Exhaust Gas Recirculation: Description and Operation

Fig. 125 EGR System Turbo:

SYSTEM OPERATION

The exhaust gas recirculation (EGR) system is used to reduce oxides of nitrogen (NOx) emissions. The atmosphere is made up of mostly Nitrogen, with a smaller percentage of oxygen, and a mixture of other gases. Oxygen and Nitrogen do not normally combine except at very high temperatures and pressures, conditions which are present in the combustion chamber especially during hard acceleration. When the engine is under load, the EGR valve admits a small amount of exhaust gas into the intake manifold to mix with the air/fuel charge. The exhaust gas is essentially inert (contains no fuel or oxidizer) and reduces peak combustion temperatures and pressures by absorbing some of the heat of combustion without participating in the actual burn. Greater amounts of exhaust gas are metered in as engine speed and load are increased.

The primary components of the EGR system are: the EGR control valve (EGR valve), EGR vacuum and vent solenoid valves, vacuum reservoir, EGR valve position sensor, and the engine control unit (ECU).

EGR (CONTROL) VALVE

The EGR valve allows a portion of the exhaust gas into the intake manifold to reduce the amount of NOx produced by the engine. It consists of a vacuum diaphragm and spring loaded valve. When vacuum is applied to the diaphragm chamber, the valve opens allowing a regulated amount of exhaust into the intake manifold.

EGR POSITION SENSOR

The EGR position sensor is a variable resistor (potentiometer) that measures how far the EGR valve is open. The ECU monitors the change in resistance to determine whether or not the EGR valve is operating properly.

ENGINE CONTROL UNIT

The engine control unit monitors signals from the distributor (engine speed), air flow sensor and throttle position sensor (engine load), and coolant temperature sensor. The ECU uses this information to determine when to allow EGR system operation and energizes the EGR vacuum or vent solenoid valves accordingly.

EGR SOLENOID VALVE

The EGR solenoid valve controls the vacuum signal to the EGR control valve. The vent side controls the vent line and the vacuum side controls the vacuum side.

VACUUM STORAGE CHAMBER

The vacuum storage chamber stores vacuum supply for use under turbo boost conditions. A one-way check valve is built in, to prevent vacuum loss.