BMW N57TU engine Common Rail fuel system with a twin-piston high pressure pump Bosch CP4.2
The N57TU engine is fitted with a twin-piston high pressure pump with the designation CP4.2.
The high-pressure pump is located on the flywheel side (rear) of the engine and is driven via the timing chain by the crankshaft with 1:1 ratio. Although it is capable of generating a pressure of 2000 bar, the N57TU uses 1800 bar of injection fuel pressure.
The pump is lubricated by the fuel.
Fuel quantity control valve
The fuel-quantity control valve (also known as flow regulating valve) is mounted on the CP4.2 highpressure pump . The fuel-quantity control valve is controlled using a pulse-width-modulated signal. The DDE controls the fuel-quantity control valve at various pulse-duty factors while the fuel-quantity control system is active. The system adjust rail pressure to the specified level as calculated by the DDE. The fuel-quantity control valve regulates the fuel flow from the low-pressure side to the highpressure side of the high-pressure pump. This is how the desired fuel-rail pressure is achieved. Progressive reductions in the amount of fuel that the fuel-quantity control valve allows to flow to the high-pressure side produce corresponding reductions in the level to which the high-pressure pump charges the cylinders. The consequence is that less rail pressure is built up. The high-pressure pump generates pressure to maintain a constant level in the rail.
When volumetric flow regulation is active, the DDE controls the fuel-quantity control valve (flow regulating valve) by means of a variable pulse duty factor in order to set the rail pressure to the level calculated by the DDE. The fuel-quantity control valve controls the flow of fuel into the high pressure pump from the low-pressure side, thereby controlling fuel-rail pressure as required. The less fuel the flow regulating valve allows to flow into the high pressure side, the less the radial cylinder of the highpressure pump is filled. The consequence is that less rail pressure is built up.
Volumetric flow regulation by the quantity control valve: The quantity control valve only allows the amount of fuel to flow into the high pressure pump from the low-pressure side that is required in order to generate the required fuel rail pressure. The high-pressure pump cylinder is not completely filled with fuel. The higher the control signal current, the lower the rail pressure that is generated. The rail-pressure regulating valve is not supplied with current for the maximum pressure. The rail-pressure regulating valve is supplied with slightly higher current than would be necessary for setting the target pressure. • Pressure regulation by the rail pressure control valve: The high-pressure pump provides a constant supply of highly-pressurised fuel to the rail. The rail pressure regulating valve diverts excess fuel arriving in the rail into the return line. The higher the control signal current, the higher the rail pressure that is maintained. The flow regulating valve is set to maximum flow. • Combined closed-loop control with simultaneous regulation from the rail pressure regulating valve and the quantity control valve: At extremely low fuel-injection quantities of less than roughly 4 mg (on trailing throttle/overrun) the rail pressure regulating valve must discharge a certain amount of fuel from the high-pressure system. The reason is that the high-pressure pump cannot run with zero delivery. This means that the high-pressure pump supplies fuel to the high-pressure system even when the quantity control valve (flow regulating valve) is closed. This would lead to excessive rail pressure and thus to a control deviation.
Fault handling.
If activation of the flow regulating valve is interrupted or has a short circuit to ground, the flow regulating valve switches to full delivery. The DDE detects the fault, limits the fuel injection rate and switches to pressure regulation. As of a fuel temperature of 60 °C (140 °F), the DDE reduces the fuel injection rate and the rail pressure to protect the rail-pressure regulating valve against overheating. In the event of a short circuit to positive, the engine shuts down.
Rail pressure sensor
A diaphragm with strain resistors is integrated in a metal housing. The measuring range of the sensor is dependent on the thickness of the diaphragm. The thicker the diaphragm, the higher the measurable pressure. The rail pressure sensor detects the current pressure in the rail. The pressure in the rail is of crucial importance to fuel injection. This measurement must be performed with great precision and within a suitably short period of time. If the rail pressure sensor fails, a fault is stored in the Digital Diesel Electronics and pressure regulation is activated "blind" by means of default values.
Injector CRI2.5
As with the N47TU the new solenoid injectors (common rail injector CRl2.5) are installed on the N57TU. They are produced by Bosch and operated with up to 1800 bar fuel pressure. Along with the increased maximum pressure, the number of possible switching actuations over the service life was also increased by 50%. It has multiple injection capability and also allows the implementation of very short switching times. The new solenoid injector delivers improved HC and CO emissions with the same power and consumption data as the 1800 bar piezo injector. An additional advantage of the solenoid type injector is the low manufacturing costs compared to piezo injectors.
The nozzle geometry has also been adapted to satisfy the more exacting requirements of exhaust emissions legislation.
Important modifications were done on the solenoid valve injector when compared with the N57 engine (not available in the US). Instead of a ball shaped control valve a ring design is now used. The main concern is the permanent leakage of the pressure compensated valve. With the new shape, a larger valve lift is achieved with low opening cross-section.
Due to the tolerance during manufacture of the injectors, the actual injected fuel volume deviates only slightly from the calculated fuel volume. This deviation is determined after manufacturing each injector by measurements in several operating conditions. For each injector an adjustment value (code) is generated from these measurements. For vehicle assembly, the compensation value of each injector is stored in the control unit after the installation of the Digital Diesel Electronics.
The compensation values are assigned to the individual cylinders according to the installation of the injectors. The DDE corrects the calculated injection quantities slightly with these compensation values and so reduces the cylinder specific deviations of the injection volume.
When injectors are replaced or exchanged, it is essential to ensure that the alphanumeric code printed on each injector is assigned in the Digital Diesel Electronics to the correct cylinder.
