Our Point of View: Atkinson Meets Otto: Why the Prius is So Efficient

September 08, 2008

by Toyota Open Road Blog

Folks interested in our Prius may have noted that its engine is referred to as an Atkinson-cycle engine. Some of you probably don’t care about that, and are just happy that the Prius is rated by the EPA at 48 city/45 highway mpg.

But maybe some of you wonder just what in the dickens an Atkinson-cycle engine is, and how it’s different from the standard engine – also known as the Otto-cycle engine. So here’s a quick primer.

There are two definitions of an Atkinson engine. The first is that in very early examples of the Atkinson design, all four strokes of the Otto cycle (intake, compression, power, exhaust) happen in a single revolution of the crankshaft, rather than the usual two. This is accomplished via a complex linkage between the piston and the crankshaft.

But there’s a subsidiary and more modern definition, and that’s the one in which we’re interested. This says an engine that uses the Atkinson-cycle is one in which the post-combustion expansion ratio is different from the effective compression ratio. In other words, the compression stroke of the engine’s piston(s) is, by whatever means, shorter than the power, or combustion, stroke.

This imbalanced compression/expansion ratio results in a reduction of what are called pumping losses. It produces a difference between how hard the engine works and how much power it develops.

In the case of the Prius engine , the effective compression ratio is about 8:1, while the expansion ratio is about 13:1. As a result, it is 12% to 14% more efficient, in terms of power output per fuel consumed, than the non-Atkinson engine upon which it is based.

But there’s no free lunch. Use of the Atkinson cycle results in improved efficiency, but it also results in a significant narrowing of the rpm range in which the engine makes useable power.

There are two ways to solve this problem. One way is to couple the engine to a continuously variable transmission (CVT) so that the engine always will run in its optimal rev range. The other is to give the engine supplemental power such as an electric motor. We do both those things.

Even better, by using our Variable Valve Timing-intelligent (VVT-i) system to continuously adjust intake-valve timing between Atkinson-cycle valve timing and conventional valve timing, the Prius engine can maximize fuel efficiency while still producing maximum power.

The result is the Prius Hybrid, which provides sprightly acceleration, more than sufficient highway speed and the best fuel economy ratings of any automobile available in the U.S. today. Seems like the best of all possible worlds.

Originally published on the Toyota Open Road blog on Sept. 08, 2008.

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