GB2344179A - Spark plug with combustion pressure sensor - Google Patents

Abstract

A spark plug with combustion pressure sensor in which the metal outer shell 10 of the spark plug has a diaphragm 18 exposed, in use, on its inner side 20 to pressure within a combustion chamber of an internal combustion engine. A transducer 23, preferable a thick film resistor device 24,25 responds to mechanical deformation of the diaphragm 18. The outer side 26 of the diaphragm 18 communicates with a recess 16 in the threaded portion 11 of the shell 10. The outer surface 26 of the diaphragm is exposed to combustion pressure in the bore of the spark plug. Although there is no hermetic gas seal around the spark plug threads, it has been found that the diaphragm will deform in proportion to in-cylinder pressure when the spark plug is subject to dynamic pressure changes.

Description

Spark Plug with Combustion Pressure Sensor Field of the invention This invention relates to a spark plug for an internal combustion engine with a combustion pressure sensor integrated into the spark plug.

Background of the invention Modern spark ignition internal combustion engines are controlled by an electronic control unit which receives signals from several sensors mounted on the engine and its exhaust system. The control unit processes the inputs from the sensors and controls fuel injection and ignition timing so that the engine produces the required power at acceptable levels of exhaust emissions. It is well known that a sensor which directly monitors in-cylinder pressure would enable improved engine control. At present combustion is sensed indirectly, for example by transducers mounted on the outside of the engine. Such sensors pick up noise from other sources than combustion. Noise levels can be of comparable amplitude to signals arising from combustion presenting challenging problems of discrimination between signal and noise. Known in-cylinder sensors are too expensive or insufficiently durable for volume production and are presently used mainly as development tools.

A spark plug with combustion pressure sensor is known from EP 0 441 157 A2. This patent publication describes a spark plug with central electrode offset to make room for a pressure reference chamber, a diaphragm forming a wall between the reference chamber, the opposite side of the diaphragm from the reference chamber being exposed to pressure in the combustion chamber. The diaphragm is displaced by combustion pressure variations and a strain gauge is mounted on the diaphragm to detect the diaphragm deflections.

Object of the invention It is an object of the present invention to provide a spark plug with combustion pressure sensor that is an improvement over that described in EP 0 441 157 A2, particularly as to cost of manufacture.

Summary of the invention According to the present invention, a spark plug with combustion pressure sensor comprises a metal shell having a threaded end for mating with a spark plug bore in the cylinder head of an internal combustion engine, a high tension electrode within the shell, an insulator between the shell and the high tension electrode, an earth electrode mounted on the shell and forming a spark gap with one end of the high tension electrode, the shell having a diaphragm arranged to be exposed, in use, on one side to pressure within a combustion chamber of the internal combustion engine, and transducer means responsive to mechanical deformation of the diaphragm, characterised in that the other side of the diaphragm communicates with a recess in the threaded portion of the shell.

In use, with the spark plug installed in a cylinder head of an internal combustion engine, the recess forms a reference cavity closed by the threaded bore into which the spark plug is mounted. The recess need not be hermetically sealed from the combustion chamber. The passage of gases between the threads of the spark plug and head into and out or the recess takes place sufficiently slowly that the diaphragm responds to dynamic pressure changes in the combustion chamber. The device is particularly effective at higher engine speeds. If an improved response at lower engine speeds, lower than about 1000 rpm, is required, the thread turns between the recess and the end of the shell may be coated with a sealant.

Preferably, there are at least two complete thread turns between the recess and the inner end of the shell.

The diaphragm may be formed integrally with the shell or welded to the shell.

The transducer is preferably a thick film sensor bonded or clamped to the diaphragm.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an axial section of the pressure sensor spark plug of the invention showing the location of the diaphragm; Figure 2 is a detail section of the sensor area of the spark plug of Figure 1 ; Figure 3 is view similar to that of Figure 2 but of an alternative embodiment in which the diaphragm is welded to spark plug shell; and Figure 4 shows the output signal of the combustion pressure sensor of the invention compared with that of the well known Kistler sensor over a number of cycles of operation of an internal combustion engine.

Detailed description of the drawings Referring now to Figure 1, a spark plug with combustion pressure sensor comprises a steel shell 10 having a threaded end 11 for mating with a spark plug bore 37 in the cylinder head of an internal combustion engine (not shown), a high tension electrode 12 within the shell 10, an insulator 13 between the shell 10 and the high tension electrode 12, and an earth electrode 14 mounted on the shell 10 and forming a spark gap 15 with one end of the high tension electrode 12.

As so far described the spark plug is entirely conventional and insulator and centre electrode components may be identical to those used for spark plugs without pressure sensor.

The shell 10 is formed with a recess 16 in its threaded outer surface 17. There are at least two complete turns of the thread between the recess 16 and the spark gap end of the spark plug. A diaphragm 18 separates the recess 16 from the interior 19 of the spark plug so that the inner surface 20 of the diaphragm is exposed, in use, to pressure within a combustion chamber of the internal combustion engine.

In use, with the spark plug installed in a cylinder head of an internal combustion engine (not shown), the recess 16 forms a reference cavity closed by the threaded bore 37 into which the spark plug is mounted. The recess need not be hermetically sealed from the combustion chamber.

