Exhaust Gas Recirculation: Description and Operation: Overview

In this system, exhaust gases are circulated to dilute the incoming fuel air mixture. Dilution of the incoming mixture lowers peak flame temperatures during combustion and thus limits the formation of NOx.

Fig. 41 EGR system. 4-156 engine:

Fig. 42 Dual EGR control valve. 4-156 engine:

On 4-156 engines, exhaust gas is partially recirculated from an exhaust port of the cylinder head into a port located at the intake manifold below the carburetor, while EGR flow is controlled by the EGR control valve and thermo valves, Fig. 41.
The dual EGR control valve, Fig. 42, used on 4-156 engines consists of primary and secondary valves. These valves are controlled by different carburetor vacuums in response to throttle valve openings, while the EGR flow is suspended at idle and wide open throttle operation. The primary valve controls EGR flow during part throttle operation, while the secondary valve allows recirculation of exhaust gas into the intake manifold when the throttle valve is further opened.