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.

Requirements

The engine management system in a diesel common rail engine needs to provide:

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.