Method and system for injecting a liquid

An SCR system for controlling an injector intended for injecting a pollution-control liquid additive into exhaust gases of an internal combustion engine at a flow rate governed by a given frequency and opening time of the injector, the injector having a calibration curve that establishes, at given frequency, pressure, temperature and nature of the liquid, an amount of liquid injected during an opening of the injector as a function of the duration of the opening time, the curve comprising at least one linear zone (calibration line) and a non-linear zone, said system comprising:

• an injector that injects a liquid flow rate, which varies depending on its opening time and frequency, into the exhaust gases of an internal combustion engine;
• and
• a controller configured to control the injector to independently vary either a frequency or an opening time, being in set ranges of opening times or frequencies respectively, said controller:
• determining whether a predetermined flow rate falls within the linear zone of the calibration curve;
• if said predetermined flow rate falls within the linear zone of the calibration curve, using a set frequency and determining a variable opening time to obtain the predetermined flow rate on the basis of the calibration line;
• if said predetermined flow rate does not fall within the linear zone of the calibration line, using a set opening time and determining a variable frequency on the basis of at least one calibration point;
• said controller configured to control said injector in accordance with the determined opening time and frequency,
• wherein said set opening time is greater than or equal to a minimum time (tmin) required in order for the opening of the injector to be effective, if said predetermined flow rate is lower than said linear zone, and
• wherein, to achieve increasing flow rates, use is made of the set opening time and the variable frequency until a certain flow rate is obtained, then use is made of the given frequency and the variable opening time until a second flow rate is reached;
• next, use is also made of a given, but higher, frequency and a second variable opening time and finally, if necessary, use is made of another set opening time and a third variable frequency.