Exhaust gas temperature
- 1 Exhaust gas temperature measurement
- 2 EGT enrich
- 3 Audi documentation
- 4 Sensor design
- 5 ECU signal conversion
- 5.1 ECU analog to digital conversion
- 5.2 Substitute function and self-diagnosis
Exhaust gas temperature measurement
VAG-COM blocks 034 and whatever else only display an internally calculated O2 sensor temperature (tabgbts), which is what most people think are real EGTs. That value may or may not have anything to do with the real EGTs or temperature at the primary O2 sensors; in fact, those temperature values are stored in 2 maps where the inputs are load and RPM.
Block 112 is the only block where a real EGT value (tats) is displayed, and only when the actual EGTs exceed 945°C. This is only due to the design of the sensors and their electronic modules, which makes that they can only "read" 945-1035°C and that's all. So basically if all you see is 945 all the time, it means your actual EGTs never exceed 945.
If you see block 034 EGTs hitting 750 or 800 or 850, those values have never been, are not and will never be actual EGTs. The only real EGTs are in block 112. If you want to know what they are when they're below 945, then you have to use RS6 sensors with adequate programming, so you get a range of -40 to 1100°C and then you will know what your real EGTs are.
There are two types of EGT enrich in ME7
- BTS (open loop) - when modeled EGT (block 034) reaches TABGBTS (500°C for the S4, 750°C for the RS4), the requested AFR tracks lambts if it is richer than lamfa.
- ATR (closed loop) - when measured EGT (block 112) reaches TABGSS (980°C), ATR will add fuel via dlatr, which is generated by the ATR PID.
Exhaust gas temperature control
For turbocharged engines, the maximum permissible exhaust gas temperature is a key design criterion.
To protect the exhaust gas turbocharger and the exhaust manifold, the exhaust gas temperature should not exceed 1000°C for a lengthy period of time.
Since many of the components which influence the exhaust gas temperature have tolerances, thermodynamic adaptation previously took place at 950°C for safety's sake.
This was achieved by enriching the air/fuel mixture.
The exhaust gas temperature is recorded in a cylinder-bank-specific manner by the two exhaust gas temperature senders G235 and G236.
The Motronic controls the exhaust gas temperature to 980°C by enriching the air/fuel mixture.
It is therefore possible to largely dispense with the prophylactic enrichment process that has been standard practice until now.
The mixture is only enriched
- when necessary and
- to the extent necessary.
This means that engine operation with lambda = 1 is possible up to high load and engine speed ranges.
To facilitate exhaust gas temperature control, the exhaust gas temperature must be recorded to a high degree of accuracy.
An accuracy of ±5°C is achieved in the measurement range from 950°C to 1025°C. The exhaust gas temperature sender is located inside the exhaust manifold upstream of the exhaust gas turbocharger.
It comprises a measuring sensor and evaluation electronics.
The measuring sensor consists of a thermocouple of unknown type which is placed into the exhaust stream and held into the exhaust manifold by a M12x1mm thread retainer. This thermocouple probe is attached to a control unit by means of a heat-resistant insulated cable with stainless steel over-braid shield. The sensor electrical connections are keyed such that bank 1 sensor cannot be accidentally connected to the bank 2 engine harness connector.
The evaluation electronics within the control unit convert the microvolt level thermocouple signal into a pulse-width-modulated signal (PWM signal) that can be interpreted by the ECU. The signal output consists of a square wave with a fixed frequency of 10Hz; meaning the ECU can be provided with 10 updated temperatures per second. The pulse width duty cycle is expressed as a percentage of signal high-time to the overall signal period. The sensor output ranges from a minimum pulse width of 5% to a maximum of 95%. The values above and below this range is reserved for the ECU to perform sensor diagnostics.
ECU signal conversion
ECU analog to digital conversion
The ECU interprets the signal generated by sensor package and converts it to a temperature value. The ECU first determines the period of the signal and then compares the signal high time to the period to determine the signal duty cycle. The period of the signal is converted to a temperature value against table TABGTA. The resultant temperature (TATS_W for bank 1 or TATS2_W for bank 2) is delivered in celsius units with possible ranges between 940C to 1035C.
Substitute function and self-diagnosis
The ECU evaluates the sensor output to determine the sensor integrity provided the control bit B_CDAT has been set true. The error condition must exist for three seconds (defined by TVEATSS) for the sensor to be flagged with an error code. The condition will be reset after 3 seconds (defined by TVEATSR) of error free sensor operation. If a sensor has been determined to be faulty the ECU will set an error bit true. E_ATS for bank 1 (G235) or E_ATS2 for bank 2 (G236).
In the event of a sensor fault the ECU will fail-over to the working sensor to provide enrichment. In the event that both sensors have faults the ECU will fail over to an EGT limp mode condition where emergency enrichment characteristic (engine speed-dependent) is utilized.
Fault code generation
The ECU is capable of producing four unique fault codes provided the control bit B_CDAT has been set true.
Exhaust gas temperature sensor (1 or 2) Open/Short to B+:
DTC is produced when the exhaust gas temperature value (TATS_W or TATS2_W) is greater than TATSMX. The stock value of TATSMX is 1034.51C. This correlates to an pulse width duty cycle of greater than 99% based on empirical data.
DTC 17861 indicates this fault for bank 1 (G235) (B_MXATS) while 17865 indicates this fault for bank 2 (G236) (B_MXATS2).
Exhaust gas temperature sensor (1 or 2) Short to Ground:
DTC is produced when the exhaust gas temperature value (TATS_W or TATS2_W) is less than TATSMN. The stock value of TATSMN is 940.49C. This correlates to an pulse width duty cycle of less than 1% based on empirical data.
DTC 17862 indicates this fault for bank 1 (G235) (B_MNATS) while 17866 indicates this fault for bank 2 (G236) (B_MNATS2).
Exhaust gas temperature sensor (1 or 2) Implausible Signal:
DTC is produce at engine idle (B_LL) when the exhaust gas temperature value (TATS_W or TATS2_W) is greater than TATSNP (default value of 950.00C) while the pre-catalytic converter modeled exhaust gas temperature (TABGM-W available to measure in block 034) is less than TDATSO (default value of 500C).
DTC 17863 indicates this fault for bank 1 (G235) (B_NPATS) while 17867 indicates this fault for bank 2 (G236) (B_NPATS2).
Exhaust gas temperature sensor (1 or 2) Signal Error:
DTC is produce when the signal period is outside side of the period tolerance limit defined by DTPATS (default value of 50ms).
It is assumed DTC 17864 indicates this fault for bank 1 (G235) (B_SIATS) while 17868 indicates this fault for bank 2 (G236) (B_SIATS2).