Method for determining the composition of the fuel used to operate an internal combustion engine

1. A method for operating an internal combustion engine, the method comprising:

measuring dynamic pressure oscillations, assignable to a cylinder of the internal combustion engine, in the inlet tract of the respective internal combustion engine at a defined operating point during intake-synchronous fuel injection during normal operation;

based at least in part on the measured oscillations, generating a corresponding pressure oscillation signal;

determining a crankshaft phase angle signal of the internal combustion engine;

determining an actual phase position based on the pressure oscillation signal using discrete Fourier transformation, the actual phase position of a selected signal frequency of the measured pressure oscillations determined in relation to the crankshaft phase angle signal;

determining a chemical composition of a fuel being burnt on the basis of the determined actual phase position and reference phase positions of the same signal frequency for different fuel compositions; and

adjusting operating parameters of the internal combustion engine based on the determined chemical composition.

2. The method as claimed in claim 1 , wherein the reference phase positions as a function of the fuel composition are stored in reference value characteristic maps.

3. The method as claimed in claim 1 , wherein determining the chemical composition of the fuel depends on at least one of the further operating parameters:

temperature of the intake medium in the intake tract,

temperature of a coolant used for cooling the internal combustion engine, and

engine speed of the internal combustion engine.

4. The method as claimed in claim 2 , wherein determining an actual phase position of the selected signal frequency and the composition of the fuel are performed by an electronic processing unit associated with the internal combustion engine; and

the reference value characteristic maps are stored in a memory of the electronic processing unit.

5. The method as claimed in claim 1 , wherein reference phase positions of the selected signal frequency have been determined in advance on a reference internal combustion engine as a function of different fuel compositions.

6. The method as claimed in claim 1 , wherein the fuel composition is calculated based on a model function of reference phase positions stored in an engine controller.

7. The method as claimed in claim 5 , wherein the determination in advance of the reference phase positions of the selected signal frequency includes measurement of a reference internal combustion engine at a defined operating point during intake-synchronous fuel injection, using reference fuels of different, known fuel compositions;

wherein the determination of the reference phase positions of the selected signal frequency includes measuring the dynamic pressure oscillations assignable to a cylinder of the reference internal combustion engine in the inlet tract during operation;

generating a corresponding pressure oscillation signal;

determining at the same time a crankshaft phase angle signal;

determining a reference phase position of the selected signal frequency of the measured pressure oscillations in relation to the crankshaft phase angle signal from the pressure oscillation signal by means of discrete Fourier transformation; and

storing the determined reference phase angles as a function of the associated fuel compositions in reference value characteristic maps.

8. The method as claimed in claim 1 , wherein the selected signal frequencies comprise an intake frequency or a multiple of the intake frequency.

9. The method as claimed in claim 1 , wherein the dynamic pressure oscillations are measured with a series-production-type pressure sensor in the intake pipe.

10. The method as claimed in claim 1 , wherein the crankshaft position feedback signal is generated by a toothed gear and a Hall sensor.

11. The method as claimed in claim 6 , wherein the model function is derived from the reference phase positions of the selected signal frequency and the associated fuel compositions which replicates the relationship between the reference phase positions of the selected signal frequency and the fuel compositions.