Method for measuring fluid injection

A method for measuring the injected mass of a fluid containing urea using a system for injecting said fluid into an exhaust system of a vehicle comprising a pump, an accumulator, a pressure sensor and a circuit distributing the fluid to an injector, with the pump and the injector being controlled by a computer as a function of engine operating parameters and of pressure measurements on the pressure sensor and in which injecting the fluid comprises sequences of series of injections of fluid contained in the accumulator between sequences for starting the pump upon detection of a pressure below a threshold PON and sequences for stopping the pump upon detection of a pressure above a threshold POFF, the method comprising:

• measuring the pressure after the pump has stopped before starting an injection sequence;
• measuring the pressure after the completion of the injection sequence and before starting the pump;
• computing the injected mass according to the formula:
• M
• =
• ρ
• K
• ·
• (
• P
• OFF
• -
• P
• ON
• )
• [
• Math
• .
• 9
• ]
• where ρ is the density of the fluid in kg/m3 and K is the complete circuit hydraulic stiffness in Pa/m3 and where K is computed on the basis of the formula:
• K
• =
• K
• l
• ·
• K
• D
• K
• l
• +
• K
• D
• [
• Math
• .
• 10
• ]
• where KD is the hydraulic stiffness of the accumulator and KI is the hydraulic stiffness of the remainder of the circuit;
• and
• comparing the computed injected mass with a setpoint injected mass.