EMISSION CONTROL
15
DESCRIPTION AND OPERATION
EGR System - Type 2
This system features twin modulators mounted one above the other on a metal plate located on the inner wing at
the RH side of the engine. The modulators are electrically controlled by the engine management system and are
used to modulate a vacuum source to the EGR valve and a supplementary Inlet Throttle (ILT) valve; the two
valves are controlled to operate in tandem. The ILT valve vacuum pot is mounted adjacent to the EGR valve
housing and has a linkage which connects to a butterfly valve mounted in front of the EGR valve at the air intake
manifold.
The modulator operations are electrically controlled by signals from the engine management system which
determines the required volume of exhaust gas needed in response to inputs relating to air flow, engine operating
conditions and ambient parameters such as temperature and altitude. The engine management ECM switches on
the circuit by completing the path to ground, operating the vacuum modulators.
Pre EU3 models: The exhaust gases are routed from the exhaust manifold through a shaped metal pipe which
connects to the underside of the EGR valve. The pipe is held securely in position to the front of the engine cylinder
head using a metal clamp bracket. The EGR pipe is attached to a mating port at the front end of the exhaust
manifold using two Allen screws and at the EGR valve assembly by a metal band clamp. The two Allen screws
fixing the EGR pipe to the exhaust manifold should be replaced every time the EGR pipe is removed.
CAUTION: Extreme care should be exercised when removing and refitting the EGR pipe to avoid
damage.
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.
CAUTION: 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
2
emissions to an acceptable
level. Normally, full recirculation is only applicable when the NO
2
emissions are most prevalent.