Field of the invention

The invention relates to a method for determining a gas pressure inside a reservoir in response to a change of the reservoir volume, thereby enabling to determine the gas volume inside the reservoir. If the reservoir volume is known a priori, the volume of an incompressible liquid inside the reservoir can be calculated.

Description of the related art

Detection of fluid levels in reservoirs has a multitude of applications. It is particular critical in safely operating combustion engines drawing fuel from the reservoir. Numerous variants for detecting fluid levels have been reported, e.g. measuring the float level of a float gauge, capacitive fluid level detection (e.g EP 2759 812, DE 102004047413 Al), microwave fluid level detection

(US 8,763,453 B2, US 7,843,199 B2), ultrasonic level detection based on propagation time measurements (e.g. US 7, 010,974 B2). All these methods are sensitive to changes of the orientation of the reservoir in space and or sloshing of the liquid inside the reservoir, which is often due to acceleration of the reservoir.

DE 29 53 903 suggests an indirect measurement of the fill level of a fuel reservoir by changing the fuel reservoir's volume and to measure the resulting change of the pressure inside the reservoir. Similar approaches have been suggested in US 7,347,089 Bl, DE 35 40 768 CI, DE 43 39 933 Al or DE 897 331. This approach is based on the fact, that the fuel in the reservoir is essentially an incompressible liquid (herein briefly 'liquid') and that the 'empty' space is occupied by a compressible gas (herein briefly 'gas'), usually air, having the volume V _g . The gas follows Boyle-Mariott's law (p - V _g = c; p symbolizes the pressure, V the volume, c is assumed to be constant during the measurement). Thus, the response of the pressure p (¾) to a change in volume =— c ^■ V _g ^~2 is a bijective function and accordingly, if the change in pressure dp in response to a particular change in volume dV _g can be measured, the volume of the gas V _g inside the reservoir can be determined ( ). If the total volume of the reservoir V _r is known

a priory (i.e. usually by design), the volume of the incompressible liquid V _t inside the reservoir and thus the reservoir's fill level can be calculated (V _t = V _r — V _g ). Only for completeness is it noted that 3V _g = dV _r , because during a measurement the liquid volume V _t should be at least essentially constant (and/or at least known). At least essentially constant simply means the change of the gas volume dV _g during a measurement is much bigger than the change of the liquid volume V _u or mathematically dV _r » dV _t . Between the measurements the liquid can be drawn at any flow rate from the reservoir or refilled to the reservoir at any flow rate. Although this approach is known for years and has the advantage of being independent of the orientation of the reservoir or sloshing of the liquid due to acceleration of the reservoir, there is no implementation commercially available. One drawback are simply the costs for reliably providing a well-defined change of the volume. Another problem is that with reducing liquid fill level the gas volume V _g increases and thus the change in the pressure dp/dV _g decreases. Thus, the measurement becomes more and more inaccurate, the more the reservoir becomes empty, i.e. in a situation where knowledge of the fill level becomes more and more important.

Summary of the invention

The problem to be solved by the invention is to accurately determine a pressure change in response to change of the reservoir's volume, thereby enhancing fill level detection of a liquid reservoir, wherein the liquid is exchanged with a gas when drawing or refilling the liquid.

