I was right, the Prius is not a full Atkinson cycle. It is: "But the Prius
1.5 Four is an Atkinson cycle four-stroke." Appear an Otto amened to an "Atkinson cycle four-stroke".Writeup of the Atkinson cycle used by the Prius.
Popular Science....
"It's an all-aluminum engine that produces just 70 hp. That may not sound impressive for a DOHC Four, but this powerplant was engineered for efficiency. The typical four-stroke engine works on the Otto cycle, with intake, compression, power and exhaust strokes of the same length. But the Prius 1.5 Four is an Atkinson cycle four-stroke. What's that?
It's an engine in which the power stroke is longer than the compression stroke, so it wrings every last bit of energy out of the downward push of the pistons. Of course, at the very end of the power stroke, there isn't much energy left in any four-stroke engine. So simply making the power stroke longer leaves something to be desired.
That weakness is overcome in the Prius by supplementing with electrical power when needed. (The only other production engine that's designed with the Atkinson cycle is the Mazda Millenia, which uses supercharging as its supplement. Mazda calls that engine a Miller Cycle.)
But how do the engineers make the strokes of different length? At the start of the 20th century it was tried with complex linkages that were hopelessly impractical. With the Prius's variable valve timing, however, it's all accomplished by computer control.
There is no change in the physical length of the strokes. What changes is their effective length. In practical terms, the engine is designed for the longest power stroke needed for efficiency, then the computer "shortens" the effective length of the compression stroke. On the Mazda Millenia, it's done by carefully timing the supercharger output. On the Prius, it's done by changing when and how long the intake valves are opened and closed, in accordance with engine rpm. At high rpm, the intake valve closing is delayed, so the compression stroke (when both valves are closed) effectively becomes shorter. At part throttle, the intake valve closing is advanced, so the effective compression stroke becomes slightly longer."
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