CO2 MASS ESTIMATION SYSTEM, EXHAUST GAS COMPOSITION RATIO ESTIMATION METHOD, AND CO2 MASS ESTIMATION METHOD

A CO2 mass estimation system includes: an acquisition element acquiring detected values in accordance with concentrations of oxygen, H2O, and CO2 contained in an engine exhaust gas output from a gas sensor; a setting element setting an air-fuel ratio of a mixture; and a calculation element calculating the mass of CO2 contained in the exhaust gas, wherein the calculation element calculates the concentrations of oxygen, H2O, and CO2 contained in the exhaust gas based on the sensor detected values, acquires concentrations of oxygen and H2O in air and the air-fuel ratio, calculates a composition ratio of at least C atoms contained in fuel based on the concentrations in the exhaust gas, the concentrations in the air, and the air-fuel ratio, and estimates the mass of CO2 contained in the exhaust gas based on the composition ratio and the amount of injection of the fuel into the engine.