fig1 is a system diagram of a gas engine 1 in v configuration with a generator 5 . the generator 5 is powered by the gas engine 1 via a shaft 2 , a coupling 3 , and a shaft 4 . electric power is generated by the generator 5 and fed into an electric network . the following mechanical components are assigned to the gas engine 1 : a gas throttle valve 6 for setting a supplied volume flow or fuel , for example , biogas , a mixer 7 for mixing air and fuel , a compressor 8 as part of an exhaust gas turbocharger , a cooler 9 , an a - side mixture throttle valve 10 in the a - side intake manifold 12 , and a b - side mixture throttle valve 11 in the b - side intake manifold 13 of the gas engine 1 . naturally , instead of the throttle valves , i . e ., the gas throttle valve 6 and the mixture throttle valves 10 and 11 , it is also possible to use other control devices , for example , a venturi mixer or a rotary valve . the mode of operation of the gas engine 1 is determined by an electronic engine control unit 14 ( gecu ). the electronic engine control unit 14 contains the usual components of a microcomputer system , for example , a microprocessor , interface adapters , buffers and memory components ( eeprom , ram ). operating characteristics that are relevant to the operation of the gas engine 1 are applied in the memory components in the form of input - output maps / characteristic curves . the electronic engine control unit 14 uses these to compute the output variables from the input variables . the following input variables are shown in fig1 : the a - side intake manifold pressure prra , a mixture temperature t 1 , a b - side intake manifold pressure prrb , the ( unfiltered ) raw values of the engine speed nmot of the gas engine 1 , a set speed nm ( sl ), which is preset by a system controller ( not shown ), and an input variable ein . the input variable ein is representative of the other input signals , for example , the oil temperature . the following are shown as output variables of the electronic engine control unit 14 : an adjusted set volume flow va ( sl ) for controlling the gas throttle valve 6 , an a - side mixture throttle angle dkwa for controlling the a - side mixture throttle valve 10 , a b - side mixture throttle angle dkwb for controlling the b - side mixture throttle valve 11 , and a signal aus . the signal aus is representative of the other signals for automatically controlling the gas engine 1 . the system has the following general functionality : a fuel volume flow supplied to the mixer 7 is adjusted by the position of the gas throttle valve 6 . the position of the a - side mixture throttle valve 10 defines an a - side mixture volume and thus the a - side intake manifold pressure prra in the a - side intake manifold 12 upstream of the intake valves of the gas engine 1 . the b - side intake manifold pressure prrb upstream of the intake valves of the gas engine 1 is determined by the b - side mixture throttle valve 11 . fig2 shows a block diagram for controlling the two mixture throttle valves 10 and 11 and the gas throttle valve 6 . the system controller of the generator is identified by reference number 15 . reference number 14 identifies the electronic engine control unit in the form of a reduced block diagram , in which the depicted elements represent the program steps of an executable program . the input variables of the electronic engine control unit 14 that are shown in this drawing are the raw values of the engine speed nmot , the a - side intake manifold pressure prra , the b - side intake manifold pressure prrb , the set speed nm ( sl ), and an active electric power pwel . the set speed nm ( sl ) and the active electric power pwel are supplied by the system controller 15 . the electronic engine control unit 14 computes the generator torque mgen from the active electric power pwel . the electronic engine control unit 14 uses the raw values of the engine speed nmot to compute the actual speed by means of a speed filter ( not shown ). the output variables of the electronic engine control unit 14 that are shown in fig2 are the a - side mixture throttle angle dkwa for controlling the a - side mixture throttle valve 10 , the b - side mixture throttle angle dkwb for controlling the b - side mixture throttle valve 11 , and the adjusted set volume flow va ( sl ) for controlling the gas throttle valve 6 . the set speed nm ( sl ), for example , 1500 rpm , which corresponds to a frequency of 50 hz , is preset by the system controller 15 as the desired output . at a point a , a speed control deviation dn is computed from the set speed nm ( sl ) and the actual speed mm ( ist ). a speed controller 16 in turn uses the speed control deviation dn to compute the controller torque mr as a correcting variable . in practice , the speed controller 16 is realized as a pidt1 controller . the controller torque mr is the first input variable of a consumption input - output map 17 . the second input variable is the actual speed nm ( ist ). a set volume flow v ( sl ) is determined by the consumption input - output map 17 as a function of the two input variables . the set volume flow v ( sl ) is the input variable for both a volume adjustment unit 31 and a mixture quantity unit 18 . the set volume flow v ( sl ) is adjusted by the volume adjustment unit 31 at least as a function of the generator torque mgen . the output variable of the volume adjustment unit 31 is an adjusted set volume flow va ( sl ), which is the input variable of the gas throttle valve 6 . an electronic processing unit integrated in the gas throttle valve 6 assigns to the value of the adjusted set volume flow va ( sl ) a corresponding cross - sectional area and a corresponding angle . a fuel volume flow as a gas fraction of the air / fuel mixture is set by the gas throttle valve 6 . the mixture quantity unit 18 combines the computation of the set intake manifold pressure and a cascade closed - loop pressure control system for the intake manifold . the conversion of the set volume