What actually makes hybrid cars fuel-efficient?
by Ellsworth Toohey · Boing BoingThe 2021 Toyota Sienna is a three-row AWD minivan that gets 34 mpg city and highway — beating its nearest rivals by more than 50%. Most people assume the batteries and electric motors explain that number. Alec Watson at Technology Connections has made a 55-minute deep dive on why that assumption is wrong, and the actual answer goes back to 1887.
The Sienna's 2.5-liter engine runs on the Atkinson cycle, a thermodynamic trick patented by James Atkinson in 1887. A conventional Otto-cycle engine — the four-stroke design in almost every car you've ever driven — wastes energy because the compression and power strokes are the same length. Hot gases get vented before they finish pushing the piston. Atkinson figured out that if you make the effective compression stroke shorter than the power stroke, the expanding gases do more work before you dump them. Toyota's Atkinson engine achieves roughly 41% thermal efficiency, compared with around 25% for a typical Otto engine.
The catch is that Atkinson engines are genuinely bad at making peak power. They can run beautifully in their narrow, efficient band, but ask one for a burst of acceleration, and you get nothing. That's the gap the hybrid system fills. The battery and motors paper over the Atkinson engine's weak peak power so it can stay in its efficient sweet spot the rest of the time. Regenerative braking tops up the battery using energy that would otherwise be lost as heat.
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