GB2150693A - Internal combustion engine and sensing device - Google Patents

Abstract

An internal combustion engine cylinder head has a wall 10 in which is formed an aperture 11, preferably a core aperture, opening into a coolant space. A sensor is mounted in the space and has a diaphragm 13 exposed to the coolant 12. A sensing element 15 is mounted on the diaphragm and from the element an electrical signal can be obtained representative of the combustion noise in the engine cylinders, the coolant acting to transmit such noise to the diaphragm, which is preferably tuned to the dominant acoustic resonance of the combustion chamber. <IMAGE>

Description

SPECIFICATION Internal combustion engines This invention relates to a liquid cooled moving piston internal combustion engine of the kind having associated therewith a sensing device from which can be derived a signal representative of operating conditions within an engine combustion chamber.

In the case of a spark ignition engine the sensing device is commonly known as a "knock sensor" and it is used to detect when detonation of the fuel/air mixture in a combustion chamber of the engine is taking place.

The signal provided by the sensing device is utilised to control the timing of the spark and if detonation starts to take place the timing of the spark is retarded. The prevention of detonation is particularly important in a spark ignition engine since such engines are not designed to withstand the very high pressures or the rapid rates of rise of pressure which are generated when detonation occurs. In a compression ignition engine the situation is somewhat different since detonation of the fuel/air mixture often takes place during normal operation of the engine and the engines are designed to cope with the high pressures and high rates of rise of pressure which are generated. Nevertheless, the sensing device can provide useful information such for example as the start of combustion of fuel which can be utilised to adjust the timing of fuel delivery.

Two categories of sensing device are known, the first of which comprises devices which have direct access to the cylinder or combustion space. Such devices can only provide signals relative to the particular cylinder or combustion chamber and therefore if it is required to monitor each cylinder or combustion chamber of the engine, as many devices as there are cylinders or combustion chamber of the engine must be provided.

Moreover, since they have direct access to the cylinder they are subject to the harsh working conditions in the cylinder. Furthermore, in order to mount such devices it is necessary to machine a bore in which the device can be mounted.

The other category of device senses combustion events indirectly and suffers from a poor signal/noise ratio because they tend also to pick up valve closure impacts, bearing noise and piston slap etc. Such devices are attached to the cylinder head of the engine for example on the external wall of the cylinder head. In an effort to improve the performance of such a device, it is known from European Published Application 0079042, to mount the device on a boss which is formed on the internal wall section of the engine block, the boss extending through the water space and the outer wall section having an aligned aperture. The device is screwed into the boss and has a sealing member which engages the outer wall to prevent escape of water.The device is sensitive to events in the particular cylinder with which it is associated but will in general not be responsive to events in other ones of the cylinders.

The present invention is concerned with a sensor of the second category and the object of the invention is to provide an engine of the kind specified in a simple and convenient form.

According to the invention in an engine of the kind specified said device comprises a diaphragm, means for supporting the diaphragm in a wall of the engine, the internal surface of the wall defining a liquid coolant space whereby one face of said diaphragm is in contact with the coolant, the coolant acting to transmit to the diaphragm combustion noises from the engine combustion chambers and means coupled to the diaphragm for providing electrical signals representative of the noise.

According to a further feature of the invention said diaphragm is mounted in a core aperture of the cylinder head of the engine.

According to a still further feature of the invention the device is made more responsive to combustion noise by tuning the diaphragm to the dominant acoustic resonance of the combustion chamber.

In the accompanying drawings: Figures 1-7 show examples of sensing device shown mounted in the wall of a cooling space of an engine, and Figure 8 shows a part section of an engine.

Referring to Figure 8 an engine has a cylinder block 1 upon which is mounted a cylinder head 2. A cylinder 3 in the block is occupied by a piston 4 coupled to the crankshaft of the engine in the usual manner. The cylinder head defines a combustion space 5 in which fuel is burnt. The cylinder block defines a space 6 for coolant and a similar space 7 is defined in the cylinder head. The space 7 may be constructed as a single space extending the length of the cylinder head or a series of interconnected spaces associated with the combustion spaces 5 of the engine. The exterior wall 10 of the coolant space 7 in the cylinder head is provided with an aperture 11 which conveniently is an aperture formed during the production of the cylinder head by the fact that there extends therethrough a core support. In use, coolant indicated at 12 is in contact with the wall.In conventional constructions the aperture 11 would be closed by a so-called core plug which conventionally consists of a disc of material which can be placed in the aperture and subsequently deformed so that it is self-retaining within the aperture.

