Boost Solenoid: Description and Operation
Electric Boost Pressure Adjuster
A precondition for optimum functioning of a turbocharger with variable turbine geometry in conjunction with an gasoline fueled engine is adjustment or control by an electric boost pressure adjuster. This is bolted directly onto the turbocharger and actuates the above-described adjusting ring with the integrated, electronically controlled servo motor via a short coupling rod.
The most important advantages compared with the pneumatic adjusting devices used for diesel engines are as follows:
- Fast reaction time (delay max. 100 ms).
- Optimum response characteristics.
- Freely selectable vane position independently of the pressure system.
- Optimum control quality of the desired boost pressure.
- Control possible without overshoot.
- Optimum diagnosis and fault detection.
Design Of Electric Boost Pressure Adjuster
The boost pressure adjuster consists of an aluminium housing, which accommodates the DC motor with sensor magnet as well as a two-stage spur gear drive with a drive shaft/lever. A two-stage spur gear drive is connected between the DC motor and drive shaft. An aluminium cover with glued-in bonded hybrids and a screwed-in four-pin connector is screwed onto the housing. The hybrid electronic components are designed for an operating temperature from - 40 °F (- 40 °C) to 266 °F (130 °C).
NOTE: The electric boost pressure adjuster is not serviceable and should note be dissembled.
Position Sensor
A contactless incremental Hall sensor is used to detect the position of the output shaft. The hybrid electronics integrated in the adjuster housing is responsible for position control and control of the DC motor. The sensor magnet wheel is mounted on the DC motor shaft, while the sensor (pick-up) itself is located on the hybrid electronics. The main advantage of the contactless sensor is freedom from wear.
In addition to digital position control and related setpoint/actual value evaluation and driver control, the electronic components also perform actual value output as well as diagnostic and fault detection routines.
Each boost pressure adjuster is connected to the DME control unit by two signal lines. The DME control unit sends a setpoint (PWM signal) or special pulse/duty ratios (e.g. command to travel to and learn end stops) via one of the lines. The electric boost pressure adjuster then sends the actual value (PWM signal) and special pulse/duty ratios (self-diagnosis of electric adjuster for fault memory entry in the DME) to the DME via the other line.
Control By The Motronic Control Unit ME 7.8.1
Optimum adjustment angles are set by way of functions in the engine control unit depending on the engine operating point, so that the target engine torque is reached as quickly as possible. The optimum vane positions for maximum efficiency were determined for the complete engine map as part of extensive application work. The goal was above all to optimize the response behavior particularly for dynamic acceleration. Variable turbine geometry allows a full torque characteristic to be achieved even at low engine speeds and also provides a wide power spectrum in the nominal output range.
Diagnosis Of Electric Boost Pressure Adjuster
The electric boost pressure adjuster features an integrated diagnostic function which transmits a fault to the DME control unit by way of a corresponding pulse/duty ratio.
- The nominal mechanical adjustment range or control range (R) extends from 20 % (vanes open) to 80 % (vanes closed).
- The pulse/duty ratio is approx. 40 % when the ignition is switched on.
- The pulse/duty ratios 0 % to 19 % and 81 % to 100 % are special pulse/duty ratios for diagnostic routines (D) and for teaching the adjusting device.
Boost Pressure Adjuster Test
When the ignition is switched on, the function "Test boost pressure adjuster" is available on the PIWIS Tester in the DME control unit under the system test function. A corresponding fault is entered after this test in the event of malfunctions.
In order to check functioning of the adjusting device, the electric boost pressure adjuster is supplied with a pulse/duty ratio of 16 % and 84 % after the ignition is switched off. The function test is also audible for the customer.
Teaching The Electric Boost Pressure Adjuster
It is necessary to perform adaptation of the electric boost pressure adjuster if a turbocharger or the electric boost pressure adjuster itself is replaced. When the ignition is switched on, the function "Boost pressure adjuster adaptation" is available on the PIWIS Tester in the DME control unit under the system test function. During this adaptation, the mechanical limit stops (20 % and 80 %) are taught again and stored in the boost pressure adjuster.
Adaptation Of Boost Pressure Control
The boost pressure is a variable which is influenced by engine tolerances and ambient conditions. The following ambient conditions influence the boost pressure with respect to the maximum engine torque, protection of the engine and turbocharger as well as fault entries.
- Air pressure (the boost pressure is adapted from an altitude of greater than 5,900 ft (1,800 m) in order to protect the components of the turbocharger).
