Patent Description:
Cylinder pressure is an important control parameter for closed loop control of combustion engines. Existing cylinder pressure sensors, typically based on either piezoelectric or piezoresistive measurement, suffer from poor reliability and longevity. In many cases the operational life time of existing cylinder pressure sensors is a tenth of the expected time between major engine overhaul. The poor reliability and longevity of current sensors causes drastic increases in maintenance costs and increases out of service times for engines. As an example, existing cylinder pressure sensors have a typical lifetime of about <NUM> hours and the target for marine applications of such sensors is <NUM> hours.

Typically cylinder pressure sensors of the piezoresistive varieties include a strain gauge which is attached to a bending membrane. The bending membrane bends the gauge and the electrical resistivity of the sensor gauge changes. This change in the electrical resistivity can be measured. Piezoelectrical sensors include a piezoelectrical ceramic which is compressed with pretension in between a moving membrane and a rigid support. Applying pressure to the membrane causes an electrical charge which can be measured. <CIT> discloses a piezoceramic pressure sensor for an internal combustion engine. <CIT> discloses a capacitive pressure sensor for the injection pressure detection in the fuel injection pump of a diesel engine. <CIT> discloses a capacitive pressure sensor that is specially designed for use in a food industry or in hospitals. <CIT> discloses a capacitive pressure sensor for measuring pressure in internal combustion engine combustion chamber, featuring a sensor which is encapsulated by a protective diaphragm. The combustion chamber pressure is transferred from the diaphragm as a force on a transfer plunger, for thermal decoupling. A lifting element is moved by the plunger, and its displacement alters the capacitance of a capacitor. The material of the measuring capacitor is chosen to match heat expansion coefficients, so that temperature changes produce insignificant capacitance changes.

Typically, in existing sensor designs, the measuring element is in direct contact with a hot deflecting membrane. This causes high mechanical and thermal stresses to the measuring element. Therefore there is a need for thermal insulation between the sensing element and the membrane. Such implementations are difficult and often lead to difficult mechanical solutions. In the presented solution these problems are solved and a simple and reliable pressure sensor can be implemented.

An object of certain embodiments of the present invention is to provide a simple and reliable capacitive pressure sensor. Through such embodiments, a simple, robust and reliable capacitive cylinder pressure sensor is realized.

Certain embodiments of the present invention provide for a capacitive pressure sensor which may be employed in combustion engines. Such capacitive pressure sensors provide for measurement of cylinder pressure within the combustion engine.

The high temperature and repeated stress provided by the cylinder of an internal combustion engine provide a challenging employment of pressure sensors. Some embodiments of the present invention allow for measurement of these challenging environments by insulating certain components of the pressure sensor from the high heat and pressure environment of the cylinder.

Some embodiments of the present invention comprise a housing, variable capacitor and an associated circuitry. The housing may be formed of a steel cylinder having a bottom surface and an outer surface. The outer surface may be threaded to allow for installation to the cylinder head of a combustion engine. The variable capacitor may be formed by a stationary electrode and an elastically bendable electrode. Pressure exerted on the bottom surface acts to bend the elastically bendable electrode. This bending alters the capacitance of the variable capacitor. The associated circuitry may be configured to generate a signal based on the variable capacitance of the variable capacitor. This capacitance is representative of the pressure exerted on the bottom surface. As such there is no need for mechanical contact to a hot membrane and certain problems of the existing solutions are avoided.

Some embodiments of the present invention employ ceramics such as Low Temperature Co-fired Ceramics (LTCC). These ceramics may serve to form a structure internal to the housing of the sensor.

According to a first aspect of the present invention, there is provided a capacitive pressure sensor as defined in appended independent claim <NUM>. The sensor may also comprise circuitry <NUM> configured to generate a signal based on the pressure dependent capacitance of the variable capacitor, the signal being representative of a pressure exerted on the bottom surface.

According to a second aspect of the present invention, there is provided a capacitive pressure sensor apparatus for measuring pressure in a cylinder of the engine, the sensor apparatus comprising the sensor described in appended independent claim <NUM>.

