Part 4 – OE LPF Pressure Regulator Function and Performance
The question arises, if the EKPM regulates the LP fuel pressure by pulse-width modulating the pump voltage, why does the system require a mechanical fuel pressure regulator?
The answer revealed in this part is that the “pressure regulator” would more accurately be characterized as an “overpressure relief valve”. This is because the regulator set point is significantly higher than the normal 72 psi (5.0 bar) operating pressure.
This type of over-pressure relief function is well described by Aeromotive in Part 4 of their training series:
https://aeromotiveinc.com/pages/fuel...ypes-functions
“On Direct Injected (DI) engines with mechanical pumps this (over pressure due to thermal expansion on shutdown hindering injector function, ed) is typically not a problem, but having excessive fuel pressure on lines and fittings is not a safe condition. Hence, a safety overpressure regulator is built into the modern OEM fuel modules. In the case of most OE fuel modules, this pressure is in the 75-90psi range”
My testing was accomplished on a fully installed fuel system using a Precision Raceworks Stage 2 non-modular bucketed fuel pump, which incorporates a TI/Walbro F90000274 (450 lph) fuel pump, and an OE fuel pressure regulator module. The pump was powered by a regulated dc power supply operating in programmable constant voltage mode instead of the EKPM. This power supply was wired directly to the pump with 10 AWG cables. Fuel pressure was monitored using the on-board fuel pressure sensor, accessed through OBDII live data using MHD. After initially bleeding off the fuel pressure, the pump was run in constant voltage mode at increasing voltages, with the associated voltage and fuel pressure being noted. The conclusions are not affected by the pump being used.
During this testing the engine was not running, so the fuel being pumped was being consumed by the venturi jet pumps and remaining in the tank, until the mechanical pressure regulator reached its set point, at which time it relieved sufficient flow (again back to the tank) to hold the system pressure close to the mechanical set point.
The pressure characteristics reveal a very distinct knee, showing that the set point is approximately 5.45 bar (79 psi). Under normal operating conditions the observed fuel pressure is less than 5.5 bar, meaning that under normal operating conditions no fuel flows through the mechanical pressure regulator and pressure regulation is strictly under PWM control.
As the fuel flow, as represented by fuel pump voltage, initially increases, all the flow is consumed by the jet pumps (blue line). When the fuel pressure reaches and exceeds the fuel pressure regulator set point, an increasing amount of fuel flows through the regulator bypass (red line), clamping the fuel pressure close to the set point. Assuming the EKPM can regulate the fuel pressure within 72 ±5 psi (within green dashed lines), the fuel pressure regulator will never flow any fuel.
To test this hypothesis, we went for a drive and logged the required data (the upcoming Part on PT-CAN communications will outline how that was done). In this case the fuel pump was powered normally by a Stage 2 EKPM3 and the system was well-adapted. The data includes idling, cruise and several full-power 3rd gear pulls. The data represents over 700 data points logged at 1 Hz, so more than 10 minutes of operation. Pressures are logged in increments of 1 psi and voltages in increments of 0.1 Volts, so many of the data points consist of multiple hits.
This data confirms our hypothesis that under normal operation the mechanical fuel pressure regulator bypass flows no fuel and all pressure regulation is accomplished by the EKPM and DME. The data points outside our assumed normal operating range represent single data points under transient conditions (e.g., abrupt throttle closure) and are not significant.
As we will confirm in later Parts, the DME requests a fuel flow from the EKPM, not a fuel pressure. The DME knows at every moment how much fuel the engine is consuming (it controls the fuel injectors) so when the operating conditions change it can request an appropriate incremental change in fuel flow. The EKPM controls the fuel pump by providing a specific voltage to the pump. To do this, it monitors the voltage supplied to itself and then calculates the appropriate duty cycle for its pulse-width-modulation (PWM) to produce the required voltage at the fuel pump. The specific voltage needed is a function of the voltage vs. fuel flow characteristics for the system. It learns this through the adaptation process.
That our system strives to operate at a single fuel pressure is very convenient. It allows us to derive the voltage vs. fuel flow characteristic curve for our system. Because the curve is non-linear, we need 3 data points to define it. Two of them can be obtained from the pump datasheet, and the third, we learned from our jet pump orifice flow testing. The datasheet data points are the fuel flows at 72 psi at 12.0 Volts and at 13.5 Volts.
Some interpolation and conversion of units is required, but from the datasheet we get these two points:
305 lph at 12.0 Volts and 72 psi
360 lph at 13.5 Volts and 72 psi
The third point is our measured jet pump fuel consumption for two jet pumps. The voltage at this condition we get from our fuel pressure regulator measurements:
110 lph at 8.4 Volts and 72 psi
Here we confront a fact of EFI fuel pumps – while this pump is nominally in the 450 lph class, it can only achieve that flow rate below about 40 psi. The flow capability of all electric fuel pumps drops with increasing pressure.
Plotting our three data points we get the following:
While the blue line represents the total output of the pump, for the usable flow we must subtract the 110 lph overhead flow required by the two jet pumps. This characteristic curve is what the EKPM needs to “know” to calculate the required fuel pump voltage for a given fuel flow. The characteristic curve for the OE pump is vastly different. The fact that the system can accommodate a wide range of pump characteristics through adaptation is a tribute to its robust design.