Method to determine the loading state of a particulate filter

An internal combustion engine (1) has a particle filter (10) with temperature (11,13) and pressure sensors (14,15) and an input air flow meter (6). A control unit (12) receives the data and calculates the trapped mass from a formula which links it with pressure difference and air flow and uses coefficients which take account of the accretion of incombustible material with length of use.