Engine Management Systems, FADEC
The fuel air mixture in the combustion chamber needs to be ignited at the correct moment to ensure efficient combustion and power generation by the engine. This is the job of the ignition system, be that the old fashion magneto of the good old days or a modern fully electronic microprocessor controlled FADEC injected system system we see more and more each passing year.
For safety reasons the ignition system may not rely on the aircraft electrical system and must be dual and each system operates one of the two spark plugs in each cylinder.
Magneto ignition suffers from a number of problems which have long been resolved with vacuum, RPM and mechanical ignition advancing combined with memory mapped microprocessor controlled FADEC systems or sophisticated electronic ignitions.
A Full Authority Digital Engine Control (FADEC) comes to good use in combination with a fuel injection system, only then are the advantages available to the pilot or operator of the aircraft.
Compared to the previous described ignition systems FADECs are complicated systems and you will probably need help from an engineer familiar with the system you plan on using.
Full Authority Digital Engine Control
A sophisticated ignition: full authority digital engine control (FADEC) is a memory mapped microprocessor controlled ignition with sensors measuring MAP, CHT, EGT, RPM, atmospheric and oil pressure. It controls fuel injection and ignition timing to fully optimize the power produced by the engine.
These systems add considerable complexity and additional wiring to the original electrical system as they need (among other things) a separate backup power supply from second battery. This secondary battery must be kept fully charged during flight through a special schottky diode (low voltage drop type) and must be monitored for charging.
Due to the added weight of the parts of the FADEC the weight and balance of the aircraft must be recalculated after installation.
Cockpit controls are added and you will have extra switches for a fuel pump, primary and secondary FADEC power supply and fuses to protect these systems. There is no mixture control as the FADEC controls the amount of mixture injection and timing.
The engine control unit (ECU) uses a 3D memory map to control the injector for the right amount of fuel and takes into account different ambient circumstances as outside air temperature (OAT) and air pressure (density) with regard to RPM and throttle setting and regulates this several times per second. Mixture control is automatic as the ECU senses barometric pressure and compensates the amount of fuel injected while the aircraft climbs.
The ignition timing of the spark plugs (spark advance) is also regulated depending on RPM, throttle setting for every load on the engine. This variable igniting timing results in quicker engine starts and smoother operation under variable loads.
Starting a FADEC engine is no more exciting than pressing the start button and it runs, no more choking, priming and endless battery draining starts. The ECU takes all variables into account and retards ignition and regulates fuel per cylinder for easy starts, just like any modern car these days.
With this system the carburetor is finally eliminated. This is a big advantage as the change for carburetor icing is then nil. Carb ice develops when the temperature drops (sometimes in excess of 25°C) due to evaporation of the fuel and decrease in air pressure in the venturi throat. Should water vapor be present in the air, it condenses and freezes on the throttle valve and in the venturi, restricting airflow.
This can be resolve by applying carburetor heat. Intake air is rerouted over the hot exhaust and fed to the carburetor. The result is that the ice melts and the engine loses a bit of power, as hot air is less dens and the mixture will be richer.
The inlet air to the exhaust muff is not filtered so the pilot must switch carb heat off on short final, just in case he needs to make a go-around with full engine power and secondly the air close to the ground contains fine dust and sand which increases engine wear in the pistons and cylinders.
Due to the high pressure fuel system, things as vapour lock are a non event. Even hot starting a fuel injected engine is no problem at all, the higher fuel pressure will remove any air pockets in the fuel lines.
With a FADEC the engine receives the right amount of fuel per cylinder and the spark plugs are ignited at the right time regarding RPM, throttle setting, ambient temperature and pressure. This results in fuel savings up to 15%, an easier starting and smoother running engine.
Lycoming IE2 Project
The TEO-540-A1A is controlled by a state of the art electronic fuel injection FADEC which is single lever, multi-fuel, with knock detection, turbo and propeller control, common rail fuel system and electronic injectors. More in our Lycoming IE2 Engine article.