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How Do Hybrid Cars Get Such Good Fuel Economy?

Hybrid cars are a modern-day marvel — but did you know that one of the key elements of the hybrid powertrain was invented by an engineer in Victorian Britain? His name was James Atkinson, and he figured out a way to make gasoline engines more efficient. Unfortunately, he didn’t have a simple way to implement his idea.

If you read the specs on a hybrid, chances are you’ll see the phrase “Atkinson-cycle engine.” In order to explain Victorian Jim’s big idea, we need to delve just a wee bit into how gasoline engines work. Engines are made up of a number of cylinders, each with a piston that moves up and down. The piston does the bulk of the work: First it moves down, creating a vacuum that sucks in gas and air, then the piston moves up to compress it. The spark plug lights the mixture — BOOM! — producing power that drives the piston down, and then the piston moves up again, pushing exhaust out, before the whole cycle starts all over again.

What we’ve just described, by the way, is the 4-stroke cycle (also known as the Otto cycle, after its German inventor, Nikolaus Otto).

In the early 1880s, James Atkinson thought he had a better idea. He wanted to make an engine with a power cycle (the one where the fuel/air mixture goes boom) that was longer than the intake cycle (where gas and air are sucked in). The idea was that the fuel/air mixture would have more time to expand, maximizing the amount of work it could do and making the engine more efficient.

Doing this required a Rube Goldberg-like arrangement of cranks and levers that made for an extraordinarily complex mechanism. Anyone who has ever owned an old British car can tell you why this is a bad idea — early in Brit motoring history, the English could barely make lights that work, let alone frighteningly complicated engines.

And so the idea sat on the shelf for a hundred years or so until modern engineers dusted it off.

The idea of a shorter intake-to-power-stroke ratio appealed to engine designers looking to build a more fuel-efficient engine. The question was, how to do it without the complicated mechanism that James Atkinson first came up with? Finally, some unsung genius hit upon an idea: Why not just leave the intake valve open?

When an engine is sucking in fuel and air, it does so through an intake valve at the top of the engine. The valve is mechanically linked to the motion of the pistons, so that it opens at the beginning of the intake stroke and closes as the piston gets near the bottom of its travel. The idea was to leave the intake valve open for a while longer as the piston started moving upwards again, allowing some of the fuel and air that had just been sucked in to be blown back out. The cylinder would now contain less fuel and air than it would had the valve closed on time — as much fuel and air (or, perhaps we should say, as little fuel and air) as it would contain had the intake stroke been shortened by Atkison’s mechanical nightmare. But when the time came for the boom, the mixture would have the piston’s full travel to burn and expand. (And what of the air and fuel that got blown back out? It would return to the intake manifold to be sucked into another cylinder.)

Finally, a mechanically simple Atkinson-cycle engine had been invented!

Now, we all know there’s no such thing as a free lunch, and the Atkinson-cycle engine has a trade-off: It’s less powerful for its size than an engine that uses a traditional Otto cycle. In a hybrid, efficiency trumps power, so this isn’t a big deal.

How much efficiency does the Atkinson cycle engine add on its own? Judging from some small Atkison-cycle engines that Toyota recenlty developed for the Aygo (a mini-car not sold in the U.S.), the improvement can be from 15 to 30 percent. The Aygo offers a tiny 1-liter 3-cylinder Atkinson-cycle engine rated at 78 miles per gallon, an 18-mpg improvement over the previous-generation Otto-cycle engine.

Atkinson-cycle engines first appeared on our shores in the first-generation Toyota Prius. Today, hybrid cars from Toyota, Ford, Chrysler, Honda, Hyundai, Lexus and Mercedes-Benz use Atkinson-cycle engines. Toyota’s newest Tundra pickup also uses the Atkinson cycle for its V6 engine, though it’s not a hybrid. James Atkinson died in 1914, but his brainchild is alive and well and is a key element in the fantastic fuel economy today’s hybrids generate.

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2 COMMENTS

  1. Very interesting article! So the main difference is the intake valve stays open for a short time as the piston is moving up, to let some of the mixed fuel and air back into the intake? It’ll result in less mixture in they cylinder to boom. How does it prolong the boom part of the cycle? Seems like it can just let less mixture in and can have the same result as pushing some of it back out. Also, are Atkinson cycle engines noisier than non-Atkinson? They all seem rougher and noisier.

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