In certain embodiments of the present invention, a housing of the sensor is defined by a bottom surface, side surface and top surface. The housing is cylindrical with circular top and bottom surfaces. In such instances the side surface would be the surface along the perimeter of the cylinder.

According to some embodiments of the present invention a capacitive pressure sensor is provide as comprising a housing having a bottom surface, variable capacitor and an associated circuitry. The variable capacitor is formed by a stationary electrode and an elastically bendable electrode. Pressure exerted on the bottom surface acts to bend the elastically bendable electrode. This bending alters the capacitance of the variable capacitor. The circuitry is configured to generate a signal based on the variable capacitance of the variable capacitor. This capacitance is representative of the pressure exerted on the bottom surface. The circuitry may be disposed inside the housing, outside on the housing or outside separate from the housing. The circuitry may be electrically connected to the variable capacitor by appropriate electrical connecting means, such as a coaxial wire.

<FIG> illustrates a schematic view of a capacitive pressure sensor <NUM> in accordance with at least some embodiments of the present invention. The capacitive pressure sensor <NUM> is mountable to the cylinder head of a combustion engine. The capacitive pressure sensor comprises a housing <NUM>, variable capacitor <NUM> and an associated circuitry <NUM>. The housing has a bottom surface <NUM>. The variable capacitor <NUM> is formed by a stationary electrode <NUM> and an elastically bendable electrode <NUM>. The stationary electrode <NUM> being disposed within the housing <NUM> and the elastically bendable electrode <NUM> being arranged between the stationary electrode <NUM> and the bottom surface <NUM>. The sensor may also have the associated circuitry <NUM> being configured to generate a signal based on the variable capacitance of the variable capacitor <NUM>, the signal being representative of a pressure exerted on the bottom surface <NUM>.

As shown in <FIG>, in some embodiments of the present invention the elastically bendable electrode <NUM> may form at least a portion of the bottom surface <NUM>.

In certain embodiments of the present invention an outer surface <NUM> of the housing <NUM> is cylindrical. The housing may form a cylinder. In such embodiments the bottom of the cylinder may be dimensioned such that it deflects as a function of pressure.

In certain embodiments the internal structure may be formed by ceramics, for example, Low Temperature Co-Fired Ceramics. In such instances LTCC layers may form the internal structure. This structure can provide for a gap between the stationary electrode <NUM> and the elastically bendable electrode <NUM> which form the variable capacitor <NUM>. The internal structure may also function as a heat insulator between the bottom surface <NUM> and the remaining components.

As illustrated in <FIG>, in certain embodiments, the internal structure may be mechanically compressed within the housing <NUM>. This may be accomplished by a mechanical clamping structure. The mechanical clamping structure can consist of a compressing nut <NUM>, PCB <NUM> and first conductive structures <NUM>. The compressing nut may be separated from the mechanical clamping structure by springs <NUM>. In such instances the springs <NUM> ensure the proper clamping force.

Also illustrated within <FIG> are electrical contacts <NUM> and first conductive structures <NUM>. The conductive structures may electrically connect the stationary electrode <NUM> to the circuitry <NUM>, e.g. via the printed circuit board (PCB) <NUM>. The circuitry <NUM> may be affixed to the printed circuit board <NUM>. The circuitry <NUM> may be electrically connected to the first conductive structures <NUM> through the PCB <NUM>.

As further illustrated by <FIG>, in certain embodiments the stationary electrode may be comprised of ceramic such as LTCC. This may be formed in a layered fashion.

Also illustrated in <FIG> is a wire <NUM> for transmitting of the signal generated by the circuitry. This wire <NUM> may transmit the signal to an engine control unit for close control of an engine to which the sensor is equipped.

In certain embodiments of the capacitive pressure sensor may further comprise threads <NUM> disposed on at least a portion of the outer surface <NUM> of the housing <NUM>.

In some embodiments of the present invention, the housing <NUM> may have a threaded <NUM> outer surface <NUM>.

In some embodiments of the capacitive pressure sensor the elastically bendable electrode <NUM> may be arranged to elastically bend responsive to the pressure exerted on the bottom surface <NUM>.

