In order that the engine can run with optimal combustion under all operating conditions,
the engine-ECU calculates exactly the right injected fuel quantity for all conditions. Here,
a number of parameters must be taken into account.
FUEL INJECTION QUANTITY CALCULATION
System Configuration Diagram
START QUANTITY
For starting, the injected fuel quantity is calculated as a function of engine coolant
temperature and cranking speed. Start-quantity signal is generated from the moment the ignition
switch is tuned to "START" position until a given minimum engine speed is reached. The driver cannot influence the start quantity.
DRIVE MODE
When the vehicle is being driven normally, the injected fuel quantity is a function of
the accelerator pedal position sensor and of the engine speed. Calculation depends upon maps
which also taken other influences into account (e.g. fuel and boost air temperature). This permits
best-possible alignment of the engine’s output to the driver’s wishes. However, to protect the engine against mechanical damage and to prevent black smoke, there
should be limitations on the quantity of fuel injected. For this reason, the engine-ECU calculates
a limit value for this quantity. The limit value depends on the engine speed, the air mass and
the boost pressure.
IDLE SPEED CONTROL
When the accelerator pedal is not depressed, it is the job of the idle speed control to
ensure that a given idle speed is maintained. This can vary depending upon the engine’s
particular operating mode. For instance, with the engine cold the idle speed is usually set
higher than that when it is warm. There are further instances when the idle speed is held somewhere
higher. For instance when the vehicle’s electrical-system voltage is too low, when
the A/C is switched on, or when the vehicle is rolling freely. When the vehicle is driven in
stop-and-go traffic, together with stops at traffic lights, the engine runs a lot of the time
at idle. Considerations concerning emissions and fuel consumption dictate therefore that idle
speed should be kept as low as possible. This of course is a disadvantage with respect to smooth-running
and pull-away.When adjusting the stipulated idle speed, the idle speed control must cope with
heavily fluctuating requirements. The input power needed by the engine-driven auxiliary equipment
varies extensively. At low electrical-system voltages for instance, the alternator consumes far more power
than it does when the voltages are higher. In addition, the power demands from the A/C compressor
and the high-pressure generation for the diesel injection system must all be taken into account.
Added to these external load moments is the engine’s internal friction torque which
is highly dependent upon engine coolant temperature, and which must also be compensated for
by the idle speed control. In order to regulate the desired idle speed, the controller continues to adapt the injected
fuel quantity until the actual engine speed corresponds to the desired idle speed.
SMOOTH RUNNING CONTROL
Smooth running control improves engine running at idle speed. Different cylinders in an engine can often generate different levels of torque even through
the same amount of fuel has been injected. Possible causes of this are, among other things,
differences in the tolerances of the parts, cylinder compressions, friction caused by the cylinders
and the hydraulic injector components. The effects of these differences in torque are imbalanced engine running and an increase
in exhaust gas emissions. The smooth running control is designed to detect the pulses in speed that are caused as
a result. The pulses in speed are then balanced by targeted control of the amount injected at
the affected cylinders. Detection works at idling speed via a signal from the crank angle sensor. If the signals are received in a balanced rhythm, the cylinders are all working the same
way. If one cylinder performance is less than the others, the crankshaft needs longer to reach
the next point of ignition. And in the same way, a cylinder that performs better than the others will have a shorter
path. If the engine-ECU detects a deviation, the affected cylinder will receive a smaller or
greater amount of fuel until the engine runs smoothly again.
ACTIVE PULSE DAMPING
With the active pulse damping system, there is a reduction in jolt-type vehicle movements
that are generated by local changes from different acceleration requirement. Incase of sudden changes in the torque required by the driver (through accelerator pedal),
a precisely matched filter function reduces the drivetrain excitation. The speed signals are used to detect drivetrain oscillations that are then damped by an
active control. In order to counteract the drivetrain oscillations, the active control reduces
the injected fuel quantity when rotational speed increases, and increases it when speed drops.
GOVERNOR
The governor protects the engine from over revving and thereby from damage. The engine
is therefore governed to a maximum permissible speed that cannot be exceeded for long periods
of time. After regulation has started, the amount of fuel injected is continually reduced. If the highest permitted engine speed is reached, the amount of fuel injected remains
constant until the driving conditions change again. The adaptive function is kept as smooth as possible in order that jolts in the amount
of fuel injected are not caused during acceleration.
START OF INJECTION CONTROL
The start of injection control influences a number of engine properties, such as the engine
performance, the fuel consumption, the noise emissions and equally as important, the exhaust
emissions. The start of injection control thus has the task of determining the correct point of fuel
delivery and injection. The engine-ECU calculates the start of injection. The specification depends on the engine
speed and the calculated quantity of fuel to be injected from the quantity calculation. Further
influencing factors are the engine coolant temperature sensor and the air pressure. In order that the start of delivery can be calculated optimally, the actual point at which
delivery begins must also be registered. To do this, the engine-ECU monitors the flow at the injector solenoid valve. From the
special flow pattern, the actual start of delivery, and thereby the start of injection, is determined. Start of injection is initiated when the injector solenoid valve is actuated. For actuation,
a magnetic field is created, current increases and the valve shuts. When the valve shuts on the valve seat, a distinctive jolt is noticeable in the current
flow. This is known as BIP (Begin of Injection Period). BIP signalizes complete closure of the injector solenoid valve and thereby the point of
delivery. The signal is received by the engine-ECU. If the valve is closed, current is maintained at a constant level. Once the required period
of delivery has elapsed, actuation will cease and the valve will open. The actual moment at which the injector solenoid valve closes, that is BIP, is determined
so that the point of actuation for the next injection period can be calculated. If the actual BIP deviates from the mapped details stored in the engine-ECU, the engine-ECU
will correct the point of valve actuation.
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