Boost pressure control or regulator for turbocharger
Important safety note
The turbocharger uses the energy of the exhaust gases to draw in and compress fresh air for combustion. This allows a greater volume of air and thus more oxygen to enter the combustion chamber. Engine performance and engine torque are subsequently increased. Simply explained, the exhaust gas turbocharger contains an exhaust gas turbine and a compressor turbine, which are connected to each other by a shaft. The exhaust gas flowing out of the engine drives the exhaust gas turbine and thus also the compressor turbine.
So as to adapt the boost pressure to suit any given prevailing load condition and also to protect the engine and turbocharger, a boost pressure control or regulator is required. Depending on the type of turbocharger in question, a mechanical-pneumatic control device or alternatively an electromechanical one can be used. In the further course of the chapter, we will primarily deal with electromechanical control.
The turbocharger actuator, also called control box or boost pressure regulator, is an electronic control device for adjustable turbochargers and is predominantly used for Variable Nozzle Turbine (VNT) and Variable Turbine Geometry (VTG) turbochargers.
In these turbochargers with variable turbine geometry, the actuator reliably and precisely controls the movement of the guide vanes. Through the adjustment of the guide vanes, the exhaust gas flow onto the turbine wheel is influenced, thus changing the boost pressure which can then be optimally adjusted to match all speed ranges. The required boost pressure is regulated according to a map stored in the engine control unit. The engine control unit sends the required boost pressure in the form of a signal to the turbocharger actuator via a data bus connection. The actuator adjusts the guide vanes accordingly in line with the required angle setting contained in the signal.
Advantages of an electronic control device are:
In the turbine housing, moving guide vanes are positioned in a circle on a carrier ring and connected with their shafts to the adjusting ring via guide pins. The adjusting ring is in turn connected to the turbocharger actuator via a rod.
If the adjusting ring is moved by the actuator, all guide vanes are adjusted synchronously and in this way the turbine inlet area is either reduced or enlarged. This action has, in turn, an influence on the behaviour of the exhaust gas flow and consequently on the turbine speed. The result is that the boost pressure can be purposefully increased or reduced.
The main function of the actuator is to bring the shaft into the position specified by the control unit or into that position calculated as a result of the map diagrams.
With the help of the contactless, inductive position sensor (CIPOS sensor), the position of the shaft is continuously determined and actively reported back. Angles are measured inductively using a non-contact and consequently wear-free method, thus guaranteeing high measuring precision throughout the entire lifetime. The insensitivity to magnetic fields and the high level of temperature stability, in particular, are the characteristic qualities of the CIPOS technology implemented here.
In addition to the CIPOS sensor responsible for precise positioning, the integrated electronics also include control of the electric motor and error diagnosis. In this way, errors can be detected, reported back and appropriate reactions automatically derived from them. The actuator has a flexible operating angle area and carries out controlled movement up to the limit stop.
Depending on the model, communication in the vehicle is possible both via CAN bus and also via a pulse-width modulated signal (PWM).
A failure of the electromechanical turbocharger actuator can occur in the following way:
Causes of a defect on the turbocharger actuator can be:
A defect in the gearbox of the turbocharger actuator is usually preceded by a defect in the guide vane adjustment on the turbocharger. Over time, the exhaust gas flow creates heavy contamination inside the turbocharger. Such soot formation impairs the functioning of the guide vanes. This leads to a higher torque requirement for the entire actuator and ultimately to gearbox damage at the actuator and error entry in the engine control unit.
As part of troubleshooting, a visual inspection of the turbocharger in the engine compartment should first be carried out after the ECU diagnosis.
The turbocharger with its individual components should always be considered and diagnosed as one unit. Most vehicle manufacturers do not supply spare parts for VNT/VTG turbochargers. This is not because the technicians in the workshop are not trusted to replace or rework individual components, but because the turbocharger and the electromechanical actuator have to be precisely matched to each other before installation in the vehicle. This calibration is carried out in the dismantled state on a flow bench for turbochargers. On this special test bench, the vehicle-specific flow rate (MIN/MAX flow) is determined and set as part of a basic adjustment. For this purpose, depending on the model, it may be necessary for some actuators, prior to calibration, to be first activated using a special map so that they can then be recognised in the vehicle by the engine control unit.
Although many turbocharger actuators look the same on the outside, their design and configuration can differ as regards performance and alignment depending on the vehicle type and the turbocharger unit. In their respective combinations with the turbocharger and in accordance with vehicle manufacturer requirements, two types of HELLA turbocharger actuators can be installed. A distinction is made here between the "Smart" and "Simple" versions. Thanks to an integrated control unit, the "Smart" autonomously regulates the adjustment of the guide vanes, while the "Simple" is controlled by a higher-level engine control unit. Although similar in appearance, the technical design of the electronics, of the gearbox and of the housing configuration is fundamentally different.
Therefore repair work involving the exchange of various components, such as gears or electronics, taken from different actuators is not possible.
The following diagnostic information is given as an example using a Mercedes-Benz E350 24V CDI (212) from the year 2014.
The function of the turbocharger actuator is monitored by the respective higher-level system control unit. Any occurring errors are stored in the engine control unit's error memory and can be read using a suitable diagnostic unit. Depending on the vehicle and on the system, additional functions such as parameters or actuator tests can be selected and displayed in the diagnostic device. The data from control unit communication forms the basis for actual troubleshooting and for successful repair work.
In this function the error codes stored in the error memory can be read out and deleted. In addition, information on the error code can be called up.
In our case study, the electrical plug connection on the turbocharger actuator was disconnected and consequently the error code P004500 was stored in the error memory.
P004500 / charge limiting flap
> electrical fault or interruption
In this function, current measured values such as atmospheric pressure and boost pressure can be selected and displayed.
In this function, the stop values of the new turbocharger unit can be programmed into the higher-level control unit.
In this function, the electromechanical turbocharger actuator can be controlled by the diagnostic device. This allows a functional check of the periphery and of the relevant system components to be carried out without much dismantling work.
System-specific circuit diagrams can be taken from vehicle information and used for troubleshooting purposes. Here, for example, the PIN assignment on the turbocharger actuator can be read and used for further troubleshooting.
The various diagnostic options have been illustrated using the mega macs 77 diagnostic unit as an example. The respective test depth and variety of functions can be set out differently depending on the vehicle manufacturer and these are dependent on the relevant system configuration of the control unit.
Schematic illustrations, pictures and descriptions serve to explain and illustrate the document text and cannot be used as a basis for vehicle-specific repairs.
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