Heater plug having a metal finger

This heater plug includes:

The present invention relates to a heater plug incorporating a force sensor to measure pressure inside a cylinder of an internal combustion engine.

In an internal combustion engine, in particular a diesel engine, each cylinder typically includes a heater plug that enables the combustion chamber of said cylinder to be heated, in particular when starting the engine. Such a heater plug is placed in a threaded bore passing through the cylinder head of the engine. It comprises firstly a body having an external thread able to cooperate with an internal thread formed in the bore, and secondly a finger intended to extend into the combustion chamber and in which a heating electrode is seated.

It is known to incorporate a force sensor, usually a piezoelectric sensor, in such a heater plug. Indeed, it has been noted that the knowledge of the pressure value inside each cylinder enables the combustion inside the engine to be controlled better. This information is in particular used to control the fuel injection in each cylinder. The fuel consumption of the engine can then be optimized and pollutant emissions reduced.

This sensor can be placed immediately above the heater finger of the plug.

Document FR-2 884 299 discloses a heater plug having a tubular body with a plug head and a fixing zone for fixing it to a bore, a finger mounted inside the plug at the extremity opposite the plug head and a force sensor. In such a plug, the finger is rigidly attached to the plug body in a connection zone and the plug body has, between the connection zone thereof with the finger and the fixing zone thereof in a bore, an elastically deformable part such that said connection zone is movable and can move longitudinally in relation to the supposedly fixed fixing zone. This elastically deformable part may be obtained by material thinning (in relation to the fixing zone). The force sensor is arranged between firstly an element rigidly connected to the connection zone and secondly a fixed element of the plug.

Thus, the elastically deformable part acts as a membrane which splits the plug body into two parts, a fixed part designed to be mounted in the cylinder head and a movable part subject to the pressure in a cylinder of the engine. This membrane is deformable and the movable part moves longitudinally. This movement, which is a function of the pressure in the cylinder, is then transmitted to the force sensor which can then provide an indication of the pressure exerted on the finger of the plug. The finger, the membrane and the force sensor, inter alia, thus form a force sensor. The movement of the membrane is not affected by the stresses in the cylinder head or in the rest of the plug body. Consequently, the measurement of the force sensor is independent of these stresses.

The heater plug described in document FR-2 884 299 includes a ceramic heater finger. Such fingers have a standard length of 35 mm and an outer diameter of 3.3 mm, i.e. smaller than normal metal heater fingers, which are around 50 mm long and have an outer diameter of around 4 mm.

The dimensions of normal metal heater fingers appear a priori incompatible with the incorporation of such a metal finger in a heater plug similar to the one described in document FR-2 884 299, particularly since the length of the finger protruding into the combustion chamber has to be limited. To address these problems, a new shorter metal heater finger could be designed, but the design and manufacturing costs of such a finger, combined with the probable drop in heating performance thereof, do not encourage this solution to be pursued.

The invention is intended to provide a heater plug with a built-in force sensor, which provides the advantages of the heater plug described in document FR-2 884 299 and which is more suitable for receiving a known standard metal heater finger.

Another objective of the invention is to disclose a heater plug with a built-in force sensor offering improved pressure-measurement reliability and/or accuracy and/or sensitivity compared to known plugs. Preferably, the heater plug will enable an efficient evacuation of the heat coming from the combustion chamber in order to limit the temperature around the pressure sensor. Also advantageously, the vibration of the finger of the plug will not adversely affect the pressure measurements taken by the sensor.

For this purpose, the invention discloses a heater plug comprising:A tubular body,A finger mounted inside the tubular body and protruding outside the body at one extremity of this body, andA pressure sensor,A membrane extending between the body and the finger enabling a relative longitudinal movement between the finger and the body.

According to the present invention, the pressure sensor is arranged between firstly a first bearing part connected via a first tubular part to the finger in the vicinity of the membrane and a second bearing part connected via a second tubular part to the body.

The structure of such a plug is entirely innovative. It enables a pressure sensor and the means for transmitting the pressure inside a combustion chamber to the sensor to be incorporated around a metallic finger (diameter 4 mm and length 50 mm). The structure disclosed makes it possible to not use (or hardly use) the finger, or an electrode, also referred to as a core, supplying it with electricity, to transmit the force exerted on this finger by the pressure to which it is subjected.

A preferred embodiment provides for the first tubular part to extend into the second tubular part.

The membrane may be rigidly connected to a first connection zone enabling the membrane to be fixed to the finger, and the first tubular part may be rigidly connected to the finger via a second connection zone less than 10 mm away from the first connection zone, preferably less than 7 mm from the first connection zone. Connecting the first tubular part to the finger in the vicinity of the connection zone between the finger and the second tubular part obviates the elasticity of the finger and limits the loads transmitted to the finger. Ideally, the above distance is zero.

