Electronic diesel control operation (Engine management system)

ELECTRONIC DIESEL CONTROL OPERATION

Diesel engines operate by compression ignition. The rapid compression of air in the cylinder during the compression cycle heats the injected fuel causing it to self ignite. During cold starting, automatically controlled glow plugs assist in raising the temperature of the compressed air to ignition point.

The Engine Control Module (ECM) monitors the conditions required for optimum combustion of fuel in the cylinder through sensors located at strategic points around the engine. From the inputs from these sensors, the engine control module can adjust the fuel quantity and timing of the fuel being delivered to the cylinder.

The main features are as follows:

• The ECM controls the fuel injection quantity and timing of the fuel delivered to the cylinder based on inputs from sensors located around the engine. The ECM incorporates short circuit protection and can store intermittent faults on certain inputs. TestBook can interrogate the ECM for these stored faults.
• The engine management system is a ’Drive by Wire’ system. The throttle pedal is not physically connected to the fuel injection pump, but instead connected by a linkage to a throttle potentiometer which provides the ECM with a signal proportional to throttle pedal movement. The ECM then adjusts the fuel injection pump to deliver the required amount of fuel.
• In conjunction with the throttle potentiometer the ECM uses the speed/density method of air flow measurement to calculate fuel delivery. This method measures the inlet air temperature and inlet manifold pressure and assumes that the engine is a calibrated vacuum pump, with its characteristics stored in the ECM, it can determine the correct amount of fuel to be injected.
• If certain system inputs fail, the ECM implements a back-up facility to enable the system to continue functioning, although at a reduced level of performance.
• A separate diagnostic connector allows engine tuning or fault diagnosis to be carried out using TestBook without disconnecting the ECM harness multiplug.
• The ECM harness multiplug incorporates specially plated pins to minimise oxidation and give improved reliability.

Fuel injection timing

The engine control module determines the optimum fuel injection timing for the injection pump based on the signals it receives from the following sensors:

1. Crankshaft sensor - Engine speed and crankshaft position
2. Needle lift sensor - Start of injection
3. Coolant temperature sensor - Engine temperature
4. Manifold absolute pressure - Engine load
5. Intake air temperature sensor

Basic fuel injection timing

Crankshaft sensor

The speed and position of the engine is detected by the crankshaft sensor which is bolted to, and projects through, the gearbox adapter plate adjacent to the flywheel.

The crankshaft sensor is an inductive sensor consisting of a bracket mounted body containing a coil and a permanent magnet which provides a magnetic field. The sensor is situated such that an air gap exists between it and the flywheel. Air gap distance is critical for correct operation.

The flywheel has four poles positioned equally around the crankshaft circumference at 90 degree intervals. When the flywheel rotates, as a pole passes the sensor it disturbs the magnetic field inducing a voltage pulse in the coil. This pulse is transmitted to the engine control module.

Four pulses are transmitted to the engine control module for each revolution of the flywheel. By calculating the number of pulses that occur within a given time, the engine control module can determine the engine speed. The output from this sensor when used in conjunction with that from the needle lift sensor is used for idle stabilisation and reference for injection timing.

Needle lift sensor

The needle lift sensor consists of a coil which surrounds the shaft of an extended injection needle. The coil is fed a DC supply from the control unit which produces a magnetic field.