Toyota Puts Fun in Functionality With New Prius

Wards 10 Best Engines competition has recognized outstanding powertrains for 22 years. This installment of the 2016 “Story Behind the 10 Best Engines” series looks at the development of the latest hybrid-electric powertrain in the Toyota Prius.

WardsAuto editors have been unabashed fans of Toyota’s iconic Prius hybrid-electric vehicle since it began reaching U.S. shores, primarily for its fuel efficiency and engineering innovation. And they have named previous models Wards 10 Best Engines Award winners in 2001, 2004 and 2010.

The ’16 Prius hatchback delivers the best fuel economy of any vehicle without a plug: 54/50 mpg (4.4-4.7L/100 km) city/highway and WardAuto’s Prius Two Eco test vehicle rated even higher on the official EPA scale at 58/53 mpg (4.0-4.4L/100 km).

So it’s no surprise the new fourth-generation Prius earns Toyota another trophy for 2016. But beyond this latest fuel-economy benchmark is a new dimension: unlike previous high-efficiency Prius models, this one actually can be fairly fun to drive. Not only does its Toyota New Global Architecture platform provide better ride and handling, its redesigned propulsion system adds noticeably more pep.

“With an all-new 96-hp 1.8L 4-cyl. paired with two electric motors pumping another 53 kW (71 hp) of juice from batteries stored in the rear, the new Prius delivers seamless acceleration and real-world fuel economy during our evaluations of up to 61 mpg (3.8L/100 km),” WardsAuto judges report. “It’s quieter and silkier than the old Prius, and a ‘Power’ button provides a noticeable kick on freeway entrance ramps.” Its peak torque is 105 lb.-ft. (142 Nm).

The ’16 Prius engine’s significantly higher efficiency comes mostly thanks to three primary areas of improvement: combustion process, heat management and friction reduction.

For improved combustion, new intake-port geometry increases the charge tumble ratio to 2.8 from the previous engine’s 0.8, leading to 35% quicker combustion, while exhaust scavenging is improved by optimizing the length of the exhaust manifold. New cooled EGR ups the EGR rate 67% for improved thermal efficiency, and optimized intake valve timing further improves scavenging.

“The power-split propulsion system that balances the engine and battery follows the same character as before,” says Heraldo Stefanon, manager-hybrid vehicles and drivetrain at the Toyota Technical Center.

“But we redesigned the engine and found ways of delivering the power in a more efficient way. We improved combustion by expanding exhaust gas recirculation and improving the way the air comes into the cylinders with increased tumble. And reduction of friction in everything that’s turning in the engine was very important.”

Those many friction reductions include fine grooves in the crankshaft bearings that create negative pressure to draw in oil and retain it better; lower-friction valvetrain and chain-drive systems; a new resin coating on the piston skirts; and a new 9-tooth oil pump that increases discharge volume 20% over the previous 6-tooth unit to enable the use of low-viscosity engine oil.

Also important are heat-management enhancements, including improvements to the cylinder-head water jacket, a new water-jacket spacer and drilled passages in the cylinder block to keep the engine within good operating temperatures.

Toyota claims this new Prius engine incorporates more than 50 other efficiency-enhancing technologies in addition to all these improvements. The result is the world’s first engine to achieve an amazing 40% maximum thermal efficiency, the measure of how much energy from the fuel actually reaches the drive wheels.

Internal combustion engines are typically 33-35% thermally efficient. “We are reaching new grounds in delivering that energy to the wheels, reducing the waste that goes out in heat,” Stefanon says. “That 40% efficiency was a specific target from the beginning.”

Another major goal was a 10% fuel-economy improvement.

“The previous Prius had combined EPA economy of 50 mpg (4.7L/100 km),” Stefanon says. “We wanted to improve that by 10%, but maintaining performance was a key objective. The new Prius is slightly quicker than the previous car and more agile due to the new architecture and rear suspension.”

Lighter and More Compact

Behind the new engine, the rest of the propulsion system is lighter and more compact. The transaxle was reduced in size and weight by adopting a parallel-shaft layout in place of the previous single shaft, which eliminates one planetary gear set and its mechanical friction.

The power control unit has new electronics that let it be more compact, and a number of other components also were made smaller, which opened more packaging room under the hood and more cargo volume in back.

Electrical losses also were reduced, “Instead of joints put together with screws on bus bars and connection bars, we now weld them together,” he says. “With those and a lot of other improvements combined, we were able to optimize the system to get even better fuel economy while maintaining performance and fun-to-drive characteristics. It was a whole system approach, redesigning every component for maximum efficiency, starting with blank sheets of paper.”

Among the many key suppliers, Denso and Aisin worked with Toyota to help improve the overall hybrid system from early in the program, and Prime Earth Energy (PEVE) collaborated on development of the new lithium-ion battery.

Stefanon adds the fuel-economy/fun-to-drive balance was the team’s toughest challenge, “In the previous generation, fuel efficiency was the major target,” he says. “With this one, maintaining performance with a more efficient system was a big challenge. The new double-wishbone rear suspension added weight, for example, so mass control and aerodynamic drag were important. We had the mass challenge of some added items, but by reducing the weight of some other components, the base vehicle weight stayed roughly the same.”

With each new, better and more fuel-efficient Prius, many observers have wondered how much better the next generation could be without significantly hurting its performance and drivability and/or driving its cost out of sight.

“Cost is always a big item,” Stefanon says, “so we continue working on reducing the cost. And there are always ways to improve efficiency, so we’ll keep working on that as well, finding ways to improve usage of the energy from the fuel and the batteries, how to use each one at a particular time that is best for efficiency, while still delivering the fun-to-drive (aspect) that everyone wants.”