2026-09-11
Every Prius, Camry Hybrid, and Mazda SkyActiv-G engine runs a trick that would have baffled engineers a century ago: they hold the intake valve open past bottom dead center on the compression stroke. This is the Atkinson cycle (naturally aspirated) or Miller cycle (forced induction), and it's the reason a 2.5L Camry hybrid gets 50 mpg without giving up much power.
The original 1882 Atkinson engine used a complex linkage to physically shorten the compression stroke while lengthening the power stroke. Modern engines fake it with valve timing. The intake valve stays open 60–80° of crank rotation after BDC. As the piston starts rising, it pushes some of the fresh charge back out into the intake manifold. Only when the valve finally closes does real compression begin.
The key insight: static compression ratio (measured geometrically) and effective compression ratio (measured from when the intake valve actually closes) diverge dramatically.
A Toyota Prius 2ZR-FXE runs a static compression of 13.0:1 — pump gas would knock instantly in a conventional engine. But because the intake valve closes ~70° after BDC, effective compression drops to roughly 9:1. Detonation-free on 87 octane, yet the expansion ratio during the power stroke is still 13:1 — extracting more work from the same combustion event.
Rule of thumb: effective CR ≈ static CR × cos²(intake-valve-close-angle-past-BDC ÷ 2). For 70° ABDC: cos²(35°) ≈ 0.67, so 13.0 × 0.67 ≈ 8.7:1 effective. That matches measured cylinder pressure data.
The downside: pumping air back out the intake reduces cylinder fill, so peak torque and specific output drop. A Miller-cycle 2.5L makes maybe 175 hp naturally aspirated where an Otto-cycle version would make 200+. That's why the trick lives in hybrids (electric motor fills the torque hole) and turbocharged engines like the VW 1.5 TSI Evo (boost restores charge density while retaining the expansion-ratio advantage).
Mazda's SkyActiv-G takes it further: 14:1 static compression with delayed IVC and a 4-2-1 exhaust that eliminates residual gases. The result is diesel-like thermal efficiency (~40%) burning regular gasoline — no hybrid required.