A Linear Optical Sensor for Measuring Needle Displacement in Common-rail Diesel Injectors

Alinear optical displacement sensorwas designed to overcome the problems related to the sensitivity of traditional transducers to electromagnetic disturbance, when applied to the measurement of needle lift in electrically actuated Diesel fuel injectors. The sensor consisted of a laser light emitter, a receiver, and a device used for modulating the intensity of the light reaching the receiver in function of position of the moving element. Thus, two rectangular windows were mounted on the injector body, while a third window was rigidly connected to the needle control piston, and aligned with the fixed ones, to obtain a clearance area which varied linearly with needle position. An optical fibre cable was used to transfer the laser light from the source to one of the fixed window, while a similar cable was connected between the second window and the receiver. Hence, the intensity of the light reaching the receiver was proportional to the position of the injector needle. Light intensity was converted into an equivalent voltage, and this signal was fed to the data acquisition system. The injector needle lift curves were measured and compared with those obtained with a commercial eddy current sensor. The reliability of the sensor was further improved by designing a suitable gasket that prevented fuel oil from reaching the optical windows, thus avoiding the onset of fluid cavitation, which would have resulted in bubble formation and consequent random laser light scattering, hence measurement alteration.