flow v ( sl ) in the mixture quantity unit 18 is shown in fig3 and will be described with reference to that drawing . as shown in fig2 , the other input variables of the mixture quantity unit 18 are the generator torque mgen , the controller torque mr , the actual speed nm ( ist ), and the two intake manifold pressures prra and prrb as controlled variables of the closed - loop pressure control system for the intake manifold . the a - side mixture throttle angle dkwa and the b - side mixture throttle angle dkwb are computed by the mixture quantity unit 18 . the a - side mixture throttle angle dkwa controls the a - side mixture throttle valve 10 , by which the a - side intake manifold pressure prra is automatically controlled . the b - side mixture throttle angle dkwb controls the b - side mixture throttle valve 11 , by which the b - side intake manifold pressure prrb is automatically controlled . as is clear from the figure , a central element is the parallel control of the gas throttle valve and the mixture throttle valves as a function of the same actuating variable , here the set volume flow v ( sl ). the block diagram in fig3 shows a functional block 19 for determining the set intake manifold pressure prr ( sl ), which is part of the mixture quantity unit 18 . the input variables of the functional block 19 are the controller torque mr computed by the speed controller 16 ( fig2 ), the actual speed nm ( ist ), the generator torque mgen , and the set volume flow v ( sl ). the output variable is the set intake manifold pressure prr ( sl ), which is then the reference input for the following intake manifold closed - loop pressure control system . an intake manifold closed - loop pressure control system of this type is described , for example , in de 10 2007 045 195 b3 . a computing unit 20 uses the controller torque mr and the generator torque mgen to determine a deviation ys by computing the quotient mr / mgen . the deviation ys is a measure of the amount by which the energy content of the fuel actually being used differs from the energy content of the reference fuel . the unit of energy content is given in kilowatt - hours per standard cubic meter of gas ( kwh / nm 3 ). the system is set to natural gas as the reference fuel by the manufacturer of the gas engine . if natural gas is used , then the quotient mr / mgen is equal to one . if , on the other hand , a gas with a lower energy content is used , for example , biogas , then the quotient can be in the range of 1 . 4 to 1 . 5 , depending on the inert gas fraction . a reference combustion - air ratio lamr is assigned to the controller torque mr and the actual speed nm ( ist ) by an input - output map 21 . the reference combustion - air ratio lamr is the first input variable of a correction unit 22 . the second input variable is a reference air requirement lminr , which in the present case is constant . the reference air requirement lminr represents the stoichiometric air requirement for the complete combustion of one cubic meter of the reference gas . the third input variable is the deviation ys . the correction unit 22 uses the following relation to compute the corrected combustion air ratio lamk : the output variable of the correction unit 22 , i . e ., the corrected combustion air ratio lamk , is the first corrected input variable of a computing unit 25 for determining the set intake manifold pressure prr ( sl ). a correction unit 23 computes a corrected air requirement lmink as a function of the deviation ys . the correction unit 23 is shown in fig4 and will be described with reference to that drawing . the corrected air requirement lmink is the second corrected input variable of the computing unit 25 . the third corrected input variable of the computing unit 25 is a corrected set volume flow vk ( sl ), which is computed in computing unit 24 by multiplying the set volume flow v ( sl ) by the square of the deviation ys . the computing unit 25 computes the set intake manifold pressure prr ( sl ) by the following relation : prr ( sl )= 2 · ys 2 · t 1 · p 0 ·[ 1 + lamk · lmink ]· vk ( sl )/[ lg · vh · nm ( ist )· t 0 ] ( 2 ) where ys is the deviation , t 1 is the temperature measured in the intake manifold , p0 is the standard air pressure at mean sea level ( 1013 hpa ), lamk is the corrected combustion air ratio , lmink is the corrected air requirement , vk ( sl ) is the corrected set volume flow , lg is the volumetric efficiency , vh is the stroke volume of the cylinder , nm ( ist ) is the actual speed of the gas engine , and t 0 is standard temperature ( 273 . 15k ). in the drawing ( fig3 ), the volumetric efficiency lg , the stroke volume vh , and the standard temperature t 0 are combined under the reference letter e . as is evident from fig3 , the set intake manifold pressure prr ( sl ) is corrected by using corrected input variables . the deviation ys proved to be the most useful correction variable , since it is a measure of the amount by which the energy content of the gas that is actually used , for example , biogas with a methane gas content of 45 vol . %, deviates from the energy content of the reference gas , which in the present case is natural gas . fig4 is a block diagram that shows the computation of the corrected air requirement . the input variables are the deviation ys and the ignition point zzp . the output variables are the corrected air requirement lmink and a filtered mixing parameter xsf . the corrected air requirement lmink is computed by means of a recursive loop 26 . a mixing parameter xs is computed by a computing unit 27 from the deviation ys and an efficiency ratio eta . in the first pass through the recursive loop , the initial value of the efficiency ratio is eta = 1 . the mixing parameter is computed by the following relation : where xs is the mixing parameter . the constant huo represents the greatest calorific value to be adopted for the fuel that is actually being used , for example , biogas . the constant huu represents the smallest calorific value