Advantageously, the first tubular part conducts heat less than the second tubular part. In this case, the two tubular parts may be metal circular cylindrical parts, the thickness of the wall of the first tubular part may be between 0.05 mm and 0.2 mm while the thickness of the wall of the second tubular part may be between 0.4 mm and 1 mm.

In a preferred variant, limiting in particular the number of parts, the first tubular part is blocked and welded to the finger at a first extremity, and the second extremity thereof bears the first bearing part assembled freely about an electrode supplying the finger with electricity.

The second tubular part may be welded to the membrane at a first extremity, and the second extremity thereof may bear the second annular bearing part assembled freely about an electrode supplying the finger with electricity. In this case, the second tubular part is for example rigidly connected to the body at the first extremity thereof.

To act on the frequency modes of the finger and to ensure they do not adversely affect the measurement made by the pressure sensor, a filling of a material such as silicone gel may be provided between the pressure sensor, the first bearing part and the second bearing part on one hand and the electrode supplying electricity to the finger on the other. Furthermore, a filling of a synthetic material may be provided to create a seal around an electrode supplying electricity to the finger and around the other conductors coming out of the body.

Finally, the present invention relates to an internal combustion engine, in particular a diesel engine, having a heater plug such as the one described above.

FIG. 1is a longitudinal cross section of a heater plug according to the present invention. Conventionally, this heater plug comprises a body2, a finger4, a core6and a piezoelectric sensor8.

In the remainder of this description, the heater plug is described in a position presenting a substantially vertical longitudinal direction, the finger4protruding from the body2in the lower part of the plug. Thus, the adjectives “lower” and “upper” or similar used below refer to this position of the plug.

The lower part of the plug is therefore intended to be located in a cylinder of an internal combustion engine and the upper part of the plug, also referred to as the plug head10, is intended to protrude out of a cylinder head of said internal combustion engine.

InFIG. 1, the body2is shown schematically. It is a tubular body. InFIG. 1, it has a circular cylindrical shape. The thread enabling the plug to be screwed into a threaded bore (also not shown) of a cylinder head of an internal combustion engine is not shown here. Conventionally, the plug head10also has a part having a section with a hexagonal external contour enabling the plug in a cylinder head to be gripped, screwed and/or unscrewed. Such means are known to the person skilled in the art and reference is for example made here to FIG. 1 of document FR-2 884 299 cited in the preamble and illustrating a plug head having such fixing and gripping means.

The body2, also sometimes referred to as the envelope, in the area shown inFIG. 1, i.e. in the lower part thereof, has for example a wall thickness of around 0.6 mm.

The lower extremity of the body2is prolonged by an annular part12the assembly details of which are given below. This annular part12has a conical lower edge. This conical extremity14is intended to cooperate with a corresponding cone formed in a cylinder head in order to create a seal. When the plug is tightened into the corresponding threaded bore, the conical extremity14of the annular part12bears against the corresponding cone formed in the cylinder head thereby forming, as a result of the tightening of the plug, an excellent seal between the body of the plug and the cylinder head.

The finger4is mounted coaxially in relation to the body2and projects from the body2and from the annular part12thereby forming the lower part of the heater plug. The protruding part of the finger4is designed to sit in a combustion chamber of an engine.

This finger4is a “conventional” metal heater finger. It includes a resistor16placed inside a sheath18. The resistor16is supplied with electricity by the core6that extends into the body2and exits by the upper part of this body2.

There are two more tubular parts between the finger4and the core6on one hand and the body2on the other. There is therefore a first tube, hereinafter referred to as the elastic tube20, and a second tube, hereinafter referred to as the interface22, about the upper part of the finger4and the lower part of the core6.

FIG. 3illustrates the assembly of the elastic tube20and of the interface22in the lower parts thereof.

FIGS. 1 and 3show a first embodiment of the elastic tube20(a second embodiment being shown inFIG. 5et seq.).

In the embodiment inFIGS. 1 and 3, a ring24is welded to the sheath18of the finger4. The outer surface of this ring24has a shoulder26that receives the lower part of the elastic tube20. This elastic tube20bears against the ring24and is mounted away from the finger4coaxially to this finger. Moreover, the lower edge of the elastic tube20and the ring24are welded together.

FIGS. 1 and 3show that the interface22is assembled on the finger4via a part hereinafter referred to as the membrane28. This membrane28has a circular cylindrical tubular connection zone30adapted to the outer surface of the finger4such as to enable a force fit of the membrane28on the sheath18of the finger4. A laser-beam weld is provided between this first connection zone30and the finger4, or more specifically the sheath18of the finger4.

The zone enabling the connection between the elastic tube20and the finger4is referred to as the second connection zone32. In the embodiment inFIGS. 1and3, this second connection zone is therefore located level with the ring24.

