The Atkinson cycle explained

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Atkinson engine

A number of cars are coming out that use an Atkinson cycle engine, which is similar to the traditional Otto cycle. Photo Credit: Michael Hicks/Wikimedia Commons/CC-BY

A few people might have noticed that some newer cars are said to be using the “Atkinson cycle,” especially on hybrids. The principle characteristic of the Atkinson cycle is completing all four “strokes” of an engine cycle with one turn of the crankshaft, which is more efficient.

Atkinson cycle has nothing to do with carbs

Some might have heard of an “Atkinson cycle” engine which is getting employed in hybrid cars. The typical engine uses what is called the Otto cycle or “four strokes.” The Otto cycle is four “strokes” of a piston inside a piston chamber: intake, compression, power/ignition, and exhaust.

It begins with the piston at the top of the piston chamber, or “top-dead center,” whereupon the piston descends, pulling in air and fuel. Then it returns to the top, compressing the mixture, where it is ignited by a spark plug and driven downward again. Then the piston returns to the top of the chamber, expelling the remaining gases and starting over at top-dead center.

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The Atkinson cycle is similar, but different. So in case you’re about to call off any borrowing money for a car because the “Atkinson cycle” freaks you out don’t worry, because it means the car is going to be fuel efficient.

What a crank

During an Otto cycle, the engine’s crankshaft, the rod being spun in the bowels of the motor, turns twice, meaning two strokes per rotation. The Atkinson cycle, according to HowStuffWorks, uses only one rotation of the engine’s crankshaft.

It was developed by James Atkinson, a late 19th-century British engineer, who observed that adding linking arms and a second crankshaft between the camshaft and the piston could produce a complete four-stroke cycle in one rotation of the crankshaft. He further noted a boost in fuel efficiency could be gained by decreasing the compression in the cylinder and shortening the compression stroke, which meant the engine still worked but used less fuel to do it.

The hitch was that it required more moving parts, which made it less reliable and more expensive, and also produced far less horsepower than a conventional motor. Also, according to Honda, extra parts means more friction, which wears components. Since it didn’t offer enough advantages aside from fuel economy, the Atkinson cycle engine stayed in the background until the dawn of the hybrid era.

Ultimate in steampunk is a Prius?!

Hybrid cars use an electric motor in conjunction with a regular gas motor, which adds an extra few ponies. Ergo, an Atkinson cycle engine can be used in a hybrid drive train without sacrificing too much horsepower and a modern technology uses old technology to work. Additionally, according to Honda, advances in materials have produced engine components that don’t generate as much friction, meaning an Atkinson cycle motor can be just as reliable as it’s Otto-cycling counterpart. That’s why the Toyota Prius uses an Atkinson cycle engine.

Ergo, they appear in a number of hybrid cars appearing at a car dealer near you. Just to name a few, the upcoming Ford C-Max Hybrid will have an Atkinson cycle engine, according to AutoGuide. So do the 2013 Ford Fusion Hybrid and Prius C, according to Car and Driver, the 2013 Lexus ES 300h, according to TG Daily, and the Hyundai Sonata Hybrid, according to the San Antonio Express-News.

Animation of Atkinson and Otto cycles


How Stuff Works



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