Fuel temperature sensor (Engine management system)

Fuel temperature sensor

Fuel temperature is monitored by a sensor located in the fuel injection pump. This sensor is of the negative temperature coefficient (NTC) type, designed to reduce its resistance with increasing temperature. The sensor is connected to earth and to a precision resistor inside the engine control module. A small current is fed through the precision resistor.

When the system is operating the engine control module regularly checks the sensor voltage. This is converted to a digital word that can be read by the microprocessor and converted to a temperature reading. As fuel density varies with temperature, the information received is used to calculate the correct quantity of fuel to inject.

Quantity servo control unit

The servo control unit is used to accurately control the amount of fuel delivered to the injectors. The unit consists of a rotary magnet mounted on an eccentric shaft; the shaft engages with the control spool (3) of the pump.

The rotary magnet is fitted with a return spring (4) and moves under the influence of a control coil (2). The magnet rotates through an arc of about 60 degrees moving the control spool from the closed position to the maximum fuel delivery position. The eccentric shaft engages with the control spool at one end, while the opposite end operates a rotary potentiometer (1).

When the control coil is energised the rotary magnet and eccentric shaft move against spring pressure. Rotary movement of the eccentric shaft is converted into linear movement of the control spool. This allows more fuel to be delivered to the injectors.

When the control coil is de-energised the return spring causes the rotary magnet and eccentric shaft to resume their original position. The control spool is moved to the closed position.

Quantity servo potentiometer

Mounted at the top of the servo control unit is a rotary potentiometer. As the eccentric shaft of the servo control unit moves, the wiper of the potentiometer is moved across the resistive part of the potentiometer. The voltage output from the potentiometer is proportional to the position of the control spool. From this input the engine control module can calculate the quantity of fuel being delivered by the pump and adjust the amount of fuel being delivered by energising the control coil to move the control spool.