The passage of gases between the threads of the spark plug and head into and out of the recess 16 takes place sufficiently slowly that the diaphragm responds to dynamic pressure changes in the combustion chamber. The device is particularly effective at higher engine speeds. If an improved response at lower engine speeds, lower than about 1000 rpm, is required, the thread turns between the recess and the end of the shell may be coated with a sealant.

Referring now to Figure 2, transducer means responsive to mechanical deformation of the diaphragm comprises a thick-film pressure sensor circuit 22 located within the recess 16. A substrate 23,96% alumina, has two ruthenium dioxide based thick-film resistors 24,25, one on each surface, connected by platinum/gold conductor tracks, and forming a half Wheatstone bridge circuit. The substrate 23 is covered with a glaze for environmental protection. The inner resistor 24 is used to detect diaphragm deflection, and the outer resistor 25 is used for temperature compensation (optional). The inner resistor 25 has additional coatings of glaze so that it stands proud of the surrounding circuit by 20pm. This ensures that diaphragm displacement applies pressure against the inner resistor alone rather than being distributed over the whole circuit.

The transducer circuit may be mounted on the diaphragm to pick up its deflection in alternative ways: Method A The transducer circuit 22 is glued to the outer surface 26 of the diaphragm 18 with high temperature epoxy, and covered with a protective layer of high temperature silicone potting compound. This method is not shown in the drawings.

Method B The transducer circuit 22 may be clamped to the diaphragm outer surface 26 using a plate laser welded to the shell 10 as shown in Figure 2. A screw 28 in the plate 27 is tightened down onto the central resistor, and load is transmitted through the"z"axis of the resistor (the"z" axis is perpendicular to the plane of the diaphragm 18). The additional coating of glaze over the resistor 24 on the inner side of the substrate 23 ensures load transfer through the resistor 24.

Method C In a different configuration, shown in Figure 3, a steel substrate 29 forming the diaphragm 18 bears a dielectric layer 30 on which is deposited a thick film sensor 31. The steel plate 29 is fixed to the shell in a gas tight manner by laser welding around its outer edge and the whole sensor is covered by silicone potting compound. In this embodiment, differential pressure between the spark plug interior 19 and the spark plug threads causes flexing of the diaphragm 18, and strain of the thick film resistor along both x and y axes.

In each of the embodiments described, electrical connections to the transducer 22 are provided by sensor wires 33 which pass from the transducer 22 along a groove 34 in the threaded area of the shell 10 and through a sealed bore 35 which passes behind the sealing surface 36 of the shell 10. The sensor wires are connected to a conventional resistance measuring circuit whereby variation of resistance provides an indication of thick film resistor deformation, hence diaphragm deformation and hence in cylinder pressure variation.

Figure 4 shows the results of tests of the pressure sensor spark plug of the invention. Cylinder number 2 of a spark ignition internal combustion engine was fitted with the pressure sensor spark plug of the invention and also with a Kistler sensor so that the performance of the sensor of the invention could be compared with that of the Kistler.

Comparison of the 2nd and 3rd lines of the Figure 4 graph shows clearly how well the invention correlates with a Kistler sensor.

It is a characteristic of the embodiments described that the outer surface the diaphragm is exposed to combustion pressure in the bore of the spark plug. Although there is no hermetic gas seal around the spark plug threads, it has been found that this diaphragm will deform in proportion to in-cylinder pressure when the spark plug is subject to dynamic pressure changes. The absence of a pressure reference cavity completely isolated from cylinder pressure ensures ease of fabrication and low cost.

Claims (9)

1. CLAIMS 1. A spark plug with combustion pressure sensor comprising a metal shell (10) having a threaded end (11) for mating with a spark plug bore in the cylinder head of an internal combustion engine, a high tension electrode (12) within the shell (10), an insulator (13) between the shell (10) and the high tension electrode (12), an earth electrode (14) mounted on the shell (10) and forming a spark gap (15) with one end of the high tension electrode (12), the shell (10) having a diaphragm (18) arranged to be exposed, in use, on one side (20) to pressure within a combustion chamber of the internal combustion engine, and transducer means (23) responsive to mechanical deformation of the diaphragm (18), characterised in that the other side (26) of the diaphragm communicates with a recess (16) in the threaded portion (11) of the shell (10).
2. 2. A spark plug as claimed in claim 1 in which there are at least two complete thread turns between the recess (18) and the spark gap end of the shell (10).
3. 3. A spark plug as claimed in claim 1 or claim 2 in which the diaphragm (18) is formed integrally with the shell (10).
4. 4. A spark plug as claimed in claim 1 or claim 2 in which the diaphragm (18) is welded to the shell (10).
5. 5. A spark plug as claimed in any one of claims 1 to 3 in which the transducer means (23) comprises a thick film sensor (24,25).
6. 6. A spark plug as claimed in claim 5 in which the thick film sensor (24,25) is bonded to the diaphragm (18).
7. 7. A spark plug as claimed in claim 5 in which the thick film sensor (24,25) is clamped to the diaphragm (18).
8. 8. A spark plug and claimed in any one of the preceding claims in which the threaded portion (11) is coated with a sealant.
9. 9. A spark plug with combustion pressure sensor substantially as hereinbefore described with reference to and as shown in the accompanying drawings.