Toyota took the time to explain that the Prius gasoline engine operates neither on the traditional Otto cycle, nor on Diesel's sparkless version of Otto's four-cycle, but rather on a quirky and seldom-used variation on the Otto cycle called Atkinson. The engineers explain that employing the Atkinson cycle makes the Prius' engine one "in which the post-combustion expansion ratio is different from the effective compression ratio."
Huh? In lay terms, that means the compression stroke (Otto cycle: intake, compression, power, exhaust) is shorter than the power stroke of the engine, which Toyota says reduces pumping losses and increases efficiency by 12 to 14 percent. Of note, Ford's Escape Hybrid used the Atkinson cycle in its 2.3-liter four cylinder powerplant, and presumably the same for its new-for-2009 2.5-liter four.
Limitations of this design include a somewhat narrow rev range of maximum efficiency, which Toyota says is mitigated by its continuously variable transmission (keeping the engine usually in the proper rev range) and the assistance provided by the Prius' electric motor.
Toyota wisely figures that "some of you probably don’t care about [this], and are just happy that the Prius is rated by the EPA at 48 city/45 highway mpg." But we think it's cool they put some punch behind their PR prose, and give them props for crediting their buying public with intelligence and curiosity.--Colin Mathews