Drivers everywhere recognize the Toyota Prius as the vehicle with the milage per gallon ratio on American roads. How does it achieve this great mileage, though? Toyota has publicly explained how the Prius’ Hybrid Synergy Drive uses what’s called an Atkinson-cycle engine to help the Toyota Prius’ electric motors produce great gas mileage.
using the Atkinson cycle twist to the gasoline engine makes good sense for hybrid cars. According to Toyota’s Corporate website, the Atkinson cycle transmits the traditional four-cycle engine’s method of intake, compression, power, and exhaust. The Atkinson cycle has a longer power stroke than compression stroke, which enables the engine to produce significantly higher power in a certain operating range. The twelve gain in efficiency versus an 8 cycle engine is mainly due to the reduction in pumping losses, or the amount of energy needed simply to vacuum air into the cylinders and then expel spent exhaust gases.
Since this Atkinson technology doesn’t equate to efficiency gains across the entire rev range, Toyota’s use of a continuously variable transmission–it uses belts and pulleys to arrive at a gear ratio, instead of a fixed set of gears– allows the engine to stay in its efficiency sweet spot for greater amounts of time than it would with a traditional, geared transmission. Also, when using the gas engine would cause a large drop in fuel economy, the electronic motors step in to provide the bulk of propulsion.
Toyota calculates that plenty of its Prius purchasers “probably don’t care about the underlying Prius technology, and are just happy that the Prius is rated by the EPA at 48 city/45 highway mpg.” But as hybrid lovers, we like learning about the finer points of gas/electric hybrid operation and are happy being able to talk the talk when we boast about these remarkably fuel-efficient vehicles.
