Exhaust Gas Recirculation: Description and Operation

EGR System:

PURPOSE
Exhaust gas recirculation reduces oxides of nitrogen (NOx) in the exhaust. NOx are formed at the high temperatures which occur in the combustion chambers at high engine loads.

Reintroducing exhaust gases into the combustion chamber reduces combustion temperature, with an attendant reduction in NOx content.

OPERATION
The system is not activated when the engine is cold or idling. Under these conditions, the NOx level is already low and recirculation would impair the running of the engine.

Recirculation is regulated by the E.G.R. valve which, in turn, is controlled by the Motronic Control Module via the E.G.R. frequency valve. The frequency valve is supplied, by the Control Module, with a signal which is a function of the engine speed, load and temperature. Combined with the ambient pressure and the pressure in the intake manifold, this signal is converted into a control pressure signal which operates the E.G.R. valve via a vacuum hose.