the motor vehicle 2 schematically depicted in fig1 of the drawing has a diesel engine 6 , which is selectively supplied with diesel , biodiesel or gas - to - liquid ( gtl ) fuel from a fuel tank 4 , with a common rail fuel injection system 8 , which is activated by an engine control unit 10 . the diesel engine 6 has in a known manner an intake tract 12 with an air mass flow meter 14 for measuring the fresh air mass delivered into the cylinders 16 of the engine 6 and a throttle valve 18 for controlling the fresh air supply as well as an exhaust gas tract 20 with a lambda probe 22 for measuring the combustion - air ratio ( air ratio λ ) of the combustion gases discharged from the cylinders 16 . the fuel injection system 8 comprises likewise in a known manner a plurality of injectors or injection valves 24 , which are supplied with fuel from the fuel tank 4 by a fuel pump 26 and a high pressure piston pump 28 via a common manifold ( rail ) 30 . the fuel line 32 leading from the fuel tank 4 to the fuel distributor rail 30 comprises besides a fuel filter 34 and both pumps 26 , 28 additionally a pressure control valve 36 for controlling the fuel pressure before the high pressure pump 28 , a temperature sensor 38 for measuring the fuel temperature as well as a pressure sensor 40 for measuring the pressure in the fuel distributor rail 30 ( rail pressure ). the engine control unit 10 is connected via signal lines 42 to the sensors 38 , 40 and the injectors 24 of the fuel injection system 8 , to the air mass flow meter 14 and an actuator 44 for adjusting the throttle valve 18 , to the lambda probe 22 as well as to further , unspecified sensors and actuators , as , for example , a tachometer , respectively an accelerator pedal of the motor vehicle 2 . sensor signals with information or data are evaluated in the engine control unit 10 . said data comprise among other things the fresh air mass delivered into the cylinders 16 , which is measured by the air mass flow meter 14 , the combustion - air ratio measured by the lambda probe 22 , the rail pressure measured by the pressure sensor 40 as well as the fuel temperature measured by the temperature sensor 38 . the evaluated information or data together with an actuating signal from the accelerator pedal as well as with open - loop or closed - loop variables deposited in characteristic diagrams provide for the open - loop , respectively closed - loop , control of diverse parameters , which are significant for the combustion of fuel in the cylinders 16 . these parameters can comprise among other things the quantity of the fuel delivered into the cylinders 16 during one or a plurality of pilot injections , the point in time of the initiation of the pilot injection , respectively the pilot injections , the temporal interval between the pilot injections , respectively between a pilot injection , and the main injection , the height of the rail pressure supplied by the high pressure pump 28 , the position of the throttle valve 18 , the swirl of the injected fuel , the exhaust gas recirculation rate , i . e . the quantity of the exhaust gas recirculated out of the exhaust gas tract 20 into the cylinders , as well as the supercharging pressure of a super charger in the case of supercharged diesel engines . a portion of the information , which is transferred from the measuring devices , probes , respectively sensors 14 , 22 , 38 , 40 , to the engine control unit 10 , is furthermore used in the normal driving operation of the motor vehicle 2 in order to ascertain the ignitability of the fuel delivered from the fuel tank 4 into the fuel injection system 8 without necessitating the use of additional sensors or a special operating mode . said ignitability can undergo considerable changes particularly when a change in the type of fuel is made , for example from diesel fuel to biodiesel fuel or vice versa . the ascertained ignitability of the fuel , which represents a measurement for its fuel quality and is usually expressed as the cetane index cci or cetane number , is then used by the engine control unit 10 in order if required , i . e . when a change in the ascertained ignitability occurs , to appropriately change a portion of the previously mentioned parameters , respectively their nominal value characteristic diagrams , which are significant for the combustion in the cylinders , so that once again an optimal combustion is achieved . because the cetane index can on the one hand be determined with the aid of empirical formulas , for example according to an especially simple formula : whereby the cetane index cci besides from being a function of the average boiling temperature t50 of the fuel in ° c ., corrected to the icao standard atmosphere , is only a function of the density ρ of the fuel in kg / m 3 at a temperature of 15 ° c . and because on the other hand , the average boiling temperature t50 for all of the fuels , which are normally used in diesel engines , can be considered approximately as constant , it is possible to calculate the cetane index cci with a sufficient degree of accuracy solely from the density ρ of the fuel . a method suited for this purpose is schematically depicted in fig2 , wherein the density ρ of the fuel is calculated as an absolute value from the quotient of the mass m and the volume v of the fuel delivered during one injection into one of the cylinders 16 of the engine 6 . in the process , the mass m of the injected fuel is determined using a suitable software in the engine control unit 10 in step s 1 with the aid of a model calculation constructed for this purpose from the mass of the fresh air 46 delivered for combustion into the cylinder , which is measured by the air mass flow meter 14 , and from the combustion - air ratio 48 ( air ratio λ ) in the combustion exhaust gas being discharged from the cylinder 16 ; while in step s 2 , the injected fuel volume v , which is established by the rail pressure , as measured by sensor 40 , and by the activation duration 52 of the injectors , is likewise determined with the aid of a model calculation . in order to improve the accuracy of the method , the fuel temperature 54 measured by the sensor 38 as well as the backpressure 56 in the cylinder 16 deposited for the respective operating point of the engine 6 in characteristic diagrams of the engine control unit 10 are additionally taken into account in step s 2 . after the density ρ of the injected fuel has been calculated as the quotient of the mass m and the volume v in step s 3 , the cetane index of the fuel is then calculated in step s 4 from this density ρ , for example using the formula stated above . because the fuel mass m , which was actually injected , can slowly change at constant activation parameters , as for example rail pressure 50 , activation duration 52 , fuel temperature 54 and backpressure 56 in the cylinder , as the result of drifts in the fuel injection system 8 over the service life of the engine 6 , respectively the motor vehicle 2 just as the air mass 46 measured by the air mass flow meter 14 as a result of a drift of the air mass flow meter 14 , it is possible that the absolute value of the density ρ and the cetane index determined from said absolute value can gradually deviate from the actual density ρ , respectively from the actual cetane index cci , of the fuel . in order to compensate for this gradual deviation , respectively in order to take said deviation into account when ascertaining the density ρ , a reference value 58 for the mass m of the injected fuel can be determined in a preceding step s 0 for a new motor vehicle with a fuel with a known cetane index in the manner previously described . in step 5 while using the same fuel , this reference value 58 can then be compared with the fuel mass m , which was ascertained ( fig2 ), in predetermined temporal intervals , for example during each service inspection of the motor vehicle 2 . this is done as shown in fig3 in order to derive a correction value 60 for the calculation of the cetane index cci . this correction value is then taken into account in step s 4 . on the contrary , rapid changes in the cetane index cci , which was ascertained , suggest rather a change in the density ρ of the fuel , so that the accuracy of the method previously described can thereby also alternatively or additionally be improved , in that the determination of the fuel mass m is correlated with the fuelings of the tank 62 . said change in the density ρ of the fuel can , for example , be suggested from a large change in the fill level of the fuel in the fuel tank 4 , from the opening of a fuel filler flap or from the calculated fuel consumption . as a further advantage of the method described , the fuel density ρ ascertained for the determination of the ignitability can also furthermore be taken into account as a multiplicative factor during the conversion of the fuel quantity to be fed into the injectors 24 into the activation duration 52 of the injectors 24 .