Process for transferring a liquid using a pump

1. A process for transferring liquid from a passive tank for storing the liquid to an active tank, using a transfer pump intended for pumping the liquid held in the passive tank and transferring said liquid to the active tank,

wherein the pump is commanded so as to transfer the liquid from the passive tank to the active tank when said active tank has a liquid level L below a set point level Lsp and when the passive tank contains liquid at a temperature above a freezing point of the liquid,

wherein said process comprising:

measuring a temperature Text outside of the active tank and the passive tank;

checking whether or not the liquid in the passive tank is frozen using the measured temperature Text, former values of Text, and a time constant that corresponds to the liquid in the passive tank changing from a solid state to a liquid state or vice versa;

comparing the liquid level L in the active tank to the set point level Lsp;

activating the transfer pump if the liquid in the passive tank is not frozen and L<Lsp, so as to pump the liquid in the passive tank and transfer the liquid into the active tank until a target level Lt is reached, where Lt>=Lsp, and the transfer pump is stopped; and

not activating the transfer pump if the liquid in the passive tank is frozen or if L>=Lsp.

2. The process according to claim 1 , wherein the active tank is provided with a heating device.

3. The process according to claim 1 , wherein

the liquid level L is measured using a level sensor equipped with a float and positioned in the active tank, if the float is not blocked; and

the liquid level L is deduced from a value of the liquid level L stored in a memory and from a consumption of the liquid in the active tank, if the float is blocked.

4. The process according to claim 1 , wherein a time during which the pump transfers the liquid is measured and the pump is stopped if the time is greater than a time constant TD and the liquid level L has not reached the target level Lt.

5. The process according to claim 1 , wherein the active tank comprises a level sensor of a float type which provides three indications of the level L as a function of the relative position of the float with respect to two set point levels L 1 and L 2 , where L 1 <L 2 <=Lt:

when the level L is greater than or equal to L 2 , the level sensor indicates a high level,

when the level L is less than L 2 and greater than or equal to L 1 , the level sensor indicates an intermediate level; and

when the level L is less than L 1 , the level sensor indicates a low level.

6. The process according to claim 5 ,

wherein the passive tank is located below the active tank and is filled by overflow of said active tank, and

wherein when the level L in the active tank is greater than or equal to L 2 , the pump is commanded to transfer a quantity of liquid equivalent to an increment ΔLof of the liquid level L that corresponds to an overflow volume in the active tank so that Lt=L 2 +ΔLof.

7. The process according to claim 6 , wherein when the level L in the active tank is greater than or equal to L 2 , a time for which the pump transfers liquid is measured and the pump is stopped if the time is greater than a time constant TO and the liquid level L is <(Lt=L 2 +ΔLof).

8. The process according to claim 1 , said process being performed by a selective catalytic reduction (SCR) system supplying urea to exhaust gases of an internal combustion engine and comprising said active tank, said passive tank, and said transfer pump, and said process being started only when the engine has reached a given rotation speed.