Engines · Electric11 min read

Hybrids: five places for a motor, three architectures, one clever gear

A hybrid is not half a petrol car and half an EV — it is a machine for keeping a combustion engine inside the narrow island where it is actually efficient, and harvesting back the energy braking would burn. Where you bolt the motor in changes everything, so engineers made a map: P0 to P4. And the cleverest answer of all, Toyota's power split, manages to replace the entire gearbox with one planetary gearset.

Why bolt the two together

A petrol engine is only impressively efficient in one small region of its operating map — roughly two-thirds throttle around 2 000–3 000 rpm. Real driving barely visits it: town traffic wants tiny powers at low revs, where the engine wastes most of its fuel just staying alive. An electric machine is efficient nearly everywhere, torquey from 0 rpm, and reversible — it can swallow braking energy instead of throwing it away. Every hybrid, from a mild 48-volt system to a plug-in Prius, is some arrangement of the same three tricks: let the engine off the hook where it is bad (or shut it entirely), load it into its sweet spot when it does run, and regenerate on every stop.

The P0–P4 map

Because the motor’s position in the driveline decides what it can and cannot do, the industry names the five mounting points P0 to P4, walking from the front of the engine to the far axle:

One driveline, five sockets. The further right the motor sits, the more it can do without the engine — and the less it can help the engine itself.

P0 replaces the alternator on the belt: a few kW for smooth restarts and a gentle shove — mild hybrids live here. P1bolts straight to the crank (Honda’s early IMA): stronger, but still married to the engine, so no pure-electric driving. P2 sits after a clutch that can disconnect the engine entirely — the first position that allows real EV running, which is why most plug-in hybrids choose it. P3 rides on the gearbox output for maximum regen directly at the wheels. P4 drives the otheraxle, touching the engine only through the tarmac — instant four-wheel drive, and the reason a hybrid’s front and rear can be made by different companies.

Series, parallel, power-split

Positions aside, the energy has to get from fuel to tyre somehow, and there are exactly three routes:

The three architectures. Solid lines carry torque; dashed lines carry current. Series converts everything twice; parallel keeps one honest mechanical path; power-split does both at once, in a ratio it can choose.

In a series hybrid the engine never touches the wheels: it spins a generator, and a motor does all the driving — the engine becomes a fuel-to-electricity appliance that can idle at its sweet spot regardless of traffic (this is also exactly how a diesel-electric locomotive works). In a parallel hybrid the engine keeps its mechanical connection and the motor merely helps, so motorway cruising stays efficient — but the engine is again chained to wheel speed through the gearbox. A power-split hybrid refuses the either/or: a planetary gearset divides engine power into a mechanical stream and an electrical stream, continuously choosing the blend.

The efficiency arithmetic of each routefor engineers

Series pays a double conversion on every watt: engine → generator (~94%) → electronics (~97%) → motor (~94%), roughly $0.94 \times 0.97 \times 0.94 \approx 0.86$ — before battery round-trips. That 14% tax is unbeatable in town, where it buys the engine its sweet spot (worth 2× or more against an idling, throttled engine), and unforgivable on the motorway, where a plain mechanical path would have been ~98% efficient at an operating point that was already good. Parallel is the mirror image. Power-split’s trick is that the electrical (taxed) stream carries only part of the power — about 28% at typical cruise for the Prius gearset, shrinking to zero at its "mechanical point" — so it pays the conversion tax only on the fraction it uses to buy engine-speed freedom.

Why the Prius has no gearbox

The Prius has no clutch, no torque converter, and no selectable ratios — and it is not an oversight. In their place sits one planetary gearset, the same component as inside any automatic, used in a completely different way. Three shafts share the gear: the sun in the middle, the carrier holding the planets, and the ring around the outside. The engine turns the carrier, the wheels (and the big motor, MG2) turn with the ring, and a small motor-generator, MG1, holds the sun.

The power-split device, drawn with the real Prius tooth counts. Nothing shifts, nothing slips; all three shafts are permanently geared together and the constraint below is the entire 'transmission'.

A planetary gearset enforces exactly one rule between its three speeds — with the Prius tooth counts (sun 30, ring 78):

Z_s\,\omega_{sun} + Z_r\,\omega_{ring} = (Z_s + Z_r)\,\omega_{carrier}

One equation, three speeds: fix any two and the third follows. The wheels fix the ring (road speed), and here is the trick — MG1 fixes the sun electrically, at any speed the computer likes, including backwards or zero. That makes the engine’s speed a free choice at every road speed: a continuously variable transmission with no belts, no clutches and no wear surfaces. Accelerate hard and the engine leaps to its best-power speed while the car is still crawling — the famous "rubber band" feel is the sound of the equation being solved, not of anything slipping. There is no reverse gear either: MG2 simply turns the other way.

The lever analogy, with real numbersfor engineers

Engineers rarely juggle the equation directly — they draw it. Put the three shafts on three vertical rpm axes, spacing them so the carrier sits $Z_r : Z_s$ of the way from sun to ring. The constraint above then says: every possible state of the gearset is a straight line across the three axes. The gearset is a seesaw; tooth counts are the lever arms.

Two real operating points, each a straight line. Green: pulling away on battery — ring at 500 rpm, engine parked at 0, so the lever forces MG1 to −1300 rpm (spinning backwards, unpowered). Orange: motorway cruise — ring at 2600 rpm with the engine held at a relaxed 2000 rpm, leaving MG1 generating gently at +440 rpm.

The line also explains the system’s one weakness. Torque balance on the lever is fixed by the arm lengths — the engine’s torque always splits $Z_r/(Z_s+Z_r) = 72\%$ to the ring and 28% to the sun. That sun-side torque is what MG1 must react, and reacting it while spinning means handling power: $\;P_{MG1} = T_{sun}\,\omega_{sun}$ . The further the engine’s speed strays from the "straight-through" point, the more power circulates through MG1 → battery bus → MG2, each leg taxed ~5–8%. At very high cruise speeds the line tilts so far that MG1 spins backwards while loaded — circulating power grows and efficiency sags, which is why classic power-split Toyotas feel happiest below ~130 km/h and why later versions added extra reduction stages for MG2. The lever gives you the whole machine: speeds from where the line crosses, torques from the arms, and the design trade-off from how far you dare tilt it.