前克莱斯勒公司在2011年推出Pentastar 3.6升汽油发动机，目标是为了取代七款现行V6发动机，以及3.0升发动机（中国使用）和3.2升发动机。现在，这一目标已完全达成。然而，尽管2011款Pentastar发动机设计较为灵活，但相对来说仍比较简单，它保留了进气口燃油喷射技术，无废气再循环系统（EGR）。并且，在涡轮增压技术盛行的当时，这款发动机采用的是自然吸气技术。

这种方法可以降低成本。目前，菲亚特克莱斯勒汽车公司（FCA）正在为2016款车型逐步进行Pentastar发动机的升级工作，目的也是为了控制成本。进行升级非常重要，因为对几乎所有FCA北美车型来说，包括Ram皮卡在内，V6发动机都很有价值。FCA北美发动机、传动和电气推进部门副总裁Bob Lee表示，改良的V6发动机在美国联邦综合测试循环中，燃油经济性提高了6%，最大输出功率提高5.0 hp/3.7 kW（达到295 hp/220 kW），3000 rpm转速下的扭矩提高了15%。

改良的V6发动机仍采用自然吸气技术和进气口喷射技术，而现在，还能够向直接燃油喷射技术和涡轮增压技术过度，这两项技术正在位于美国密歇根州的奥本山公司总部进行研发，污染物颗粒捕集器也在此进行研发，以满足未来的法规要求。

可变气门升程系统

二阶式可变升程进气门系统也添加到了发动机中，系统由电磁阀控制油压启动，每个气缸盖都具备两个电磁阀。一个电磁阀在前端，用于控制第一个气缸进气门；另一个电磁阀在后端，用于控制第二个和第三个进气门。每个凸轮滚轴从动件都装有一个由弹簧负载锁销固定的旋转大升程部件；大升程模式（10.3毫米/0.41英寸）设定为默认模式。

在加速状态下，一个电磁阀打开，油压推动锁销释放大升程从动部件。升程从动部件沿着套管向下旋转，滚轴从动部件配合低阀门升程（5.75毫米/0.23英寸）凸轮轴工作，在转速小于2800 rpm的切换值下，发动机都是如此运转。Lee解释说这种切换可以降低油泵能量消耗，有助于优化燃烧过程，其结果是马力增大，燃油经济性提高2.7%。

Pentastar发动机具备降低燃烧爆震的技术，因此燃气压缩比从10.2:1提高到了11.3:1。据称，在结合了新型八孔喷油嘴（之前为四孔），以及绕流进气道和高能（100兆焦）点火线圈之后，燃油经济性可以提高1%。

发动机的可变进气排气门正时系统（VVT）的开启相位已从之前的50度增加到了70度。老款发动机的VVT是控制系统的一部分，无需使用EGR。2016款Pentastar发动机的VVT结合了基于发动机扭矩的凸轮定相技术，减少了气缸泵气损失。优化的凸轮定相也在最大程度上降低了热启动时发动机的敲缸现象，从而提高燃气压缩比。

此外，VVT经过了特别校验，目的是为了结合重新设计的进气歧管上的加长进气通路，有助于提高低档扭矩。系统也扩展了怠速启停系统的操作范围，实现了双赢效果，这不仅提升了实际操作的燃油经济性，也为FCA赢得了出众的公司平均燃油经济性数据（CAFE credit）。

液冷式废气再循环系统

长期以来，Pentastar发动机都没有装备EGR。而这次，工程师们为发动机添加了一个液冷系统，该系统通常只为许多重型货车配备。Lee向《汽车工程》杂志透露，该系统可以将尾气温度从650°C降到130°C（1202°F/266°F）。更低的气体温度有助于抑制高负载下的敲缸现象，从而提高燃气压缩比。EGR系统本身增加了0.8%的燃油经济性，降低了氮氧化物排放。高绕流进气通道和覆缘阀进一步增强了燃油、空气和废气的混合程度。

Pentastar发动机的高产量（迄今为止超过500万台）凸显了其高价值含量。Lee表示，直接燃油喷射技术（DI）已在发动机设计之中，但目前尚未应用。

Pentastar首席工程师Steve Gorgas表示，只有在更高负荷时，尤其是应用于增压发动机和涡轮发动机时，或者受益于高燃气压缩比时，DI才有积极作用。他也补充道，DI硬件系统包括装在发动机上的高压燃油泵和燃油通道，这也增加了重量和成本。并且，DI也应用了更为昂贵的燃油喷嘴，每个动力冲程发出5到7次脉冲，需要非常精准的燃油喷雾分布控制颗粒生成。

