Piston Engines - Fuel Injection — Page 127, Lesson 161

We're starting a new topic now: fuel injection for piston engines. Let's look at how the fuel gets from the tank into the cylinders, and the components that control it. First, the fuel pumps. The pump supplies more fuel than the engine actually needs, and there's a recirculation path to handle the excess. Two pumps are provided, arranged in parallel. That parallel arrangement matters: when the mechanical pump isn't operating, fuel under positive pressure from the electrical pump can bypass the mechanical pump. That lets the electrical pump be used for engine priming, for starting, and in an emergency. Next, the fuel/air control unit. This unit is mounted on the intake manifold and contains three control elements: the air throttle assembly, which is the throttle valve; the throttle metering valve, which is the metering fuel valve; and the mixture control valve. The air throttle assembly contains the air throttle valve, which is connected to the pilot's throttle lever and controls airflow to the engine. Note that the intake manifold has no Venturi or other restrictions to airflow — that's a key difference from a carburetor system. The fuel control unit is attached to the air throttle assembly and controls fuel flow to the engine by means of two valves. The first, the metering fuel valve, is connected to the air throttle and controls fuel flow to the fuel manifold valve according to the position of the air throttle. So fuel flow is proportioned to airflow, giving the correct air/fuel ratio. The second valve, the mixture control valve, is connected to the pilot's mixture control lever. It bleeds off fuel pressure applied to the metering valve. That way, the air/fuel ratio can be varied from the basic setting of the metering valve, as required by operating conditions. There's also a fuel pressure gauge in the system. It indicates metered fuel pressure, and by suitable calibration, it enables the mixture to be adjusted according to altitude and power setting. Now the fuel manifold valve. This valve is located on the engine crankcase and is the central point for distributing metered fuel to the engine. When the engine is stopped, all the outlet ports are closed, and no fuel can flow to the engine. As fuel pressure builds up — from engine rotation or booster pump operation — all the ports to the discharge nozzles open simultaneously. A ball valve ensures the ports are fully open before fuel starts to flow. Finally, the discharge nozzle. A fuel discharge nozzle is located in each cylinder head, with its outlet directed into the inlet port. Nozzles are calibrated in several ranges, and they're fitted to individual engines as a set, each nozzle in a set having the same calibration. Let me show you the general arrangement of this indirect fuel injection system. So the whole picture: the pumps supply fuel under pressure, the fuel/air control unit meters it proportionally to airflow, the manifold valve distributes it evenly to all cylinders, and the discharge nozzles spray it into each inlet port.