Method for operating a hybrid drive system of a motor vehicle, hybrid drive system, and motor vehicle

A method for operating a hybrid drive system of a motor vehicle, wherein the hybrid drive system comprises an internal combustion engine with a belt-driven starter generator coupled thereto, a transmission, an exhaust system for the internal combustion engine, a control device, an output, and a first clutch arranged between the internal combustion engine and the transmission, the method comprising:

• bringing, via the control device, the motor vehicle to a first speed by controlling the hybrid drive system;
• bringing about, via the control device, an operating state of the hybrid drive system in which the first clutch is disengaged;
• shutting off, via the control device, the internal combustion engine;
• detecting, via the control device, a braking demand for braking of the motor vehicle;
• determining, via the control device, a potential crankshaft speed of a crankshaft of the internal combustion engine in the case of a potentially engaged first clutch;
• comparing, via the control device, the determined potential crankshaft speed with a predefined first limit speed;
• engaging, via the control device, the first clutch for overrunning the internal combustion engine as well as for operating the belt-driven starter generator in a recuperation mode when the crankshaft speed is lower than the first limit speed;
• and
• configuring, via the control device, the internal combustion engine such that with regard to intake valves and exhaust valves of the internal combustion engine, the intake valves alone remain closed while the crankshaft of the internal combustion engine rotates during the overrunning of the internal combustion engine.