How to choose bldc motors to use as generators?
A motor with a 40 kW rating should be a reasonable match for your engine if the motor RPM matches the engine RPM. The engine will have an RPM at which it can produce the most power and potentially a different operating point at which its fuel efficiency is maximum. Assuming that you already have or have selected the engine, you should decide your desired engine operating point - speed and torque. Then select a motor can safely operate at that point. A motor will operate safely as a generator over some range of speed below its rated speed and possibly a little above. The motor power rating will vary with speed. You need to look for a motor with detailed speed vs. torque specifications. You will also need to determine what output voltage you want and what motor (generator) speed will produce that voltage.