Oil dilution rate calculation system of internal combustion engine

An oil dilution rate calculation system of an internal combustion engine, wherein

• the internal combustion engine has an intake passage in which a throttle valve is arranged and which leads an air-fuel mixture containing air and fuel to a combustion chamber, an exhaust passage discharging exhaust gas produced by combustion of the air-fuel mixture in the combustion chamber, a blowby gas passage returning blowby gas in a crankcase to the downstream side of the throttle valve in the intake passage, and an air-fuel ratio sensor provided in the exhaust passage and detecting an air-fuel ratio of the exhaust gas flowing through the exhaust passage, and
• the oil dilution rate calculation system is configured to acquire a blowby gas flow ratio showing the ratio of the flow of blowby gas to the flow of gas to the combustion chamber and an output current of the air-fuel ratio sensor during fuel cut control in which the internal combustion engine stops the feed of fuel to the combustion chamber and at a plurality of points of time of different flows of blowby gas passing through the blowby gas passage and flowing to the downstream side of the throttle valve in the intake passage, and calculate an oil dilution rate based on the blowby gas flow ratio and output current, and
• the plurality of points of time are a plurality of points of time at a single cycle of fuel cut control.