Solutions of the problem are described in the independent claims. The dependent claims relate to improvements of the invention. Briefly summarized, the invention provides a possibility to preferably periodically alter the volume of the reservoir in particular by using a modified valve. The periodic alteration of the volume provides a periodic response of the pressure. By including the information about the temporal modulation of the gas volume (Vg (t), t symbolizing the time) into signal processing of the pressure response signal (t) , the pressure response signal can be distinguished from noise signals, i.e. the pressure response signal can be retrieved even if it is only of the order of magnitude of the noise and so to speak 'hides in the noise'. As it will become apparent below, altering of the volume does not necessarily need to be periodically, but this embodiment is particular instructive. The apparatus for altering the volume of a liquid reservoir, comprises at least a valve. The valve has at least a first port being in fluid communication with the liquid reservoir and a second port. The second port can be connected e.g. with another reservoir, a vent opening, a combustion engine's manifold or the like. The valve has a conduit for enabling a liquid communication between the first and the second port. The liquid communication can be controlled, i.e. the valve can be opened or closed by moving, e.g. displacing or rotating a valve member. The valve member has an open position in which the first port and the second port are in fluid communication via said conduit. The valve member further has a first closed position, in which the fluid communication between the first port and the second port is blocked by the valve member. Further, the valve member is moveable into at least a second closed positon, in which the fluid communication between the first port and the second port remains closed, i.e. blocked, preferably as well by the valve member. Moving the valve member or a part thereof between the first and second closed positions changes the volume of a chamber being in fluid communication with the first port and thus with the reservoir. In other words, by simply moving the valve member forth and back from the first closed position to the second closed position the volume of the compressible gas inside the reservoir is iteratively changed and the

corresponding pressure response can be measured using an available pressure sensor. As apparent, during this movement, the valve remains closed. Briefly summarizing, the valve member or a part thereof restricts a chamber being in fluid communication with the reservoir, e.g. via the first port. Moving of the valve member thus changes the chamber volume and thus the effective volume of the reservoir. Accordingly, the first port is preferably in fluid communication with a chamber, into which the valve member protrudes, when the valve member or at least a part thereof protrudes when it is moved from its first to its second closed position.

As usual, the valve member and/or a movable part thereof can be operated by a drive, i.e. an actuator, e.g. a solenoid drive, a rotary drive or the like.

Briefly summarizing, the valve has a valve member being movably supported, e.g. in a valve housing, thereby enabling its movement for opening or closing of the valve has at least two closed positions. By iteratively moving the valve member or at least a part of it from one of these two closed positions into the other closed position, the volume of the chamber and thus the reservoir can be iteratively altered. Iteratively moving the valve member between these two closed positions thus enables to e.g. periodically alter the volume V _g (t) and thus the pressure p(t) of the compressible gas inside the reservoir as function of time t. The pressure sensor can be integrated in the apparatus, thereby reducing manufacturing cost as well as installation costs. For example, the apparatus may comprise a sensor, said sensor measuring a signal being representative for the force F required the move the valve member against the pressure in the reservoir from the first to the second closed position and/or from the second to the first closed position. As the surface A of the valve member being exposed to the pressure p(t) in the reservoir is known by design, the pressure can be calculated simply using p = -. In a particular preferred example, the signal being representative for the force F is the voltage and/or the current feed to a drive for moving the valve member between the first and second closed position.

In a preferred application, the reservoir is a fuel tank of a combustion engine. The valve can be e.g. a Fuel Tank Isolation Valve (commonly FTIV) for controlling a fluid communication between the reservoir and the environment (usually via a filter, e.g. an activated charcoal filter, like an activated carbon canister) or a Fuel Tank Vent valve for controlling a fluid communication between the reservoir and an engine air intake, e.g. the engine's intake manifold. Valves like these are already standard in modern combustion engine powered apparatuses, like cars, piston aircraft, motorcycles, power stations. Thus, by simply replacing the prior art valves, the fill level of the fuel reservoir can be determined easily. There are no additional costs for extra power supply cables or other installation costs that would be required if a separate volume modulation unit would be used. Beyond, the costs for the valve according to the invention are, if at all, only slightly higher than the costs for the prior art valves. In any case, the additional costs are significantly lower as for prior art volume modulators. In a further preferred application, the reservoir is a reductant reservoir for providing a reductant to an exhaust gas for denitrification of the exhaust gas. Typical reductants are anhydrous ammonia, aqueous ammonia or an aqueous urea solution as required for selective catalytic reduction of nitrogen oxides. Such liquids are commonly referred to e.g. as 'AdBlue ^® ' (Europe) or Diesel Exhaust Fluid, briefly DEF (USA).

