新型发动机总会或多或少地带来新的设计和制造难题，保时捷(Porsche)最新718 Boxster(http://articles.sae.org/14582/)所采用的涡轮增压平置4缸发动机，更是给冷却带来了一个巨大挑战。为了解决这一问题，保时捷的工程师最终拿出了34个可能的解决方案。

这个难题是：如何才能在一款专为自然吸气平置6缸发动机设计的发动机室内，安装车辆的中冷器和两个横向冷却系统之间的管道？

高级工程师Michael Wessels在保时捷的职位非常特别，他是公司水平对置发动机(boxer engine)汽车周边设计部研发经理。Wessels表示，“我们必须确保所有元件都各司其职，而且不能影响效率。”

他解释说，我们的工程师会不断对比各种封装解决方案，直至角逐出最后两个“优胜者”。保时捷对此称得上不遗余力：“最终，我们确定了一个能够满足所有要求的方法。”

这款发动机从概念到实现真正量产总共花了4年时间，还有一款与其平行研发的发动机，即专为保时捷911（见http://articles.sae.org/14336/）设计的全新3.0L bi-turbo双涡轮平置6缸发动机，代号B6。在整个研发过程中，Wessels和他的20人团队一直将精力集中在车辆的中冷器、空气控制和热需求上。

他解释说，为了满足所需的设计和空气动力学要求，我们采用了一种间接冷却系统，会使用一段延长的循环回路。

车门后沿区域背后的横向进气口可用于车辆的间接冷却系统。车辆可通过冷却系统中的一段辅助冷却循环，降低增压空气的温度。车辆发动机上方安置了一个换热器，来自涡轮增压器的压缩空气可通过该装置将部分热量传递至冷却剂，而后冷却剂液体将继续流过每个进气口的散热器。

车辆采用了两套冷却系统，Wessels表示这款中冷器完全可以应付车辆的道路行驶和跑道行驶。

涡轮增压器的“预处理”

新Boxster采用的发动机是自保时捷911E问世40年来，保时捷推出的首款量产4缸发动机，现已更名为B4。这款发动机使用的材料中，有40%与保时捷911所采用的B6发动机相同。新款Boxster发动机有两个版本：一款2.0L版本，输出功率为220 kW (295 hp) ；另一款2.5L版本，输出功率为257 kW (345 hp)，用于S版Boxster。新款发动机的输出功率较之前采用的6缸发动机提升了26-kW(35-hp)。

新B4发动机的扭矩输出表现也相当不错，其中2.5L版本在转速1900-4500rpm之间时的扭矩输出为420 N·m (310 lb·ft)，较B6提升了60 N·m (44 lb·ft)；2.0L版本更是取得了100 N·m (74 lb·ft)的惊人提升，峰值扭矩输出可达380 N·m (280 lb·ft)。这两款发动机的冲程均较短，为76.4 mm (3.0 in)，最高安全转速为7500rpm。

保时捷称，如果搭配可选的PDK（双离合）7速变速器和Sport Chrono运动套装，2.0L版B4发动机可在4.7秒内静止加速至100 km/h (62 mph)，较现行版本快0.8秒。S版Boxster则能在4.2秒内静止加速至100 km/h (62 mph)，较上一代车型快0.67秒。保时捷称，2.0L Boxster和2.5L S版Boxster的最高时速分别可达275 km/h (171 mph)和285 km/h (177 mph)。如果搭配PDK变速器，车辆的燃料经济性最高可提升13%。Boxster的标配为6速变速器。

为了配合增加的扭矩，Boxster的传动轴比之前大约厚100 mm(3.9 in)。

保时捷认为，单涡轮增压器的设计完全可以满足发动机的需求。2.0L发动机采用了传统的废气门设计，而2.5L发动机则采用了911 Turbo使用的VTG（可变几何形状涡轮机）技术。保时捷称，公司是唯一一家为量产汽油车型采用VTG技术的汽车厂商。保时捷的工程师表示，为了对废气流进行定向控制，从而取得最佳性能，这款VTG还搭配了第二个尾气排气口。

此外，当驾驶员选择Sport或Sport Plus，发动机处于部分负载状态时，这款涡轮增压器还拥有一种“预处理”模式。此时，发动机的旁通气门闭合、点火正时延迟，节气门略微打开。这种模式可以提升气流通量，并提高增压。因此，一旦驾驶员踩下油门，已经过增压的气体就可以立刻提供更高的扭矩，从而减少涡轮延迟。

当驾驶员松开油门时，车辆的Dynamic Boost功能还可以保证节气门仍处于打开状态，但此时并不会继续给油。增压在这种情况下并不会立刻完全消失，此时的涡轮增压发动机可以像自然吸气发动机一样，对油门的操作做出立刻反应。

工程师指出，为了保证涡轮增压器在各种条件下的表现都能使驾驶员满意，这给设计带来了额外挑战。与保时捷911一样，Boxster也有20秒的Sport响应按钮，可以实现保时捷所说的“自发响应(spontaneous responsiveness)”

与新3.0L保时捷911一样， Boxster的发动机也采用了置于中心位置的喷油器。系统压强可提升至250 bar (3626 psi)。废气凸轮轴采用了可变气门提升控制，进气一侧采用了保时捷现有的VarioCam Plus系统。

保时捷的设计师非常注重减少摩擦损失和提高功率输出。B4发动机的气缸套特别采用了铁等离子涂层表面，而且还搭配了完全可变机油泵(fully variable oil pump)和可切换水泵。

