Sensor – Electronic Diesel Control (Troubleshooting)

EDC – Electronic Diesel Control

At some point in the course of development of diesel engines, mechanical control was no longer sufficient to keep pace with technical process. More and more stringent exhaust gas standards and the wish to both reduce consumption and increase engine power made the development of an electronic control system necessary for diesel engines. The first EDC (Electronic Diesel Control) was used in 1986. Today, EDC is a standard component in modern high-pressure diesel injection systems. Without it, realising convenient and powerful diesel injection systems would be impossible.

How does the EDC work? 

Basically, it can be compared with an injection system in petrol engines. The EDC can be divided into three component parts: 

■ Sensors 

■ Control unit 

■ Actuators 

The sensors

The sensors map all actual and reference states. This means, for example, that engine temperature and fuel pressure are mapped as actual values at the same time as reference values such as the accelerator pedal position. The sensors map the operating conditions and convert measured physical or chemical values into electrical signals, which they then forward to the control unit. The demanding requirements made on sensors have caused them to become increasingly smaller and more powerful over the past few years. Conventional sensors are usually individual components that transmit an analogue signal to the control unit where it is then processed further. New sensors in the EDC are equipped with signal processing, an analogue/digital converter and sometimes even evaluation electronics. Signal transmission to the control unit is digital. This results in numerous advantages: 

• The sensors can map smaller measured values. 
• Transmission to the control unit is immune to interference. 
• The computer capacity of the control unit can be reduced. 
• The sensors are databus-capable and their information can be used for several applications.

The various sensors 

• Speed sensors

Depending on the injection system, the speed sensors map the speeds and positions of various rotating shafts. The most important sensor is the engine speed sensor. This records the engine speed and position of the crankshaft. The speed sensor is usually an inductive sensor (passive sensor). It consists of an iron core with a coil wound around it and is connected to a permanent magnet. If the trigger wheel turns, the magnetic flow in the coil changes, inducing a sine-shaped voltage. The frequency and amplitude are proportional to the engine speed. By changing the tooth spacing on the trigger wheel, the signal can be changed and provide information about the position of the crankshaft. 

Some vehicle manufacturers also use active sensors. These sensors work according to the Hall sensor principle. Pairs of magnetic poles (one north pole and one south pole alternately) are attached to the trigger wheel in place of the teeth. Here, too, the reference mark to the crankshaft position is produced through a changed spacing. As opposed to the inductive sensor, the Hall sensor generates a rectangular signal, the frequency of which is also proportional to the engine speed.

• Air mass sensor

In order to determine the exact injection quantity and exhaust gas return rate, the control unit requires information about the quantity of intake air. The mass air flow is measured using the air mass sensor installed in the intake manifold.

• Temperature sensors

Temperature sensors are usually designed as NTC. This means that there is a precision resistor made of semi-conductor material with a negative temperature coefficient (NTC) in the housing. These have a high resistance at low temperatures, with resistance decreasing as temperature increases. The engine temperature sensor is installed in the engine coolant circuit. It maps the coolant temperature, which provides information about the engine temperature. The control unit requires the engine temperature as a corrective value for calculating the injection quantity. 

The fuel temperature sensor is installed on the low-pressure side of the fuel system. It records the fuel temperature. As the temperature changes, the fuel density also changes. The control unit requires the fuel temperature to precisely calculate the injection starting point and quantity. Any fuel cooling is also controlled using the value measured by the temperature sensor. The air temperature sensor maps the temperature of the intake air. The intake air temperature sensor can be installed in the intake tract as a separate sensor or is integrated in the intake pipe pressure sensor. As with the fuel, the density of the air also changes as its temperature changes. The control unit uses the information about the intake air temperature as a corrective value for charge air control.

•  Pressure sensors 

There is an electronic evaluation unit and a measuring cell in the pressure sensor housing. This measuring cell contains a membrane that encloses a reference pressure chamber to which four expansion resistors are attached in a bridge circuit. Two of these expansion resistors are used as measuring resistors and are in the centre of the membrane. The two other resistors are attached to the outside of the membrane and are used as reference resistors to compensate temperature. If the shape of the membrane changes due to the pressure applied, the conductivity of the measuring resistances changes and thus the measuring voltage. This measuring voltage is processed by the evaluation electronics and forwarded to the engine control unit. The charge pressure sensor records the pressure in the intake pipe between the turbocharger and the engine. The charge pressure is not measured against environmental pressure but rather against a reference pressure in the sensor. The sensor provides the control unit with information about the charge pressure. The reference and actual values are compared in the characteristic diagram for charge pressure regulation, and the charge pressure is adapted to the engine requirements via the charge pressure limitation.

The environmental pressure sensor (height sensor) maps the environmental pressure. Since this fluctuates depending on altitude, the control unit uses this value to correct the charge air regulation and the exhaust gas re-circulation system. The environment pressure sensor is often integrated in the control unit, but can also be housed in the engine compartment as a separate sensor. The fuel pressure sensor maps the fuel pressure. There are two applications here: The fuel pressure sensor in the low-pressure area, in the fuel filter for example. This allows the fuel filter soiling to be monitored. The second application is monitoring the fuel pressure on the high-pressure side. The rail pressure sensor is used here in the common rail system.

