While we have not flight tested these ourselves, they have proven very reliable in race cars and other aircraft applications. We plan to use them in our new RV10 project. All of the pumps we sell have built in check valves.
In-tank pumps can also be used to eliminate the low pressure pumps and header tank. It may be harder to source and fit these types and of course repair is far more difficult. There would be weight, cost and power savings on the other side of the coin. On shallow wing tanks, the pump inlets may become uncovered in non-level attitudes, so they should be boxed in to avoid starvation. This may be difficult to do in many designs and still have pump access for maintenence.
Header Tanks
Most designs will need a small header tank mounted on the firewall. These serve an important function for for entrained air to exit the system from the low pressure and return system and to provide a gravity fed fuel supply to the EFI pumps. The header tank should be mounted as high as possible on the firewall with the EFI pumps mounted directly below it, inlets facing upwards.
Header tanks are easily constructed from 3 or 4 inch steel or aluminum tubing. Caps are welded on each end with provisions for threaded fittings top and bottom. 4 fittings on the top of the tank and 3 on the bottom work well. A tall design is best as this allows the best chance for air bubbles to float to the top, away from the EFI pump inlets on the bottom. This design also allows good gravity feed to the EFI pumps even at extreme bank and pitch angles. A quick drain fitting in the tank bottom can be used to check for water. Tank volumes of 1 to 2 liters/quarts are more than adequate. To calculate tank volume for cylindrical tanks: tube diameter divided by 2. Square this dimension and multiply by 3.14. Multiply this figure by the length of the tube. 1 US quart equals about 58 cubic inches. The schematic below shows a typical design:
Fuel that the engine doesn't consume is routed through the pressure regulator, back to the top of the header tank. If the tank is full, the fuel exits the top of the header tank and runs back to the main tanks through return lines.
Fuel Tanks, Filters, Lines and Return Lines
It is important to note that with the system depicted, returned fuel will enter both tanks simultaneously. If you have selected the left tank to feed from, the right tank will receive a large portion of the returned fuel and will start to fill up. You may want to consider designing your fuel system so that you always draw fuel from both tanks simultaneously and return fuel to both as well. This involves the least system management. A disadvantage to this is if one tank runs dry before the other, air will be sucked into the pump and the engine will shortly die. Both tanks need to be filled with the same amount of fuel also.
Alternately, you could design a system to return fuel only to the selected tank although this is slightly more complicated but fuel management is simplified. You could install 2 fuel selectors, one for feed and one for return. Turn both to left or both to right or if you want to do this right, Andair in the UK builds a very nice selector designed for fuel injected engines. This is a beautiful (and expensive) piece with Left, Right and Off selections. It has a feed and return port for each tank. This is the one we use in our RV6A. It is available from Aircraft Spruce. The Part number is FS 20-20-D2 for the female 1/4 NPT version.
Return lines should ideally exit to a location within the tanks which is away from the feed fittings to prevent entrained air from re-entering your low pressure pumps. There is very little pressure acting on the return system so virtually any fuel safe hose or line can be used here. Because of the high pressure involved, HP fuel lines can be smaller than in an equivalent carbureted installation. 5/16 inch lines are more than enough for a 300 hp injected engine. Lines should be routed away from hot items or air to minimize fuel boiling problems. Fire shield sheathing should be used to protect flexible lines from exhaust system components. Fuel filters are important and are best placed before the fuel pumps to keep debris out of them. Roller vane type EFI pumps are not very dirt tolerant. Metal mesh screen filters or high quality paper element types are both suitable. Fuel system checks are even more important on an EFI equipped aircraft because of the high pressures involved. A line or component failure on the HP side will likley create a big fire in a big hurry.
We prefer to mount the header tank and fuel pumps inside the cabin to minimise the posibilities of ignition in the event of a leak. Conventional aircraft typically have the fuel lines and selector already cabin mounted so there is little difference. We don't like the idea of engine heat running over the pumps when mounted firewall forward as this can and has caused vapor lock problems.
Earl's 230206 has a stainless steel screen and can be disassembled for cleaning. Available in -6 or -8 AN.
Measuring Fuel Flow
Because of the amount of returned fuel on an EFI engine, conventional flow meters will not work unless another meter is placed in the return line and your system can subtract returned fuel from delivered fuel to arrive at consumed fuel. On engines equipped with SDS engine management, you can order our $50US fuel flow option. This displays through the programmer LCD. The computer measures injector duty cycle and with the injector flow rate entered, can calculate fuel burned in pounds per hour. It is real-time only so it cannot calculate total fuel burned. On engines equipped with other systems, accurate fuel gauges are very important.
Electrical System Considerations
If we look at the current draw for various components, we can arrive at a total current draw for the system:
Injectors (4)-------------------3-4 amps
Coil pack ignition-------------2-3 amps
Low pressure pumps (each)-1-2 amps
High pressure pumps (each)-4-6 amps
ECU-----------------------------0.2 amps
In cruise flight, with one LP and one HP pump running at medium engine rpm, we are probably consuming around 10-13 amps. With everything running at high engine rpm, we are probably using around 17 to 20 amps. It is important to be aware of this extra current draw over a carburetor system with an engine driven pump. You may wish to upgrade your alternator with a larger capacity one. A small supplemental battery, isolated from your main battery, will give you 15 to 30 minutes to look for a safe landing spot in the unlikely event that both your alternator and main battery expire. It is an excellent idea to have a voltmeter or warning lights on your electrical system when using EFI. No electrical power and your engine stops. A small sealed battery or gel cell as shown below can give you some piece of mind for only 4-7 lbs. This one weighs 4 lbs. and is rated at 8.0 amp hours. They are available in many different voltages and capacities. It should give you 15-20 minutes with minimal radios and reduced power. We run an 18 amp/ hour one at 12.6 lbs. 12 amp hour ones are also available. It should be isolated from the main battery with diodes or switches to protect it from a main battery failure. It can be kept charged at all times from the alternator.
Moisture Concerns
EFI engines are far more tolerant to water in the fuel system than a carbureted engine. This is because of the high volume of returned vs. burned fuel and the extreme dynamic motion of the fuel in the system. This basically mixes the water up in the fuel, reducing the chances of the engine ingesting a big slug of water. This fact does not mean that you should be any less careful about checking for water during preflight. With enough water in your system, even EFI engines can stop. Water sitting in the pumps and injectors can also cause corrosion and cost hundreds of dollars to replace. I should be noted that some inspectors insist on a gascolator even on EFI engines. This is probably a result of ignorance or just part of outdated regulations. We suggest a fuel quick drain in your header tank and each fuel tank to check for any water. Hopefully this will satisfy this requirement. A conventional gascolator at the pressures involve with EFI would be useless and possibly a liability. Andair does build a high pressure unit if you must have one.
Aerobatics
Normal aerobatics, pulling positive G at all times should present no problems with EFI. If you intend to perform inverted or negative G aerobatics, all tanks will require flop tubes. Just remember that even though your engine might run inverted with EFI, you do need an inverted oil system as well.
Flight Testing
Flight testing of your new system should always be performed cautiously. Maintain low deck and bank angles until you reach safe altitudes. Start by performing shallow turns, climbs and descents , then progressing to steeper banking turns and steep pitch attitudes both up and down with all pumps on. Then try the same things with only one set of pumps on. Be aware that it may take several minutes to use up the fuel supply in the header tank if the flow from the main tanks to it are interrupted. You should hold the steep attitudes if possible for 3-5 minutes at moderate power settings to ensure that you system will supply fuel under all flight conditions.