Continuously variable transmission and control method thereof

1. A continuously variable transmission installed in a vehicle, comprising:

a continuously variable transmission mechanism (to be referred to hereafter as a “variator”) which modifies a speed ratio continuously;

a subtransmission mechanism provided in series with the variator and including, as forward gear positions, a first gear position and a second gear position having a smaller speed ratio than the first gear position; and

a shift control unit which modifies a through speed ratio, which is an overall speed ratio of the variator and the subtransmission mechanism, by modifying the speed ratio of the variator and changing the gear position of the subtransmission mechanism on a basis of an operating condition of the vehicle,

wherein the through speed ratio when the speed ratio of the variator is a Highest speed ratio and the gear position of the subtransmission mechanism is the first gear position (to be referred to hereafter as a “low speed mode Highest speed ratio”) is smaller than the through speed ratio when the speed ratio of the variator is a Lowest speed ratio and the gear position of the subtransmission mechanism is the second gear position (to be referred to hereafter as a “high speed mode Lowest speed ratio”),

a midway value between the low speed mode Highest speed ratio and the high speed mode Lowest speed ratio is set as a mode switch speed ratio, and

when the through speed ratio varies across the mode switch speed ratio, the shift control unit changes the gear position of the subtransmission mechanism and modifies the speed ratio of the variator in an opposite direction to a variation direction of the speed ratio of the subtransmission mechanism.

2. The continuously variable transmission as defined in claim 1 , wherein, when the through speed ratio varies from a larger speed ratio than the mode switch speed ratio to a smaller speed ratio than the mode switch speed ratio, the shift control unit changes the gear position of the subtransmission mechanism from the first gear position to the second gear position and modifies the speed ratio of the variator to a large speed ratio side.

3. The continuously variable transmission as defined in claim 1 , wherein, when the through speed ratio varies from a smaller speed ratio than the mode switch speed ratio to a larger speed ratio than the mode switch speed ratio, the shift control unit changes the gear position of the subtransmission mechanism from the second gear position to the first gear position and modifies the speed ratio of the variator to a small speed ratio side.

4. A continuously variable transmission installed in a vehicle, comprising:

a continuously variable transmission mechanism (to be referred to hereafter as a “variator”) which modifies a speed ratio continuously;

a subtransmission mechanism provided in series with the variator and including, as forward gear positions, a first gear position and a second gear position having a smaller speed ratio than the first gear position; and

shift control means for modifying a through speed ratio, which is an overall speed ratio of the variator and the subtransmission mechanism, by modifying the speed ratio of the variator and changing the gear position of the subtransmission mechanism on a basis of an operating condition of the vehicle,

wherein the through speed ratio when the speed ratio of the variator is a Highest speed ratio and the gear position of the subtransmission mechanism is the first gear position (to be referred to hereafter as a “low speed mode Highest speed ratio”) is smaller than the through speed ratio when the speed ratio of the variator is a Lowest speed ratio and the gear position of the subtransmission mechanism is the second gear position (to be referred to hereafter as a “high speed mode Lowest speed ratio”),

a midway value between the low speed mode Highest speed ratio and the high speed mode Lowest speed ratio is set as a mode switch speed ratio, and

when the through speed ratio varies across the mode switch speed ratio, the shift control means changes the gear position of the subtransmission mechanism and modifies the speed ratio of the variator in an opposite direction to a variation direction of the speed ratio of the subtransmission mechanism.

5. The continuously variable transmission as defined in claim 4 , wherein, when the through speed ratio varies from a larger speed ratio than the mode switch speed ratio to a smaller speed ratio than the mode switch speed ratio, the shift control means changes the gear position of the subtransmission mechanism from the first gear position to the second gear position and modifies the speed ratio of the variator to a large speed ratio side.

6. The continuously variable transmission as defined in claim 4 , wherein, when the through speed ratio varies from a smaller speed ratio than the mode switch speed ratio to a larger speed ratio than the mode switch speed ratio, the shift control means changes the gear position of the subtransmission mechanism from the second gear position to the first gear position and modifies the speed ratio of the variator to a small speed ratio side.

7. A control method for a continuously variable transmission installed in a vehicle and having a continuously variable transmission mechanism (to be referred to hereafter as a “variator”) which modifies a speed ratio continuously and a subtransmission mechanism provided in series with the variator and including, as forward gear positions, a first gear position and a second gear position having a smaller speed ratio than the first gear position; the control method comprising:

a shift control step for modifying a through speed ratio, which is an overall speed ratio of the variator and the subtransmission mechanism, by modifying the speed ratio of the variator and changing the gear position of the subtransmission mechanism on a basis of an operating condition of the vehicle,

wherein the through speed ratio when the speed ratio of the variator is a Highest speed ratio and the gear position of the subtransmission mechanism is the first gear position (to be referred to hereafter as a “low speed mode Highest speed ratio”) is smaller than the through speed ratio when the speed ratio of the variator is a Lowest speed ratio and the gear position of the subtransmission mechanism is the second gear position (to be referred to hereafter as a “high speed mode Lowest speed ratio”),

a midway value between the low speed mode Highest speed ratio and the high speed mode Lowest speed ratio is set as a mode switch speed ratio, and

when the through speed ratio varies across the mode switch speed ratio, the shift control step changes the gear position of the subtransmission mechanism and modifies the speed ratio of the variator in an opposite direction to a variation direction of the speed ratio of the subtransmission mechanism.

8. The control method as defined in claim 7 , wherein, when the through speed ratio varies from a larger speed ratio than the mode switch speed ratio to a smaller speed ratio than the mode switch speed ratio, the shift control step changes the gear position of the subtransmission mechanism from the first gear position to the second gear position and modifies the speed ratio of the variator to a large speed ratio side.

9. The control method as defined in claim 7 , wherein, when the through speed ratio varies from a smaller speed ratio than the mode switch speed ratio to a larger speed ratio than the mode switch speed ratio, the shift control step changes the gear position of the subtransmission mechanism from the second gear position to the first gear position and modifies the speed ratio of the variator to a small speed ratio side.