In certain embodiments of the present invention the measurement resolution of the capacitive pressure sensor can be estimated with the aid of the following equations: ΔPmin = (d/β) * ( V/V), where β = ( d )/ P. Within the equations ΔPmin refers to the measurement resolution, d is the distance between the stationary electrode <NUM> and elastically bendable electrode <NUM> and d is the displacement of elastically bendable electrode, P is a pressure change, V is representative of electronics noise and V is the applied measurement voltage. As an example of estimation as per the supplied equations, if the mechanical sensitivity is β=7n/bar, electrode gap is d = <NUM>, electronics noise is ΔV =3nV/√Hz and measurement voltage is V = 3V the resolution of the <NUM> band is ≈1mbar. For example, the required measurement resolution for diesel engine having a capacity greater than <NUM> MW is <NUM> bar@<NUM> BW.

In certain embodiments the stationary electrode <NUM> may be disposed substantially perpendicular relative to the outer surface <NUM> of the housing <NUM>.

In some embodiments of the present invention, the bottom surface <NUM> may be configured to be exposed to the cylinder of the combustion engine.

In certain embodiments of the present invention the elastically bendable electrode <NUM> may be configured to be exposed to the cylinder of the combustion engine.

<FIG> illustrates a schematic view of a capacitive pressure sensor <NUM> according to certain exemplary embodiments of the present invention wherein a compressing nut <NUM> may provide a mechanical clamping force. The clamping force affixes the stationary electrode <NUM> to the housing <NUM>. Insulators <NUM> may be positioned at least partially between the stationary electrode <NUM> and the housing <NUM>. The insulators may be ceramic. The insulators may also be MICA. The insulators electrically insulate the stationary electrode <NUM> from the housing <NUM>.

Some embodiment may comprise springs <NUM> disposed at least partially between the compressing nut <NUM> and stationary electrode <NUM>. The stationary electrode <NUM> may be electrically insulated from the springs <NUM> by further insulators <NUM>.

A thermal insulating component may be disposed between the compressing nut <NUM> and support plate <NUM>.

<FIG> illustrates a schematic view of a capacitive pressure sensor <NUM> according to certain embodiments of the present invention. The stationary electrode <NUM> may be comprised of steel and insulated with insulators <NUM>. The insulators may be ceramic. The insulators may also be MICA. The insulators electrically insulate the stationary electrode <NUM> from the housing <NUM>. The stationary electrode <NUM> may be fastened to the housing via tightening bolts or screws <NUM>. The tightening bolts or screws <NUM> may be also insulated from the stationary electrode <NUM> by the insulators <NUM>. Springs <NUM> may be disposed between the stationary electrode <NUM> and the tightening bolts or screws <NUM>.

A thermal insulating component may be disposed between the stationary electrode <NUM> and a support plate <NUM>. The PCB <NUM> may be supported at least partially by the support plate <NUM>. Affixed to the PCB may be circuitry <NUM> configured to generate a signal based on the variable capacitance of the variable capacitor <NUM>. The circuitry <NUM> may alternatively be disposed in various other ways inside the housing, outside on the housing or outside separate from the housing. A wire <NUM> may provided that is electrically connected to the variable capacitor and/or the PCB <NUM> and may be connected to another component or to the circuitry <NUM> disposed separate from the housing.

A coaxial wire <NUM> may be provided for electrically connecting the stationary electrode <NUM> with the circuitry <NUM>. The circuitry may be disposed inside the housing, outside on the housing or outside separate from the housing. The coaxial wire <NUM> may be connected to the stationary electrode <NUM> via a clamped contact <NUM>. The coaxial wire <NUM> or corresponding connecting electrode and clamped contact <NUM> may be an integrated part of the stationary electrode <NUM>.

The variable capacitor <NUM> may be formed by the stationary electrode <NUM> and the elastically bendable electrode <NUM> which forms a portion of the housing <NUM>.

As illustrated by <FIG>, in certain embodiments of the present invention a thermal insulating component <NUM> may be disposed at least partially between the bottom surface <NUM> and the circuitry <NUM>. This thermal insulating component may be, for example, stone wool.

According to certain embodiments of the present invention the capacitive pressure sensor is mountable to the cylinder head of an internal combustion engine.

