How to identify transfer function system SISO

I'm having a problem with my project. I have a generator system, my experiment is as follows: I will open a fixed valve angle to fuel the engine and record the speed then over time will increase the valve angle to a further value. follow and record. My sampling time is 0.1s. My problem is determining the transfer function that describes the system to match the obtained data> 95%. Hope to have the help of all of you.