Boost pressure control, function

The boost pressure is regulated with the aid of the wastegate and is regulated by the CDI control unit (N3/21) depending on the following values:

- Coolant temperature, through the coolant temperature sensor (B11)

- Charge air temperature, through the charge air temperature sensor (B17/9)

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- Atmospheric pressure, via the intake manifold pressure sensor (B28)

- Engine speed, via the crankshaft sensor (L5/7)

- Actual boost pressure, via the boost pressure sensor (B5/1)

- Injection quantity which is determined via the injection time of the fuel injectors (injector, cylinder 1 (Y76/1), injector, cylinder 2 (Y76/2), injector, cylinder 3 (Y76/3) and injector, cylinder 4 (Y76/4)) and the fuel pressure in the rail, by the rail pressure sensor (B4/6)

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After evaluating the input signals the CDI control unit actuates the wastegate pressure transducer (Y77) with a Pulse Width Modulation (PWM) signal depending on the boost pressure characteristic map.

The vacuum produced by the vacuum pump which is applied to the boost pressure control valve vacuum unit (wastegate ) is adjusted by the wastegate pressure transducer. The boost pressure control valve vacuum unit (wastegate) operates the control unit which adjusts the boost pressure control valve (wastegate) to a greater or lesser extent. By diverting the exhaust flow via a bypass channel the boost pressure control valve (wastegate) can regulate the exhaust gas quantity and thus the boost pressure.

If the actual boost pressure is lower than the specified boost pressure (boost pressure performance map), the boost pressure control valve (wastegate ) is closed further (pressure buildup). If however the actual boost pressure is higher than the specified boost pressure, the boost pressure control valve (wastegate ) is opened correspondingly further (depressurization). One part of the exhaust flow is routed via the turbine of the turbocharger and the other part is routed past the turbine. In this way the gas pressure in the turbine is adjusted to the requested boost pressure. The turbocharger speed is used to determine the quantity of compressed air and thus the boost pressure. The actual boost pressure is constantly being monitored by the boost pressure sensor and passed on to the CDI control unit.