For example, the conduit may comprise a valve seat and the valve member closes the valve seat in its first and second closed positions and opens the valve seat when moved in its open position. An elastic gasket may seal the valve seat in the closed positions. In other words the elastic gasket is positioned between a gasket facing contact surface of the valve member and the valve seat. The valve member presses the gasket against the valve seat. By displacing the valve member from the first closed position into the second closed positon the elastic gasket is compressed. If the valve member is positioned between the first port and the valve seat, compressing the gasket augments the volume of the chamber being in fluid communication with the first port and thus reduces the pressure in the chamber and thus the gas volume. When moving the valve member from the second closed position to the first closed position, the gasket expands and thus the volume of the chamber is reduced. Accordingly, the pressure in the chamber and thus as well in the reservoir is augmented. If the valve member is positioned at the other side, i.e. between the second port and the valve seat, compressing the gasket results in a reduced volume of the chamber being in fluid

communication with the first port and thus the reservoir and thus essentially of the reservoir. Accordingly, the pressure in the reservoir is augmented. Moving the valve member back in its first closed position, the gasket expands;

accordingly the gas volume is augmented and the pressure reduced. In this example, we assumed for simplicity only that the valve member does not protrude through the valve seat, but of course it can. In this case one would have to consider the position of a gasket facing contact surface for contacting the gasket and not the complete valve member.

In another example, the valve member comprises at least a section of the conduit. The conduit has at least a first opening being positioned in front of the first port and a second opening being positioned in front of a second port if the valve member is in its open position, to thereby provide said fluid

communication. Of course the conduit openings do not need to be directly in front of the respective ports, but the ports should be in liquid communication with openings of the conduit and thus with each other. As already explained above, the first port is preferably in fluid communication with a chamber, into which the valve member protrudes, when the valve member is moved from its first to its second closed position. The boundary of the chamber closes at least one of said first and/or second openings of the conduit, when moving the valve member from its open position into its first and/or second closed positions. For example, the valve member could be or resemble a piston, being movably supported in a cylinder. The conduit may extend e.g. perpendicular (but of course as well oblique) through the piston. The first and second ports can be provided by through holes in the cylinder wall. The through holes are positioned to be connected by the conduit, if the valve member is in its open position. If the valve member is advanced or retracted, i.e. axially moved in the cylinder, and/or rotated the through holes are closed by the piston. The cylinder provides a chamber, being enclosed by the piston. Said chamber is connected, e.g. by a tube or any other kind of conduit, to the first port. Thus advancing (or retracting) the valve member, briefly any axial movement alters the volume of the chamber and thus of the gas. The pressure response can be detected by any pressure sensing means and thus the gas volume V _g can be determined. Summarizing, the volume of the chamber can be augmented or reduced. For exa mple the volume may be oscillated, by oscillating the valve member. The corresponding pressure thus oscillates accordingly.

In a further example, the valve comprises a chamber being in fluid

communication with the first port. The valve member is rotably supported and comprises a conduit for connecting said first and second ports. By rotating the valve member, a ring segment is inserted into said chamber, thereby closing the second port. Further rotation of the valve member reduces the volume of the chamber and thus enables to compress the gas in the chamber and thus in the reservoir. The pressure response can be detected as explained above.

The above explained valves enable to repetitively compress and decompress a gas being confined in a reservoir according to an arbitrary but well known signal S(t), wherein S(t) symbolizes a measure for the change of the volume relative to a reference volume V ₀ as a function of time (in the simplest case V _g (t) = V ₀ + V ^■ S(t)). This signal is subsequently referred to as initial signal S(t) or reference signal 5(t). The initial signal 5(t) may be periodic but is not necessarily periodic, but is not constant during a measurement. Accordingly, the method for determining a pressure response p(t) of a gas being confined in a reservoir comprises repetitively reducing and augmenting the gas volume V _g (t) as function of time. For example the gas volume may be altered periodically, e.g.: V _g (t) = V _Q + AV ^■ sin(eL>t) (i.e. in this example 5(t) = sm(a>t), wherein ω is the angular frequency and t the time).

