It is well known that a combustion condition of an internal combustion engine varies according to fuel property (for example, cetane value). When an actual fuel property is largely different from a predetermined fuel property, it is likely that a combustion condition of the internal combustion engine becomes unstable, so that emission and fuel economy may be deteriorated.
JP-2006-16994A shows that a combustion condition such as ignition timing is detected by a combustion pressure sensor and the fuel property is detected based on this detected ignition timing. JP-2009-180174A (US-2009-0198456A1) shows that a small quantity of fuel is injected while a vehicle is decelerated, and the fuel property is detected based on a variation in engine speed due to the small injection.
However, in JP-2006-16994A, it is unclear whether an ignition timing retard is caused by a combustion condition or other reason. It is likely that the cetane value can not be correctly obtained. For example, due to exhaust gas recirculation (EGR), a deviation in ignition timing due to a difference in fuel property can not be obtained with high accuracy.
Also, in JP-2009-180174A, since the fuel property is detected only when the vehicle is decelerated, the detection frequency of the fuel property is relatively low. In order to improve the detection accuracy, it is necessary to increase the detection frequency. If the fuel is injected to detect the fuel property while the vehicle is not decelerated, it is likely that drivability may be deteriorated.