With 3.5 million Prius hybrids sold worldwide since its launch, Toyota is understandably keen to maintain the iconic hybrid-electric’s momentum. To that end, the company has relied on upgrades familiar to automotive orthodoxy rather than the upstart electric drive crowd.
In the case of the 2016 Prius, that means bodywork that is longer, lower, and wider. There’s also special attention to the gasoline portion of the car’s gas-electric drivetrain. Toyota claims 40% thermodynamic efficiency for the Atkinson cycle 2ZR-FXE 1.8-L I4 naturally aspirated gasoline engine under the Prius’s hood, a number that rivals turbodiesels.
In an Automotive Engineering Q&A, Assistant Chief Engineer Kazuaki Shingo discussed the challenges of developing the 2016 Prius on the Toyota New Global Architecture (see http://articles.sae.org/14485/).
Toyota’s first order of business was to make the Prius a better car to try to escape its niche as a rolling political statement. It is the first vehicle on the Toyota New Global Architecture, though it returns on exactly the same 106.3-in (2700-mm) wheelbase as the outgoing car.
Even with that similarity, the 2016 Prius is 2.4 in (61 mm) longer, 0.8 in (20 mm) lower, and 0.6 in (15 mm) wider than the outgoing car. The peak of the roof is moved forward 6.7 in (170 mm), providing for a longer taper to the rear. Automatic grille shutters close off airflow to the radiator when it isn’t needed, and underbody covers smooth the flow of air around its suspension. These changes whittle the car’s coefficient of drag down another notch to 0.24 from the old car’s 0.25. The driver’s H-point sits 2.3 in (58 mm) lower than before, imparting a more car-like feel behind the wheel.
A double-wishbone rear suspension replaces the old car’s torsion beam axle, with the predictable improvement in both ride and handling quality from this change to truly independent rear suspension. The front suspension continues with ubiquitous MacPherson struts.
Engineers trimmed unsprung weight by replacing cast iron brake calipers with aluminum ones and by carving 1.5 lb (0.7 kg) from the weight of each of the car’s 17-inch aluminum wheels.
Toyota uses a wide base of suppliers for its tires, as carmakers recall the pinch of supplies that can occur due to defects, as with the Ford Explorer/Firestone recall, or with manufacturing disruptions such as the earthquake in Kobe, Japan, interrupted production of Dunlop tires there. So the Prius comes on Dunlop Enasave, Yokohama Avid, Bridgestone Ecopia, and Toyo NanoEnergy tires.
The new suspension is tied together by a unibody that has 60% better torsional stiffness than before. Increased use of high-strength steel contributes to the improved stiffness, and it combines with aluminum in the hood and rear door frames to also save weight. The lightest Prius Eco is 3010 lb (1365 kg), while the most thoroughly equipped Prius Four Touring weighs 3080 lb (1397 kg).
The Prius gains the suite of Toyota Safety Sense P crash prevention technologies including pre-collision braking, pedestrian detection, lane departure alert and assist, dynamic cruise control, and automatic high beams. The "P" in the system's name differentiates it from a lesser system on Toyota's compact models designated "C," which lacks the forward radar of the P system.
There is also a color headup display providing vehicle data directly in the driver’s line of sight, which is good because the car’s instrument cluster is located in the center of the dashboard.
Toyota addresses cabin comfort with a thicker, more complete one-piece sound-deadening mat for the floor, with none of the gaps of the previous model’s mat. The air conditioner features a quieter electric compressor and for heat, the car has an electric ceramic heater to warm the cabin more quickly than the main engine-coolant heater does.
The Eco model employs a heat-resisting infrared-cut windshield that substantially reduces transmission of infrared rays to the cabin for a lighter air conditioning load in summer.
The car’s powertrain has a feature to assist with cabin comfort, a heat exchanger in the exhaust system that transfers exhaust heat to the coolant circuit that is routed to the car’s heater core. This shortens the time needed for the engine to provide heat and quickens the availability of fuel-saving engine stop/start in cold weather.
Thermal control is a key aspect of the engine’s ability to achieve 40% thermal efficiency. The car runs as much as 20% exhaust gas recirculation. Using EGR reduces pumping losses because the engine can run with its throttle plate open wider, according to Kentaro Tomo, Toyota’s Group Manager for the hybrid vehicle system planning department.
“In order to do that, before we return it, we need to lower the temperature of the exhaust gas,” he said. So the Prius uses the return line of the heater core circuit to flow through a heat exchanger for the EGR just before exhaust gas flows into the engine’s intake side.
However, cooling recirculated exhaust isn’t enough. “Combustion is not stable if you pay attention to that only,” Tomo warned. “For that reason, we made improvements to the intake port to make sure that turbulence is brought to the combustion chamber.” This is done though the design of the port, and not through the use of flaps that operate only when needed.
Having achieved 40% efficiency, Toyota’s sights are now set on reaching 50%efficiency for gasoline engines, according to Tomo. The company is researching techniques that might let it reach that level, even without employing heterogeneous charge compression ignition, he said.
Since it is a Prius, Toyota did not ignore advances on the electric drive side. The car’s electric motor is now mounted parallel to the gasoline engine, rather than to the end of its crankshaft with the flywheel. This reduces package length of the combined motors and it gives the electric motor a higher speed range through a reduction gear. Toyota claims this arrangement cuts parasitic losses by 20% compared to the old layout.
The DC-DC boost converter can now reduce its output current in low-load situations, which also aids in fuel efficiency. The car’s inverter is smaller than before, allowing space under the hood for the auxiliary battery, which used to consume space at the rear of the car.
The 56-cell lithium-ion battery pack is 38% smaller than the old nickel-metal hydride pack and now fits entirely beneath the rear seat, permitting an unobstructed flat load floor when the rear seats are folded.
In the U.S., the base Prius Two will sell with a NiMH battery like the old car to provide a low entry price. This battery will continue to be popular in other markets, Toyota said.
Because of difficulty determining peak system power in a hybrid-electric drivetrain there is some debate over official rating, Toyota said. The company has chosen the Japanese Auto Institute standard as its benchmark, and according to that rating, the Prius’s gasoline and electric engines produce a total of 121 hp (90 kW) when working together.
SAE net power for the gasoline engine is 95 hp (71 kW) and 105 lb·ft (142 N·m), while the electric motor is rated at 71 hp (53 kW) and 120 lb·ft (163 N·m).
Together, the changes produce EPA fuel economy ratings of 54 mpg city and 50 mpg highway for all Prius models other than the Eco. That one is rated at 58 mpg city and 53 mpg highway as a result of lighter weight from less equipment and harder tires with a higher specified air pressure.
This increased efficiency should continue to satisfy the car’s core environmentalist customer base, but it will remain to be seen whether the improved driving dynamics and lower-slung looks expand the car’s appeal to customers with less ideological motivation.Continue reading »