Toyota details powertrain advances in Gen4 Prius; available E-Four system for all-wheel drive (not for US)
Toyota made significant improvements in its new generation full hybrid powertrain applied in the fourth generation Prius (earlier post), reducing losses, redesigning and relocating key components and reducing weight. Extensive changes to the transaxle, engine and combined hybrid system, generate an expected improvement in EPA estimated MPG on core models of up to 10% (i.e., up to ~55 mpg US combined), with an Eco model that will achieve an even greater improvement.
In a presentation outlining the changes to the new Prius system, Shunsuke Fushiki from Toyota’s Hybrid Engineering Management Division noted that the new engine contribues about 28% of the improvement in fuel efficiency; hybrid system management contributes about 26%; the new electric motor, 16%; new transaxle and PCU, 13% each; and new battery packs, 4%.
Engine. Although the new 2016 Prius retains the established 1.8-liter VVT-i Atkinson cycle gasoline engine (2ZR-FXE), Toyota completely re-engineered the engine, delivering results in terms of performance, fuel economy, and reductions to size and weight.
The new SULEV engine achieves a high maximum thermal efficiency of 40%. This improvement was enabled in part through the use of a large-volume exhaust gas recirculation (EGR) system and improvements in combustion efficiency and knock control.
Additionally, a redesigned air intake port improves airflow inside the combustion chamber, while the coolant passages have been redesigned to optimize internal engine temperature. The friction created by the piston skirts, rotating parts and the oil pump has been reduced, while the use of low-viscosity oil reduces friction between sliding engine components.
|2016 Toyota Prius 1.9 2ZR FXE cutaway. Click to enlarge.|
Fuel efficiency is also boosted by improvements to heating performance. An active grille shutter has been added to open and close the grille as necessary in response to outside temperatures. The exhaust heat recirculation system has also been improved and made more compact, lighter and efficient. This recycles exhaust heat from the engine into the coolant to be reused in the heaters and to warm the engine. Cold weather fuel efficiency has also been improved by introducing an engine coolant selector valve and adding a new system which accelerates engine heating.
Transaxle and Electric Motor. Toyota engineers also redesigned the transaxle and motor, delivering a reduction in their combined weight of 6%. The motor itself is considerably more compact (approximately 12%) and gains a better power-to-weight ratio (weight is reduced some 20%). Notably, there is an approximate 20% reduction in mechanical losses through friction compared to the previous model. The planetary gear arrangement in the reduction gear has been replaced with parallel gears, which further helps loss reduction.
|2016 Toyota Prius Transaxle. Click to enlarge.|
Power Control Unit. The use of a loss-reduction device in the power control unit cuts losses by about 20%. With a more compact (33% size reduction) and lighter (11% weight reduction) design, the unit can now be positioned directly above the transaxle.
|2016 Toyota Prius PCU. Click to enlarge.|
New Lithium-Ion or Nickel-Metal Hydride Hybrid Batteries. The new Prius will be available with either a new lithium-ion or a new nickel-metal hydride hybrid battery. While the batteries use established technology, Toyota has succeeded in increasing their energy density, which means more power can be obtained from a smaller unit.
The Li-ion pack is 31% lighter, with a 6% reduction in size; the NiMH pack is 2% lighter, with a 10% reduction in size. This in turn has allowed the battery to be relocated beneath the rear seat, increasing cargo space.
E-Four. For the first time, the Prius will feature an available E-Four (electronic four-wheel drive) system, which uses a high-output rear motor to assist the engine and front motor as needed and continuously optimizes the allocation of power between the front and rear wheels.
With a compact size and lightweight design that minimizes any negative impact on fuel efficiency, the system is positioned snugly in the rear of the vehicle to ensure plenty of legroom and luggage space. The luggage area capacity of the model featuring E-Four (with spare tire) is 457 liters, the same as the 2WD model (with spare tire).
Toyota is not offering E-Four in the US.
TNGA and Handling. The 2016 Prius is the first global vehicle to implement Toyota’s New Global Architecture (TNGA). (Earlier post.) TNGA aims to greatly improve core vehicle performance and enhance product appeal through an integrated development program for powertrain components and vehicle platforms.
The Toyota TNGA concept delivers an increase of more than 60% in body torsional rigidity in the new Prius, compared to the previous model, by using Laser Screw Welding (with a greater number of weld points), structural adhesives and uniquely shaped frame structures. In addition, the amount of high-tensile strength steel has increased from 3% to 19%. Combine these enhancements with its new double wishbone rear suspension and a lower center of gravity, and the result is a hybrid with a sporty ride.
|2016 Toyota Prius package. Click to enlarge.|
New updates to the hybrid system software improve the feel of acceleration, giving a smooth and direct response in a lower rpm range. Toyota has added a new feature to enhance the driving experience when Power mode is selected. An adaptive system continuously monitors the acceleration pattern and lateral G-force readings to determine if the driver is adopting a more enthusiastic driving style. This system will then adapt and implement a sportier acceleration and deceleration.
To improve brake feel and control while reducing noise reduction, the all-new Prius utilizes a newly-developed active hydraulic booster for its regenerative brakes.
Body styling. The TNGA chassis allows for lower lines, while retaining the Prius’s signature triangular silhouette.
The body height has been reduced by 20mm and the high point of the roof has been moved 170mm forward, ensuring plenty of headroom for front seat passengers while also contributing to excellent aerodynamics and a 0.24 coefficient of drag (Cd). The lower center of gravity has also allowed the nose height to be brought down by 70mm and the rearmost section of the hood by 62mm, improving the driver’s forward view.
Available Advanced Technologies. The new Prius will make a significant advance in active and preventive safety measures with the available Toyota Safety Sense P package that includes a Pre-collision System with Pedestrian Detection. The system uses a millimeter-wave radar and a single-lens camera with integrated control to detect both cars and pedestrians. The package also comes with Full-speed Dynamic Radar Cruise Control.
Drivers can also receive parking assistance with the Intelligent Parking Assist, which uses ultrasonic sensors to detect surrounding objects and identify parking spaces. The driver stops the car before the open parking space and by pushing a single button, the system guides drivers to the right position for reverse parking and assists drivers in backing into the space.
Following its unveiling in Las Vegas last month, the all-new Prius will go on display from October 28 at the Tokyo Motor Show, and is slated to go on sale in Japan in December. The new Prius will arrive in dealerships in the US early next year.