Last week we looked at the mechanical make-up of
the common rail diesel fuel injection systems that have revolutionised
diesel-powered cars (see
Common Rail Diesel Engine Management, Part 1). The
systems used extremely high fuel pressure, electronically controlled injectors
and complex exhaust aftertreatment to provide very high specific torque outputs
with low fuel consumption and low emissions.
But how does the electronic control system work?
In this article we look at the electronics of the system.
The engine management system in a diesel common
rail engine needs to provide:
Very high fuel injection pressures (up to 2000
Variation in injected fuel quantity, intake
manifold pressure and start of injection to suit engine operating conditions
Pre-injection and post-injection
Temperature-dependent rich air/fuel ratio for
Idle speed control independent of engine load
Exhaust gas recirculation
Long term precision
As with current petrol engine management systems,
the driver no longer has direct control over the injected fuel quantity.
Instead, the movement of the accelerator pedal is treated as a torque request
and the actual amount of fuel injected in response is dependent on the engine
operating status, engine temperature, the likely affect on exhaust emissions,
and the intervention by other car systems (eg traction control).
This diagram shows the inputs and outputs of a
typical Bosch common rail diesel injection system.
The injected fuel quantity and start of injection
timing required for starting are primarily determined by engine coolant
temperature and cranking speed. Special strategies are employed for very cold
weather starting, especially at high altitudes. In these conditions, the
turbocharger operation may be suspended as its torque demand – although small –
may be sufficiently great as to prevent the car from moving off.
In normal driving, the injected fuel quantity is
determined primarily by the accelerator pedal sensor position, engine speed,
fuel and intake air temperatures. However, many other maps of data also have an
effect on the fuel injection quantity actually used. These include strategies
that limit emissions, smoke production, mechanical overloading and thermal
overloading (including measured or modelled temperatures of the exhaust gas,
coolant, oil, turbocharger and injectors). Start of injection control is mapped
as a function of engine speed, injected fuel quantity, coolant temperature and
The set idle speed depends on engine coolant
temperature, battery voltage and operation of the air conditioner. Idle speed is
a closed loop function where the ECU monitors actual engine speed and continues
to adjust fuel quantity until the desired speed is achieved.
Unlike a petrol engine management system which
usually cuts fuel abruptly when the rev limit is reached, a diesel engine
management system progressively reduces the quantity of fuel injected as the
engine speed exceeds the rpm at which peak power is developed. By the time
maximum permitted engine speed has been reached, the quantity of fuel injected
has dropped to zero.
Sudden changes in engine torque output can result
in oscillations in the vehicle’s driveline. This is perceived by the vehicle
occupants as unpleasant surges in acceleration. Active Surge Damping reduces the
likelihood of these oscillations occurring. Two approaches can be taken. In the
first, any sudden movements of the accelerator pedal are filtered out, while in
the second, the ECU detects that surging is occurring and actively counteracts
it by increasing the injected fuel quantity when the engine speed drops and
decreasing it when the speed increases.
Because of mechanical differences from cylinder to
cylinder, the development of torque by each cylinder is not identical. This
difference can result in rough running and increased emissions. To counteract
this, Smooth Running Control uses the fluctuation in engine speed to detect
output torque variations. Specifically, the system compares the engine speed
immediately after a cylinder’s injection with the average engine speed. If the
speed has dropped, the fuel injection quantity for that cylinder is increased.
If the engine speed is above the mean, the fuel injection quantity for that
cylinder is decreased.
Closed Loop Oxygen Sensor
As with petrol management systems, diesel
management system use oxygen sensor closed loop control. However, in diesel
systems a broadband oxygen sensor is used that is capable of measuring air/fuel
ratios as lean as 60:1. This Universal Lambda Sensor (abbreviation in German:
LSU) comprises a combination of a Nernst concentration cell and an oxygen pump
Because the LSU signal output is a function of
exhaust gas oxygen concentration and exhaust gas pressure, the sensor output is
compensated for variations in exhaust gas pressure. The LSU sensor output also
changes over time and to compensate for this, when the engine is in over-run
conditions, comparison is made between the measured oxygen concentration of the
exhaust gas and the expected output of the sensor if it were sensing fresh air.
Any difference is applied as a learned correction value.
Closed loop oxygen control is used for short- and
long-term adaptation learning of the injected fuel quantity. This is especially
important in limiting smoke output, where the measured exhaust gas oxygen is
compared with a target value on a smoke limitation map. Oxygen sensor feedback
is also used to determine whether the target exhaust gas recirculation is being
Fuel Pressure and Flow
The pressure in the common rail is regulated by
closed loop control. A pressure sensor on the rail monitors real time fuel
pressure and the ECU maintains it as the desired level by pulse width modulating
the fuel pressure control valve. At high engine speeds but low fuel demand, the
ECU deactivates one of the pistons in the high pressure pump. This reduces fuel
heating in addition to decreasing the mechanical power drawn by the pump.
Other Management System Outputs
In addition to the control of the fuel injectors,
the diesel engine management system can control
Glow plugs for sub-zero starting conditions
Glow plugs that heat the coolant, providing
adequate cabin heating in cold climates
Switchable intake manifolds, where at low loads
air is forced through turbulence ducts to provide better in-cylinder swirl
Turbocharger boost pressure control
Switching of radiator fans
The triggering of the injector can be divided into
The second phase is termed ‘pick-up current’. In
this phase, the current supply for the injector switches from the capacitor to
the battery. In this phase, peak current continues to be limited to 20A.
European car manufacturers and consumers have
thrown their weight heavily behind passenger cars equipped with diesel engines.
The major improvement in specific torque outputs and the reduction in fuel
consumption and emissions have been achieved with sophisticated electronic
control of very high pressure, individually controlled injectors.