by Mike Millikin and Jack Rosebro
|The 2010 Prius. Click to enlarge.|
In preparation for the global launch of the much-anticipated 2010 Prius hybrid, informally referred to as “Generation III” or “Gen III”, Toyota has been staging a series of media briefings, bringing in key engineers from Japan as well as its North American technical center in Southern California, to discuss technical improvements of the new hybrid over its immediate predecessor, the 2004-2009 Generation II Prius. Toyota also previewed the vehicle this weekend in Los Angeles via its “Prius Connection” program for select early adopters and enthusiast forum participants, attracting potential buyers from across the US and Canada.
The Gen III Prius offers an EPA-rated combined cycle fuel economy of 50 mpg (now revealed to comprise 50 mpg city and 49 mpg highway). This is about a 10% improvement in EPA-rated combined cycle fuel economy over the older generation. Of that 10% improvement, about 6% can be attributed to the new hybrid system (including the larger combustion engine) and about 4% to efficiency improvements in other aspects of the vehicle, such as better aerodynamics, said Chief Engineer Akihiko Otsuka.
Hybrid Synergy Drive
The newest version of Toyota’s Hybrid Synergy Drive as applied in the 2010 Prius is more than 90% newly developed, and has produced more patents than previous Prius generations combined. Maximum system output, including the engine, is controlled to about 100 kW (134 hp).
|The 2010 Prius 1.8L Atkinson cycle engine. Click to enlarge.|
2ZR-FXE 1.8L DOHC 16-valve VVT-i engine. The 2010 Prius features a new 1.8-liter engine (up from 1.5 liters in the older Prius) that provides greater torque at lower RPM for improved fuel economy, especially on the highway. The 1.8L Atkinson cycle engine delivers 98 hp (73 kW) @ 5,200 rpm, and 142 Nm (105 lb-ft) of torque @ 4,000 rpm.
With the replacement of the engine’s belt-driven water pump, all engine coolant pumps are now electrically operated, reducing mechanical losses and providing engine coolant flow rate control independent of engine RPM at all times. As Prius power steering and air conditioning systems remain electrically driven, the 2ZR-FXE powerplant is Toyota’s first beltless engine.
As with the Generation II, cabin heating and air conditioning can continue operating with the engine stopped; the latter is a capability further leveraged by the use of Toyota’s optional Solar Roof system (below).
The complex engine coolant heat storage system utilized by the Gen II Prius has been replaced by an exhaust heat recirculation system that shuttles catalyst heat to a heat exchanger which interfaces with the engine cooling system, reducing engine-on time and improving cabin heater system performance. An exhaust gas recirculation (EGR) system has also been added. The 2ZR-FXE engine is SULEV-rated (with AT-PZEV) in California, and has a Federal Tier 2 Bin 3 rating.
|Major components, including motor-generators, from the 2nd generation Prius transaxle (left) and the new 2010 Prius transaxle (right). Click to enlarge.|
Transaxle. The all-new hybrid transaxle adopts improvements first used in previous post-2004 Toyota/Lexus hybrids such as the Camry hybrid, Highlander hybrid, and Lexus RX 400h, as well as refinements not previously employed. The result is 20% lighter in weight compared to the previous model.
The transaxle’s reduction chain assembly has been deleted in favor of a reduction gear. Power output of the more compact transaxle traction motor (motor-generator 2, or MG2) has been increased to 60 kW (80 hp), and torque has been reduced by about half to 207 Nm (153 lb-ft). (The revised gearing compensates for the new motor-generator’s torque reduction.) Maximum speed on MG2 has been increased to 13,500 rpm, according to Toyota, up from 6,400 rpm, and its maximum operating voltage has increased from 500V to 650V.
(Waste material from the construction of the 2010 Prius generator—most likely center cuttings from production of the generator’s stamped steel stator plates—is reused in the construction of the electric motor that drives the vehicle’s air conditioning compressor.)
The multifunction gearset incorporates two sets of planetary gears: a power-split planetary gearset and a speed reduction planetary gearset. Both gearsets share the same ring gear, which drives the transaxle’s counter gear. The sun gear of the power-split gearset is connected to the 42 kW motor generator 1 (MG1); the planet carrier is connected to the engine. In the speed reduction planetary gear set, the sun gear is connected to MG2, the carrier is fixed, and the ring gear connects to the counter gear. A parking pawl is also part of the assembly, which is connected to a differential with a final drive ration of 3.267:1.
Battery pack. Nominal pack voltage of the nickel-metal hydride (NiMH) battery remains at 201.6 VDC. The more compact battery pack and system main relay assembly were repositioned, resulting in increased cargo volume and cabin comfort. Cooling air velocity and volume have been increased for increased cooling efficiency, resulting in overall improved battery efficiency and fuel economy. The pack is warranted for 10 years/150,000 miles.
