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.