Toyota Prius THS (1997) — the power split

1997·1.5 L Atkinson four·2 motor-generators·One planetary — no shifts·~half the fuel of its peers

· THE STORY

Two engines, one clever gear

The 1997 Prius put a small petrol engine and two electric motor-generators around a single planetary gearset, and let a computer decide — hundreds of times a second — where every kilowatt should go. The engine can drive the wheels, spin a generator, or switch off entirely; braking runs a motor backwards and refills the battery. It looked like an economy statement; it was actually a new drivetrain architecture, and every hybrid since is its descendant.

1.5 L Atkinson fourEngine2 motor-generatorsMotors

· WHY IT MATTERED

One gearset, every ratio

A planetary gear has three shafts: sun, ring and carrier. Put the engine on one, a motor-generator on another and the wheels on the third, and varying the generator’s speed smoothly varies the effective gear ratio — a gearbox with no gears to change. The engine can sit at its most efficient rpm while the road speed does whatever it likes.

· WHY IT MATTERED

The engine escapes traffic

Stop-and-go driving — the petrol engine’s worst habitat — is handed to the battery and motor, and the engine simply switches off. When it does run, it runs loaded and efficient, on a gentle Atkinson cycle that trades peak power (the motor covers that) for thermal efficiency.

· WHY IT MATTERED

Braking becomes fuel

Every stop in an ordinary car converts motion into brake heat. The Prius runs its drive motor backwards instead, pumping the energy into the battery and saving the friction brakes for emergencies. Regenerative braking is why hybrids thrive precisely where other cars suffer — in town.

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· TRY IT

Split the power

Choose a driving moment and follow the kilowatts: through the planetary to the wheels, backwards into the battery, or both at once — the blend the computer redraws every millisecond.

Readout

Engine22 kW

MG1 (generator)generating

MG2 (drive motor)assisting lightly

Batterycharging

Illustrative kilowatts for a first-generation Prius. The trick is that no mode is special — the planetary blends them continuously, so the car slides between these pictures without a single gearshift or a driver noticing.

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