An engine, which is mounted on a vehicle as a driving power source, also serves as a driving power source for auxiliary machines, such as a compressor of an air conditioner, an oil pump of a power steering, and the like. For this reason, a load of an internal combustion engine tends to fluctuate depending on operating conditions of these auxiliary machines, and thus an output torque of the internal combustion engine also tends to fluctuate. For instance, when these auxiliary machines are driven, the load of the internal combustion engine increases and hence the output torque of the internal combustion engine decreases.
As a countermeasure against such load fluctuations of the internal combustion engine, an intake air quantity may be controlled according to the load fluctuations such that the output torque of the internal combustion engine changes. In this manner, it is possible to cope with such a situation. However, when changing the output torque of the internal combustion engine by controlling the intake air quantity, a certain response delay occurs until a change in throttle opening emerges as a change in the quantity of air with which the combustion chamber is filled. Therefore, it is impossible to compensate for output torque fluctuations of the internal combustion engine, occurring owing to the load fluctuations of the internal combustion engine, with a better response by intake-air-quantity control. Also, at idling operation during which the output torque of the internal combustion engine becomes small, the influence of the load fluctuations of the internal combustion engine on the output torque of the internal combustion engine becomes relatively great.
Therefore, Patent document 1 teaches an internal combustion engine that, at idling operation during which the output torque of the internal combustion engine becomes small, ignition timing is shifted to an ignition-timing change side on which the output (the efficiency) is decreased, while supplying an intake air quantity exceeding a given quantity corresponding to an actual output torque, so as to cope with the load fluctuations of the internal combustion engine by ignition-timing adjustment. That is, a predetermined margin torque is preset as a torque that can be increased only by ignition-timing adjustment. Hence, when the load of the internal combustion engine has fluctuated during idling operation, it is possible to change the output torque of the internal combustion engine with a better response only by ignition-timing adjustment.
However, in the above-mentioned Patent document 1, the margin torque is set to increase, when the load of the internal combustion engine is high during idling operation.
The load acting on the internal combustion engine during idling operation tends to increase, when the auxiliary machines and the like come into operation. Thus, when the load during idling operation is high, it is considered that many auxiliary machines are in their active states. Conversely when the load during idling operation is low, it is considered that many auxiliary machines are in their inactive states. That is, in the case that the load of the internal combustion engine during idling operation is high, it is considered that the total auxiliary load of auxiliary machines which may possibly act on the internal combustion engine tends to decrease from such a high-load condition. Conversely in the case that the load of the internal combustion engine during idling operation is low, it is considered that the total auxiliary load of auxiliary machines which may possibly act on the internal combustion engine tends to increase from such a low-load condition.
For the reasons discussed above, there is a problem that such margin-torque setting as disclosed in the above-mentioned Patent document 1, in which the margin torque is set to increase when the load during idling operation is high, does not accord with the actual situation.