METHOD FOR DETERMINING THE THERMODYNAMIC STATE OF THE FUEL IN A FUEL SYSTEM

Abstract

Determining the thermodynamic state of fuel includes opening the venting connection to release the tank pressure while monitoring the derivative pressure (dP/dt), closing the venting connection when one of the following conditions is met, the derivative pressure (dP/dt) is lower than a predetermined threshold DP1 or the opening time t1 reaches a predetermined value, if the closing of the venting connection occurs when the opening time t1 reaches the said predetermined value, determining that the fuel is boiling and aborting the method if the closing of the venting connection occurs when the derivative pressure (dP/dt) is lower than the said threshold DP1, measuring an initial tank pressure at the closing of the venting connection, measuring the final tank pressure after a closure time t2, calculating the pressure variation (P/t2), comparing the pressure variation (P/t2) with a first threshold PV1, if the pressure is lower then aborting the method.

Claims

1. A method for determining the thermodynamic state of the fuel in a fuel system mounted on board of a vehicle comprising a pressurized fuel tank and a closable venting connection between the tank and the atmosphere, characterized in that the method comprises the following steps: [-] a) opening the venting connection to release the tank pressure for an opening time t1 while monitoring the derivative pressure (dP/dt); [-] b) closing the venting connection when one of the following conditions is met: the derivative pressure (dP/dt) is lower than a predetermined threshold DP1 or the opening time t1 reaches a predetermined value; [-] c) if the closing of the venting connection occurs when the opening time t1 reaches the said predetermined value, determining that the fuel is boiling and aborting the method; [-] d) if the closing of the venting connection occurs when the derivative pressure (dP/dt) is lower than the said threshold DP1, measuring an initial tank pressure at the closing of the venting connection, measuring the final tank pressure after a closure time t2, calculating the pressure variation (P/t2); [-] e) comparing the pressure variation (P/t2) with a first threshold PV1. [-] f) if the pressure variation (P/t2) is lower than the first threshold PV1, determining that the fuel is not boiling and aborting the method.

2. Method according to claim 1, wherein after step e), if the pressure variation (P/t2) is higher than the first threshold PV1, determining that the fuel is boiling and aborting the method.

3. Method according to claim 1, wherein after step e), if the pressure variation (P/t2) is higher than the first threshold PV1, the method further comprises steps as follows: [-] g) comparing the pressure variation (P/t2) with a second threshold PV2; [-] h) if the pressure variation (P/t2) is higher than the second threshold PV2, determining that the fuel is boiling and aborting the method; [-] i) if the pressure variation ((P/t2) is lower than the second threshold PV2, repeating steps a), b), c) and d) a number of time N, analyzing the pressure variation (P/t2) after each repetition and determining if the fuel is boiling.

4. Method according to claim 3, wherein the number of time N is a predetermined number, for example 5.

5. Method according to claim 3, wherein the number of time N is calculated according to the first two pressure variations (P/t2).

6. Method according to claim 1, wherein in step d), the measuring of the final tank pressure is taken out when one of the following conditions is met: the tank pressure reaches a predetermined value P1 or the closing time t2 reaches predetermined value.

7. Method according to claim 1, wherein the predetermined threshold DP1 is between 5 Pa/s and 50 Pa/s, preferably 10 Pa/s.

8. Method according to claim 1, wherein the predetermined value for the opening time t1 is between 2 s and 10 s, preferably 5 s.

9. Method according to claim 1, wherein the first threshold PV1 is 25 Pa/s.

10. A method for determining a leak present in a fuel system comprising the following steps: [-] j) carrying out the method for determining the thermodynamic state of the fuel in the fuel system according to any one of preceding claims, [-] k) carrying out a diagnostic for the presence of leaks in the fuel system only if the fuel is not boiling.

11. Method according to claim 10, wherein the method is carried out when the vehicle is key-off since at least two hours.

12. Method according to claim 10, wherein the method begins only if the relative internal tank pressure is between 1500 Pa and 10000 Pa.

13. Method according to claim 10, wherein the method begins only if the internal tank temperature is below a threshold T1, for example 29 C.

Description

[0040] The accompanying drawings are used to illustrate presently preferred non-limiting exemplary embodiments of the present invention. The above and other advantages of the features and objects of the invention will become more apparent and the invention will be better understood from the following detailed description when read in connection with the accompanying drawings, in which:

[0041] FIG. 1 is a schematic view of a fuel tank connected to an inlet duct, an electronic control unit and an outlet duct;

[0042] FIG. 2 is a graph showing an example of pressure increase after the first pressure release;

[0043] FIG. 3 is an algorithm schema showing an embodiment of the determination method;

[0044] FIG. 4 is an algorithm schema showing another embodiment of the determination method.

