Pressures to perform
FAR23.955(c) states that a pump-fed fuel system must be capable of supplying 125 percent of the takeoff fuel consumption requirements of the engine. Since the pump is mechanically driven from the accessory housing by a direct-drive type coupling, any change in engine speed directly impacts pump speed. Thus, fuel pressure becomes proportionate to engine rpm.
Altitude and ambient air temperatures also play an important role and can limit the amount of fuel reaching the cylinders. It is precisely for these and other reasons that the pump must deliver fuel in excess of engine requirements. So, how does the pump compensate for this overabundance of fuel when it's not needed? A re-circulation path is provided within the pump body itself - a provision that ensures proper pressures at all operating speeds (see diagrams of fuel flows above). The relief valve and seat become a variable orifice, a simple, but effective means of accurately tailoring pump output pressures. The relief valve is in parallel with the rotor and vanes (the pumping mechanism). Adjusting the spring tension on the relief valve controls discharge pressure generated by the pump. As pump output increases beyond what the engine requires, the relief valve opens and regulates the re-circulation of fuel through the pump. Surplus fuel is routed back to the intake side of the pump.
The precise metering of fuel is accomplished downstream of the pump within a Fuel Injection Servo. The servo, in turn, feeds fuel to a flow divider atop the crankcase or divider blocks positioned beneath each bank of cylinders. Fuel in equal volume is then fed via injection nozzles to the individual cylinders where it becomes mixed with induction air.
A diaphragm in the relief valve housing is essential for two reasons:
1) It provides venting to turbo discharge pressure or to the atmosphere; and by its balancing action,
2) it helps to maintain a constant output pressure regardless of variations in pressure on the suction side of the pump.
The relief valve must be vented to either atmospheric or upper deck pressure (pressure between the turbo and throttle plate) to assure that fuel pressure is maintained and fed at a constant pressure to the fuel servo or carburetor. Failure to reference the relief valve to turbo pressure would result in decreased fuel pressures at higher altitudes. In a normally aspirated installation, the vent is referenced to atmosphere and must remain unobstructed to prevent fluctuations in fuel pressures. In a turbocharged application, this vent fitting is orificed (restricted to .014-.017). If the diaphragm fails, the restricted fitting prevents massive amounts of fuel from entering the upper deck air reference lines to the injection nozzles. Such a leak would cause a rich mixture creating a roughness in engine performance and a corresponding loss of power at any power setting. No indication of this additional fuel consumption would be indicated on the fuel flow gauge. Should such a symptom occur, simply remove the hose feeding this fitting and look for evidence of fuel in the line. Another problem in relation to the reference line and fitting occurs if either becomes blocked or leaks. Turbo pressure to the relief valve would be reduced, resulting in a reduction in pump output pressure during climbs to altitude.
In the unlikely occurrence of pump failure, a bypass valve within the pump body allows the fuel to flow around the pump vanes under boost pressure from the electrically- driven auxiliary pump. This bypass valve also provides a conduit for fuel when the boost pump is engaged for starting the engine. This check valve feature is designed to sense output pressures from both the injection pump and the boost pump. Once the engine starts and the pump pressure rises above that of boost pressure, the valve is forced closed.
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