Lee表示，到2018年，颗粒物标准将更加严格，增加污染物微粒捕集器的潜在需求也需要纳入考量之中，虽然汽油进气通道喷射系统能效较低，但燃料在进气门背面雾化的时间得以延长，这会最大化降低颗粒物的生成量。

Gorgas说，涡轮增压并非完全的自由动力源。尽管总体效果很好，但在2000 rmp转速下，会产生更多附加损失。所以对于Pentastar发动机来说，涡轮增压机并不适用。但是，与DI相同，涡轮增压机仅在设计阶段，尚未开始实际应用。

轻量化，减小摩擦

升级所涉及的各部件总共为发动机增重13磅（6公斤），其中EGR和可变升程系统重量超过7.5磅（3.4公斤）。Gorgas的研发团队尝试了各种方法减轻重量，气缸盖、机壳前罩采用了薄壁铸造工艺。在全系列装备中，升级版V6发动机的整备重量在326-332磅（148-151公斤）之间，比老款Pentastar发动机平均降低了4磅(1.8公斤)。

除越野道路评级的吉普车型外，油底壳尺寸都有所减小，机油容量从6夸脱减小到5夸脱（5.7到4.8升），节省了约2磅(0.9公斤)的重量。发动机机体有所改进，目的是维持结构强度，并减小一部分加强肋板的尺寸。空气泵的结构区域从未使用，已被去除。整体而言，机体重量降低了近2磅（0.9公斤）。主轴承盖以球墨铸铁为原材料，而之前是粉末铸铁。Gorgas说，虽然球墨铸铁成本更高，但其可以成形性能更好，因而能减轻0.8磅（0.4公斤）的整体重量。

为加长气道长度，进气歧管进行了重新设计，由之前的三阶式设计改为两阶式，降低了0.7磅（0.3公斤）的重量。有两处前端可见的明显改动：曲轴阻尼器固定螺栓头凹陷下去，减重43克（1.5盎司）；水泵皮带轮正面钻了一些孔，减重近110克（4盎司）。

很多改进降低了阻力，以此提升1%的燃油经济性，其中主要改进是曲轴轴颈宽度（从72毫米下降到68毫米），以及曲柄稍长度（从59毫米下降到54毫米）。圆角滚压有所增加，从而保证曲轴强度；经证实，整体的重新设计也维持了之前的NVH特征。正时链条导板采用了一种具有超低摩擦系数的HDR-1塑料.

最后，气门机构也进行了超级抛光处理，气门弹簧负载得以降低，两冲程的机油泵也有所改良，从而减小了附加损失。

作者：Paul Weissler

来源：SAE《汽车工程杂志》

翻译：SAE 上海办公室

2016 Pentastar V6 adds new VVT, cooled EGR

When the former Chrysler Corp. introduced the Pentastar 3.6-L gasoline V6 in the 2011 model year, it was engineered to replace seven incumbent V6 engines, and along with 3.0-L (for China) and 3.2-L variants. It has done exactly that. However, despite all the flexibility built into it, the Pentastar remained relatively uncomplicated—port fuel injection, no exhaust gas recirculation (EGR), and it was naturally-aspirated at a time when turbochargers were proliferating elsewhere.

This approach kept costs down, and for 2016 Fiat Chrysler Automobiles is phasing in a Pentastar update in the same cost-conscious vein—important because the V6 serves as a value offering in virtually all FCA's North American nameplates, including the Ram pickup. The revamped V6 delivers a 6% increase in fuel economy on the combined U.S. Federal Test Cycle, along with a 5.0 hp/3.7 kW bump (to 295 hp/220 kW) and 15% more torque under 3000 rpm, according to Bob Lee, FCA North America Vice President of engine, powertrain and electrified propulsion.

Still naturally aspirated and port injected, the upgraded V6 also is now capable of transitioning to direct fuel injection and turbocharging—both technologies are "on the shelf" in Auburn Hills—as well as particulate traps, should future regulations require them. 

Variable valve lift

A two-step variable-lift intake valve system (https://youtu.be/YZtqCq9TXZg) was added, activated by oil pressure under control by solenoid valves—two for each cylinder head. One solenoid is in front, for the first cylinder intake valve; the other is at the rear, for the second and third intakes. Each roller cam follower incorporates a pivotable high-lift section held in place by a spring-loaded lockpin; the high-lift mode (10.3 mm/0.41 in) is the default.

On acceleration, a solenoid valve opens and oil pressure pushes the lockpin, releasing the high-lift follower section. It pivots down on a bushing, and the roller follower runs on low-valve-lift (5.75 mm/0.23 in) cam lobes, in which the engine stays through to the 2800-rpm switchover point. Lee explained that this switchover reduces pumping power draw and contributes to improved combustion, which delivers both the modest increase in horsepower and boosts fuel economy by 2.7%.

