Exhaust Gas Recirculation: Description and Operation

Negative Backpressure EGR Valve:

The EGR system routes exhaust gases from the exhaust manifold to the intake manifold at specified engine operating conditions. The EGR valve is opened by ported manifold vacuum to let exhaust gas flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. Since there is less air/fuel mixture to burn in the combustion chamber during EGR operation, the temperature is reduced enough to limit NOx formation. If too much exhaust gas enters combustion will not occur. Because of this very, little exhaust gas is allowed to pass through the valve, especially at idle. the valve should only open when the engine is at operating temperature and above idle speed. Variations in vacuum and in some cases, exhaust backpressure, control the amount of exhaust gas recirculation.

VALVE OPERATION
The valve used on this system is a negative backpressure valve. It varies the amount of exhaust gas flow into the manifold depending on the manifold vacuum and variations in exhaust backpressure. The diaphragm on this valve has an internal vacuum bleed hole which is held closed by a small spring when there is no exhaust backpressure. Engine vacuum opens the EGR valve against the pressure of a large spring. When the manifold vacuum combines with negative exhaust backpressure, the vacuum bleed hole opens and the EGR valve closes. The valve will open if vacuum is applied with the engine not running.