PRESSURE SENSOR This invention relates to a pressure sensor. In particular it relates to a pressure sensor for use for measurement of pressure in combustion chambers of engines. Electronic engine management is increasingly used in engines of modern vehicles to maximise power output and minimise fuel consumption and the output of emissions. Typically, the engine management system takes information including engine speed, position, temperature, inlet air pressure/temperature or inlet air mass from sensors connected to the engine. This information is used to calculate a "best guess" at the operating conditions within the combustion chambers (cylinders) of the engine which is a guess which is averaged over many cycles of the engine and over the different cylinders of the engine. Engine load is typically determined from measurements of the mass of air drawn into the inlet manifold of the engine. The ignition spark timing and fuel delivery are determined from algorithms that process external data such as engine speed, engine position and engine load. These methods result in some error in controlling the engine spark and fuel delivery. The error is worst during transients in the engine load.

The present invention seeks to provide better real time information relating to the operating conditions within the combustion chamber and information which can be specific to particular cylinders of the engine since in an engine all the cylinders do not behave identically.

Thus, the present invention broadly relates to a pressure sensor for insertion into a combustion chamber which is capable of measuring high gas pressures at high temperatures to produce a signal which is directly related to the pressure in a combustion chamber of that engine.

The provision of such a signal measuring the rise of pressure within the cylinder from the start of an intake

stroke through the entire power stroke of the engine provides a powerful tool for the designer of engine management systems to allow the engine management system to know the operating conditions within each cylinder of the engine in real time: such a sensor providing in-cylinder information from each cylinder of the engine can replace less accurate mass intake air flow measurement techniques. This allows more accurate calculation of critical engine management parameters such as fuel/air ratio and ignition spark timing. Improvements would be most evident during sudden engine transients when the advantage of real time data allows the engine management system to track the requirements of the engine with only a one cylinder cycle delay. In cylinder pressure data can also be used to determine the onset of engine knock or detonation. This allows the engine to develop maximum power without detonation.

In one aspect of the present invention there is provided a sensor including: a photoelastic medium; means for producing polarised light and directing the same through said photoelastic medium; and detecting means for measuring the alteration in polarisation of the light passing through said photoelastic medium, the arrangement being such that in use the pressure sensor can be disposed so that the photoelastic medium is subject to the pressure in the combustion chamber, changes in pressure on the photoelastic medium causing changes in the light transmission properties of the medium and hence the light detected by the detecting means produces a signal indicative of the pressure in the combustion chamber. The sensor is based on the photoelastic effect in

which an isotropic material can become anisotropic when subjected to an applied stress or induced strain through an elastooptic interaction known as stress birefringence. The sensor utilises the change in polarisation of light as the polarised light passes through the photoelastic material or medium exposed to the pressure to be measured. The light is initially polarised before entering the medium. The polarisation may be set such that components of the polarisation are both perpendicular and parallel to the direction of the applied stress on the medium.

The birefringence induced by the applied pressure causes the components parallel to the direction of the applied stress in the material to see a different refractive index to the components perpendicular to the direction of applied stress. This results in the two components travelling at different speeds. This induces a net rotation in the plane of polarisation of the light which rotation is proportional to the distance travelled through the stress medium.

The light is then passed through an analyser consisting of a second polariser. The intensity of the light passing through the analyser is dependent on the plane of polarisation of the light exiting the stressed medium, hence the intensity of light out of the analyser is dependent on the pressure applied to the medium.

In a preferred embodiment the medium is a block of glass. Most glasses are suitable for use as the medium including fused silica, however, one preferred material is quartz.

In a preferred embodiment the quartz is in the form of a rod defining a longitudinal axis, disposed inside a generally tubular metal housing, the rod being fixed in a first end of the housing at which end the means for producing polarised light and the detecting means are

located, the second end of the housing being open to allow pressure to act along the longitudinal axis of the rod and compress the rod in that longitudinal direction.

In use, the pressure sensor is screwed or otherwise fitted into a combustion chamber with the second end located inside the chamber. Typically, the means for producing polarised light comprise a light source such as a light emitting diode (led) and a polarising film.

