A petrol engine and two motor-generators blended through one planetary gear — a computer picks the mix every millisecond.
1997·1.5 L Atkinson four·2 motor-generators·One planetary — no shifts·~half the fuel of its peers
1· 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
2· 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.
3· 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.
4· 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.
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.
Deep dive · The power split· opened from Toyota Prius (THS)
Timelines · Engines · go deeper · Toyota Prius (THS)
The power split1997
One planetary gear, three shafts — and suddenly the engine never needs a gearbox again.
one planetary·on the carrier·sun / ring·all of them, continuously
1· THE IDEA
Sun, planets and ring: engine on the carrier, motors on the other two shafts
A planetary gearset has three connection points: the sun gear in the middle, the ring gear around the outside, and the carrier holding the planets between them. Their speeds are locked to one simple rule — a weighted average. Put the engine on the carrier, a motor-generator on the sun and the wheels on the ring, and something remarkable falls out: by varying the generator’s speed, ANY engine speed can serve any road speed. A continuously variable transmission with no belts, no clutches and no shifts — just one gear, used cleverly.
2· WHY IT MATTERS
The speed rule
The planetary’s law is that the carrier’s speed is a fixed blend of sun and ring speeds. Fix any two and the third follows. So when the computer wants the engine at its most efficient rpm while the wheels do whatever the road demands, it simply commands MG1 on the sun to spin at whatever closes the equation — hundreds of times a second, seamlessly.
3· WHY IT MATTERS
Power takes two paths
Engine power leaves the planetary partly as mechanical drive to the ring and partly as electricity generated by MG1 — which flows to MG2 to be pushed back into the wheels, or into the battery for later. The split is why the Prius feels like neither a petrol car nor an electric one: it is both at once, in a proportion redrawn every instant.
4· WHY IT MATTERS
Why it conquered
No clutch to wear, no gearshift to interrupt torque, an engine that either runs loaded and efficient or not at all — and regenerative braking folded in naturally, because MG2 is already on the axle. Every full hybrid since is a variation on this one gearset, and Toyota rode it to the top of the industry.
5· TRY IT
Close the equation yourself
Pick a road speed and an engine speed — the sun gear’s rpm follows from the planetary’s one rule. Watch MG1 spin, stop, even reverse, while the engine stays wherever it is happiest.
Readout
MG1 (sun)1,020 rpm
MG1 isgenerating forward
Gearshifts used0
Notice MG1 absorbing every mismatch — even spinning backwards when the wheels outrun the engine’s contribution. That freedom is the entire transmission: no clutch, no shift, just one equation closed hundreds of times a second.