Engine starting system

An engine starting system for a vehicle, the vehicle including an engine and a transmission, and the engine including a plurality of cylinders and a throttle valve, the engine starting system comprising:

• a starting device;
• and
• an electronic control unit configured to:
• automatically stop the engine in response to a request to stop the engine;
• calculate, in a course of automatically stopping the engine, an allowable throttle opening degree based on at least one of a vehicle speed of the vehicle or an input rotational speed of the transmission, such that the calculated allowable throttle opening degree, when the at least one of the vehicle speed or the input rotational speed is higher than a predetermined threshold, is larger than the calculated allowable throttle opening degree when the at least one of the vehicle speed or the input rotational speed is lower than the predetermined threshold;
• determine whether the engine can be started through ignition-based engine starting, when a torque that is required to restart the engine is higher than an estimated generation torque based on an estimated intake pipe pressure based on the calculated allowable throttle opening degree, an estimated stop-time in-cylinder air density based on a crank angle at which a crankshaft of the engine is estimated to stop, and an estimated temporal change in an in-cylinder air density;
• carry out scavenging of each of the cylinders of the engine by opening the throttle valve to the calculated allowable throttle opening degree in the course of automatically stopping the engine, when the electronic control unit determines that the engine can be restarted through ignition-based engine starting;
• restart the engine, which is at a standstill after being automatically stopped, in response to a request to restart the engine through ignition-based engine starting, when the electronic control unit determines that the engine can be restarted through ignition-based engine starting;
• and
• restart the engine, which is at a standstill after being automatically stopped, in response to the request to restart the engine by using the starting device without opening the throttle valve, when the electronic control unit determines that the engine cannot be restarted through ignition-based engine starting.