- Ambient temperature and intake air temperature (charge air temperature) (the boost pressure is reduced at an ambient temperature less than approx. 32 °F (0 °C), or a charge air temperature less than approx. 50 °F. (10 °C).
- Fuel quality (knock resistance under thermodynamic loading).
The adaption ranges 0 to 11 are available for boost pressure adaptation.
These are divided into 3 load ranges (boost pressure) and 4 engine
speed ranges. Adaptation of ± 15 % is possible for each range before
a fault is entered.
Table Of Boost Pressure And Speed Thresholds
plsol - Boost pressure in mbar
nmot - Engine speed in rpm
Partial Load Adaptation
The adaptation ranges 0 / 3 / 6 / and 9 are available for partial load or reduced full load due to ambient conditions (e.g. poor fuel quality).
Full Load Adaptation
The adaptation ranges 1 / 4 / 7 / and 10 are provided for full load under normal conditions.
Full Load With Overboost
The adaptation ranges 2 and 5 are available for full load with Overboost (Sport Chrono activated). The adaptation ranges 8 and 11 are not normally adapted.
Sport Chrono Package Turbo (Optional)
The Sport Chrono Package Turbo is offered for the first time for the new 911 Turbo. The package contents correspond to those of the Sport Chrono Package Plus from the current 911 Carrera (997) generation, but are supplemented by the functions, Overboost, PTM control and modified starting program for the optional Tiptronic S.
The "Sport Chrono Package Plus" offers the driver distinctively sporty settings for various vehicle functions, therefore providing a completely new sporty driving experience. The accelerator pedal characteristic, engine behavior at the speed limit and in the event of load changes, PSM intervention thresholds as well as the characteristics of PASM and Tiptronic S are all changed at the push of a button. The Sport Chrono program also allows the driver to control these advantages as required to take into account contemporary conditions.
Additional Function - Overboost
Overboost is a brief excess increase in the boost pressure (max. 10 s) for acceleration under full load (fully depressed accelerator pedal). The function is activated after operation of the Sport button on the center console and fast depression of the accelerator pedal. Overboost is performed with the assistance of the boost pressure control and results in an increase in the maximum boost pressure by approx. 2.9 psi (0.2 bar) (20 %). As a result, the maximum torque is increased from 460 ft lb (620 Nm) to a maximum of 505 ft lb (680 Nm) between 2,100 rpm and 4,000 rpm (the torque is regulated linearly to the "normal" full-load value between 4000 and 5000 rpm) and permits a significant improvement in acceleration performance and elasticity. The higher boost pressure does not just result in an abrupt increase in mechanical loading of the components, but above all also significantly increases thermal stressing. This is due in particular to the continuously rising charge air and combustion chamber temperatures.
The Overboost function is restricted to 10 seconds in order to take these factors and the maximum component loading into account. After this time, the original full-load boost pressure without Overboost is restored. The Overboost function can be reactivated again as soon as the engine load is relieved briefly by the throttle closing (e.g. after a gearshift).
Overboost operation is indicated by an arrow symbol in the boost pressure display of the instrument cluster. The arrow next to the boost pressure display indicates the brief Overboost operation, which is possible only in conjunction with activated Sport function (text "SPORT" shown on display). The boost pressure indication is shown as a digital value and as a graphic representation in the multi-function display. The boost pressure can increase up to approx. 1.2 bar (17.4 psi) when the Sport button is pressed; a pressure of approx. 1.0 bar (14.5 psi) is reached when this function is not activated.
Exhaust Temperature Sensors, Banks 1 and 2
The signal from the exhaust temperature sensors of banks 1 and 2 installed at the turbine inlet is used in the DME control unit for component protection. The exhaust temperature is evaluated for each bank in the DME control unit, and is restricted to a maximum temperature of 1,767 °F. (980 °C) The two sensors are combined into one component unit together with their control unit. The measured signals are evaluated in this control unit.
The control unit sends a pulse/duty ratio (PWM) to the DME. The pulse/duty ratio range is from 4 % (corresponds to 25 °F/- 4 °C) to 96% (corresponds to 2,030 °F/1,110 °C). The control unit has a self-diagnosis function. If the control unit detects a cable break/short-circuit to the sensor or in the evaluation circuit, it sends the special pulse/duty ratio of 98 % to the DME and this leads to an entry in the fault memory.