OPTICAL PRESSURE SENSOR FOR DETERMINING THE VARIATION OF THE PRESSURE IN AN INTERNAL COMBUSTION ENGINE

Abstract: An optical pressure sensor for determining the variation of the pressure in an internal combustion engine, comprising a bolt (1) having a through bore (2), a membrane (3) which is mounted in the bore or at one end of said bore and at least one surface of which is reflective, means (5) for causing light to fall on the reflective membrane part and means (7) for measuring the light reflected at the membrane or guiding it to a light-measuring device. In order to achieve the result that the pressure sensor is not sensitive to the intensity of the light source, the distance between the fibre assembly and the membrane, the light loss in the said means, variations in the reflecting power of the membrane surface and the ambient temperature, the means for causing light to fall on the reflective membrane have the form of a ring or sheath (5), means (7 or 6, respectively) being situated both inside said ring or sheath (5) and around said ring or sheath (5) for measuring light reflected at the membrane or guiding it to a light-measuring device. The pressure drop across the membrane is determined from the ratio of the reflected light which is received via the means (6) provided around said ring or sheath (5) and the reflected light which is received via the means (7) provided in the ring or sheath (5).

Description: The invention relates to an optical pressure sensor for determining the variation of the pressure in an internal combustion engine, comprising a bolt having a through bore, a membrane which is mounted in the bore or at one end of said bore and at least one surface of which is reflective, means for causing light to fall on the reflective membrane part and means for measuring the light reflected at the membrane or guiding it to a light-measuring device. Such a sensor is disclosed by GB-A-2,186,360. In that case, both the means for causing light to fall on the reflective membrane part and the means for guiding the light reflected at the membrane to a light-measuring device consist of separate bundles of optical fibres. Said sensor is cheap to manufacture and easy to install. The susceptibility to malfunction is also low. A drawback of the known sensor is that it is sensitive to the intensity of the light source, the spacing between the bundles of optical fibres and the membrane, the light loss in the input fibres, variations in the reflecting power of the reflective membrane surface and the ambient temperature. In view of this, the bundles of optical fibres will have to be accurately positioned. The object of the invention is to avoid these drawbacks and for this purpose the pressure sensor referred to in the preamble is characterised in that the means for causing light to fall on the reflective membrane part have the form of a ring or sheath, in that means are situated both inside the ring or sheath and around the ring or sheath for measuring light reflected at the membrane or guiding it to a light-measuring device, and in that the pressure drop across the membrane is determined from the ratio of the reflected light which is received via the means provided around said ring or sheath and the reflected light which is received via the means provided in the ring or sheath. Since the pressure difference is determined from the abovementioned ratio, the measurement result is independent of an accurate positioning of the said means. Preferably, use is made of an optical fibre assembly consisting of three concentric bundles of light-transmitting fibres. In a variant, use is made of an annular light source and two concentric photodiodes. The invention will now be explained in greater detail by reference to the figures. Figure 1 shows a section through a first embodiment of an optical pressure sensor for an internal combustion engine. Figure 2 shows a second embodiment. Figures 3 a, 3b and 3c show diagrammatically the operation of the pressure sensor in three positions of the membrane. Figure 3d shows a section of the fibre assembly. The pressure sensors shown comprise a bolt 1 to be screwed into a threaded opening in the wall of an internal combustion engine, in particular a diesel pump, which bolt is provided with a through bore 2, a membrane 3 provided at one end of the bolt and an optical fibre assembly 4 which in Figure 1 does, and in Figure 2 does not, project into the bore 2. The membrane surface facing the fibre assembly is reflective. As is evident from Figure 3, the optical fibre assembly comprises three sheath-type optical fibre groups: a light-input fibre group 5 having a light-output fibre group 6 or 7, respectively, on either side. Figures 3 a, 3b and 3c consecutively show the case where the pressure in the pump is higher than, equal to, and lower than the atmospheric pressure. The bulging of the membrane is a measure of the pressure in the internal combustion engine. In the case of Figure 3 a, the membrane reflects appreciably more light via the innermost fibre sheath 7 than via the outermost fibre sheath 6. In the case of Figure 3c, it is the other way round, while in Figure 3b equally as much light is reflected via both fibre sheaths 6 and 7. The ratio of the light reflected via the sheath 6 to the light reflected via the sheath 7 is a measure of the bulging of the membrane, that is to say, of the pressure difference on either side of the membrane. The sensor is relatively insensitive to vibrations and emminently suitable for pressure measurements in an internal combustion engine, in particular a diesel engine. In order to prevent contamination of the reflective membrane, the bore 2 will be sealed by a plug or head. This is temporarily removed during measurement. Because the fibre assembly is not inserted into the bore 2 in the embodiment according to Figure 2 (the membrane is provided at the head of the bolt), the fibre assembly can be replaced by a central light source and two concentric photodiodes. The fibre assembly or the central light source with photodiodes will normally only be placed in or near the bolt during measurement.

Claims: Optical pressure sensor for determining the variation of the pressure in an internal combustion engine, comprising a bolt (1) having a through bore (2), a membrane (3) which is mounted in the bore or at one end of said bore and at least one surface of which is reflective, means (5) for causing light to fall on the reflective membrane part and means (6, 7) for measuring the light reflected at the membrane or guiding it to a light-measuring device, characterised in that the means for causing light to fall on the reflective membrane part have the form of a ring or sheath (5), in that means (7 or 6, respectively) are situated both inside the ring or sheath (5) and around the ring or sheath (5) for measuring light reflected at the membrane or guiding it to a light measuring device, and in that the pressure drop across the membrane is determined from the ratio of the reflected light which is received via the means (6) provided around said ring or sheath (5) and the reflected light which is received via the means (7) provided in the ring or sheath (5). Optical pressure sensor according to Claim 1, characterised by an optical fibre assembly consisting of three concentric bundles of light-transmitting fibres (5, 6, 7). Optical pressure sensor according to Claim 1, characterised by an annular light source and two concentric photodiodes.

Inventors: KOELLING, ARIE JAN, Kölling, Arie Jan
Applicants: TEXAS INSTRUMENTS HOLLAND B.V.
Application year: 1990
Main classification: F02B77
IPC codes: G01L23, F02B3