Thereby the gas is compressed and/or decompressed according to the initial signal S(t). During said altering of the gas volume according to the initial signal 5(t) a pressure signal p(t) being representative for the pressure inside the reservoir is measured as function of time t. For example, the pressure signal p (t) may be the signal provided by a pressure sensor, e.g. after an optional amplification. This pressure signal p (t) is demodulated, e.g. using the initial signal 5(t) as reference signal and thereby a demodulated pressure signal p _d (t) is obtained. In particular, if the initial signal is periodic, a previously detected pressure signal p(t— τ) may be used for demodulation the pressure signal as well. As usual, τ denotes a time shift. Subsequently, the demodulated signal can be subjected to a low pass and/or a band pass filter, thereby eliminating remaining noise. The such obtained pressure signal p _d (t) can be used to determine dp/dV and thus V _g . Due to the correlation of the signal 5(t) and the pressure signal p(t) pressure changes far below the noise level (i.d. signal to noise levels of down to 1 to le6) can be detected with high accuracy. In other words, the pressure amplitudes can be significantly smaller compared to prior art techniques. For producing these low amplitude volume variations the above explained valve is sufficient, whereas according to the prior art much bigger volume modulations have been necessary (and of course could still be used, provided the volume is altered as explained above). In other words, the reservoir volume is preferably in fluid

communication with the first port of the valve explained above. Repetitively altering the volume according to the initial signal 5(t) can be obtained by altering the position of the valve member between at least two of said closed positions.

Preferably, the initial signal 5(t) is a periodic function. In a simple preferred embodiment is 5(t) = V ₀ ^■ sin(cL>(t + 5t)), wherein ω denotes an angular frequency, and St a time shift. More generally the initial signal can be expressed as

5(t) =∑ ^max aiSin(a)i(t + 5^) + PiCos(a)i (t + 8t ), wherein i is an integer index, ι, βι, ωι and Sti are constants.

Preferably the pressure signal p(t) is amplified prior to its demodulation to thereby simplify processing of the pressure signal p(t).

The step of demodulation may preferably comprise multiplying the initial signal 5(t) with the pressure signal p(t), thereby obtaining a demodulated pressure signal. Multiplication can be performed digital, i.e. numerically or by an analog mixer. In a preferred embodiment the initial signal 5(t) may be a square signal and demodulation may comprise synchronous amplification of the pressure signal p(t) and subsequent integration, thereby essentially eliminating the noise from the signal. Integration of the multiplied pressure signal p _m (t) eliminates noise in the response signal, as the integral over a noise signa l vanishes.

To determine an estimate for dp/dV _g it is advantageous to perform the measurement of p(t) twice but with different volumes changes ΔΙ^ and V ₂ . The estimate for dp/dV _n can be obtained by simply calculating— « ^Pd " ^{1 Pd,2} _t

a ^& dV _g AV ₁ -AV ₂ wherein Pa,i _> Pd,2 ^{a re} the demodulated pressure signals corresponding to ΔΙ^ and AV ₂ , respectively.

In another example, the angular frequency of the reference signal 5(t) is varied. Observing the frequency dependency of the demodulated pressure signal enables to determine the resonance frequency of the volume modulation, i.e. the frequency where the absolute value of the pressure response shows a maximum. This resonance frequency enables to determine the gas volume inside the reservoir as well, e.g. using a look-up table.

Above, it has been assumed that it is not necessary to explicitly explain that the movement and thus the position of the valve member can and in practice will be controlled electrically, e.g. by a (micro) controller operating a drive being operationally connected with the valve member to move it according to the controller's commands.

Further, pressures and forces are vectors, but for simplicity they have been treated like scalars.

As apparent for the skilled person and only mentioned for completeness, the reservoir should be closed when altering the volume to measure the response of the pressure. At least, the leakage should be small compared to the change in volume, or in other words the mass of the gas and the liquid in the reservoir should be kept at least almost constant during a measurement. Further, the method does not only enable to determine the fill level of a reservoir, but as well to determine the volume of an 'empty' enclosed reservoir, by simply connecting the first port to the reservoir and operating the apparatus as set out above. 'Empty' means the there is only a compressible fluid inside the reservoir.