A third connection zone is provided to join the membrane28to the interface22. This third connection zone34is formed at the level of a circular cylindrical tubular part of the membrane28. This third connection zone34is of greater diameter than the first connection zone30and is arranged above the first connection zone30. The first connection zone30is joined to the third connection zone34by the membrane itself. This latter has an inflection zone to join the first connection zone30to the third connection zone34.

The membrane itself, on account of the shape thereof and the thickness of the metal sheet used to make it, provides a relative flexibility enabling a movement of the first connection zone30in relation to the third connection zone34, which, as will be shown below, is assumed to be fixed. The flexibility of the membrane itself therefore enables a longitudinal movement of the finger4in relation to the body2, which is also assumed to be fixed.

By way of illustrative example, it can be seen that the relative movement between the first connection zone30and the third connection zone34, when the ring is mounted in an engine, is around 2 μm. More generally, the elasticity of the membrane is such that it enables a movement of the first connection zone30in relation to the third connection zone34of around 0.01 μm if a pressure of 1 bar is exerted on the finger rigidly connected to the first connection zone30. More generally, the first connection zone30may for example move relative to the third connection zone34by between 0.005 μm and 0.02 μm for one bar of pressure exerted on the finger4.

FIG. 2illustrates the assembly of the piezoelectric sensor8.

This latter comprises a piezoelectric ceramic36assembled between a first retaining ring38and a second retaining ring40.

The first retaining ring38is a circular cylindrical ring mounted like a cork at the upper extremity of the elastic tube20. A shoulder is formed around the periphery of the first retaining ring38to enable a perfect positioning of this ring in relation to the elastic tube20. These two parts are welded together, for example using a laser-beam weld. The first retaining ring38is tubular and enables the core6to pass freely through the middle thereof.

The piezoelectric ceramic36bears directly against the first retaining ring38. Above the piezoelectric ceramic36, there is conventionally a contact42enabling in particular the recovery of the electric signal provided by the piezoelectric ceramic36. In this case, the contact42is tube-shaped and flared at the lower extremity thereof, the flaring coming into contact with the piezoelectric ceramic36.

The second retaining ring40bears against the contact42via a grommet44.

The second retaining ring40has an external diameter that fits the internal diameter of the interface22and is mounted inside the upper extremity of this interface22. The interface22and the second retaining ring40are welded together, for example using a laser-beam weld. The second retaining ring40is prestressed before and during such welding such as to prestress the piezoelectric ceramic36.

It can be seen here how, compared to most heater plugs fitted with a pressure sensor in the prior art, the forces are not transmitted by the finger4and the core in this case. The piezoelectric ceramic36is not prestressed by pulling the core6or by tightening a nut engaged on the core6, but by bearing directly on the sensor.

FIG. 2, which illustrates a preferred alternative embodiment of the present invention, shows how a filling is provided around the piezoelectric sensor8. In the preferred embodiment shown, a silicone gel46is poured around the core6in particular to fill the free spaces above the finger4between the core6on one hand and the upper part of the elastic tube20, the first retaining ring38, the piezoelectric ceramic36and the contact42on the other.

In the upper part of the body2of the plug, a filling48of synthetic material is also provided to create a seal between the body2of the heater plug and the conductors coming out of this plug (including the core6).

To better illustrate a structure of a plug according to the present invention,FIGS. 4 to 10illustrate a part of a method used to create a preferred embodiment of such a plug.

The method illustrated starts with a finger4and the core6thereof. As mentioned above, the finger is a metallic finger, as opposed to a ceramic finger. Metal fingers are generally more bulky than ceramic fingers, but have the benefit of being cheaper. An elastic tube20′ is then slipped onto the finger4. An alternative embodiment is shown here. The elastic tube20′ in fact corresponds to the elastic tube20and to the ring24inFIGS. 1 and 3. In this case, the elastic tube20′ is restrained at the lower extremity thereof. As a result, in terms of operation, this is exactly the same as the first alternative embodiment described above. The elastic tube20′ is interference fitted to the finger4and is welded, for example using a laser-beam weld, to this finger4.

The membrane28is then placed on the finger4. The first connection zone30of this membrane28is then welded, for example by laser-beam welding, to the finger4. This first connection zone30is preferably as close as possible to the second connection zone32which is at the level of the weld between the elastic tube20′ and the finger4. Ideally, the first connection zone30overlaps the second connection zone32. Failing this, the distance separating the first connection zone30from the second connection zone32should be minimized. This distance is for example less than 10 mm, and preferably less than 7 mm. These remarks concerning the relative position between the first connection zone30and the second connection zone32also apply to the embodiment inFIGS. 1 to 3. In this first embodiment, there is therefore preferably a distance between the first connection zone30and the second connection zone32of less than 10 mm, and preferably than 7 mm. The distance separating the welds on the finger4may be used in this case.