The pressure of combustion and compression inside the chamber may be directly exposed to the second end of the quartz rod or alternatively may be transmitted to the rod by a mechanical linkage such as a push rod, lever system or by a fluid link.

Suitable light sources can include light incandescent globes or laser diodes as well as leds. The photoelastic effect is dependent upon the wavelength of light passing through the medium so single wavelength sources are preferred. The light source may be mounted inside the metal housing of the sensor or, alternatively it may be fed to the sensor by an optical fibre.

The polarising and analysing elements may be provided by separate sheets of polarising material placed adjacent opposite sides of the medium; alternatively they may be fabricated onto the sides of the medium. The sensor may use only one polariser that doubles as an analyser by reflecting the light by fresnel reflection and/or coatings applied to the opposite side of the material. The light would then pass through the block twice and could leave the sensor along the same optical fibre on which it entered.

If the sensor uses a separate polariser and analyser rotated with respect to each other by 90 then the sensor would give zero output intensity for zero applied pressure. If the sensor uses only one polariser which doubles as an analyser then the sensor would give maximum

output for zero applied pressure.

The light intensity out of the analyser could either be converted into an electrical signal by, for example, standard photo detectors such as pin diodes or phototransistors or alternatively it could be further processed as an optical signal.

The detector may be mounted inside the sensor housing; alternatively it may be remotely mounted and fed the light from the analyser by an optical fibre. In one particular preferred embodiment the pressure sensor may be incorporated into an ignition spark plug. In this embodiment the material may be a cylinder of quartz having a central bore in which the copper core of the spark plug passes and may be surrounded by a metal housing which also forms part of the spark plug.

Optical fibres or the like may be incorporated into the housing to provide a path for light passing through the quartz tube. Because of the location of the copper core running through the centre of the quartz tube the optical path is offset from the centre of the tube. The optical fibre may be contained in the high tension leads that connect the ignition cords or distributor to the ignition spark plugs.

In a further preferred embodiment the ignition driver, ignition coil, spark plug and pressure sensor are integrated into a single assembly so that the spark may be automatically triggered at some predetermined in cylinder pressure.

Specific embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which:-

Figure 1 shows a first pressure sensor in which a polarised light source and detector are located in a single sensor housing; Figure 2 is a schematic representation of components

for a pressure sensor in which a light source and a detector are remotely mounted; and

Figure 3 shows a pressure sensor incorporated into a spark plug; and Figure 4 shows a similar pressure sensor to that of figure 3.

Referring to the drawings, Figure 1 shows a pressure sensor 1 for insertion into the combustion chamber of an engine. The sensor includes a metal housing 2 having a generally annular cross-section inside which there is a 12.5mm diameter quartz rod 4 defining a central longitudinal axis 5.

At a first end 3 of the sensor on one side of the quartz rod 4 there is an led 6 to provide a light source and a first polariser 8 comprising a film of polarising material; on the opposite side of the rod there is a second polariser 10 which acts as an analyser and a photovoltaic cell or detector 12.

The other (second) end 14 of the housing is covered by a copper crush washer 16, the diameter of the central hole 18 of the copper crush washer being smaller than the diameter of the quartz rod which helps retain the quartz rod in the housing but which allows pressure to impinge directly on free end 19 of the quartz rod though the central hole 18. Blow holes 20 are provided to relieve any pressure built up between the quartz rod and the inside of the housing. The housing includes means (not shown) for screwing or otherwise fitting the pressure sensor into a combustion chamber as well as means for compressing or prestressing the quartz rod by means of an adjustment bolt 21 which is provided for that purpose.

The pressure sensor is fitted in a combustion chamber with the second end of the housing including the blow holes 20 inside the chamber and the first end 3 located outside the chamber.

