This disclosure relates to fuel pumps, and to fuel pumps used in aircraft engines. More particularly, this disclosure relates a system and method for providing adequate and reliable fluid flow to a journal bearing of the fuel pump during different modes of operation.
Aircraft engine fuel pumps are often sized at engine start conditions, that is windmill re-light. Using this parameter for design purposes results in a fuel pump that is unnecessarily oversized at other conditions. By turning off journal bearing lubrication flow, more flow can be delivered at the pump discharge. However, this journal bearing lubrication flow must be turned back on at high-speed/high-pressure conditions. A reliable assembly and method of turning this flow off and on is required.
In addition, there is a desire to protect against pump operation at higher speed/higher pressure conditions if the journal bearing lubrication flow has been turned off. Thus, a need exists for a system and method of reliably selecting the use or non-use of bearing flow, and in inhibiting pump/engine fuel system operation at higher speed/higher pressure conditions should the user bearing flow not be selected.
In a typical aircraft engine fuel pump system, there is a constant supply of bearing lubricant flow. More particularly, there is inlet flow to a boost stage where the flow is raised to a low pressure state and then output to a high pressure pump, usually a gear pump. A portion of the high pressure flow from the gear pump is recirculated to a relief valve. The relief valve is normally spring biased toward a closed position so that flow is directed to a downstream end use at a predetermined flow and pressure. A signal or reference pressure from the boost stage at a location upstream of the gear pump is provided to the relief valve. The relief valve also receives pressure from the discharge side of the high pressure pump and when the pressure reaches a level that is above the reference/signal pressure from the boost stage (in combination with any biasing force provided by the spring, for example), then the relief valve opens and allows flow to circulate around the recirculation path or loop. The recirculating flow from the relief valve combines with the outlet tlow from the boost stage and the combined flow is inlet to the high pressure pump. The relief valve caps the pressure level.
One example of controlling flow to the bearings during different pump modes or operation is shown and described in U.S. Pat. No. 6,526,743—Maker, et al. Particularly, the fuel control includes a metering or selector valve that regulates a desired flow of fuel to an engine. The metering valve provides a low, regulated flow during low speed operation of the engine. When an intermediate to high flow is required, the position of the valve member of the metering valve is altered to provide high pressure auxiliary flow to the pump rotor bearing surfaces.
A need exists for the ability to switch bearing flow between low and high pressure in a manner that protects the pump from not switching on the bearing flow at a predetermined system level. For example, when the engine is accelerated, pressure increases and gets capped by the relief valve function being at a low setting based on the low-pressure bearing supply. In that case, the engine will reach a certain level of acceleration (certain speed level) and stay at that level. In this manner, the system is not switched to a high setting when there has not been a corresponding increase in the bearing feed pressure. Since the selector valve provides the feed to the bearings, a need exists to inform the relief valve whether it should be operating at a low or high pressure. Moreover, the need to reliably switch between at least two pressure levels of the relief valve should be reliable and economical to implement.