At end of life, every part of the battery is recycled or processed for disposal at a dedicated facility in Japan.
|The 2nd generation inverter (left) vs. the 2010 Prius inverter (right). Click to enlarge.|
Inverter. System operating voltage is now boosted from battery voltage to as high as 650 VDC by the vehicle’s boost converter before being converted into three-phase alternating current by the inverter, which Toyota refers to as a Power Control Unit (PCU). This represents a 30% increase over the boost employed by the Gen II Prius. All Hybrid Synergy Drive vehicles released subsequent to that vehicle have employed a boost converter capable of up to 650 VDC output to the inverter.
Similar in size and shape to current Toyota/Lexus hybrid inverters, the new unit features direct liquid cooling of the IGBT power transistors (developed in-house by Toyota), reducing energy losses. Company engineers will present a technical paper on the new inverter at SAE World Congress in April.
Fuel tank. A new resin 11.9-gallon fuel tank does away with the bladder used in previous North American iterations of the Prius, and utilizes an enhanced evaporative emissions system that complies with AT-PZEV zero fuel evaporation regulations.
The new Prius offers four driving modes: the default (normal) mode and three others (EV, Eco and Power) selectable by buttons on the center console. EV mode helps keep the vehicle in electric-only operation for up to about 1 mile at speeds less than 25 mph. Among other parameters, the state-of-charge (SOC) level of the vehicle’s high-voltage battery pack, as shown on the energy monitor display, must be four bars or more for the Prius to operate in EV mode.
Eco mode reduces throttle opening angle to a maximum of 11.6%. Eco mode also modifies air conditioning operation, and improves performance in low-traction conditions such as ice and snow, as the reduced output helps to minimize wheel slippage.
Power mode essentially does the reverse of Eco mode, increasing mid-range throttle response to optimize acceleration performance.
Toyota will offer the 2010 Prius with four option packages, including a Advanced Technology Package and a Solar Roof Package. The Advanced Technology Package includes safety options that recently debuted in the North American market with the Lexus LS series: a pre-collision system that utilizes seatbelt pre-tensioners and brake assist, a millimeter-wave radar system that regulates the vehicle’s distance from cars ahead of it when cruise control is engaged, a Lane Keep Assist (LKA) system that reduces driver effort during straight-line driving and warns the driver if the vehicle begins to drift out of its lane, and a semi-automatic parallel parking system called Intelligent Parking Assist.
A solar roof is available on the 2010 Prius as part of a Solar Roof Package. 36 polysilicon solar cells in the roof generate up to 59 W of electricity, powering an interior ventilation system. One minute after the vehicle’s Solar Ventilation Switch is activated, the interior air intake is switched to outside ventilation, and blower mode is switched to the face vents. Cooling begins ten minutes after the vehicle is shut off so that air that was already cooled is not expelled, and continues until the system is manually switched off or the vehicle is started.
The solar-powered ventilation system is paired with a remote air conditioning system that allows the driver to remotely command the vehicle’s electric air conditioning system on for up to three minutes when the vehicle is parked. The air conditioning system is powered by the vehicle’s hybrid battery pack during this time.
|“The body size and overall weight were optimized to achieve excellent environmental performance, fuel efficiency and cabin comfort. To achieve this, our thoughts were ‘Outside Minimum, Inside Maximum’.”|
—Chief Engineer Akihito Otsuka
Although the vehicle is roughly the same size as its predecessor, its interior is slightly larger. Select interior parts in the new Prius are made of Toyota’s new “ecological plastic” which the company estimates to have around 20% less CO2 associated with its production. Coefficient of drag is Cd 0.25, aided in part by modifications to the body such as clean bumper edges accentuated by flat wheel flare surfaces to improve airflow and a large grille opening. Extensive aerodynamic undertrays reduce turbulence underneath the vehicle.
The 2010 Prius is 110 lbs (50 kg) heavier than the second generation. The decrease in the weight of the hybrid components is offset by the addition of new safety equipment and improvements to the body structure.
Bob Carter, Toyota Motor Sales USA Group Vice President and General Manager, Toyota Division, said that Toyota is projecting sales of about 100,000 units for the 2010 Prius in 2008, and of 180,000 for the car’s first full calendar year on the market.
“We think it will appeal to current Prius owners,” he said. “More than 90% of Prius owners tell us they intend to buy another Prius. We also are confident that it will attract more buyers We have two main challenges: the economy, and the [Honda] Insight.”
Carter said that Toyota believes there is ample room in the market for both the mid-sized Prius and the compact Insight, and that the Prius could achieve about a 7% share in the mid-sized segment and that the Insight could garner about 5% share in the compact segment. The 2010 Prius is scheduled to go on sale this May. Toyota has yet to announce the manufacturers’s suggested retail price (MSRP).