Exhaust Gas Recirculation: Description and Operation

Exhaust Gas Recirculation:

SYSTEM OPERATION

This system is composed of an EGR valve, and a Thermal Vacuum Valve (TVV). The EGR system is an emission reducing system that concentrates primarily on the reduction of oxides of nitrogen (NOx). The purpose of the EGR system is accomplished by allowing a specified amount of hot exhaust gas to flow from the exhaust manifold into the intake manifold to mix with the incoming air/fuel mixture. This mixture enters the combustion chamber and lowers the peak flame temperature during combustion. This reduction in combustion temperature reduces the possibility of ignition knock and the level of NOx output.

The EGR valve is held normally closed by a flexible diaphragm which is spring loaded. Carburetor vacuum applied to the top side of the diaphragm overcomes the spring pressure and opens the valve. This allows exhaust gas to be pulled into the intake manifold and enter the engine cylinders.

THERMAL VACUUM VALVE (TVV)

The operation of the EGR valve is controlled by the opening and closing of the TVV. It opens when the coolant temperature of the engine reaches normal operating temperature. It is mounted on the intake manifold and controls the carburetor vacuum that operates the EGR valve.

If too much exhaust gas is allowed to flow into the combustion chamber, combustion will not occur. For this reason, very little exhaust gas is allowed to pass through the valve, especially at idle. The EGR valve is usually open under the following conditions:

^ Warm engine operation.
^ Above idle speed.

The amount of exhaust gas recirculated is controlled by variations in vacuum and exhaust back pressure.