Method and device for operating a gas internal combustion engine

1. A method for operating a gas internal combustion engine having a gas mixer, an intake path and an engine having a number of cylinders, the method comprising the steps of:

supplying the engine with a gaseous fuel mixture comprising a charge mixture and operating the engine in gas operation with gas as fuel in the charge mixture;

determining an output mixture fraction, assigned to a later mixture state, of the fuel gas-air mixture by an input mixture fraction, assigned to at least one earlier mixture state, of a fuel gas-air mixture, wherein the determining takes place by an intake path model serving as a basis for a computational model for the intake path, wherein the output mixture fraction of the fuel gas-air mixture is determined at an engine supply;

determining the input mixture fraction from the output mixture fraction via a number of intermediate states of the mixture fraction in a number of assigned volumes of the intake path in real-time simultaneous with operation of the engine, wherein the input mixture fraction of a fuel gas-air mixture is determined at the gas mixer and an air flow and/or a fuel gas flow at the gas mixer is set as a function of the input mixture fraction.

2. The method as claimed in claim 1 , wherein

the input mixture fraction of a gas-air mixture is determined at an outlet of the gas mixer and/or

the output mixture fraction of a gas-air mixture is determined at a cylinder or a cylinder inlet of the engine or a receiver.

3. The method as claimed in claim 1 , wherein determining a mixture fraction comprises:

determining a mixture mass flow by a throughflow equation for a volume of the intake path, wherein a mixture mass flow is assigned to the input mixture fraction at the gas mixer and/or a mixture mass flow at an engine supply is assigned to the output mixture fraction;

and/or

determining a mixture state includes at least determining a temperature and/or a state pressure of the mixture fraction for an assigned volume of the intake path by a thermodynamic state equation for real or ideal gases in the assigned volume.

4. The method as claimed in claim 3 , wherein the mixture fraction is determined in the assigned volume of the intake path for a number of theoretical computational volumes of the intake path and/or for a number of real housing volumes of the intake path, wherein the number of intermediate states of the mixture fraction in the intake path is assigned to at least one large volume of the intake path.

5. The method as claimed in claim 4 , wherein a number of large volumes of the intake path comprises at least one component volume of the intake path chosen from the group consisting of:

at least one receiver volume,

at least one cylinder volume in an engine block,

at least one intercooler volume, and

at least one compressor volume.

6. The method as claimed in claim 1 , wherein a combustion air ratio is assigned to the input mixture fraction, wherein a gas metering unit of the gas mixer is controlled by a stoichiometric air requirement and/or by the combustion air ratio.

7. The method as claimed in claim 1 , wherein a state pressure is defined as a receiver pressure in a receiver volume upstream of a cylinder of the engine, which is arranged upstream of the cylinder and is arranged downstream of a forced-induction unit and/or of a bypass path.

8. The method as claimed in claim 1 , wherein a state pressure is determined virtually based on a computational model of the intake path comprising at least one computational volume of a receiver volume and/or of an intercooler.

9. The method as claimed in claim 1 , wherein the number of at least one large volume(s) of the intake path comprises at least one component volume of the intake path chosen from the group consisting of:

at least one compressor bypass volume; and

at least one intake path volume.

10. The method as claimed in claim 9 , wherein the compressor bypass volume is at a bypass pipe section and/or a compressor bypass flap, and the intake path volume is at an intake pipe section and/or at an engine throttle flap and/or inlet throttle flap.

11. The method as claimed in claim 1 , wherein a mixture mass flow is determined at a throttle, wherein a throughflow of a return flow and/or of a forced-induction flow is determined by a throughflow equation for compressible media at an ideal or real nozzle assuming a flow of ideal or real gases which is frictionless or is subject to friction.

12. The method as claimed in claim 1 ,

wherein the gas operation is a spark-ignition gas operation.

13. An internal combustion engine designed as a gas internal combustion engine, comprising:

a gas mixer;

a gaseous fuel intake path;

an engine having a number of cylinders and a receiver volume arranged upstream of the cylinders;

a forced-induction unit for the fuel intake path;

a bypass path for the fuel intake path for bypassing the forced-induction unit,

wherein the engine is operable in gas operation with gas as fuel with a supplied fuel mixture comprising a charge mixture; and

a control system configured:

to determine an output mixture fraction, assigned to a later mixture state, of the fuel gas-air mixture by an input mixture fraction, assigned to at least one earlier mixture state, of a fuel gas-air mixture,

to determine the input mixture fraction in the earlier mixture state via the output mixture fraction in the later mixture state by an intake path model serving as a basis for a computational model for the intake path,

to determine the output mixture fraction of the fuel gas-air mixture at an engine supply, and

to determine the input mixture fraction from the mixture state of the output mixture fraction via a number of intermediate states of the mixture fraction, in a number of assigned volumes of the intake path in real-time simultaneous with operation of the engine, wherein

the input mixture fraction of a fuel gas-air mixture is determined at the gas mixer and an air flow and/or a fuel gas flow is set at the gas mixer as a function of the input mixture fraction.