to be adopted for the fuel that is actually being used . the constant hur represents the calorific value of the reference fuel , here : natural gas . to determine the constants huo and huu , it is necessary to know the provenience and the family of gases to which the fuels belong . these constants are not varied in the operation of the gas engine . the mixing parameter xs is then supplied to an efficiency input - output map 28 , which determines a new efficiency ratio eta as a function of the mixing parameter xs and the ignition point zzp . the efficiency ratio eta can be obtained as the ratio of an actual efficiency to a reference efficiency determined on the test bench with the use of the reference fuel ( natural gas ). the new efficiency ratio eta is then fed back to the computing unit 27 , in which the mixing parameter xs is then recomputed from the new efficiency ratio eta by formula ( 3 ). the recursive loop is repeatedly passed through until a termination criterion is recognized . a termination criterion occurs when the recursive loop 26 has been passed through i times . alternatively , a termination criterion is present when the difference between two recursively computed mixing parameters is less than a limit . when the termination criterion has been recognized , the last mixing parameter computed is set as the valid value . the recursive loop 26 is followed by a filter 29 , typically a pt1 filter , which filters the mixing parameter that has been set as valid . a computing unit 30 uses the filtered mixing parameter xsf and constant values k to compute the corrected air requirement lmink , which is further processed in the functional block 19 shown in fig3 . the computation is carried out with the following relation : where lminu is the minimum air requirement of the fuel that is actually used and lmino is the maximum air requirement of the fuel that is actually used , which are the constants . the mixing parameter xsf is further processed internally , for example , for adjustment of the fuel density and the ignition point . fig5 is a program flowchart of the method of the invention . at s 1 the actual speed nm ( ist ) and the set speed nm ( sl ) are read in , and then at s 2 they are used to compute the speed control deviation dn . at s 3 the speed controller uses the speed control deviation dn to determine the controller torque mr as a correcting variable , for example , by means of a pidt1 algorithm . at s 4 the consumption input - output map 17 ( fig2 ) computes the set volume flow v ( sl ) as a function of the controller torque mr and the actual speed nm ( ist ). at s 5 the deviation ys is determined from the controller torque mr and the generator torque mgen by computation of the quotient mr / mgen . at s 6 a reference combustion air ratio lamr is assigned to the controller torque mr and the actual speed nm ( ist ) by an input - output map 21 ( fig3 ). at s 7 formula ( 1 ) is then used to determine the corrected combustion air ratio lamk . then at s 8 control passes to a subroutine up 1 for computation of the corrected air requirement lmink . the subroutine up 1 is shown in fig6 and will be described in connection with fig5 . after control passes back to the main program , then at s 9 the corrected set volume flow vk ( sl ) is computed from the set volume flow v ( sl ) and the deviation ys . at s 10 formula ( 2 ) is used to compute the set intake manifold pressure prr ( sl ) as a function of the corrected set volume flow vk ( sl ), the corrected air requirement lmink , and the corrected combustion air ratio lamk . the program then ends . fig6 shows the subroutine up 1 for computing the corrected air requirement lmink by a recursive method . at s 1 a running variable i and the efficiency ratio eta are set to the initial value of one . at s 2 the mixing parameter xs is computed by formula ( 3 ). the constant huo represents the greatest calorific value to be adopted for the fuel that is actually being used , for example , biogas . the constant huu represents the smallest calorific value to be adopted for the fuel that is actually being used . the constant hur represents the calorific value of the reference fuel , here : natural gas . to determine the constants huo and huu , it is necessary to know the provenience and the family of gases to which the fuels belong . these constants are not varied in the operation of the gas engine . then at s 3 an input - output map 28 ( fig4 ) assigns a new efficiency ratio eta to the mixing parameter and the ignition point zzp . the efficiency ratio eta can be obtained as the quotient of an actual efficiency and a reference efficiency . at s 4 a new mixing parameter is determined with the previously computed new efficiency ratio eta and set as the new mixing parameter . on the first pass through the loop , the new mixing parameter is then xs ( 2 ). the running variable i is then increased by one at s 5 , and its value is interrogated at s 6 . if the running variable i is less than four ( interrogation result at s 6 : yes ), then the program flow continues at s 3 . if the interrogation result at s 6 is negative , the last computed mixing parameter is set as the valid mixing parameter . the valid mixing parameters computed in this way within an interval of time are filtered at s 7 , for example , by a pt1 filter . the result is the filtered mixing parameter xsf . at s 8 the corrected air requirement lmink is computed as a function of the filtered mixing parameter xsf . the filtered mixing parameter xsf is further processed internally , for example , for adjustment of the fuel density bd and the ignition point zzp . control then returns to the main program of fig5 at s 8 . the invention was described with reference to a gas engine that powers a generator . instead of a generator , a standby - ready unit or a combined heat and power ( chp ) installation can also be used . in this case , the generator torque mgen then corresponds to the torque delivered by , for example , the standby - ready unit .