The following stage then involves installing the first retaining ring38which is slipped onto the top of the plug about the core6to fit into the upper part of the elastic tube20′. A weld, preferably a laser-beam weld, is then created between the first retaining ring38and the elastic tube20′. The next stage involves installing the interface22which bears against the membrane28and is welded thereto, for example by laser-beam welding, level with the third connection zone34. Once the interface22is in place, the piezoelectric sensor8can be assembled. The piezoelectric ceramic36, the contact42, the grommet44and the second retaining ring40are then slipped in that order onto the core6. The second retaining ring is assembled inside the interface22, and it is prestressed downwards, i.e. towards the first retaining ring38. A weld between the interface22and the second retaining ring40enables a prestress to be maintained on the piezoelectric ceramic36.

The drawings do not show installation of the body2, the annular part12or the fillings provided around the plug head10.

As shown inFIGS. 1 to 3, the lower part of the interface22has a peripheral band50. The body2is mounted above this peripheral band50and butts against it while the annular part12is mounted above this peripheral band50and also butts against this band.

The operation of this heater plug with a pressure sensor according to the present invention is described below also with reference toFIG. 11. This latter illustrates very schematically the different elements used to measure the pressure in the cylinder. The pressure to be measured is symbolized by an arrow52. It is exerted on the finger4symbolized here simply by a line. In this case, it is assumed that the cylinder head of the engine is fixed. The heater plug is in contact with this cylinder head level with the thread formed on the outside of the body2and the conical extremity14.

The cylinder head is represented here on the left-hand side ofFIG. 11and is marked with reference54.

In this case, it is assumed that the body2is fixed since no stress is being exerted on it and it is mounted between two fixed zones, specifically the annular part12and the thread thereof (not shown). Consequently, the lower part of the interface22is fixed just like the third connection zone34(due to direct proximity of the body2).

FIG. 11also shows a very schematic representation of the elastic tube20(or20′), the piezoelectric sensor8and the interface22. The membrane28is also shown on another side ofFIG. 11.

FIG. 11shows that the membrane28joins the cylinder head54to the finger4, the third connection zone34being rigidly in contact with the cylinder head54. The interface22, the piezoelectric sensor8and the elastic tube20(or20′) are parallel to the membrane28.

The orders of magnitude below are given by way of non-limiting numerical examples. The following assumptions have been made:The tube20(or20′) has a measurement sensitivity K20 of between 10 and 20 N/μm,The piezoelectric sensor8has a measurement sensitivity K8 of between 200 and 250 N/μm,The interface22has a measurement sensitivity K22 of between 50 and 100 N/μm, andThe membrane28has a measurement sensitivity K28 of between 100 and 150 N/μm.

To obtain these values, it is assumed for example that the elastic tube20(or20′) has a wall thickness of around 0.1 mm and that the interface22has a wall thickness of around 0.5 mm. In this case it is important that the elastic tube20(or20′) be thinner than the interface22.

If it is also assumed that the membrane28enables a movement of the first connection zone30in relation to the third connection zone34of around 0.01 μm per bar of pressure, for a maximum pressure of 200 bars in the combustion chamber, a force of around 30 N may be exerted on the piezoelectric sensor.

A piezoelectric ceramic36having a sensitivity of around 20 pC/N ensures a system having a sensitivity of around 3 pC/bar.

The fact of having an elastic tube20(or20′) having a thin wall enables the heat conduction of this elastic tube20(or20′) to be limited. In the embodiment shown, it is estimated that around 90% of the heat coming from the combustion chamber is evacuated through the membrane28to the cylinder head. This protects the piezoelectric sensor8.

Moreover, with regard to measurement, the prestress exerted on the piezoelectric ceramic36is oriented in the opposite direction to the force to be measured. This limits the overall stress on the piezoelectric ceramic36when measuring pressure.

The presence of a silicone gel46and of the filling48enable the modes of resonance of the finger4and of the core6to be influenced. In this case, it is possible to discard the modes of the passband of the piezoelectric sensor8which is usually located below 5 kHz.

The present invention is not limited to the preferred embodiment and the variants thereof described above. It also concerns all of the alternative embodiments available to the person skilled in the art.

So for example, as suggested above, the elastic tube and the membrane could be joined to a single point on the finger of the heater plug.

Just as the ring enabling connection of the elastic tube to the finger of the plug can be removed by modifying the shape of the elastic tube, the membrane could also be incorporated into the interface.

In the embodiment described, it is assumed that the outer envelope of the plug is in two parts, the body and the annular part. A one-piece external envelope could also be used for a plug according to the present invention.