Last month we looked at the mechanical make-up of the common
rail diesel fuel injection systems that have revolutionised diesel-powered cars.
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 Bar)
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 starting
Idle speed control independent of engine load
Exhaust gas recirculation
Long term precision
Fig.1: an overview of a common rail diesel engine management system. The input signals to the ECU are on the left and include accelerator pedal position, intake mass airflow, fuel rail pressure and engine speed. Not shown here but also often included is a wideband exhaust gas oxygen sensor. The outputs (right) include the control of the fuel injectors, exhaust gas recirculation (EGR) and fuel rail pressure. Inside the ECU (middle) control strategies are implemented for idle speed, smooth running control, quantity of fuel injected, starting point of injection, and many others. [Bosch]
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).
Figure 1 shows an overview of the inputs, outputs and internal
processes in the Bosch common rail management system.