The i nvention relates to such a method for any conceivable use of such a piston displacement pump, but for i llumi nating the invention and the problems to be addressed thereby the use of such a piston displacement pu mp in a motor veh icle for feeding liquid, such as Ad blue or fuel , will hereinafter be discussed without restricti ng the present invention to that application .

It is of course of interest to be able to detect leakages on the suction side of such a pump, for instance to become aware of why something downstreams the pump is not as it should be, for example the pressure of the liquid pumped, so that measures may be taken to stop the leakage. BACKG ROU N D ART

Several methods are known for detecting leakages on the suction side of a piston displacement pu mp. However, most of them are intrusive and often involve having a sensor located in the suction part of the liquid feedi ng system to which the pu mp belongs. Such methods cou ld for example involve closi ng the end of the suction line and generati ng a pressure or vacuu m in the line that is then measured over a period of ti me or fitti ng a sight-glass or similar in the suction line and check if there is bubble formation i n the liquid.

SE 1 350438 discloses a method of the type defined in the introduction i n which the pressu re in a position downstreams the pump is measured and the pump is then switched off and the pressure decrease in said position is measu red and compared with values of such a pressure decrease stored so as to determine whether a leakage is located in the line upstreams or downstreams the pump. However, th is method may not be carried out during normal operation of the pu mp and it is not su ited for estimating a value of a possible leakage on the suction side of the pump but only to determine whether a leakage is located on the suction side or the exhaust side of the pump.

Fitting extra sensors and/or havi ng to perform i ntrusive tests on a system to detect leakages is mostly undesirable. It would be a great advantage if the leakages could be detected using the hardware already available i n the system , possibly even u nder normal operation .

SUMMARY OF TH E INVENTION

The object of the present invention is to provide a method and a device of the type defined in the introduction being i mproved in at least some aspect with respect to such methods and devices already known . This object is with respect to the method obtained by providing such a method with the features listed in the characterizing part of appended patent clai m 1 . The invention is based on the understanding that the geometric compression ratio known for a piston displacement pump may be used i n combination with the value of the maximum attainable outlet pressure of the pu mp measured for providing an estimated val ue of a possible leakage on a suction side of the pump. Th is may be achieved by assu ming that the gas temperature is constant allowing the general gas law to be used for calculating the ratio of air vol ume to pressure chamber volu me of the pu mp at the bottom dead point of the piston . The val ue of the air volu me ratio so calculated is then compared with a characteristic value thereof for the pu mp without leakage on the suction side thereof enabli ng determi ning of an esti mated value of a possible leakage on the suction side of the pu mp throug h the result of this comparison .

A sensor measu ring the outlet pressure of the pump is in most systems including such a pump already there, and the steps b)-d) of the method may be carried out by an electronic control unit already present, such as for the control of components in a veh icle. This means that i n such a case the hardware already avai lable in the system may be used for detecting and estimati ng a value of a possible leakage on the suction side of a piston displacement pu mp under normal operation of the pu mp. The maximu m attainable outlet pressure may either be actively achieved by driving the pump at a maxi mum in the case th is pressure is h igher than the normal operation pressure of the system or under normal operation of the pu mp in case the maxim um attainable pressu re in fact is lower than the normal operation pressure and the pu mp works at a maxi mum for trying to reach the normal operation pressure. Accordi ng to an embodi ment of the invention a knowledge of the flow rate of the pump is utilized i n the determi ning step d) to determine said estimated value of a possible leakage as an air leakage flow rate. This determi nation may form the basis for a decision whether any measure is to be taken for stopping the leakage.

According to another embodiment of the invention the method is carried out for a pu mp i n a low pressure part of a system for feedi ng liquid in a motor veh icle. "Low pressure" does here normally mean a pressure in the region of 5-30 bars, and a piston displacement pump is suitable to be used in such a low pressu re part of a system for feeding liqu id in a motor veh icle and it wi ll be crucial for the operation of the vehicle to detect and provide an estimated value of a possible leakage on the suction side of such a pump.

According to another embodiment of the invention the method is carried out for a piston displacement pump connected between a tank of Ad bl ue to pump Ad blue to be downstreams of the pump sprayed into the exhaust gas system of the vehicle upstreams the catalytic converter thereof. In such an application of a piston displacement pu mp there is already a pressure sensor arranged downstreams the pu mp for measuring the pressure of Ad bl ue to be sprayed into the exhaust gas system , so that no extra sensor is needed for carrying out the method accordi ng to the present invention .

Accordi ng to another embodiment of the invention the method is carried out for a piston displacement pump connected to a fuel tank of the vehicle to pu mp fuel towards a hig h pressure part of a system for i njection of fuel into an internal combustion engine of the vehicle. This constitutes another application of a piston displacement pump where it is important to be able to provide an estimated value of a possible leakage on the suction side of the pump.

The object of the present i nvention is with respect to the device obtai ned by providing a device with the features listed in the appended independent device claim . The advantages of such a device and the embodiments thereof defined in the dependent device clai ms appear clearly from the above discussion of the method according to the i nvention . The i nvention also relates to a computer program , a computer- readable medium , an electronic control unit and a motor veh icle according to the appended clai ms directed thereto.

Further advantages as well as advantageous features of the invention will appear from the description following below.

