In this entry – about some specific nuances of LPFP management.
The data of the LPFP sensor are received in an analog way. In the situation, when the car is woken up (it was in sleep mode before), DME evaluates the indications of the LP sensor, before starting the LP pump.
If the indications of the sensor are unreliable (the sensor indicates increased pressure (significantly different from 0hPa), DME records the error message regarding the error of the sensor data and ignores the sensor. If the sensor is ignored, DME gives the command to the EKP work with full power (PWM 100%).
If the indications of the sensor correspond to the allowed corridor, the value, appropriate to its conditions of 0hPa, is evaluated. If the indications of the sensor are “close” enough to 0hPa (Offset) value, DME (re)adapts the indications of 0hPa pressure of the sensor.
In such a way DME avoids the parameter offset from the perfect value of the sensor itself and also the Offset errors of the DME ADC hub.
Similarly, the 0pHa value of the HP sensor is adapted. Additionally, the indications of both sensors are controlled in exact conditions, for example: at the moment, when LPFP is turned on, but HPFP (yet) is not turned on – readings of both pressure sensors are compared.
Returning to the LP contour.
As we know, the target pressure of LPFP is 5000hPa. The pressure is adjusted, using a closed-loop system, it means: the data of the pressure sensor are used to stabilize the pressure of the LP contour.
To avoid the situations, when, for example, the defect of the pressure sensor causes a total crash of the system, the following actions are performed:
- EKP knows the approximate total flowrate (mostly the flow-rate of the fuel pumping between sides of the fuel tank, HFPF flowrate makes the smallest part) to ensure the necessary target pressure (overpumping flowrate is defined by the system of fuel pumping tubes and valves, instead the necessary flowrate of HPFP is known/calculated from required torque and RPM) and according to this flowrate the U management map of output voltage, supplied to the LP pump (dependance of the EKP Uout from the total flow-rate, which is pumping flowrate + HPFP flowrate);
- during a longer time (reacting slowly), taking into account indications of LP pressure sensor, EKP completes the LPFP adaptations map and the corridor of its allowed influence, which includes the true impact of the LP system (electrical parameters of the pump + pumping system parameters);
- in a relatively narrow range, Uout is changed depending on data of the LP sensor, inversely proportional its readings (it means if the sensor indicates increased pressure, Uout is reduced; if the sensor indicates reduced pressure, Uout is increased).
Accordingly – in the short-term, EKP is guided by management + adaptation map data, also – adjusting Uout according to the sensor indications in a narrow range. LP sensor is used also to adjust the adaptation map in the long-term (slowly). Why such (mainly – slow) reaction is OK and is needed?
If the pumping system and LPFP have no defects, it “behavior” can be projected very easily – there is no need to change the voltage supplied to the pump in a wide range and very swiftly – its flowrate and the pressure of the system are stable.
Instead, if the LP pressure sensor reports sudden and swift differences of the pressure from ideal or the fuel pump consumes increased or reduced current, the following options are possible:
- damage to the sensor itself. EKP evaluates the current, consumed by the pump. If the current (and accordingly – the power) is not changed and correspond the planned – obviously, the pressure sensor itself is damaged;
- problems of the pump. If EKP identifies inadequacy of the current of the LP pump, there is reason to believe, that the problems have affected the pump itself;
- if the drop of the LPFP pressure corresponds the data of HPFP regarding increased fuel flow-rate thru HPFP (PWM, supplied to the HPFP pump, should be reduced to maintain necessary HP pressure), DME /EKP obtains the confirmation to the increased fuel consumption via HPFP contour (obviously – either the system is not airtight or the injectors have stuck in open condition);
d) if there is less than 5 .. 7 liters of the fuel in the fuel tank and EKP identifies reduced current of the LP pump (obviously, the pump is in a dry situation), in addition to the “usual” error messages regarding misfires, problems of the pump, etc., also appropriate error messages with note regarding low fuel level in the fuel tank are stored. In this situation, it has to be evaluated if the cause of all problems is the low level of fuel in the fuel tank.
Note: if in LP contour (due to lowered fuel level in the tank) the air is pulled in, its flowing out requires a certain time because the LP system is organized similarly than for diesel cars (not as in “regular” petrol cars, where the circulation of LP contour swiftly removes the air from the system).
Thanks to the previously mentioned mechanism of maintaining the pressure, LP system works very stable: the problems of the pump and LP pressure sensor are detected easily.
Several non-standard situations:
- separate security systems (for example, IGLA) tend to block DME in such a way, that sometimes (very rare, but events are identified) DME performs a check of LP and HP sensors Offset (it means, according to the pressure of 0hPa) “untimely”. This check is finished unsuccessfully and in the DME error message memory false error messages regarding LP and HP sensors are recorded (in the attributes of the error messages can be seen, that exactly the measurements of 0hPa have failed);
- after readapting the engine and also after replacing the LPFP pump or pumping system, the error message 2AAF (LPFP, plausibility) is recorded. In the attributes, usually correct LP contour pressure (4500 .. 5500hPa) can be seen. Reason – the creation of the LPFP adaptation maps requires a longer time. Accordingly – if the conditions (pump, fuel filter, pumping system, etc.) have changed, it will take a long time until EKP will upgrade LPFP adaptation maps. If 2AAF is recorded after repair of the LP system – there is no reason to worry. Instead, if 2AAF is recorded regularly, the performance of all LP contour (pumping system, valve, pump) has to be checked.
Note: 2AAF causes no devastating consequences in the performance of EKP. EKP continues to stabilize the pressure of the LP contour. 2AAF means: to reach the necessary pressure (indicated in the attributes of the error message), EKP was forced to apply Uout PWM, which “fell” out the required U corridor.
If the LP pressure will be different from the required one (5000hPa), not taking into account the significantly changed Uout for a longer time (longer than 5 seconds), the error message 2AAE will be recorded. This already is a critical error message, and after recording the EKP will be switched to the emergency mode (with PWM 100%).
Image of the LPFP management:
 HPFP flowrate
 fuel pumping flowrate
 total LPFP flowrate
 LP pump management map
 LP pump management + adaptation map
 LP pump management + adaptation maps + correction corridor
Note: Flowrate and Uout values are indicative and can be different for the exact car