Akihino Otsuka was thrilled when Toyota management handed him the lead to create the company’s third-generation Prius hybrid a couple of years ago. It was the engineer’s first time heading up “the development of a high-volume car…and a hybrid icon” at that. He knew that he’d have a lot less time for the two vintage Alfa Romeos in his garage or his rock band.
Right from the start, the development team faced the basic issue: what type of hybrid to build? After considering the spectrum from mild to plug-in, “we realized that we could save money by going toward a mild hybrid,” recalled the chief engineer. “But we opted instead to stay with the Prius’ traditional full-hybrid package, which reliably provides the best fuel economy and greatest environmental benefit.”
Then began the group’s laborious effort to realize their 50-mpg combined mileage rating target, 4 mpg better than the previous Prius. Reaching the 50-mpg mark meant concentrating on making all aspects of the system more energy efficient, Otsuka said. The painstaking effort focused on cutting the car’s energy use by smoothing the aerodynamics and systematically reworking the energy-consuming subsystems as well as the vital interactions among them.
“Overall, we achieved a 10% improvement in energy consumption,” he reported. Four percent came from more efficient interiors and better aerodynamic performance. The Prius’ slippery body shape features squared-off leading and trailing edges, and nearly full-length under panels that help it get a good 0.25 drag coefficient. The remaining 6% efficiency boost derived from a thriftier electrical system. Otsuka added that the new hybrid also features low rolling-resistance tires.
To shave off as much gas mileage as feasible, the time-tested Toyota Synergy Drive propulsion system was mostly redesigned. “Ninety percent is new,” Otsuka said.
The team first enlarged the Atkinson-cycle base engine. The new, more powerful 1.8-L, four-cylinder engine produces 98 hp (73 kW) at 5200 rpm and provides 20% greater maximum torque. Contrary to conventional wisdom, the bigger engine actually helps improve highway mileage, he said. By making more torque, the engine can run at lower average rpm on the highway, so it consumes less fuel. Fuel economy is especially improved in cold-start conditions and at higher speeds.
The new engine is beltless to reduce parasitic losses, Otsuka continued. All auxiliary systems—water pump, air conditioning compressor, etc.—are electrically driven, which enables optimal, independent operation and less potential maintenance. Contributing to the engine’s efficiency is an exhaust gas recirculation (EGR) system that reduces pumping losses.
The electromechanical part of the Toyota hybrid powertrain was optimized in a similar fashion. The revised transaxle is lighter in weight and reduces torque losses by as much as 20% compared to the previous model. The inverter, which converts direct current to alternating current, now cools its transistors more efficiently, allowing size and weight reductions. Taken together, the inverter, motor, and transaxle are smaller and one-fifth lighter. In addition, the new Prius better captures stopping energy with an electronic system whose control logic optimizes the brake regeneration process.
The permanent magnet synchronous electric motor, which provides the car’s instant torque and power boost during hard acceleration, is also more powerful. Otsuka’s group doubled its rpm output via a reduction gear to boost maximum power output from 50 to 60 kW. Together, the electric motor and gasoline engine generate a combined net power of 134 hp (100 kW), an improvement of 24 hp (18 kW).
The one major powertrain component that is not all new is the battery. “The previous technology is still good,” he asserted, noting that “we’ve had no problems with nickel/metal hydride batteries.” Otsuka said that more effort is needed to better understand the performance of next-generation lithium-ion batteries. Lithium batteries are, however, to power 150 plug-in Priuses that the company is making available later this year to U.S. universities and fleet customers for testing.
Encouraging drivers to acquire the economical driving habits that can make the most of the Prius’ fuel-stingy ways meant that the development team devoted considerable effort to perfecting the car’s display panel, which employs several different screens to provide real-time feedback to the driver on fuel and energy consumption.
Likewise, the motorist can choose from several driving modes. A center console switch changes the normal mode to the gas-saving “Eco” mode. This setting is especially useful for stop-and-go conditions because it counters wasteful driving habits by damping the throttle response. “It cuts the corners off the throttle opening angle during revving,” Otsuka said. Eco mode also optimizes the air-conditioning load and maximizes the electric-assist operations. An EV (some say “sneak-home”) mode provides 1 mi (1.6 km) of silent, all-battery power, and a Power setting is for sporty or mountain driving.
In keeping with the green theme, the new Prius includes solar power generation. A sliding glass moon roof that is lined with photovoltaic panels is an option. The silicon solar cells power a cabin-air-circulation fan/vent that lessens the need for air conditioning on startup. Otsuka said that the engineers originally studied using the 59-W solar array to at least symbolically recharge the 1300 W·h battery, but progress was blocked by an unforeseen “technical issue.” It seems that when the battery is connected to the solar array it acts as an antenna, radiating electrical noise (from the battery reactions) that interferes with nearby radios and mobile phones.
Another nod to being green is the Prius’ use of “eco-plastics” in its scuff grids, side trim, finish plates, and seat cushions. Composed of recyclable polymers made with 15% plant-based oils from corn and castor beans, the lifetime use of the environmentally friendly plastic emits roughly 20% less carbon dioxide compared to that of its petroleum-derived counterparts, Otsuka stated.
To achieve a superior safety rating, he continued, Toyota engineers installed improved passenger safety systems and used thicker steel gauges and more high-tensile steel alloys in the car structure, resulting in a total weight increase of 45 lb (20 kg), which marginally reduces fuel economy. The gain occurred even though the team cut the weight of the hybrid powertrain by 65 lb (30 kg).
Despite the vehicle’s green focus, “we also wanted to expand the range of customers by making it fun to drive and by providing comfort and high-quality NVH,” he concluded. “We’re happy if the customer thinks of the Prius as a regular midsize car.”
Today, the Prius is one of the carmaker’s top three best-selling models; it seems that hybrid technology has truly gone mainstream.