A sensing device comprising a diaphragm 1 3 having an integral skirt portion 14, is a push fit within the aperture and forms a liquid tight seal with the wall thereof. An adhesive incorporating filler material may be utilized to retain the diaphragm in position. The diaphragm is constructed from material which is corrosion resistant and may for example be formed from metal, ceramic, glass, hydrocarbon plastics, silicone elastomers etc.

Secured to the diaphragm is a sensing element 1 5 which can be of any convenient type for example, a piezo-electric, piezo-resistive or foil strain gauge. The thickness and form of the diaphragm is adjusted to provide a natural resonant frequency which coincides as nearly as possible with the natural frequency of the dominant acoustic resonance of the combustion spaces. For diesel engines, the dominant resonance occurs at 5 kHz in engines with a bore diameter of 100 mm and at proportionally higher frequencies for engines with smaller cylinder bores. The natural frequency of this resonance might rise to 6.5 kHz for a spark ignited petrol engine with a 76 mm bore. In larger engines particularly where the combustion chamber is hemispherical, the major acoustic mode has a wavelength equal to twice the diameter of the combustion chamber.In engines which employ pre-chambers, the pre-chamber may act as a Helmholz resonator with the gas in the main chamber providing an extra stiffness component. Calculation of these natural frequencies is possible but complicated and verification by experiments is advisable.

The construction of device as shown in Figure 1 limits the sensing element 1 5 to one which can withstand flexure of the diaphragm when the device is located in the aperture.

Furthermore, if the diaphragm becomes dished during assembly, its natural frequency is raised. One way of overcoming this difficulty is to utilise the construction shown in Figure 2 in which the diaphragm is initially provided with a dished portion 16. In this case a piezoelectric ceramic sensing element is located in the dished portion of the diaphragm and may be retained in position by means of a light coiled spring 18, the latter also serving to effect electrical connection to the element. As shown in Figure 2, the spring engages a support member 1 9 which is insulated to allow an electrical connection to be made to the spring. Before the ceramic element is located in position, an electrically conductive adhesive is placed in the dished portion of the diaphragm, the adhesive being chosen to cure above 50'C.Moreover, the adhesive is such that it will accommodate distortions such as occur during assembly of the diaphragm in the aperture 11. When the engine is operated the increasing temperature of the coolant causes the adhesive to cure.

The effect therefore is that the ceramic element is not subject to high stress as a result of assembly.

An alternative arrangement is seen in Figure 3 where the diaphragm 20 is isolated from the flange 14 by a convoluted portion 21 which ensures that when the device is inserted into the aperture the diaphragm 20 is not subject to any significant stress. In this case the ceramic element 1 7 can be secured to the diaphragm before assembly. The constructions of Figures 2 and 3 illustrate covers 22 which carry electrical connectors 23 whereby electrical connections can be made to external apparatus. It is possible however to accommodate a head amplifier beneath the cover, the head amplifier having a low output impedance so as to minimise pick-up from electrostatic and electro-magnetic fields through which the connections to the external apparatus might pass. Preferably the head amplifiers provide some band pass filtering.

Alternative constructions are seen in Figures 4 and 5. In Figure 4 the skirt portion 14 is not used for mounting purposes but is provided with an annular outwardly extending flange 24 which can be secured by suitable fastening devices to the exterior surface of the wall of the cylinder head. A sealing ring 25 is provided between the junction of the skirt and the flange to prevent escape of coolant.

In the construction shown in Figure 5, the diaphragm 26 is mounted on s cup-shaped support member 27, by means of an annular elastomeric mounting member 28 which isolates the diaphragm from any slight distortion of the support member 27 when it is pressed into position in the aperture.