[0045] FIG. 1 illustrates a fuel tank 1 containing an amount of fuel and different sensors such as a volume sensor 2, a temperature sensor 3 and a pressure sensor 4. The volume sensor 2 and the temperature sensor 3 are connected to an electronical control unit (ECU) 5 which is placed outside the tank. The tank 1 comprises a filling duct 6 presenting a fuel inlet opening 7 and a discharging duct 8 in which are arranged a first valve 9 which is a closable venting connection between the tank and the atmosphere, a canister 10 configured to absorb impurities in the fuelcapture the fuel vapor and prevent their release to the atmosphere and a second valve 11. The pressure sensor 4 is also connected to the electronical control unit which commands the opening and the closing of the first valve 9 and the second valve 11.

[0046] Referring to a first embodiment of the determination method as illustrated in the algorithm schema in FIG. 3, before starting a fuel leak detection, several conditions should be met in order to obtain more accurate measurements and avoid unnecessary energy cost. These conditions could be: the key-off time of the vehicle, the relative internal tank pressure and/or the internal tank temperature, etc. Once the conditions are met, the venting connection to atmosphere which is originally closed is opened to release the tank pressure during an opening time t1.

[0047] The derivative pressure and the passed opening time are monitored. Two possibilities may occur: if the derivative pressure is always higher than a threshold DP1 even when the passed opening time reaches the predetermined value, it means that the fuel is boiling and the leak detection does not need to be done after the closing of the venting connection; If, within the predetermined time frame, the derivative pressure falls below a threshold DP1, the venting connection is closed and it can be known that the thermostatic state of the fuel may meet the conditions for a leak detection test. In this case, an initial tank pressure is measured at the closing of the venting connection. After a predetermined closure time t2, the final tank pressure is also measured for calculating the pressure variation (P/t2).

[0048] In an analysis step, the pressure variation (P/t2) is compared to a first threshold PV1, if the pressure variation (P/t2) is lower than the first threshold PV1, the fuel is determined to be not boiling which means that the fuel is in a good condition for a leak detection. The FIG. 2 shows an example of two possibilities of pressure increase after the first pressure release : the curve A shows the position of the venting connection (closed/open), the curve B shows the pressure variation of a boiling fuel and the curve C shows the pressure variation of a fuel which is not boiling.

[0049] Now referring to a second embodiment of the determination method as illustrated in the algorithm schema in FIG. 4, the method is identical to the first embodiment until the step of closing of the venting connection after the first pressure release. The particularity of this embodiment is that no predetermined closure time is disposal and the measuring of the final tank pressure is taken out when one of the following conditions is met: the tank pressure reaches a predetermined value P1 or the closing time t2 reaches predetermined value. Even though two different fuels cannot be compared anymore together in a schema as illustrated in FIG. 2 (since the closing time may change from one fuel to another), the pressure variation of each fuel can still be calculated based on the initial pressure, final pressure and measured closing time. This value is then compared to the first threshold PV1 as in the preceding embodiment to determine the thermostatic state of the fuel.

[0050] When the first measured pressure variation is higher than the first threshold PV1 but lower than a second threshold PV2, a repetition of opening-closing of the venting connection can be considered in order to observe the pressure variation of each repeated process and thus to determine whether the fuel is boiling.

[0051] The tableau below shows a comparison between two fuels 1 and 2. A predetermined value P1 has been set to 5000 Pa and for each repetition the venting connection is closed only when the internal tank pressure reaches this value. For the fuel 1, the time to reach the value P1 is 1 second at the 1.sup.st time of repetition, 2 seconds at the 2.sup.nd time of repetition and 2 seconds at the 3.sup.rd time of repetition. It can be observed that the pressure increases very quickly to reach the predetermined value. It's easy to infer that the fuel is boiling and the test can be stopped. This method is very reliable because the result corresponds to the RVP of the fuel 1 which is 90 kPa (13 psi). This value is higher than 69 kPa (10 psi) and the fuel is considered to be boiling according to the definition at the beginning of this application.

TABLE-US-00001 time to increase of 5000 Pa(sec) RVP Fuel Pinit before Canister 1st 2.sup.nd 3rd (kPa) Liter test (Pa) load (g) time time time Fuel 1 90 27 119 74 1 2 2 Fuel 2 55 27 16 11 60 286 413

[0052] Concerning the fuel 2, the same predetermined value of pressure P1 has been set and the time to reach this value P1 is much longer at each repetition (60 s for the 1st time, 286 s for the 2nd time and 413 s for the 3rd time). It shows that there are almost no volatile compounds in the fuel 2 and it's not boiling. This observation is also confirmed by the RVP of the fuel 2 which is 55 kPa (8.3 psi) (i.e. less than 10 psi or 69 kPa).

[0053] A method for determining a leak present in a fuel system (not shown) is also improved with a preliminary step of carrying out the method for determining the thermodynamic state of the fuel in the fuel system as described above. Therefore, the diagnostic as mentioned above or another diagnostic which is described in the European application N 17305638.3 for the presence of leaks in the fuel system is started only if the fuel is not boiling which allows obtaining more accurate results and avoiding unnecessary energy consumption.