Emission control - Td5 (EGR systems): Description and operation (page 2 of 2)

EU3 models: The exhaust gases are routed from the exhaust manifold through the EGR cooler to the underside of the EGR valve. The EGR cooler is bolted to the front of the engine cylinder head. An EGR pipe connects the EGR cooler to the exhaust manifold and is secured by two Allen screws. The two Allen screws fixing the EGR pipe to the exhaust manifold should be replaced every time the EGR pipe is removed. A second pipe connects the EGR cooler to the EGR valve; this pipe is secured to the EGR valve by a clip, and to the cooler by two Allen screws.Extreme care should be exercised when removing and refitting the EGR pipe to avoid damage. When refitting the EGR cooler, always tighten the pipe connections BEFORE tightening the bolts securing the cooler to the cylinder head.

When a vacuum is applied to the EGR suction port, it causes a spindle with sealing disc (EGR valve) to be raised, opening the port at the EGR pipe to allow the recirculated exhaust gas to pass through into the inlet manifold. The valve is spring loaded so that when the vacuum is removed from the suction port, the valve returns to its rest position to tightly close the exhaust gas port.

A vacuum is simultaneously applied to the inlet throttle (ILT) valve suction port which causes the butterfly valve in the inlet manifold to close by means of a spindle and lever mechanism. Closing the butterfly valve limits the supply of fresh intercooled air entering the inlet manifold and causes a depression within the inlet manifold to create a greater suction at the open port to the EGR delivery pipe. In this condition a greater mass of recirculated exhaust gas is drawn into the inlet manifold for use in the combustion process. When the vacuum is released from the ILT valve suction port a spring returns the butterfly valve to its fully open position.

By controlling the quantities of recirculated exhaust gas and fresh intake air available in the inlet manifold, the optimum mix for the prevailing engine operating conditions can be maintained which ensures the intake gas to the combustion chambers will have burning rate properties which will reduce the NO_x emissions to an acceptable level. Normally, full recirculation is only applicable when the NO_x emissions are most prevalent.