Connecting means here to enable a fluid communication between the reservoir and the first port.

Description of Drawings

In the following the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment with reference to the drawings. Figure la to lc show a first a valve,

Figure 2a to 2c show a second valve,

Figure 3a to 3c show a third valve,

Figure 4a to 4c show examples of a method,

Figure 5 shows a control circuit diagram, and Figure 6 shows an application of the control circuit diagram in figure 5.

Figure 1 shows a valve 10 with a valve housing 12 having a first port 21 and a second port 22. At least one of the ports can be connected to a reservoir 5. Subsequently we assume that the first port 21 is connected to the reservoir 5 as indicated by a dotted line 5. The reservoir 5 is not drawn to scale, in practice its volume V _r is much larger than the volume enclosed by the valve. An

incompressible liquid 7 may occupy a volume V _t inside the reservoir 5. The remaining volume V _g is filled with a compressible gas 6. If the valve 10 is open, as shown in figure la, the reservoir 5 is in fluid communication with the second port 22. The reservoir 5 may have additional openings as indicated by a second valve 8.

The valve 10 has a valve seat 20 with an opening 24 providing a conduit having a longitudinal axis 25. A valve member 30 is movably supported inside the housing, enabling a movement along or at least essentially parallel to the longitudinal axis 25. The valve member 30 has a valve seat facing side 32 supporting an elastic gasket 35. A translation, i.e. a movement of the valve member 30 enables to bring the gasket 35 in contact with the valve seat 20, thereby closing the opening 24. The valve 10 is thus closed and the valve member 30 is now in a first closed position, which is depicted in Fig. lb. Preferably the valve member 30 pushes the gasket with a first force f against the valve seat 20, to ensure a sealing contact between the gasket 35 and the valve seat 20 as well as between the valve member 30 and the gasket 35. As can be seen in figure lc, the force for pressing the gasket 35 against valve seat 20 can be augmented to a force f ₂ until the elastic gasket is deformed as depicted in figure lc. Now, the valve

member 30 is in a second closed position. Due to the corresponding movement, i.e. the stroke of the valve member, the gas volume V _g is slightly increased, e.g. by a few milliliters. Accordingly, a slight pressure drop can be measured inside the reservoir 5, e.g. using a pressure sensor 51 being coupled to a

controller 50 (for a more detailed example, see figure 5). By reducing the force which is applied to the gasket back to the first level f _lt the valve member reciprocates back into its first closed position, resulting in a slight pressure rise, being measurable as well by the pressure sensor 51.

The valve member 30 can be actuated by a linear drive as indicated in figurela to figure lc. The linear drive has a drive housing 40 supporting a spring 41 being coupled to the valve member 30. The spring 41 can be a pull-spring as indicated or as well a push spring. In the latter case is should be repositioned accordingly. Further, at least one coil 42, 43 (depicted are two coils, a first coil 42 and a second coil 43) is attached to the drive housing. The valve member 30 comprises a plunger of a dia- or ferromagnetic material as indicted by reference numeral 44. By applying a current ^ to at least one of the coils 42, 43 the valve member can be moved against the spring force into its first closed position indicated in figure lb. By increasing the current to a second current I ₂ and/or by applying the current to both coils, the force exerted by the valve member 30 to the gasket 35 can be increased to the higher value f ₂ znd thus the valve member translates into its second closed position (as can be seen in figure lc). The current through the coils is preferably controlled by the controller 50 as indicated by corresponding lines. Reducing the current back to ^ (and/or switching the second coil off, respectively) enables to reduce the force back to f _lt i.e. the spring and the elastic gasket 35 move the valve member 30 back into its first closed position (see figure lb). Switching the coils 42, 43 off completely, results in opening the valve 10 (see figure la). figure 2a to 2c show a further embodiment of an apparatus according to the invention: A valve 10 has a valve housing 12 having a first port 21 and a second port 22. At least one of the ports 21, 22 can be connected to a reservoir 5.

