Vehicle control system

1. A vehicle control system comprising:

a controller configured to obtain an index on the basis of a running condition of a vehicle and configured to vary a running characteristic of the vehicle on the basis of the index, wherein

the controller is configured to relatively delay a variation in the index in response to a variation in the running condition when the variation in the index decreases quickness of a behavior of the vehicle as compared with when the variation in the index increases the quickness of the behavior of the vehicle, and to correct the running characteristic on the basis of the index so that energy efficiency of a driving force source of the vehicle varies within a predetermined range depending on control over power output from the driving force source.

2. The vehicle control system according to claim 1 , wherein

the running condition includes a resultant acceleration that includes a longitudinal acceleration component and lateral acceleration component of the vehicle,

the controller is configured to set the index so as to be larger as the resultant acceleration increases, and

the controller is configured to set the running characteristic such that energy efficiency of the driving force source is better when the index is obtained on the basis of a resultant acceleration smaller than a predetermined value than when the index is obtained on the basis of a resultant acceleration larger than or equal to the predetermined value.

3. The vehicle control system according to claim 1 , wherein

the vehicle includes an internal combustion engine as the driving force source,

the controller is configured to control a rotational speed of the internal combustion engine, and

the energy efficiency of the driving force source is fuel efficiency of the of the internal combustion engine.

4. The vehicle control system according to claim 3 , wherein

the vehicle further includes a transmission of which a speed ratio is varied to vary the rotational speed of the internal combustion engine, and

the controller is configured to control the speed ratio of the transmission such that the rotational speed of the internal combustion engine becomes a rotational speed at which the fuel efficiency falls within the predetermined range.

5. The vehicle control system according to claim 3 , wherein

the vehicle further includes a power split mechanism that has a differential function for splitting power output from the internal combustion engine and a generator that is coupled to the power split mechanism and that is configured to vary a rotational speed of the internal combustion engine on the basis of an amount of electric power generated, and

the controller is configured to control a rotational speed of the generator such that the rotational speed of the internal combustion engine becomes a rotational speed at which the fuel efficiency falls within the predetermined range.

6. The vehicle control system according to claim 5 , wherein

the vehicle further includes an electrical storage device that stores electric power generated by the generator, and

the rotational speed at which the fuel efficiency falls within the predetermined range is lower when an amount of electric power stored in the electrical storage device is large than when the amount of electric power stored in the electrical storage device is small.

7. The vehicle control system according to claim 1 , wherein

the running condition includes at least one of a longitudinal acceleration and lateral acceleration of the vehicle.

8. The vehicle control system according to claim 1 , wherein

the controller is configured to acquire information about a running environment that is an environment outside the vehicle, including a road surface on which the vehicle runs, and to correct the running characteristic on the basis of the index and the information about the running environment.