A combustion state estimation method comprising:
- acquiring a relationship between a first combustion period length and maximum heat release rate timing in a reference state as a reference relationship with estimation or actual measurement, the reference state being a state where a rotation speed of a spark-ignition internal combustion engine is a predetermined reference rotation speed and a valve opening characteristic of an intake valve of the engine is set to a predetermined reference valve opening characteristic, the first combustion period length being a duration of a first combustion period, the first combustion period being a period from ignition timing of fuel in a combustion chamber of the engine to the maximum heat release rate timing, the maximum heat release rate timing being timing at which a rate of release of heat resulting from combustion of the fuel is maximum;
- estimating a relationship between the maximum heat release rate timing and the first combustion period length in a specific state on the basis of the reference relationship, the specific state being a state where the rotation speed of the engine is the reference rotation speed and the valve opening characteristic of the intake valve is set to a specific valve opening characteristic different from the reference valve opening characteristic;
- acquiring the first combustion period length at the time when the maximum heat release rate timing is predetermined first timing in the reference state as a first reference period length on the basis of the reference relationship or with actual measurement;
- acquiring the first combustion period length at the time when the maximum heat release rate timing is the first timing in the specific state as a first specific period length with actual measurement;
- acquiring a first differential value by subtracting the first reference period length from the first specific period length or acquiring a second differential value by subtracting the first specific period length from the first reference period length;
- acquiring the first combustion period length at the time when the maximum heat release rate timing is second timing different from the first timing in the reference state as a second reference period length on the basis of the reference relationship;
- and
- estimating the first combustion period length at the time when the maximum heat release rate timing is the second timing in the specific state by adding the first differential value to the second reference period length or subtracting the second differential value from the second reference period length.