I designed one like Ross', but much smaller. And no auto start.
Think 1hp ECM or 2 and a little Honda?
LM3914/3915, using 4 outputs, that turn mosfets on and off.
Lowest LED output turns linear actuator fastest, speeding up the motor.
Next lowest turns up the speed a little slower. Series resistor.
Voltage gap when the battery is about right.
2nd highest turns the speed down slowly. Series resistor.
Highest voltage turns it down fast.
Simple, electricity.
Never actually built one because I don't have a use for it.
G-