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451_078
451_077
Engine load sensing
The load on the 2.5l R5 TFSI engine is determined via the engine
speed and the air mass. Since no air mass meter is installed, two
identical combined temperature/pressure sensors are used for air
mass metering:
• Charge pressure sender G31, intake air temperature sensor 2
G299
• Intake manifold pressure sender G71, intake air temperature
sensor G42
Two sensors are used because the throttle valve must be regarded
as a source of interference - very different pressures can simulta-
neously exist upstream and downstream of the throttle valve.
Another reason is the use of the cast aluminium intake manifold.
This manifold accumulates heat and could, therefore, potentially
falsify the temperature signal generated by the temperature sen-
sor (G71/G42). The temperature signal generated by the sensor
upstream of the throttle valve (G31/G299) is utilised for this pur-
pose.
First sensor:
Task: measurement of pressure and temperature upstream of the
throttle valve
Designation: charge pressure sender G31 and intake air tempera-
ture sensor 2 G299
This sensor is the charge pressure sensor, upon whose signal the
charge pressure is controlled. Since the throttle valve will be oper-
ating wide open as much as possible, there is no need for this valve
because charge pressure equals intake manifold pressure.
To achieve better response, however, the charge pressure control
unit runs before the throttle valve is wide open. In principle, there-
fore, it works against the throttle valve.
Failure of this sensor
If the sensor fails, the charge pressure control system will go into
limp-home mode, which means that the engine will be running
naturally aspirated. In addition, the EPC and MIL lamps will be
activated and a corresponding fault code will be stored in the fault
memory.
Second sensor:
Task: measurement of pressure and temperature in the intake
manifold
Designation: Intake manifold pressure sender G71 and intake air
temperature sensor G42
This identical combined sensor is the so-called main charge sen-
sor, which replaces the hot-film air mass meter. The air mass flow-
ing through the engine is determined at every operating point
from the pressure and temperature signals generated by this sen-
sor. For this purpose, a corresponding quantity of fuel is injected.
Failure of this sensor
If this sensor fails, the engine will go into limp-home mode.
Engine power output will be reduced. In this case, air mass is
determined in "alpha-n mode", i.e. based on the throttle valve
angle (α) and engine speed(s). In addition, the EPC and MIL lamps
will be activated and a corresponding fault code will be stored in
the fault memory.
Charge pressure sender G31,
intake air temperature sensor 2
G299 (first sensor)
Intake
manifold
Throttle valve control unit J338
Intake manifold pressure sender G71,
intake air temperature sensor G42
(second sensor)