In use, the light source 8 and polariser 6 combine to produce polarised light such that the components of the polarisation are perpendicular and parallel to the longitudinal axis of the rod. When the light passes through the quartz rod through the second polariser to the detector 12 pressure acts on the free end 19 of the rod and this compresses the quartz rod along the longitudinal axis and causes the components of polarisation parallel to the direction of the axis to see a different refractive index to the components perpendicular to the axis which induces a net rotation in the plane of polarisation of the light. After travelling through the rod the light passes through the analyser 10, the intensity of the light passing through the analyser is dependent on the plane of polarisation of the light exiting the quartz rod hence intensity of light out of the analyser 10 is dependent on the pressure applied to the quartz rod, in the manner discussed in the introduction to this application. The light may then converted to an electrical signal by the photovoltaic cell 12 whose signal depends on the light intensity and the resultant electrical signal can then be analysed by an engine management system.

Figure 2 illustrates one possible design of pressure sensor where the source and detector are remotely mounted from the sensor. A light source 24 passes a light through a 50% transmission/50% reflection mirror 26 angled at 45 to the direction of travel of the light beam, to a lens 28 which focuses the light and then passes it into a multimode optical fibre 30 and the light is transmitted via the fibre to a GRIN (graded refractive index) lens 32 where it passes through a polariser 34 through a quartz sensor element 36. After passing through one side of the element the light is reflected back through the sensor element by means of a mirror 38 disposed on the opposite side of the sensor element, the light passes back through

the polariser 34 back through the lens 32 through the fibre 30 to lens 28 where it is reflected by the mirror 26 to a detector 40.

In a more practical embodiment the mirror and lens 28 which act as an optical splitter would be replaced by a twisted fibre optic equivalent to make the system less vibration sensitive.

Figure 3 shows a combined pressure sensor and spark plug 50 in which a 3mm diameter core 52 of the spark plug 50 is encased in a quartz tube 54 having a 9mm outside diameter and a 3mm inside diameter which is in turn mounted in a metal housing 55. An optical path 56 is provided through the housing to allow light to pass through the quartz sensor element 54 via polarisers (58, 60) placed on opposite sides of the quartz tube. The optical path is offset from the diameter of the quartz tube due to the location of the copper core passing through the centre of the tube. A metal C ring 59 located just above the optical path, as oriented in Figure 3, fixes the quartz tube in the housing to enable the tube to be compressed by pressure acting on the free end 61 of the rod.

Figure 4 shows a combined pressure sensor and spark plug 60. This is similar to that shown in figure 3 except that the light to and from the sensor is carried in multimode optical fibres incorporated into the high tension leads of the ignition system.

The high tension spark is carried though a conducting core 61 of a high tension cable 62. Light passes from a remotely mounted source (not shown) through an optical fibre 63 to a grin lens 64. The grin lens 64 expands the light beam from the optical fibre. The light is then turned through 90 degrees by a prism 65 to pass through a polariser 70 and then through a quartz rod 67 along optical path 66 which is at 90 degrees to the axis of

applied pressure on the quartz rod. After passing through the quartz rod, the light passes through polariser

(analyser) 71 before being turned through a further 90 degrees by a further prism 68 and collected by grin lens 69. The grin lens 69 focuses the light into optical fibre

72 which passes the light to a remotely mounted receiver

(not shown) .

This embodiment of the sensor also shows a ceramic end 73 attached to the quartz rod. The ceramic acts as a push rod which applies pressure on the quartz rod while protecting the end of the quartz rod from damage. The ceramic could be bonded to the quartz rod using techniques such as fritting or gluing.

The ceramic and quartz can be bonded to the conducting core with a gap in the core allowing both movement and conductivity achieved by mechanical means such as insertion of a spring.

The quartz may also be rigidly attached to the outer metal housing with pressure being transferred at the centre of the quartz by means of a push rod to a fraction of that part of the cross-sectional area designed to accept pressure and compression at the centre within the material to provide the required bi-refringence effect. Although the design shows a spark plug in which remotely mounted optical sources and detectors would be used it would also be possible to use directly mounted optical sources and detectors.

The devices described above are particularly suited for testing and research and development purposes. The data provided by the devices can give a means to calculate engine power.

The invention has been described with reference to the use of the pressure sensor in combustion chambers/cylinders of engines. However the sensor can be adapted for use in other control or measurement

applications where a pressure sensor of rugged design is required.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.