In the arrangement shown in Figure 6, the diaphragm 29 is supported relative to a rigid cupshaped backing plate 30. by an annular column of piezoelectric material in this case comprising two rings 31. 32 of ceramic material. The face of the ring 31 is electrically coupled to the backing plate 30 and the opposite side face of the ring 32 is electrically connected to the diaphragm which is also connected at its centre to the backing plate by means of a central screw 33. A central annular collecting member 34 is interposed between the two rings which are so polarised that upon compression they both contribute to the signal available to the ring 34. Sealing and damping of the diaphragm is provided by an annular gasket 35 interposed between the rings 31 and 32 and the skirt of the backing plate 30. In the modification shown in Figure 7 the diaphragm 36 is of cup-shaped form with its skirt portion 37 terminating short of the base wall of the backing plate 30. In this case sealing is achieved by means of an annular gasket 31A. If desired however the diaphragm may be convoluted as in the example of Figure 3 and the peripheral edge of the diaphragm secured directly to the skirt portion of the support member.

The sensing element may be formed from semiconductor material mounted on the dia phragm. Such material while providing a high output signal does suffer from the disadvantage that its output varies with temperature.

This may be minimised by supplying the sensing element with current by way of the collector emitter path of a transistor the base of which is held at a constant potential. The transistor is housed adjacent the element and conveniently is formed from the same material as the element or from a material which provides compensation by the change in transistor parameter "Vbe" with change in temperature, for the change in the gauge factor of the semi-conductor element with temperature.

An amplification stage may be provided in close proximity to the element, the amplifying stage having an input impedance at least 100 times the resistance of the element by way of a capacitor. The value of the capacitor and the input impedance can be chosen to provide a high pass filter to prevent low frequency signals overloading the amplifying stage.

Claims (11)

1. A liquid cooled moving piston internal combustion engine having a sensing device from which can be derived a signal representative of operating conditions within an engine combustion chamber, said sensing device comprising a diaphragm, means for supporting the diaphragm in a wall of the engine, the internal surface of the wall defining a liquid coolant space whereby one face of said diaphragm is in contact with the coolant, the coolant acting to transmit to the diaphragm combustion noises from the engine combustion chambers and means coupled to the diaphragm for providing electrical signals representative of the noise.

2. An engine according to Claim 1 in which said diaphragm is mounted in a core aperture of a cylinder hnead of the engine.

3. An engine according to Claim 1 or Claim 2 in which the diaphragm is tuned to the dominant acoustic resonance of the combustion chamber.

4. An engine according to Claim 1 in which said diaphragm has an integral skirt portion which is a push fit in said aperture and forms a liquid seal therewith.

5. An engine according to Claim 4 including a sensing element secured to the diaphragm.

6. An engine according to Claim 5 in which the diaphragm is of dished form, the dished portion of the diaphragm locating said sensing element, and an adhesive mass for securing the sensing element within the dished portion of the diaphragm the curing temperature of the adhesive mass being such that the adhesive mass is cured only when the engine is first operated.

7. An engine according to Claim 5 in which the diaphragm is connected to the skirt portion by a convoluted portion which acts to isolate the diaphragm from stress generated when the device is fitted within the aperture.

8. An engine according to Claim 1 in which said device comprises a backing plate of cupshaped form the skirt of said backing plate being secured within the aperture, the diaphragm comprising a disc, a screw connecting the centre of the diaphragm to the centre of the cup-shaped member, an annular sensing element located between the diaphragm and the base wall of the backing plate and means for effecting a seal between the periphery of the diaphragm and the backing plate, said means also acting to damp the movement of the diaphragm.

9. An engine according to Claim 8 in which said annular sensing element comprises a pair of rings of ceramic material positioned on opposite sides of an annular collecting member said rings being polarized so that upon movement of the diaphragm, both rings contribute to the output signal developed at the collecting member.

10. An engine according to Claim 1 in which said diaphragm is of cup-shaped form having an outwardly directed flange which is secured to the exterior wall of the engine surrounding said aperture.

11. A liquid cooled multi-cylinder reciprocating piston internal combustion engine substantially as hereinbefore described with reference to the accompanyinq drawings.