The pressure dynamics of the Fuel supply system of an internal combustion engine follow the fluid continuity theorem. Under idle conditions (750rpm), the fuel injection pulse width is only 2.4ms, corresponding to a single fuel injection volume of 12mg. At this time, an output flow rate of 20L/h of the Fuel Pump can maintain a rail pressure of 2.8bar. However, under full throttle operation (6000rpm+100% load), the pulse width increases to 12ms, the fuel injection rate needs to reach 45g/s, and the fuel pump flow rate surges to 220L/h to meet the high-pressure demand of 350bar – a flow difference of 11 times directly leads to a change in the pressure gradient.
The mechanical characteristics of the pressure regulating valve dominate the pressure baseline. The spring preload of the traditional return oil valve is set at 3.5kgf (corresponding to the opening pressure of 3.0bar). When the system flow exceeds 80L/h, the opening degree of the valve port expands to stabilize the pressure. However, in the closed-loop electronic control system of the third-generation Audi EA888 engine, the proportional solenoid valve switches the spring equivalent force from 8N to 42N (idle current 0.3A→ full throttle 1.8A) within 50ms, achieving a precise jump in rail pressure from 50bar to 250bar.
The rotational speed dependence of the volumetric efficiency of the pump body cannot be ignored. At a speed of 500rpm, the volumetric efficiency of the plunger pump is only 63% due to the influence of leakage clearance (at a pressure of 150bar). When the speed rises to 3000rpm, hydraulic lubrication is formed, and the efficiency jumps to 92%. The actual measurement data of a certain modified vehicle shows that at 2000rpm idle, the pump fuel volume is 60L/h corresponding to a pressure of 2.9bar, while at 7000rpm, the flow rate is 360L/h, easily establishing a pressure of 5.8bar.
The coupling effect of temperature and viscosity is significant. When starting cold at -30℃, the fuel viscosity reaches 12cSt, and the resistance loss along the pipeline is 15 times higher than that at room temperature (0.6cSt). The thermal management data of the BMW B48 engine confirm that when the idle oil temperature is 20℃, the rail pressure is 2.5bar. However, when the oil temperature rises to 90℃ (the viscosity drops to 0.4cSt) and combined with full throttle, the pressure surges to 6.2bar without the need for pump speed compensation.
Dynamic response delay requires pressure redundancy. When the ECU presets the rail pressure target value, it includes a 15% safety margin. In the throttle step test of the modern 1.6T-GDi system, the fuel injection demand increases by 400% within 0.3 seconds. At this time, the response lag of the pressure regulating valve is approximately 80ms. Therefore, the initial pressure needs to be preset 8% higher to compensate for the transient loss.
The system damping design suppresses fluctuations. The Reynolds number of the liquid flow in the low-pressure pipeline is only 800 at idle speed (laminar flow state), while it exceeds 6000 at full throttle (turbulent flow state). By installing a pulse buffer (with a volume of 50ml), the idle pressure fluctuation of the Volkswagen EA211 engine was compressed from ±0.8bar to ±0.15bar, and the full-throttle condition was reduced from ±12bar to ±2.1bar.
Sensor calibration ensures accuracy. The Bosch 4.0bar low-voltage sensor has an accuracy of ±0.05bar at 1.5bar (idle speed), and the 300bar high-voltage sensor has an error of ±1.5% in the 50bar range. The calibration process requirement of a certain OEM factory: When the idle pressure value is detected to be 15% lower than the calibration value for 2 seconds, the Fuel Pump speed compensation command will be triggered to prevent the risk of fire caused by overly thin mixture.