The invention is directed toward methods for operating a series
hybrid vehicle in a manner that responds to the operator's demand for
power output, while maximizing
engine efficiency and minimizing disruptions
in vehicle drivability. According to principles of the present invention, when the driver of a series
hybrid vehicle makes a demand for
power output, whether the secondary power source(s) is supplied with
secondary energy stored in an
energy storage device(s), direct input energy generated by an engine(s), or both, depends on the amount of available
secondary energy stored in the vehicle's secondary storage device(s) alone, and in combination with vehicle speed. During the time that the engine is used to generate
secondary energy, the power efficiency level at which the engine is operated also depends on the vehicle speed and the amount of available secondary energy stored in the vehicle's secondary storage device alone, and in combination with vehicle speed. Further, in some embodiments, when the engine is not generating secondary energy, the engine is selectively turned off or idled in response to various operating conditions.