BRI EF DESCRI PTION OF TH E DRAWINGS

With reference to the appended drawi ngs, below follows a specific description of an embodiment of the invention cited as an example. is a schematic simplified view showing a possible use of a device according to the invention, is a simplified view of a piston displacement pump used for explaining the method according to the present invention,

Fig.3 is a graph of the maximum attainable outlet pressure P of a piston displacement pump versus ratio of air volume to pressure chamber volume of the pump at the bottom dead point of the piston of the pump,

Fig.4 is a flow chart illustrating the steps carried out in a method according to an embodiment of the invention, and

Fig.5 is a schematic view illustrating an electronic control unit for implementing a method according to the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

A method and a device according to an embodiment of the invention will now be described while making reference to Figs. 1- 3. It is very simplifiedly shown in Fig. 1 how a piston displacement pump 1 is arranged to feed Ad blue from a tank 2 of Ad blue to a nozzle 3 spraying Ad blue into a pipe 4 of the exhaust gas system 5 of a vehicle 6 at the location between an internal combustion engine 7 and a catalytic converter 8 of the exhaust gas system.

A member 9 configured to measure the outlet pressure of the pump in operation thereof in the form of a pressure sensor is arranged in the line 10 from the tank 2 to the nozzle 3 downstreams the pump 1. This pressure measuring member 9 is there for checking that the liquid pressure at the nozzle 3 is as desired. The pressure sensor sends during normal operation of the pump 1 pressure values so measured to a first unit 11 configured to calculate the ratio of air volume to pressure chamber volume of the pump at the bottom dead point of the piston. How this will be done will now be explained while making reference to Figs.2 and 3.

The bottom dead point of the piston 12 is indicated at 13 in Fig.2 defining the maximum pressure chamber volume V _ma x of the pump, whereas 14 indicates the upper dead point of the piston defining the minimum pressure chamber volume V _m in of the pump. The geometric compression ratio r _geo of the pump is Vmax Vmin and is known by the unit 11. Assuming that both the pump chamber and the liquid 15 inside it are incompressible, as soon as the pump chamber volume decreases below the liquid volume, liquid will be expelled from the pump. If an elastic element, for example a gas, is present in the pump chamber, that elastic element will have to be compressed enough to generate sufficient pressure to overcome the back pressure at the outlet of the pump before any liquid can be expelled. If we assume that the only (significant) elasticity is caused by an ideal gas inside the pump chamber, the ideal or general gas law can be used along with a known geometric compression ratio to determine the amount of gas in the chamber at maximum chamber volume if the maximum attainable outlet pressure of the pump is known. It is then assumed that the gas temperature is constant. Said maximum attainable pressure at the outlet of the pump is for a pump with a given geometric compression ratio a function of the ratio of gas, here air, volume to the total chamber volume V _ma x and this is for a certain pump shown by the graph in Fig. 3. It is there shown how a maximum attainable pressure of about 3.5 bars results in a calculated ratio of air volume to chamber volume of 0.5, whereas this ratio is 0.42 for a pressure of 9 bars normally obtainable without any leakage. A second unit 16 of the device according to the invention is configured to compare the value of the air volume ratio calculated by the first unit and shown in Fig. 3 with a characteristic value thereof for the pump without leakage on the suction side thereof and to determine an estimated value of a possible leakage on the suction side of the pump through the result of this comparison. Accordingly, no extra hardware, such as sensors, or invasive measure, such as by blocking/disconnecting components, is required for providing an estimated value of a possible leakage on the suction side of the pump. The method simply relies on getting the opportunity to measure the maximum obtainable pressure on the outlet side of the pump, where pressure might already be measured for control purposes.

Fig. 4 illustrates a flow chart of a method according to an embodiment of the present invention carried out for a liquid feeding system of the type shown in Fig. 1. The method is started with the step Si of continuing to drive the pump. While driving the pump the maximum attainable pump outlet pressure is measured in a step S2, whereupon in a step S3 the general gas law and a knowledge of geometric compression ratio of the pump is utilized to calculate the ratio of air volume to pressure chamber volume. Then the value calculated is in a step S ₄ compared with a characteristic value for the pump without leakage, and finally and estimated value of a possible leakage on the pump suction side is in a step S5 determined based on the comparison.

Computer program code for implementing a method according to the invention is with advantage included in a computer program which can be read into the internal memory of a computer, e.g. the internal memory of an electronic control unit of a motor vehicle. Such a computer program is with advantage provided via a computer program product comprising a data storage medium which can be read by a computer and which has the computer program stored on it. Said data storage medium is for example an optical data storage medium in the form of a CD ROM disc, a DVD disc etc., a magnetic data storage medium in the form of a hard disc, a diskette, a cassette tape etc., or a flash memory or a memory of the ROM, PROM, EPROM or EEPROM type. Fig. 5 illustrates very schematically an electronic control unit 17 comprising an execution means 18, e.g. a central processor unit (CPU), for execution of computer software. The execution means 18 communicates with a memory 19, e.g. of the RAM type, via a data bus 20. The control unit 17 comprises also a non-transitory data storage medium 21, e.g. in the form of a flash memory or a memory of the ROM, PROM, EPROM or EEPROM type. The execution means 18 communicates with the data storage medium 21 via the data bus 20. A computer program comprising computer program code for implementing a method accordi ng to the invention , e.g . in accordance with the embodiment ill ustrated in Fig . 3, is stored on the data storage medium 21 .

The invention is of course in no way restricted to the embodiments described above, since many possibi lities for modifications thereof are likely to be obvious to one skilled in the art without havi ng to deviate from the scope of i nvention defined in the appended claims.

The fi rst and second u nits may of course be combined in one single member, such as the ECU of a vehicle.