Hybrid vehicle and control method for same

1. A hybrid vehicle comprising:

an engine;

a motor connected to the engine; and

an electronic control unit configured to control the motor to execute motoring to rotate a crankshaft of the engine, wherein the electronic control unit is configured to execute speed-drop offset control when a rotation speed of the engine falls below a first rotation speed that is lower than a self-sustaining rotation speed of the engine while the engine is operated in a self-sustaining manner at the self-sustaining rotation speed, the speed-drop offset control being control for executing the motoring such that the rotation speed of the engine does not fall below the first rotation speed, and the speed-drop offset control being control for executing the motoring such that the rotation speed of the engine does not fall below a second rotation speed that is higher than the first rotation speed and lower than the self-sustaining rotation speed.

2. The hybrid vehicle according to claim 1 , wherein the electronic control unit is configured to execute the speed-drop offset control when the rotation speed of the engine falls below the first rotation speed before a reference time elapses from a start of the engine.

3. The hybrid vehicle according to claim 2 , further comprising

a coolant temperature sensor configured to detect a coolant temperature of the engine, wherein the electronic control unit is configured to set the reference time such that the reference time increases as the coolant temperature decreases.

4. The hybrid vehicle according to claim 1 , wherein the electronic control unit is configured to end the speed-drop offset control when one of a first end condition and a second end condition is satisfied while executing the speed-drop offset control, the first end condition being that the rotation speed of the engine has reached a third rotation speed that is higher than the second rotation speed, the second end condition being that the rotation speed of the engine has not reached the third rotation speed even after the speed-drop offset control is continued for a predetermined time.

5. The hybrid vehicle according to claim 1 , further comprising

a coolant temperature sensor configured to detect a coolant temperature of the engine, wherein the electronic control unit is configured to set the second rotation speed such that the second rotation speed increases as the coolant temperature decreases.

6. The hybrid vehicle according to claim 1 , further comprising

an alcohol concentration sensor configured to detect an alcohol concentration of a fuel, wherein:

the engine is configured to operate with the fuel containing alcohol; and

the electronic control unit is configured to set the second rotation speed such that the second rotation speed increases as the alcohol concentration of the fuel increases.

7. A control method for a hybrid vehicle, the hybrid vehicle being configured to execute motoring to rotate a crankshaft of an engine by a motor, the control method comprising

executing speed-drop offset control when a rotation speed of the engine falls below a first rotation speed that is lower than a self-sustaining rotation speed of the engine while the engine is operated in a self-sustaining manner at the self-sustaining rotation speed,

wherein the speed-drop offset control is control for executing the motoring such that the rotation speed of the engine does not fall below the first rotation speed, and

wherein the speed-drop offset control is control for executing the motoring such that the rotation speed of the engine does not fall below a second rotation speed that is higher than the first rotation speed and lower than the self-sustaining rotation speed.