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.
Engine Sensing |
In Part two we are going to look at how engine condition is sensed and how this
information is used to control the fuel supply.
We will start by looking at the air intake. Depression of the accelerator pedal causes
the throttle plate to rotate, and allows air to enter the plenum chamber.
The airflow is measured by a flap inside this metering unit, located between the air
filter and the throttle plate.
Rotation of the flap spindle operates a potentiometer. This supplies a signal,
corresponding to the flap position, to the ECU. The signal is indicative of engine load
and is used to determine the injector opening period.
Excessive fluttering of the flap is eliminated by a compensating flap, moving at the same
time, in an arc shaped chamber.
Air density variations, caused by temperature change, would affect flap deflection.
Compensation is made by a thermistor located in the airflow meter inlet. It modifies the
Rotation of the flap spindle also operates a pair of contacts. They ensure the fuel pump
is switched on only when the engine is running.
A second potentiometer connected to the throttle plate spindle provides the ECU with
information about the position and movement of the plate.
For example during sudden opening of the throttle plate, the ECU will sense a rapid
voltage change. When this occurs the unit opens all eight injectors once simultaneously,
thus ensuring adequate engine response.
Engine Speed |
We have seen how air intake is used to control the air fuel mixture, but engine speed must
also be accounted for.
A signal from the coil negative terminal, sensed through a protective resistance in the
ignition ballast resistor, determines the frequency of injector opening. There is however,
no fixed relationship between injection timing and engine timing. The actual injection
point is unimportant.
The ECU carries fuelling information in a memory, so that any combination of intake-air
and engine speed can be compared with the theoretical values. This enables the fuel
supply to be controlled precisely.
CLOSED LOOP MIXTURE CONTROL |
Now let's see how the closed loop system maintains the stoichiometric air/fuel mixture.
Exhaust oxygen, which relates to the air/fuel mixture is measured by two oxygen or lambda
sensors, one ahead of each catalyst.
An electrical signal from each sensor is processed by the ECU and is used to correct the
mixture, by extending or shortening the injector opening period. Too high an exhaust
oxygen level causes mixture enrichment, too low a level causes the mixture to be weakened.
Incidentally, the nature of both the catalyst and the oxygen sensors dictates the use of
unleaded fuel as this is currently unavailable in Europe, closed loop mixture control
will not be impaired. The clased loop system is only fitted to satisfy exhaust emission
Cranking Enrichment |
First cranking, the ECU provides an enrichment by extending the injection period and
providing the engine is cold, the cold-start injector, sprays additional fuel into the
As you saw earlier it is controlled by this thermal time switch.
The switch contacts are operated by a bimetallic strip. The strip is subjected to two heat
sources, the engine coolant and an electrical heating coil. When the contacts open, the
cold-start injector is de-energised, as its earth path is broken.
Now warm-up enrichment, this sensor. located between the cylinder heads provides the ECU
with coolant temperature information., At low engine temperatures the injection period is
As the engine warms up the sensor resistance decreases, and mixture enrichment, which is
proportional to this, is also decreased.
It is worth noting that if the sensor is ever disconnected, while the engine is running,
following the warm-up period, flooding will occur and the engine will stop. The ECU senses
infinite resistance, thinks the engine is cold, and causes excessive mixture enrichment.
Finally the extra air valve, it is mounted above a water passage in the inlet manifold,
and allows metered air to bypass the closed throttle plate during engine warm-up, thus
increasing idle speed.
Inside the air valve is a spring loaded blocking plate and an electrically heated
bimetallic strip. As the strip heats up, the plate rotates, progressively closing the
throttle plate bypass channel. When the engine reaches normal running temperature,
the heating coil earth path is broken, inside the ECU, by the coolant temperature sensor.
However the extra air valve is still subjected to engine heat and the air channel remains