Subsequently we assume that the first port 21 is connected to the reservoir 5 as indicated by a dotted line. The reservoir 5 is not drawn to scale, in practice its volume V _r is much larger than the volume enclosed by the valve. An

incompressible liquid 7 may occupy a volume V _t inside the reservoir 5. The remaining volume V _g is filled with a compressible gas 6. If the valve 10 is open, as shown in figure la, the reservoir 5 is in fluid communication with the second port 22 via a conduit 31. Again, the reservoir 5 may have additional openings as indicated by a second valve 8.

The valve member 30 has a conduit 31. In the position as shown in figure 2a the conduit 31 connects the first and the second ports 21, 22, or in other words it provides a liquid communication between the first and second ports 21, 22. By moving the valve member 30 forth or back in the housing, the liquid

communication can be interrupted as shown in figure 2b and figure 2a. The valve member 30 is mechanically coupled to a drive M for pushing or pulling, i.e. moving the valve member as required. A controller 50 controls the e.g. linear drive M and thus the movement of the valve member 30

The valve housing 12 has a chamber 23 being in fluid communication with the first port 21. When moving the valve member 30 inside the housing, the volume of the chamber 23 and thus of the first port 21 can be reduced as shown in figure 2b and figure 2c. Thus reduction leads to an augmentation of the gas pressure pressure p inside the chamber 23, the first port 21 and the reservoir 5. This change in pressure can be detected using a pressure sensor 51 being connected to a controller 50 as already explained with respect to figure la to figure lc.

The valve in figure 3a to figure 3c has like the valves in figure la to figure 2c a valve housing 12 herein with a first port 21 and a second port 22. Like in the examples above, the first port 21 is configured to be connected to a reservoir 5. The first port 21 and the second port 22 can be connected, or in other words set in fluid communication by positioning a conduit 31 of a ring segment like valve member 30 in between of the ports 21, 22 as shown in figure 3a. By rotating the valve member 30, it blocks the second port 22 as shown in figure 3b and figure 3c: By rotating the valve member 30, the valve member enters a chamber 23 being in fluid communication with the first port 21 and thus with the reservoir 5. Thus, depending on the (angular) position of the valve member 30, volume V _g can be varied. The pressure p (t) inside the chamber 23 and thus the reservoir 5 varies inversely proportional (p(t) = c/V _g (t)), (provided reservoir 5 is connected and closed). The pressure p(t) can be detected using a pressure sensor 51 and the corresponding pressure signal p(t) can be evaluated by a controller 50. The controller may control a drive M for moving the valve member in addition.

As apparent from the above the term 'port' does not only refer to an opening in an enclosure of some space enabling a fluid communication with e.g. between the space and the reservoir or the environment, but as well denotes the enclosed space or so to speak a conduit enabling the fluid communication, e.g. between a chamber inside the valve housing and the reservoir.

Referring to figure 4a the method according to the invention is explained based on an example 100. The method comprises, varying the volume V _g (t) of a gas being a confined in a reservoir according to a reference signal 5(t),

mathematically speaking V _g (t) = V ₀ + AV ^■ S(t), wherein V ₀ is an offset and AV a measure for the amplitude (Step 110). As explained above, the reference signal is not necessarily periodic, but preferably. For example, the reference signal 5(t) can be square-wave signal (like e.g. 5(t) = Sign(sm(a>t + φ)), a sinusoidal signal (e.g. 5(t) =∑ _£ α _έ 5ϊη(ω^ + φ )) or a superposition of different signals. The reference signal S(t) can even be a noise signal. In any case, the theoretic pressure s p _th (t) = c/(V _Q + AV ^■ S(t)). In step 120, the pressure p(t) is measured using a pressure sensor providing a first pressure signal p^t). (As apparent we do not distinguish here between the pressure and a signal representing the pressure.) For example, the pressure sensor may provide a voltage being proportional to the pressure. This voltage signal can be amplified and/or filtered and/or provided to an analog digital converter as required. In any case, the pressure signal p^t is demodulated using the reference signal 5(t— τ) in step 130. Demodulation can be obtained for example using a mixer or numerically. Demodulation provides a demodulated pressure signal P _d ,i( with a significantly enhanced signal to noise ratio. Mathematically the demodulated pressure signal can be expressed e.g. as p _d ( ) =— J ^to+At 5(t— τ) · p(t—