Thanks to steps taken to reduce combustion knock, the Pentastar compression ratio was raised from 10.2 to 11.3:1. Combined with new eight-hole injectors (vs. the previous four-hole), tumble intake ports and high-energy (100 mJ) ignition coils, the result is a claimed 1% fuel economy improvement.

The engine's variable intake and exhaust valve timing (VVT) authority has been increased to a range of 70°, vs. 50° previously. VVT on the old engine was part of the control system that eliminated the need for EGR. On the 2016 Pentastar, VVT incorporates an engine-torque-based cam phasing strategy, which reduces cylinder pumping losses. The enhanced cam phasing also is one contributor to the compression ratio boost by minimizing engine knock during hot starts.

In addition, the VVT was specifically calibrated to combine with longer intake runners in a redesigned intake manifold, contributing to the improved low-range torque. And the system expands the operating range of the idle stop/restart system, a carryover feature that not only improves fuel economy in real-world operation but also gives FCA a substantial CAFE credit.

Liquid-cooled EGR

After the long period without EGR, engineers added a liquid-cooled system that has normally been a design feature only on many heavy-duty trucks. It reduces temperature of the exhaust gases from 650°C to 130°C.(1202°F/266°F), Lee toldAutomotive Engineering. The reduced gas temperature helps enable the higher compression ratio by suppressing knock at higher loads. And the EGR in itself delivers a 0.8% improvement in fuel economy and low NOx emissions. High-tumble intake ports and shrouded valves further enhance fuel, air and exhaust mixing.

The Pentastar's high production volume (over 5 million units to date) puts an exclamation point on the value factor. Direct fuel injection (DI) was engineered into the engine design but it remains "on the shelf," said Lee.

DI is a positive addition only where loads are higher, particularly in applications with supercharged and turbocharged engines and when it also can be justified by the benefit of a higher compression ratio, noted Steve Gorgas, the Pentastar Chief Engineer. He added that a DI hardware package includes an engine-mounted high-pressure fuel pump and rail, which adds both weight and cost. Further, DI also uses more expensive fuel injectors that deliver 5-7 pulses per power stroke and requires very precise distribution of the fuel sprays to control formation of particulates.

Lee explained that as particulate standards tighten by 2018, the potential need to add particulate traps also has to be considered. Gasoline port-injection systems have lower efficiency, he admitted, but they also provide some additional time for the fuel to atomize on the back of the intake valves, which minimizes particulates.

Turbocharging, Gorgas said, isn't all free power. Although the overall result is a plus, there is greater parasitic loss at under 2000 rpm. So for the Pentastar, a turbo doesn't fit yet. However, like DI, a turbo was engineered in and is "on the shelf."

Weight, friction reductions

The various upgrades added 13 lb (6kg) to the engine's overall mass, with the EGR and variable lift system accounting for over 7.5 lb/3.4 kg. Gorgas' development team used a "look everywhere" approach to weight reduction, and employed thin-wall castings for the heads, windage tray and front cover. Across the range of installations, the upgraded V6 weighs about 326-332 lb/148-151 kg fully dressed, an average of 4.0 lb/1.8 kg less than the previous generation Pentastar.

Except for trail-rated Jeep applications, the oil pan is smaller, and oil capacity drops from six to five quarts (5.7 to 4.8 L), saving about 2.0 lb/0.9 kg. The block was modified to maintain structural strength while thinning out some of reinforcement ribs. The structural provision for an air pump, which has never been used, was eliminated. Overall, block weight was pared by  almost 2.0 lb (0.9 kg). The main bearing caps are nodular cast iron, vs. the previous powdered cast iron. Nodular iron costs more, Gorgas noted, but it permits a level of contouring that saved 0.8 lb (0.4 kg) overall.

The intake manifold, as part of the redesign that lengthened the runners, is now a two-piece design, vs. the previous three section, for a 0.7 lb/0.3 kg reduction. There's a pair of visually obvious frontal modifications: the crankshaft damper retaining bolt has a recess in the head, saving 43 g/1.5 oz and the water pump pulley has drilled holes in the front face, saving almost 110 g/4.0 oz.

Many improvements reduced friction to improve fuel economy 1%, led by reductions in crankshaft journal width (from 72 to 68 mm) and crankpins (from 59 to 54 mm). Fillet rolling was increased to maintain strength and the overall redesign was verified to maintain the previous NVH characteristics. The timing chain guide is faced with HDR-1, an ultra-low- friction plastic.

Finally, the valve train was given a super-polish treatment, valve spring loading was reduced, and the two-stage oil pump was refined to reduce parasitic loss.

Author: Paul Weissler

Source: SAE Automotive Engineering Magazine