It’s interesting to compare those figures with the original Japanese-market model, which we’ll call “Generation Zero.” By today’s EPA standards, its fuel-economy ratings would have been 35 mpg city/37 highway. When we tested that car in ’99, we measured 35 mpg, a far cry from the 47 we saw in the new Eco, especially when the gen-four model is so much faster and roomier. Squeezing efficiency gains of this magnitude from a complex hybrid powertrain is an exercise in continuous detail development.
For example, the new Prius has a revised version of Toyota’s 1.8-liter 2ZR-FXE Atkinson-cycle engine used by previous hybrids. Redesigned intake ports promote faster combustion. More exhaust-gas recirculation, cooled by a dedicated heat exchanger, allows the engine to run at larger throttle openings for reduced pumping losses. Then there are some reduced-friction components, tighter control of coolant temperature through active grille shutters, and an improved exhaust-heat recirculation system. The result is a peak thermal efficiency reaching a diesel-like 40 percent.
To trim friction, the planetary gearset between MG2 and the final drive has been replaced by helical gears. A second electric motor/generator, called MG1, still starts the engine, controls the drive ratio, and generates electricity to recharge the battery and to power MG2. Overall, the new lighter and more compact Prius powertrain is more like the Ford hybrid transaxle arrangement, which is based on Toyota patents.
Toyota’s hybrid mavens also massaged the power-control electronics to cut their energy losses by about 20 percent. This circuitry is now sending power back and forth to a new lithium-ion (Li-ion) battery that replaces the nickel metal-hydride (Ni-MH) unit on all Prius models except for the base Prius Two. This battery is smaller and lighter than the Ni-MH one, though even the latter has been compacted so that both batteries now fit under the back seat.
The Prius Two Eco achieves exemplary mileage thanks to a number of measures. Its Li-ion battery and a tire-inflator kit rather than a spare shave 65 pounds versus the Prius Two. That’s enough to drop it one 125-pound weight class on the EPA fuel-economy tests. It also gets special Dunlop Enasave 01 A/S low-rolling-resistance tires, inflated to slightly higher pressures, and a solar-reflecting windshield to reduce the air conditioner’s workload.
Though this Prius is built on Toyota’s new TNGA platform, its wheelbase is still 106.3 inches, a dimension seen on the 2004 model. But it uses more high-strength steel than the 2015 Prius (up from 3 to 19 percent); is assembled with more adhesives plus something called laser screw welding; and has several reinforcements in the cowl area. Toyota claims 60-percent greater torsional rigidity than the old model. There’s also more sound insulation and a floor silencer pad constructed without seams to more effectively curb road noise.
One other new platform feature is a control-arm rear suspension, replacing the previous trailing twist axle. Toyota touts this as making the Prius more “fun to drive,” though that’s like saying Finnegans Wake is more fun with pictures. But the new model is indeed more dynamically capable than the last one.
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