Method for metering a reducing agent, method for setting up a control unit for a metering device and motor vehicle having a metering device

1. A method for metering a reducing agent from a metering device to an exhaust gas treatment device, the metering device having an injector and a delivery unit with a pump and a pressure sensor, the delivery unit and the injector being connected by a line leading from the delivery unit to the injector, the method comprising the following steps:

a) determining at least one operating parameter of the metering device the at least one parameter being a system pressure in the delivery device determined by the sensor and providing the at least one operating parameter to a control unit;

b) calculating in the control unit, from the at least one operating parameter, an injector pressure at the injector for feeding the reducing agent into the exhaust gas treatment device while considering at least one of a length of the line, a volume of the line, a diameter of the line, or a flexibility of the line;

c) calculating, in the control unit, an opening time and an opening instant for the injector using at least the injector pressure determined in step b); and

d) opening the injector for the opening time at the opening instant calculated in step c).

2. The method according to claim 1 , which further comprises using at least one of at least one opening period or at least one closing period of the injector as the operating parameter in step a).

3. The method according to claim 1 , which further comprises specifying the opening instant and a closing instant to ensure that no pressure fluctuation initiated by the pump reaches the injector during an opening time between the opening instant and the closing instant.

4. The method according to claim 1 , which further comprises taking at least one of the following properties of the metering device into account in step b):

at least one property of the delivery unit of the metering device;

at least one property of the pump of the metering device; or

at least one property of the control unit configured to control the metering device.

5. The method according to claim 4 , which further comprises taking an elasticity and a length of the line into account as the property of the metering device.

6. The method according to claim 5 , which further comprises adapting the elasticity of the line, taken into account, at regular intervals in order to take ageing of the line into account.

7. The method according to claim 1 , which further comprises carrying out at least the calculation of the injector pressure in step b) without feedback.

8. The method according to claim 1 , which further comprises using at least one of the following further parameters in step c) for calculating the opening time:

a temperature of the reducing agent;

a viscosity of the reducing agent;

at least one physical property of the reducing agent;

at least one chemical property of the reducing agent; or

a required amount of reducing agent.

9. The method according to claim 1 , which further comprises taking at least one of the following parameters into account as a disturbing influence in step b):

a level of reducing agent in a reducing agent tank;

a temperature of the reducing agent;

a density of the reducing agent;

a viscosity of the reducing agent;

at least one physical property of the reducing agent;

at least one chemical property of the reducing agent; or

a required amount of reducing agent.

10. A motor vehicle, comprising:

an internal combustion engine;

an exhaust gas treatment device for purifying exhaust gases from the internal combustion engine;

a metering device for metering a reducing agent into the exhaust gas treatment device; and

a control unit configured to carry out the method according to claim 1 .