<FIG> shows a schematic view of a capacitive pressure sensor according to certain embodiments not being part of the present invention. In the illustrated embodiment the internal structure of the sensor may be adhered to the housing via an adhesive <NUM>. This adhesive may be applied to the housing such that it is between the bottom surface <NUM> and the internal structure. In such applications a high temperature glue may be used, for example a ceramic adhesive.

When the portion of the housing forming the bottom surface <NUM> elastically bends, it causes the elastically bendable electrode to bend and the variable capacity of the variable capacitor to change.

As also shown in <FIG>, the elastically bendable electrode <NUM> may be comprised of ceramic such as LTCC.

As shown in <FIG>, in certain embodiments of the present invention there are first conductive structures <NUM> electrically connected to the stationary electrode <NUM> and a second conductive structure <NUM> electrically connected to the elastically bendable electrode <NUM>. These structures electrically connect the electrodes to the PCB <NUM> and/or circuitry <NUM>.

In some embodiments of the present invention a thermal insulating component <NUM> may be disposed at least partially between the bottom surface <NUM> and the elastically bendable electrode <NUM>.

Also shown in <FIG> are structural material layers <NUM>. These layers may also be comprised of a ceramic such as LTCC.

<FIG> illustrates a capacitance readout solution according to certain embodiments of the present invention.

In certain embodiment of the present invention electrical conducting components are integrated within ceramic, for example, LTCC.

In some embodiments of the present invention, the elastically bendable components may be elastically bendable when exposed to pressures seen within the combustion chamber of combustion engine. In some instances the elastically bendable electrode <NUM> may be elastically bendable at pressures exerted on the bottom surface <NUM> below <NUM> bar, <NUM> bar, <NUM> bar, <NUM> bar, <NUM> bar, <NUM> bar, or <NUM> bar.

In certain embodiments of the present invention the circuitry <NUM> may be thermally isolated from the bottom surface <NUM>. The thermal conductivity between the bottom surface <NUM> and the circuitry <NUM> may be, for example, less than <NUM> (W / (m*K)).

In some embodiments not being part of the present invention the bottom surface <NUM> of the housing <NUM> may be elastically bendable. In such instances the components between the bottom surface <NUM> and the elastically bendable electrode 24may also be elastically bendable.

A method of installation of the capacitive pressure sensor according to an embodiment not being part of the present invention comprises the steps of affixing the capacitive pressure sensor to a cylinder head of combustion engine.

An internal combustion engine according to some embodiments not being part of the present invention comprises a cylinder head having the capacitive pressure sensor according to the present invention affixed to said cylinder head.

A cylinder head for a combustion engine according to some embodiments not being part of the present invention comprises the capacitive pressure sensor according to the present invention affixed to said cylinder head.

At least some embodiments of the present invention find industrial application in control systems of combustion engines. Some embodiments comprise pressure sensor applications in other harsh environment at high temperature and/or at high pressure such as air compressors and hydraulic systems.

Claim 1:
A capacitive cylinder pressure sensor (<NUM>) for measuring cylinder pressure within a combustion engine, the sensor comprising:
- a cylindrical housing (<NUM>) made of steel and mountable to a cylinder head of an internal combustion engine, said cylindrical housing (<NUM>) having an outer surface (<NUM>) and a bottom surface (<NUM>) that is configured to be exposed to a cylinder of the internal combustion engine;
- a variable capacitor (<NUM>) having a stationary electrode (<NUM>) and an elastically bendable electrode (<NUM>), the stationary electrode (<NUM>) being disposed within the cylindrical housing (<NUM>) parallel to the elastically bendable electrode (<NUM>), and the elastically bendable electrode (<NUM>) forming at least a portion of the bottom surface (<NUM>) of the cylindrical housing (<NUM>) made of steel and being arranged to be bent in response to a pressure exerted on the bottom surface (<NUM>) and thereby the variable capacitance of the variable capacitor (<NUM>) being representative of the pressure exerted on the bottom surface (<NUM>); and
- an insulator structure (<NUM>, <NUM>) insulating the stationary electrode (<NUM>) from the cylindrical housing (<NUM>) and defining a gap between the stationary (<NUM>) electrode and the elastically bendable electrode (<NUM>).