自上市以来，丰田普锐斯（Toyota Prius）的全球总销量已经超过350万辆。不难想象，丰田汽车公司肯定希望能够延续这款标志性混合动力轿车的迅猛势头。为了实现这一点，丰田选择将筹码押在汽车行业都认可的正统升级上，而非完全投入到突然兴起的电驱动大潮之中。

经过升级，2016款普锐斯拥有更长、更低，而且更宽的车身。此外，丰田还特别处理了车辆混动系统中的汽油机部分。丰田称，普锐斯采用了一款1.8升的自然吸气阿特金森汽油发动机（型号：2ZR-FXE），其热力学效率高达40%，完全可以与涡轮增压柴油机媲美。

在接受《汽车工程杂志》采访时，丰田的副总工程师Kazuaki Shingo介绍了公司在借助丰田全新的全球平台（Toyota New Global Architecture，下简称TNGA平台），研发2016款普锐斯时曾遇到的种种挑战（见 http://articles.sae.org/14485/）。

丰田的首要任务是对普锐斯进行优化，使其摆脱固有形象，不再仅仅是一款强调“环保优势”的车辆而已。2016款普锐斯是丰田首款采用TNGA平台的车型，但完全保留了现行版本2700mm的轴距。

虽然轴距相同，但2016款普锐斯的尺寸比之前长61mm、低20mm、宽15mm，车顶最高点也前移了170mm，车辆尾部线条因此更加平缓。该车的自动格栅（automatic grille shutter）可按要求阻断流向散热器的气流，车辆底板的底壳则能使下方气流顺畅地通过车辆悬挂。正因这些改变，新款普锐斯的风阻系数（coefficient of drag）得到了进一步降低，从现行版本的0.25CdA成功降至0.24CdA。此外，车辆座椅H点的位置也下移了58mm，可以给驾驶员带来更像“汽车”的驾乘感受。

新款普锐斯摒弃了之前的后横梁设计，采用了一款双叉臂后悬挂系统。与真正的独立后悬挂相比，这种悬挂系统能够给驾驶员带来更棒的驾乘和操控体验。前悬挂则延续了现行版本的设计，采用了相当常见的麦弗逊。

丰田的工程师用铝制制动卡钳替代了原先的铸铁元件，从而减轻了车重，而且他们还从车辆的17吋铝制车轮中成功削减了0.7kg的重量。

丰田对轮胎的选择相对自由，不会局限在某一家供应商。这是因为只选择“一家”供应商意味着可能面临更多风险，比如福特（Ford）就曾因为供应商的产品存在缺陷，而不得不召回旗下Explorer和Firestone车型，又比如邓禄普（Dunlop）轮胎在日本的生产就曾因神户地震而中断，从而引发供货不足。很显然，丰田并不想受到供应商的牵制。因此，普锐斯可以搭配的轮胎种类非常丰富，包括邓禄普Enasave、优科豪马Avid、普利司通Ecopia，以及通伊欧NanoEnergy轮胎。

2016款普锐斯的车身采用一体化设计，其扭转刚度比之前提升了60%。这主要是由于丰田增加了这款车中的高强度钢用量。此外，该车的发动机盖和后门框还结合使用了铝材，进一步减轻了车辆重量。总体来说，不同版本的普锐斯，重量也存在一定差异。目前最轻的版本为普锐斯Eco，重约1365kg，而最重的则为顶配普锐斯Four Touring，重约1397kg。

普锐斯采用了丰田的碰撞预防技术——主动安全P套装（Toyota Safety Sense P），其功能包括碰撞前制动、行人监测、车道偏离预警与协助、动态巡航控制，以及自动远光灯开启等功能。除了完备的P套装外，丰田公司还推出了一款适用于紧凑型车辆的主动安全C套装（Toyota Safety Sense C），与P套装相比，C套装不具备前向雷达功能。

2016款普锐斯还配备了一款彩色平视显示器（HUD），可直接在驾驶员的视线内显示车辆数据。这非常有用，因为一般来说，这些信息都是显示在位置较低的仪表盘上，而驾驶员必须下移视线才能看得到。

丰田还有效提升了普锐斯的车内舒适度，在地板上铺了覆盖更全面，也更厚的一片式吸音垫，解决了之前存在的吸音垫缝隙问题。车辆空调系统采用的是电动压缩机，在运行时更加安静。在取暖方面，该车还采用了一款电陶瓷加热器，与主发动机冷却加热器相比，其加热速度更快。

