While hybrid drivetrains have led to significant fuel economy gains for cars, it’s not necessarily the same equation for a city bus.

A typical bus gets only between four and six miles per gallon. About half of that fuel is never used to put the bus in motion.

Instead, that energy is gobbled up powering other equipment, namely systems to cool the engine and keep a window-lined interior the size of a small studio apartment at a comfortable temperature.

A year ago, two of the world’s most advanced ‘super hybrid