The Prius's Not So Secret Gas-Mileage Secrets

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Open Doors - 2004 Toyota Prius 5dr HB (Natl)

Open Doors - 2004 Toyota Prius 5dr HB (Natl)

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Drivers everywhere recognize Toyota's Prius as the vehicle with the best fuel economy in America. How does it achieve that mileage, though? On its corporate blog, Toyota recently explained how the Prius' Hybrid Synergy Drive uses what's called an Atkinson-cycle engine to help the Prius' batteries and electric motors produce world-class gas mileage.

Employing the rarely-used Atkinson cycle twist to the four-stroke gasoline engine makes lots of sense for a gas/electric hybrid. According to Toyota, the Atkinson cycle shifts 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 more power in a certain operating range.  The 12- to 14-percent gain in efficiency versus an Otto-cycle engine is  mainly due to the reduction in pumping losses, or the amount of energy needed simply to suck air into the cylinders and then expel spent exhaust gases.

Toyota's use of a electronic continuously variable transmission--it uses a planetary gear set to vary the power split among the engine, primary, and secondary electric motors, instead of a fixed set of gears-- allows the engine to stay in its high-revving efficiency sweet spot for greater amounts of time than it would with a traditional, geared transmission. Additionally, the electric electronic motors provide the bulk of propulsion from rest, because they generate maximum torque from 0 rpm.

As mentioned, Ford's Escape Hybrid presumably uses the Atkinson cycle in its new-for-2009 2.5-liter four as it did in its 2.3-liter four cylinder powerplant used in 2008.

Toyota figures that plenty of Prius purchasers "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 as hybrid aficionados, we appreciate being reminded about the finer points of gas/electric hybrid operation and appreciate being able to talk the talk when we boast about our remarkably fuel-efficient vehicles.

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Comments (4)
  1. "Prius Owner"

    Toyota's continuously variable transmission does not use belts or pulleys. It has a fixed-ratio planetary gear set, and adjusts a generator's resistance to turning to vary the effective drive ratio.

  2. I have a 06 prius and would like to know 2 things,why didn't they put a tach.on it and does it have any type of transmission that has fluid in it.Thanks

  3. coop - the prius does not actually have a transmission. instead it has a 'psd' which is much less complicated. the speed of the car is governed by the electric motor and the engine working in combination through the psd. the engine rpm is quite different to a conventional car - it is spun up by one of the electric motors to around 800 rpm before starting and does not rev very high. You are not using the engine in any sort of comparable drive train, so rpm is irrelevant. visit for more info. it takes a bit of study but is quite interesting.

  4. Marty, the real "secret" of Prius fuel efficiency is tapping into a free force that has been present for 4.6 billion years: gravity. Each time the road goes even slightly downhill, a Prius electronic system uses the opposite of our boyhood electric train set engines--magnetism + wire coil--to put electrons INTO the storage battery, for use as motive force to go horizontally. Our 2007 Prius has 152,000 miles on it, and our 2012 has 35,000 miles; I have observed this gravity "sipping" for seven years! Voila.

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