Air supply

Air supply

The 4-cylinder petrol engine is charged by an exhaust turbocharger. Here, the channels of 2 cylinders are each grouped separately in the exhaust manifold and in the exhaust turbocharger. This technology is referred to as ”twin-scroll”. Increasing the gas dynamics in the exhaust manifold at low engine speeds means that the energy of the pulsing columns of air is better exploited. This sets the maximum torque even at 1600 rpm.
The effect is clearly noticeable. The 'turbo hole' that otherwise frequently occurs is almost completely avoided.

Brief description of components

The following components are described for the air supply:

Intake temperature and charge-air pressure sensor

The combined sensor delivers the following information to the DME control unit: temperature and pressure of the charge air before the throttle valve (absolute).
The intake temperature and charge-air pressure sensor are used for boost pressure control.
The DME control unit also uses the signal of the intake-manifold pressure sensor to calibrate the position of the throttle valve.

The intake temperature and charge-air pressure sensor is located in the air duct after the charge air cooler.

Intake-manifold pressure sensor

The intake-manifold pressure sensor measures the (absolute) pressure in the intake system. The DME uses the signal from the intake-manifold pressure sensor to calculate the air mass taken in. The pressure also serves as a substitute value for the load signal.

The intake-manifold pressure sensor is located on the air collector for intake air.

Throttle-valve actuator

The digital engine electronics control unit calculates the position of the throttle valve: from the position of the accelerator pedal as well as the torque request from other control units. The position of the throttle valve is monitored in the throttle-valve actuator without contact by 2 Hall sensors. The throttle-valve actuator is opened or closed electrically by the DME control unit.
The position of the throttle valve is also influenced by the boost pressure.

The throttle-valve actuator is secured to the air collector for intake air.

Blow off valve

In order to avoid the occurrence of strong vibrations at the impeller in the case of suddenly closing of the throttle valve (e.g. during gearshift), the blow off valve opens. This creates a circuit around the compressor. The blow off valve prevents ”pumping” against the closed throttle valve: improved engine acoustics.
Additional effect: the exhaust turbocharger reacts quickly when the throttle valve is opened again. Without the blow off valve, the exhaust turbocharger would work against the backpressure of the closed throttle valve and become slower. On opening the throttle valve, the exhaust turbocharger would react with a delay.

The blow off valve is attached with the wastegate valve to the exhaust turbocharger.

Exhaust turbocharger with wastegate valve

The engine is equipped with a so-called ”twin scroll” exhaust turbocharger. Here, the channels of 2 cylinders are each grouped separately in the exhaust manifold and in the exhaust turbocharger: cylinders 1 and 4, cylinders 2 and 3. Increasing the gas dynamics in the exhaust manifold at low engine speeds means that the energy of the pulsing columns of air is better exploited. This sets the maximum torque even at 1600 rpm. The 'turbo hole' that otherwise frequently occurs is almost completely avoided.

The boost pressure is regulated by the DME via a wastegate valve. The wastegate valve is adjusted pneumatically by a diaphragm can. An electropneumatic pressure converter applies a partial vacuum to the diaphragm can.

There are 2 connections each for cooling lubrication of the exhaust turbocharger. Two connections for the engine cooling circuit as well as 2 connections for the oil circuit. The exhaust turbocharger is cooled by a separate pump. The DME control unit switches on the turbocharger coolant pump after stopping the engine.

Hot-film air-mass sensor, only US version

The hot-film air-mass sensor is used in the US version. This increases the accuracy of the load identification. The measure is necessary due to exhaust emissions legislation. The signal of the intake air temperature sensor in the HFM is not used.
The hot-film air-mass sensor is behind the intake muffler.

System functions

The following system function is described for the air supply system:

Calculation of the air mass

The air mass that is taken in is no longer measured directly with the mass air flow sensor, rather it is calculated by the DME. A filling calculation (filling model) has been programmed in the DME for this calculation. The following signals are included in this calculation:

• VANOS setting (load identification)
  
• Position of the throttle valve (choke action)
  
• Intake-air temperature (correction of air density)
  
• Engine speed (cyinder fill levels)
  
• Air intake pressure (correction for choke action)
  
• Ambient pressure (air density divided by altitude correction)
  

The air mass calculated in this way is synchronised with:

• Signal of the oxygen sensor (fuel-air ratio)
  
• Fuel injection period (volume of fuel)
  

If necessary, the calculated air mass is corrected. In the event of failure of the oxygen sensor, a fault is entered in the fault code memory of the DME (plausibility check of the air mass). In this case, there is no calibration of the calculated air mass.

Charge-air-pressure control

The boost pressure is controlled by the DME by means of a wastegate valve to a maximum of 8 bar. A portion of the exhaust gases is fed via the wastegate valve to the turbine. The wastegate valve is adjusted pneumatically by a diaphragm can. The wastegate valve can be set variably. An electropneumatic pressure converter applies a partial vacuum to the diaphragm can. The DME controls the electropneumatic pressure transducer.

An additional function is available for boost pressure control. Here, the boost pressure is briefly increased by approx. 150 mbar (approx. 12 seconds). This increase in the boost pressure (overboost) is available between approx. 1600 rpm and approx. 5000 rpm. This enables a torque increase and power output increase with the engine speed remaining the same.

The increase in the boost pressure is activated by the DME when the accelerator pedal is pressed very quickly.

Idle speed control

The DSC control unit delivers the driving speed signal across the PT-CAN to the DME control unit. The signal is required for a number of functions, e.g. for idle speed control. For idle speed control, the DME activates the throttle-valve actuator.
If the vehicle is not stationary, the idle speed is regulated to a fixed value (slightly above the engine speed when the vehicle is at a standstill). If the driving speed equals 0 kph, the idle speed is regulated (depending on air-conditioning compressor ON, engaged drive position for automatic gearbox, light ON).

No liability can be accepted for printing or other errors. Subject to changes of a technical nature