At to

T _d )dt. The bigger At, the better is the noise reduction. The delay-offset τ _ά is a parameter showing that the measured signal has a phase offset due to electronical and physical delays. The offset τ is to be adjusted to maximize the signal to noise ratio. Or in other words, S(t— τ) and p (t— τ _ά ) should be in phase. Since the offset τ _ά is usally not known a priori nor is it necessarily constant over all environmental parameters, e.g. like environmental

temperature or atmospheric pressure, it is favorable to perform two calculations with the above formula for two different phase-offsets τ, one can be performed e.g. with τ ₁ = 0 and one with e.g. τ ₂ = π/2. Thus one gets two orthogonal vector components X = p _d ( , ^d ^{= T} i) ^{a n} d Y ⁼ Pd ( > ^T d ^{= T} ₂ ) which can be further processed to an absolute signal value p _{d l} = ΛΙΧ ² + Y ² and a phase value φ = tan ^-1 Q.The corresponding pressure is briefly referred to as simply p _{d l} .

Preferably, the method further comprises varying (modulating) the gas volume as well using a different second V, i.e. V _g (t) = V ₀ + V ₂ · S ₂ (t), wherein V = AV-L≠ V ₂ and 5 ₂ (t) is a second reference signal (Step 110', see figure 4b) and to measure the corresponding second pressure signal p ₂ (t) (Step 120' see figure 4b). The corresponding pressure signal p ₂ (t) can as well be demodulated (Step 130') but of course the second reference signal 5 ₂ (t) has to be used to obtain p _d>2 .

In a particular preferred example (cf. figure 4c), the first and the second modulation of the gas volume are performed at the same time, but with different reference signals S ₁ (t , S ₂ (t), i.e V _g (i) = V ₀ -Ι- ΔΙ^ · S^t) + AV ₂ ^■ 5 ₂ (t), wherein 5(t) = S^t)≠ γ · S ₂ (t) Vy and y is a constant (Step 110"). The corresponding pressure signal p (t) is again measured (Step 120") but demodulated twice, once using the first reference signal S^t (Step 130a) and once using the second reference signal S ₂ (t) (Step 130b). Summarizing, demodulation takes place using only the first or second reference

signal 5 ₁ ( _t ) _{J 2} (t), respectively, to thereby obtain the respective first and second demodulated pressure signals V _d ,i _> V _d ,2 - Thus, an estimate for dp/dV _g can be obtained as— Pd, ^■ i -Pd, -2,■ i.e.— dp ~— Pd, ^■ i ~Pd, -2.

AV ₁ -AV ₂ dV _g AV ₁ -AV ₂

The above explained method(s) may be implemented using a circuit according to the simplified circuit diagram depicted in figure 5. As already indicated above, the circuit comprises a controller 50 configured for digital signal processing. The controller may thus comprise a microprocessor or a number of interacting microprocessors. The controller 50 comprises a signal generator 55 configured to provide at least a first reference signal S(t) to a driver 56. The driver 56 drives and thus controls the movement of a valve member 30 of the valve 10. The valve 10 can be e.g. one of the valves as depicted in figures 1 - 3. Briefly, the valve 10 comprises a movable valve member 30 with an open position in which the first port 21 and the second port 22 are in fluid communication via a conduit. From this open position, the valve member 30 is movable into a first closed position, in which the fluid communication between the first port 21 and the second port 22 is blocked by the valve member 30 (cf. e.g. figures lb, 2b and 3b). In addition, the valve member is movable into at least a second closed positon, in which the fluid communication between the first port 21 and the second port 22 is as well blocked and in which the volume of a chamber being in liquid communication with the first port 21 is altered when moving the valve member 30 from the first closed position to the second closed position and/or from the second to the first closed position. As explained above, the movement of the valve member30 enables to changes the volume V _r of a reservoir 5 being connected to the first port of the valve 10. In fact, of course the volume V _r of the reservoir remains constant, but as the first port 21 of the valve 10 is connected to the reservoir volume the movement of the valve member has the same effect, i.e. the pressure inside the reservoir changes accordingly. This change in pressure is detected by a pressure sensor 51 being in fluid communication with the reservoir 5. The corresponding pressure signal p(t) is amplified by an amplifier 52 and optionally filtered, e.g. by a low pass filter 53 to avoid aliasing effects when subsequently digitizing the signal in a digital to analog converter 54 (AD- converter). The digitized pressure signal can be processed by the controller 50. In the figure, the AD-converter 54 and as well the signal generator 55 are included in the controller 50, but they do not need to be integrated into a single chip or on a single PCB, but may be separate units.

