FIELD OF THE INVENTION:
The invention relates to a device for dosing of liquids.
Particularly it relates to a device for dosing of reducing agent in the exhaust path.
BACKGROUND OF THE INVENTION:
Systems using the Water-hammer effect for controlled injection tasks are already known. The fluid being pumped is suddenly stopped by closing the valve in the path. This results in a pressure impulse being generated in the fluid being pumped. The resulting pressure impulse is transferred to a spring loaded needle. When the opening pressure for the needle is exceeded, then the injection occurs. Such systems are also known for dosing of reducing agent in Selective Catalytic Reduction (SCR) applications.
Patent publication DE102008018945 discloses a device for dosing of urea solution in the exhaust path. This device uses two actuators for operation in form of a motor for the pumping of urea solution and a second actuator for controlling the opening and closing of a valve to generate the pressure impulse in the fluid flow. Some applications where precise metering for the dosing is required are provided with pressure relieve valves to compensate for tolerances in pump flow rate which may result in different dosing pressures and quantities. The valve that is typically used for these applications is an ON-OFF valve and does not allow control over the supply pressure.
DESCRIPTION OF THE INVENTION:
The present invention discloses a device for dosing liquids. The device comprises a pump driven by a motor to pump the liquid from a liquid storage tank to a first port of a T-joint, a normally closed dosing valve connected to a second port of the T-joint and receiving the liquid being pumped. A comparator compares pressure measured by the pressure sensor mounted in the liquid flow path to a threshold pressure value. A second valve with a stepped opening behaviour is connected to a third port of the T-joint and actuated by a

valve controller based on the output of the comparator. The stepped opening behaviour of the second valve gives the advantage of having better control on the supply pressure.
Generally the liquid dosing systems based on water hammer effect include a pressure relieve valve. But the pressure relieve valve adds hydraulic losses and cost to assembly of a water-hammer system. Depending on the pump and flow path configuration there are waves of low frequency in the system. These, waves affect the maximum injection pressure and hence injection quantity. The present invention improves the functioning of the system by controlling the initial pressure via a proportional valve or valve with a stepped opening behaviour.
BRIEF DESCRIPTION OF THE DRAWINGS:
The invention is now described in reference to the accompanying drawing
Fig. 1 shows a block diagram illustrating the device for dosing of liquids in accordance with
this invention; and
Fig. 2 illustrates a flowchart showing the steps in the operation of the device in accordance
with this invention.
DETAILED DESCRIPTION OF THE DRAWINGS:
Fig. 1 of the accompanying drawings illustrates a device for dosing liquids. The device includes a pump (12) driven by a motor (10) to pump (12) the liquid from a liquid storage tank to a first port of a T-joint (14). A normally closed dosing valve (16) is connected to a second port of the T-joint (14). A pressure sensor (18) is mounted in the liquid flow path either in the path between the pump (12) and the T-joint (14) or in the path between the T-joint (14) and the dosing valve (16). A comparator (20) compares the pressure measured by the pressure sensor (18) with a threshold (22) pressure value. A second valve (24) is connected to a third port of the T-joint (14) and actuated by a valve controller (26) based on the output of the comparator (20). The second valve (24) has a stepped opening behaviour.

Fig. 2 of the accompanying drawings illustrates a method for controlling supply dosing of liquids. The liquid is pumped from a tank by a pump and motor assembly to a dosing valve via a liquid flow path (SO). The pressure in the liquid flow path is measured by a pressure sensor (SI). The pressure measured by the pressure valve is compared with a threshold pressure value (S2). The opening of a second valve is controlled by a valve controller based on the comparison of the pressure in the flow path with the threshold (S3). The liquid is dosed by opening of the dosing valve due to the pressure impulse generated by the opening of the second valve (S4).
The liquid dosing systems available presently use a shut off valve to generate a pressure impulse for the water hammer effect. The shut off valves do not allow control of the liquid supply pressure. In accordance with the present invention a second valve (20) with at least two stage opening behaviour or a proportional valve is provided in place of the shut off valve.
In one embodiment the pressure sensor is mounted in the liquid path between the pump and the T-joint. In another embodiment the pressure sensor is mounted at the dosing valve. A comparator compares the measured value of the pressure in the liquid flow path with the pre stored threshold. When the second valve starts to close the pressure in the liquid flow path starts to increase. When the pressure reaches the threshold the second valve closes completely and the pressure in the flow path peaks generating the pressure impulse to open the dosing valve. The duration and opening of the second valve is controlled based on the deviation of the pressure from the threshold value. Thus, the stepped opening of the second valve allows better control on the supply pressure for dosing of the liquid.
It should be understood that embodiments explained in the description above are only illustrative and do not limit the scope of this invention. Many such embodiments and other modifications and changes in the embodiment explained in the description are envisaged. The scope of the invention is only limited by the scope of the claims.

WE CLAIM:
1. A device for dosing liquids, said device comprising a pump (12) driven by a motor
(10) to pump (12) the liquid from a liquid storage tank to a first port of a T-joint (14); a
normally closed dosing valve (16) connected to a second port of the T-joint (14) and
receiving the liquid being pumped; said device characterized by
a pressure sensor (18) mounted in the liquid flow path;
a comparator (20) to compare pressure measured by the pressure sensor to a threshold (22)
pressure value;
a second valve (24) connected to a third port of the T-joint (14) and actuated by a valve
controller (26) based on the output of the comparator (20), said second valve (24) having a
stepped opening behaviour.
2. A device for dosing liquids as claimed in claim (1) wherein said second valve (20) is a proportional valve.
3. A device for dosing liquids as claimed in claim (1) wherein said second valve (20) has at least two stage opening behaviour.
4. A device for dosing liquids as claimed in claim (1) wherein in first stage the second valve is partially open.
5. A device for dosing liquids as claimed in claim (1) wherein in second stage the second valve is completely open.
6. A device as claimed in claim (1) wherein, the pressure sensor is mounted in the liquid path between the pump and the T-joint.
7. A device as claimed in claim (1) wherein, the pressure sensor is mounted at the dosing valve.
8. A method for controlling supply dosing of liquids, said method comprising the following steps
pumping the liquid from tank by a pump and motor assembly to a dosing valve via a liquid
flow path (SO);
measuring the pressure in the liquid flow path (SI);
comparing the measured pressure with a threshold pressure value (S2);

controlling the opening of a second valve based on the comparison of the pressure in the flow path with the threshold (S3); and
dosing of said liquid by opening of the dosing valve due to the pressure impulse generated by variations in the opening of the second valve (S4).
9. A method as claimed in claim (3) wherein, the duration and opening of the second valve is controlled based on the deviation of the pressure from the threshold value.