Toyota Prius 2016

The original, relentlessly honed over the years for efficiency. Toyota’s hybrid system is quite hard to visualise, but here goes.

The engine, drive motorand a separate generator are mounted in a planetary gearset. The generator is on the sun wheel, the engine drives the planet cage and the drive motor is the outer annulus. As the electronics vary the torque and speed of the motor and the inverse torque of the generator, the effective gearing of the engine changes, and the with it the split of its output between generator and motor. Did we mention it’s hard to imagine? The result is probably the most efficient hybrid system out there, but its performance seems strange, as the engine speed is unrelated to road speed.

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The 1.8 l engine from Toyota Prius

The Prius, like many hybrids, uses an Atkinson Cycle petrol engine. The inlet valve stays open after the piston starts rising. This allows air back out of the engine. Afterward, when the spark fires, the expansion stroke is effectively longer than the intake stroke, and eking out all the expansion energy extracts more energy from the fuel. The Prius engine converts 40 percent of the energy of the fuel into useful work, which might not sound much but is actually a record high. The disadvantage of the Atkinson cycle is lower power for the size of engine, here a 1.8-litre petrol. But the added electric power makes up for it: 0-62mph acceleration is a not too sluggish 10.6 seconds.

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Fuel economy on the official EU cycle is an amazing 94.2mpg, for a CO2 rating of just 70g/km. In the real world, its economy isn’t hurt too much by congestion, but it suffers greatly with hard acceleration and braking and sustained high speed. That’s the same for any engine, though. If you want saintly numbers, you know how you have to behave.

Like the previous Prius, the new one will shortly be available as a plug-in hybrid, called Prime. The Prime has a bigger battery at 8.8kWh, chargeable in two hours from a 240V socket for about 22 miles of electric range. To improve electric-only acceleration over the regular Prius’s electric mode, the generator switches into a motor.

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