Exhaust gas temperature
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 and the control unit are permanently connected by means of a shielded, heat-resistant wire.
The evaluation electronics convert the signal which the measuring sensor generates into a pulse-width-modulated signal (PWM signal). This is a square-wave signal with a fixed frequency and a variable pulse duty factor. The pulse duty factor is expressed as a percentage . The measurement range extends from 10% to 90%. A specific pulse duty factor is assigned to each temperature (refer to diagram).
Substitute function and self-diagnosis
A pulse duty factor of <1% or >99% is recognised as a fault. A fault is detected as of a certain enrichment quantity. If a sender fails, the charge pressure is reduced to a safe level and an emergency enrichment characteristic (engine speed-dependent) is used.