Hi, I work on an engine which when the throttle is not active seems to have like a constant frequency, I have some examples to analyze.
But with the natural inertia at low velocity it’s hard to have a constant sound because the RPM goes down naturally. 
So this is what I try: find the frequency range and try to make it long and to have a non-mecanical engine sound sample (like the “Generate” menu).
Some examples here:
I’ve also find a constant RPM period to make a sample, but as you see before I try to reproduce virtually the sound 
Here is it: https://www.sendspace.com/file/saqx1v
If you have some ideas to get a good result (not perfect, just good and listenable) without external noises it would be nice
Thanks !