BOOST PRESSURE CONTROL/REGULATOR FOR THE TURBOCHARGER: BASICS

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.

Electromechanical control device for the exhaust gas turbocharger
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:
• Faster response of the turbocharger already at low engine speeds
• Exact adjustment of the guide vanes in all speed ranges
• Improvement of the emission values

Adjustment of the guide vanes
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 behavior of the exhaust gas flow and consequently on the turbine speed. The result is that the boost pressure can be purposefully increased or reduced.