A typical application of a valve 10 is depicted in figure 6. In general, the valve 10 is connected to a reservoir 5 and a controller 10 as already explained with reference to figure 5. Thus the description of figure 5 can be read as well on figure 6. Different from figure 5, the reservoir 5 is a fuel tank e.g. for supplying a combustion engine 70. The second port 22 of the valve 10 is connected to a filter 61, e.g. an inlet of an activated charcoal filter 61. Thus, if the valve 10 (below 'first valve 10') is open, gas from the fuel tank 5 can enter the activated charcoal filter 61. Volatile organic compounds are essentially removed by said In activated charcoal filter 61. The cleaned gas leaving the activated charcoal filter 61 may either escape to the environment via a second valve 62 (and a subsequent dust filter 63) or be provided via a third valve 64 to the air intake system of a combustion engine 70. If the gas 6 from the fuel tank 5 is provided to the combustion engine 70, the first valve 10 and the third valve 64 are each open. If the gas is provided to the combustion engine, the active charcoal filter may be bypassed as indicated by the dashed line. If fuel 7 is drawn from the fuel tank 5, the corresponding volume is to be replaced by air or another fluid to obtain a pressure compensation. Pressure compensation is as well essential in case of temperature changes. For pressure compensation (indicated by a double headed arrow) the first valve 10 and the second valve 62 are each opened. Air can enter the reservoir 5 by passing a dust filter 63, the second valve 62, the activated charcoal filter 61 and the first valve 10 or escape into the environment, depending on the pressure gradient. In some legislations, the second valve 62 can be omitted. In the figure, only the first valve 10 is controlled by the controller 10 (via the driver 56), but of course the other valves can be controlled in essentially the same may. The controller 50 is preferably configured to communicate via a bus system, e.g. via a CAN bus, Flexray or the like with other controllers, like e.g. an engine controller.

List of reference numerals

5 reservoir

6 gas

7 incompressible liquid

8 valve (optional)

10 valve

12 valve housing

20 valve seat

21 first port

22 second port

23 chamber in fluid communication with the first port

24 opening / conduit

25 longitudinal axis

30 valve member

31 conduit

32 valve seat facing side

35 gasket

40 drive housing

41 spring

42 coil

43 coil

44 plunger

50 controller

51 pressure sensor

52 amplifier

53 filter (e.g. low pass filter /anti aliasing filter ), optional

54 AD converter

55 signal generator

56 valve member driver 61 activated charcoal filter

62 second valve

63 filter (e.g. dust filter)

64 third valve

70 combustion engine

100 method

110 method step: volume modulation

120 method step: pressure measurement

130 method step: demodulation of pressure signal using first and/or second reference signal

130a method step: demodulation of pressure signal using first reference signal 5]_(t)

130b method step: demodulation of pressure signal using second reference signal 5 ₂ (t)

Pi (t) pressure / pressure signal (optional index / ^' is used only to distinguish different signals)

P _d ,i (j) demodulated pressure (optional index / ^' is used only to distinguish demodulated pressures)

M motor / linear drive

Si (t) initial/reference signal (optional index / ^' is used only to distinguish different signals)

Vg/V _g (t) gas volume

Vi liquid volume