Needle movement sensor

The needle movement sensor maps the actual opening time-point of the injection nozzle. The control unit needs this information in order to compare the start of injection with the data from the characteristic diagram so that injection always takes place at exactly the right moment.

The needle movement sensor is made up of a pressure bolt surrounded by a magnetic coil. If the pressure bolt is mechanically actuated by the nozzle needle opening, the magnetic field in the magnetic coil changes. This in turn changes the voltage applied in the coil, which has a constant voltage supply from the control unit. From the time lag between the information of the needle movement sensor and the OT signal of the speed sensor, the control unit can calculate the real start of injection. 

Accelerator pedal sensor (pedal sensor)

The accelerator pedal sensor records the position of the accelerator pedal. This can be done by measuring path or angle of the accelerator pedal. The accelerator pedal sensor can be attached directly to the accelerator pedal (accelerator pedal module) or located in the engine compartment. In this case, it is connected to the accelerator pedal sensor via a Bowden cable. There are different kinds of accelerator pedal sensors. Some work with a potentiometer that forwards different voltages to the control unit and which are then compared with a characteristic curve. 

The control unit calculates the position of the accelerator pedal on the basis of the characteristic curve. Inductive sensors have a permanently installed Hall sensor instead of the potentiometer. There is a magnet on the accelerator pedal, which changes its position depending on the position of the accelerator pedal. The signal thus produced is amplified and forwarded to the control unit as a voltage signal. The advantage of these inductive sensors is that they are not subject to wear. The idling switch is integrated in the accelerator pedal sensor, as is the kick-down switch in vehicles with automatic transmission.

Brake switch 

The brake switch is on the foot pedal and is usually combined with the stoplight switch. It passes a signal on to the control unit when the brake pedal is pressed. This results in the control unit reducing engine power to prevent simultaneous braking and accelerating.

Clutch pedal switch  

The clutch pedal switch is also located on the foot pedal. It informs the control unit whether the clutch pedal is being pressed or not. If the control unit receives the information that the clutch pedal is being pressed, it reduces the fuel injection quantity briefly in order to achieve "smooth" gear changing.

Air conditioner

The EDC control unit receives a signal indicating whether the air conditioner is switched on or off. This information is required in order to increase the idling speed with the air conditioner switched on. This prevents the idling speed decreasing too much when the compressor clutch is applied.

Speed signal

The EDC control unit requires information about current speed in order to control the radiator fan (radiator fan run-down), to dampen jolting during gear changing and for the speed control system, if fitted.

Speed control system

The EDC control unit receives information from the speed control system as to whether the system is switched on or off, whether the driver would like to accelerate, slow down or maintain speed. 

The EDC control unit

All the information provided by the sensors is processed in the EDC control unit, and outputted as control signals for the actuators. The actual control unit, a PCB with all electronic components, is mounted in a metal housing. Sensors and actuators are connected by means of a four-pin plug-type connection. The power components necessary for the direct triggering of the actuators are installed on heatsinks in the metal housing in order to dissipate the heat that builds up.

Further requirements have to be taken into account with the design. These requirements concern the environment temperature, mechanical load and humidity. Just as important is resistance to electromagnetic interference and the limitation of radiated high-frequency interference signals. The control unit has to work perfectly at temperatures from -40 °C to approx. +120 °C. To enable the control unit to output the correct triggering signals for the actuators in every engine operating state, the control unit must be “realtime-capable”. This requires high computer power and computer architecture.

The sensor input signals reach the control unit in different forms. For this reason, they are routed via protective circuits, and amplifiers and signal converters if necessary, and then processed directly by the microprocessor. Analogue signals indicating the engine and intake air temperature, the amount of air suctioned in, the battery voltage, oxygen sensor etc. are converted into digital values in the microprocessor by an analogue/digital converter. To prevent interference pulses, signals from inductive sensors, such as speed mapping and reference mark sensors, are processed in a part of the circuit. 

The microprocessor needs a program in order to process the input signals. This program is stored on a read-only memory (ROM or EPROM). In addition, this read-only memory contains the engine-specific characteristic values and curves required for engine control. To be able to realise the function of some vehicle-specific features or engine variants, the vehicle manufacturer or garage carries out a variant coding. This is necessary if the control unit is to be replaced as a spare part or if individual sensors or actuators are renewed. To keep the number of different control units at the vehicle manufacturers to a minimum, the complete data records with some unit types are not installed on the EPROM until the end of production (EOL = End Of Line Programming). As well as the ROM or EPROM, a read/write memory (RAM) is required. This has the task of storing calculated values, adaptation values and any faults that may occur in the whole system so that they can be read out using a diagnostic unit. 

This RAM memory requires a permanent power supply. If the power supply is interrupted because the battery is disconnected, for example, the stored data are lost. In this case all adaptation values have to be determined again by the control unit. To avoid the loss of variable values, these are stored in an EPROM instead of a RAM in some unit types. Signal output to control the actuators takes place through final stages. The microprocessor controls these final stages that are powerful enough to be directly linked to the individual actuators. These final stages are protected in such a way that they cannot be destroyed by short-circuits to ground and battery voltage or excess electrical load. Thanks to self-diagnosis, any faults occurring at any of the final stages can be recognised and the output switched off if necessary. This fault is then stored in the RAM and can be read out in the garage using a diagnostic unit.