Fuel property determination apparatus for internal combustion engine

A fuel property determination apparatus for an internal combustion engine, the internal combustion engine including a spark plug, the spark plug including a spark generating portion, the fuel property determination apparatus comprising:

• an electronic control unit configured to:
• (i) perform a feedback control such that an engine speed converges to a target speed by correcting ignition timing during the feedback control, the ignition timing being a timing when the spark generating portion generates a spark,
• (ii) make a determination of a fuel property that fuel supplied to the internal combustion engine is a heavy fuel when an ignition fulfillment rate is equal to or greater than a predetermined threshold value within a predetermined period after the internal combustion engine is started, the ignition fulfillment rate being a ratio of an advance correction amount of the ignition timing to a maximum ignition correction range, the maximum ignition correction range being a maximum range of a correction range in which the ignition timing is corrected during the feedback control, and
• (iii) set the threshold value to a value that depends on a temperature representative value of the internal combustion engine, the temperature representative value being correlated with the temperature of the internal combustion engine.