Method of operating a dosing device

A method of operating a dosing device (15) for introducing a liquid medium into an exhaust-gas stream of an internal combustion engine (1) of a motor vehicle, the method comprising:

• providing a dosing valve (13) with at least one low-pressure chamber (47) and at least one high-pressure chamber (41), wherein the low-pressure chamber (47) and the high-pressure chamber (41) are delimited by a piston (49) and a part of the piston which delimits the low-pressure chamber has a larger diameter than a part of the piston which delimits the high-pressure chamber, and the piston (49) travel is limited to a predefined maximum stroke;
• operating a pump (23) to deliver the liquid medium at a pre-delivery pressure into a delivery line (19);
• actuating the dosing valve (13) under direction of a control unit (31) by opening a 2/2 directional valve (17) along the delivery line (19) to direct the pumped liquid medium at the pre-delivery pressure into the low-pressure chamber (47) of the dosing valve (13), and in response, the piston (49) initiating a stroke;
• and
• injecting the liquid medium from the high-pressure chamber (41) into the exhaust-gas stream during the stroke of the piston (49), the injection stopping when the piston (49) reaches the maximum stroke,
• wherein the control unit (31) holds the 2/2 directional valve (17) open for longer than the piston (49) requires to perform its maximum stroke so that the dosing valve (13) delivers a predetermined volumetric dose of the liquid medium into the exhaust-gas stream, the predetermined volumetric dose corresponding directly to the maximum stroke of the piston (49).