This part of site is an attempt to organize and add to the web resources for the Rover SD1
to form a cohesive and easily usable guide for those of us without easy access to
expert repair and/or advice. It is not offered in any way as a definitive source
and we take no responsibility for any errors that may exist.
the webmaster Rene Winters.
ANCILLARY EMISSION CONTROL EQUIPMENT|
To end this second part of the programme let's look briefly at the ancillary emission
control equipment. First exhaust gas recirculation, the EGR valve dilutes the air/fuel
mixture by returning a proportion of the exhaust gas to the inlet manifold. The lower peak
combustion temperatures reduce the formation of nitrogen oxides.
The valve is operated by manifold depression and exhaust back pressure. It is therefore,
sensitive to engine load.
Depression is sensed at this throttle edge tapping and is transmitted to the valve,
through a hose.
Overrun Valve |
Next the overrun valve, its function is to maintain satisfactory combustion during
deceleration. The valve is mounted on the plenum chamber and is operated by manifold
When depression reaches a critical level, a disc valve opens against spring pressure,
and allows metered air to bypass the closed throttle plate. When depression falls,
the spring ensures the bypass channel is closed.
Evaporative Emission Control
3 Finally, to prevent fuel vapour entering the atmosphere, two charcoal canisters are
fitted in the fuel tank vent pipe.
A vapour separator prevents liquid fuel entering the pipe.
The canisters are also connected to the crankcase and to the throttle body. This
arrangement not only ensures combustion of both oil and fuel vapour, but also, after
the engine has stopped, any resultant vapour is absorbed by the canisters.
The Electrical Circuit |
First let's introduce the combined relay, it is the link between the ECU and the injection
When the ignition is switched on, the main section of the combined relay is energised.
It supplies current to the ECU switching it on.
When the engine is cranked, the main section of the relay supplies current, via the power
resistor pack, to the injectors. The injectors open as their separate earth paths are
completed by the ECU.
At the same time, current from the starter relay energises the pump section of the combined
relay. The relay completes the circuit through the inertia switch to the fuel pump; this
providing the initial fuel supply.
Apart from this, the pump section of the relay instructs the ECU to extend the injection
period, thus enriching the mixture, it also completes the circuit through the extra air
valve heater coil, to earth, via the ECU: and it supplies current to the thermal time
switch and the cold-start injector. Providing the engine is cold, finely atomised fuel is
sprayed into the plenum chamber.
As soon as the engine fires, air drawn into the manifold causes the flap in the airflow
meter to be deflected. The pump contacts close and energise the pump section of the
combined relay. This replaces the initial cranking current and provides a supply to the
fuel pump and the heater coil in the extra air valve.
While the engine is running, the injectors open at a frequency determined by the pulse
chain from the ignition coil negative terminal, sensed at the ballast resistor.
The duration of each injection being determined by the signals received by the ECU, from
the airflow meter and air temperature sensor; the throttle potentiometer, the coolant
temperature sensor and the lambda sensors.
As the engine warms up, the electrical resistance of the coolant temperature sensor drops,
and the ECU gradually weakens the mixture by shortening the injection period.
At normal running temperature, the only effect of the coolant temperature sensor, is to
instruct the ECU to cut the power supply to the extra air valve. The coolant temperature is
now sufficient to keep the throttle bypass channel closed.
As you know the output voltage from a battery depends on its state of charge and the
To compensate for this, the ECU continuously monitors battery voltage and generates a
correction factor. which is then applied to the injector opening pulses. This means that
fuel delivery is unaffected over a wide voltage range.