I am part of a team developing a hydraulic system to power a liquid propane fuel pump (we need hydraulics because the pump is too large to be run off the alternator). I want to model the response of the system as the engine RPM increases from approximately 2400 to 3600. Therefore, I need curves of fuel pressure, fuel pump RPM, fuel flow rate, and engine RPM plotted against time as the engine RPM increases. I am using a hydraulic pump to model the fuel pump, and I modified the hydraulic fluid properties in order to simulate liquid propane. The fuel pressure is the input to a PID control loop used to control a proportional valve.
My problem seems to be in fuel pump circuit. The fuel pump is wired in series to the two sensors, the orifice, and the reference tank. When I run the simulation, the fuel pressure increases to the value I expect, and yet the flow sensor records a flat zero. I cannot understand how I am getting pressure in the fluid without any flow; even when I remove the orifice entirely, the flow meter shows zero flow. I suspect there may be a problem with assumptions made by the Simulink code; perhaps there is leakage in the fuel pump that is causing this problem?
Additionally, I have observed a strange trend that the fuel pressure recorded by the pressure sensor decreases as the displacement of the motor and pump increase. I have had to use motor and pump sizes of 5% and 3% or their expected required size in order to observe the required fuel pressure. This runs counter to what I expect – should not the hydraulic pressure increase as displacement increases?