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| Aircraft fuel injection system |
Aircraft fuel injection systems, like most thing on reciprocating aircraft engines, are designed to be incredibly simple at the price of efficiency, the theory being that it’s better to have a functioning but inefficient system in flight than it is to have an elegant system that is prone to dying at inopportune time (where “inopportune” any time the gear isn’t firmly on the ground”).
Aircraft fuel injection systems are continuous flow; fuel is delivered under pressure via an engine driven pump with electrical backup to a metering unit, which is nothing more than a flow control valve that is operated by the throttle with an upstream valve to control the delivery rate of the fuel from the pumps that is used as the mixture control; from there it flows to a distributor (basically a small manifold with n+1 ports, where n is defined by the number of cylinders) and hence to the injectors, which constantly mist fuel onto the intake valves. There is no engine speed compensation, no altitude compensation, no MAP (vacuum or boost) compensation; that’s all done by the human with their hand on the mixture control and an eye on the EGTs; in fact the only concession to boost is found in using bleed air to provide a form of compensation at the injectors.
There are 3 separate systems in a fuel injection engine of an aircraft.
*Fuel pumps,*Fuel*Air control unit
Fuel Manifold valve connected to discharge nozzles. There are two pumps. An electrically driven pump and an Engine Driven Pump (EDP). They are both independent. If one fails, the other will continue to function. In normal operations the electric pump is used during start up. The fuel pumps supply fuel to the fuel and air control unit.
In the fuel air control unit, we have the throttle control, which controls the throttle butterfly used to vary the air flow. It also has a valve control which can be used to vary the fuel flow according to the air flow into the engine. In addition to this, we have a mixture control lever, which is the first obstruction the fuel faces in the fuel air unit. This lever can be used much like a carburettor engine mixture lever to vary the fuel air ratio and also as a fuel cut off.
The fuel manifold valve is used to distribute the fuel around the engine. When the fuel pressure builds up, the manifold lets the fuel pass through a link of piping to discharge nozzles. The discharge nozzles then sprays fuel into the intake port which is close to the engine intake valves. The discharge nozzles are designed in such a way that the fuel particles change into a fine mist. To achieve this, air is passed through the fuel as it passes through the nozzles, vaporizing the fuel.
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