A method for determining the deviation of the quality of a fuel from a reference quality, and the resulting change in exhaust-gas emissions, through evaluation of pressure waves in the fuel system, wherein - the internal combustion engine is equipped with discontinuous fuel conveyance and/or fuel withdrawal, - during operation, the pressure oscillations which form in the fuel system are detected by means of at least one sensor, - the deviation from a fuel reference quality is ascertained through the analysis of this pressure profile in an electronic control unit, characterized in that - deviations of the untreated emissions of the internal combustion engine from the untreated emissions in the case of the reference fuel being used and/or influences on the changed behaviour of an aftertreatment system connected downstream of the internal combustion engine in relation to operation with reference fuel are determined from the deviation from a fuel reference quality, and - in that, in the event of a deviation from the reference fuel quality being ascertained, a change of operating parameters of an exhaust-gas aftertreatment system positioned downstream of the internal combustion engine is performed, wherein the operating parameters of the aftertreatment system are changed, in a manner dependent on the information items obtained from the pressure oscillations, such that the emissions are at least approximated to those in the case of the reference fuel being used; - wherein the emissions in the case of a non-reference fuel being used are adapted to the emissions in the case of the reference fuel being used a) in that, in the case of a catalytic converter for selective catalytic reduction of the nitrogen oxides being used, the supplied reducing agent quantity is changed; and/or b) in that, in the case of a NO x storage catalytic converter for the reduction of the nitrogen oxides being used, the ratios between rich and lean operating phases are changed; and/or c) in that, in the case of a particle filter for separating off soot particles being used, the intervals between soot accumulation and particle filter regeneration are changed; and/or d) in that, in the case of an aftertreatment system being used, the exhaust-gas temperatures are changed by changing the operating parameters of the internal combustion engine. - the internal combustion engine is equipped with discontinuous fuel conveyance and/or fuel withdrawal, - during operation, the pressure oscillations which form in the fuel system are detected by means of at least one sensor, - the deviation from a fuel reference quality is ascertained through the analysis of this pressure profile in an electronic control unit, characterized in that - deviations of the untreated emissions of the internal combustion engine from the untreated emissions in the case of the reference fuel being used and/or influences on the changed behaviour of an aftertreatment system connected downstream of the internal combustion engine in relation to operation with reference fuel are determined from the deviation from a fuel reference quality, and - in that, in the event of a deviation from the reference fuel quality being ascertained, a change of operating parameters of an exhaust-gas aftertreatment system positioned downstream of the internal combustion engine is performed, wherein the operating parameters of the aftertreatment system are changed, in a manner dependent on the information items obtained from the pressure oscillations, such that the emissions are at least approximated to those in the case of the reference fuel being used; - wherein the emissions in the case of a non-reference fuel being used are adapted to the emissions in the case of the reference fuel being used a) in that, in the case of a catalytic converter for selective catalytic reduction of the nitrogen oxides being used, the supplied reducing agent quantity is changed; and/or b) in that, in the case of a NO x storage catalytic converter for the reduction of the nitrogen oxides being used, the ratios between rich and lean operating phases are changed; and/or c) in that, in the case of a particle filter for separating off soot particles being used, the intervals between soot accumulation and particle filter regeneration are changed; and/or d) in that, in the case of an aftertreatment system being used, the exhaust-gas temperatures are changed by changing the operating parameters of the internal combustion engine. a) in that, in the case of a catalytic converter for selective catalytic reduction of the nitrogen oxides being used, the supplied reducing agent quantity is changed; and/or b) in that, in the case of a NO x storage catalytic converter for the reduction of the nitrogen oxides being used, the ratios between rich and lean operating phases are changed; and/or c) in that, in the case of a particle filter for separating off soot particles being used, the intervals between soot accumulation and particle filter regeneration are changed; and/or d) in that, in the case of an aftertreatment system being used, the exhaust-gas temperatures are changed by changing the operating parameters of the internal combustion engine.