普锐斯Eco采用了可以阻隔红外线的耐热挡风窗，可在夏季时持续减少射入车内的红外线，减缓车内温度的上升，从而降低空调的负载。

车辆的动力系统也有助于维持一个舒适的车内环境，其尾气系统中的热交换器可将尾气热量传送至冷却回路，而冷却回路则与车辆加热系统相连。这种设计可以缩短发动机开始供热所需的时间，而且还能在寒冷天气条件下加快车辆启停系统的启动。

为了达到40%的热效率，关键手段之一是对发动机进行有效的热控制。2016款普锐斯对废气再循环（Exhaust Gas Recirculation，下简称EGR）系统的利用率高达20%。丰田混合动力车辆系统规划部集团经理Kentaro Tomo表示，使用EGR系统可以降低发动机的泵送损失，因为发动机的节气门可以开的更大。

他说，“为了实现这一应用，我们在往回送气前，必须先降低尾气的温度。”因此，普锐斯在将尾气送回发动机进气口前，会先将其导入加热器核心回路中的回流管，使气体先通过ERG系统的热交换器。

然而，仅对尾气进行循环冷却还不够。“如果仅进行循环冷却，不能保证发动机燃烧性能的稳定。”Tomo提醒说，“因此，我们还对发动机的进气口进行了一些改进，确保湍流可以进入燃烧室内。”这里的改进指的是优化进气口的设计，而非导片的使用，导片仅在需要时起用。

Tomo介绍说，在达到40%的热效率后，丰田又开始着眼于更高的目标，希望将这一数字提升到50%。目前，丰田正在探索各种新技术，试图在无需采用非均质充量压燃技术的情况下，实现50%的热效率目标。

普锐斯首先是一辆混合动力车，因此丰田也没有忽略这款车的电动特性优化。新款普锐斯采用了电机与发动机平行的设计布局，而不是将电机安装在曲柄一端，并为其搭配飞轮。这种设计能缩短电机的封装长度，而且还可搭配减速齿轮，提高电机的最高转速。丰田称，与之前相比，新设计可以将系统的寄生损失降低20%。

新款普锐斯的DC/DC逆变器可在低负载的情况下减小自身的电流输出，从而协助提升车辆的燃油经济性。新车型的逆变器体积比之前更小，可以在发动机舱内腾出更多空间，用于放置之前一直装在车尾的辅助电池。

车辆的锂离子电池组拥有56个单体电池，其体积比过去的镍氢电池组小38%，可以直接安装在车辆后座之下，而且后座地面无需抬高，在座椅收起后可以用于载货。

为了使入门级车型价格更具竞争力，在美国境内出售的基本款二代普锐斯仍将采用与老版相同的镍氢电池。丰田称，镍氢电池在其他市场内也将继续受到欢迎。

丰田称，由于混合动力系统的峰值功率很难判断，在普锐斯的燃油经济性官方数据上一直存在一些争论。丰田的官方数据是以日本自动车研究所(Japanese Auto Institute)的标准为基础进行测量的。按照该标准，普锐斯汽油机和电动机协同工作时的输出功率为90kW（121hp）。

但按照SAE的标准，普锐斯汽油发动机的输出功率为53kW（95 hp），输出扭矩为142 N·m，电机的输出功率则为53kW（71 hp），输出扭矩为163 N·m。

总的来说，普锐斯（Eco版本除外）的EPA认证燃油经济性分别为54mpg（城市工况）和50mpg（高速公路工况）。而在车辆装配更少部件、车重更轻，并且采用硬质轮胎，从而胎压更高的情况下，普锐斯能够取得58mpg（城市工况）和53mpg（高速公路工况）的燃油经济性成绩。

二代普锐斯的燃油经济性有一定程度的提升，应该可以继续满足该车型关注环保的核心用户需求。但是新版车型更优越的行驶动态性能和低悬挂外观是否能够赢得其他用户的青睐，仍有待验证。

作者：Dan Carney
来源：SAE《汽车工程杂志》
翻译：SAE 上海办公室

Prius is re-engineered on Toyota New Global Architecture

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, YokohamaAvid, 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 theJapanese 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.

Author: Dan Carney
Source: SAE Automotive Engineering Magazine