继承了标志性发动机声音 

新保时捷718的外形看起来与目前的版本比较相似，但事实上新一代车型进行了大量改变。除了可折叠式织物材质车顶（车顶打开/关闭所需时长：9秒，仅可在车速50 km/h以下时进行操作）和挡风窗外，新保时捷718的所有面板和外部元件均有所不同，车辆因此看起来更加整洁，在路上也更加显眼。新车型的前脸位置更低，前方采用了非常大的进气口设计。挡泥板和侧槛均经过重新造型。车辆标配为19吋车轮，也可升级为可选20吋车轮。

后稳流板比之前版本宽，可根据车顶的开启或折叠状态进行调整，优化车辆的空气动力学性能。当车顶开启时，稳流板延长45mm；当车顶折叠时，则延长55mm（分别折合1.77和2.16英尺）。该车的风阻系数为0.31 Cd。

Boxster的底盘经过重新调试，新增了一个横向部件，用以巩固后副车架并提升硬度。车辆减震器采用了更大的活塞和气缸管径，还搭配了额外的反弹缓冲弹簧。保时捷911 Turbo的转向系统经过调整，据称转向性能比现行版本高10%。

保时捷还可提供一款保时捷主动悬挂管理（Porsche Active Suspension Management，下简称PASM）系统，悬架高度可调低10mm(39 in)。S版（运动版）Boxster还可提供一款可选运动底盘，比标配低20mm。

全新4缸boxer的表现怎么样？的确非常棒。本文作者曾在法国马赛附近的Fontage米其林测试中心(Fontage Michelin Test Center)短暂接触过两个版本的保时捷Boxster，其中2.5L S版本在长直车道上的时速甚至可以超过官方公布的250 km/h (155 mph)。

保时捷的狂热粉丝曾担心4缸保时捷是否会丢失品牌的标志性发动机声响，结论是他们无需再担心了。2.0L和2.5L 718 Boxster的发动机“声音”均非常有说服力。2.0L版本采用了一个椭圆排气管，2.5L的S版本则采用了两个圆型排气管。这两个版本均可提供一款可选的驾驶员选择运动排气系统(driver-selected sport exhaust system)，这个系统在发动机运转时带来如同赛车般的轰隆声。

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

How Porsche met cooling challenges on the new 4-cylinder Boxster

New engines always bring fresh design and engineering challenges, but Porsche’s switch to a turbocharged flat 4-cylinder for the latest generation 718 Boxster(http://articles.sae.org/14582/) created one that resulted in 34 possible solutions.

The challenge: How to install the car’s intercooler and associated pipework for two lateral cooling systems in an engine bay designed for a naturally aspirated flat-six?

“We needed to ensure all components were in the right place, with no compromise of efficiency,” said Senior Engineer Michael Wessels, who has the unusual job title of Manager, Vehicle Periphery Design, Boxer Engines.

The potential packaging solutions list was narrowed again and again until there were two, he explained. These were rigorously pursued: “Eventually a final decision was reached that met all criteria.”

The engine, designed in parallel with the new 3.0-L bi-turbo flat-six, coded B6, for the Porsche 911 (see http://articles.sae.org/14336/), took four years to complete from conception to production. Wessels and his team of 20 were occupied with the intercooler, its air control and its thermal requirements throughout that period.

He explained that indirect cooling using an extended circulation loop was used to cohere with required design and aerodynamics criteria.

The lateral air intakes behind the trailing edge of the car’s doors are used for indirect cooling. Reducing the temperature of charge air is via an auxiliary loop of the cooling system. A heat exchanger is positioned over the engine in which compressed air from the turbocharger transfers some of its heat to the coolant, the liquid then flowing through one radiator per air intake.

There are two lateral cooling systems and Wessels said the intercooler is sufficient for both road and track work.

Turbocharger "preconditioning"

The Boxster’s engines—Porsche’s first production 4-cylinder units since the 911E 40 years ago, although the hybrid 919 race car has only four—are codenamed B4. They share about 40% of their bill of material with the 911's B6 unit. The new Boxster engines include a 2.0-L that is rated at 220 kW (295 hp) and a 2.5-L for the 'S' version of the car rated at 257 kW (345 hp). These represent about a 26-kW (35-hp) gain over the previous Boxster sixes.

Also impressive is the B4s' torque production. The 2.5-L achieves 420 N·m (310 lb·ft) from 1900 to 4500 rpm, an improvement of 60 N·m (44 lb·ft), while the 2.0 L gains an extraordinary 100 N·m (74 lb·ft) to reach a peak 380 N·m (280 lb·ft). Each engine has a short stroke of 76.4 mm (3.0 in) and are redlined at 7500 rpm.

The 2.0-L with optional PDK (dual-clutch) 7-speed transmission and Sport Chrono Package reaches 100 km/h (62 mph) in a claimed 4.7 s—0.8 s quicker than the outgoing comparable model. The S achieves it in 4.2 s, 0.67 s ahead of the old S. The 2.0-L Boxster’s Vmax is 275 km/h (171 mph) and the 2.5-L S can do 285 km/h (177 mph), claims Porsche. Fuel economy with PDK improves by a best 13%. A 6-speed gearbox is standard.

To handle the additional torque, the Boxster's driveshafts are thicker in cross section by 100 mm (3.9 in).

Porsche decided a single turbocharger for each engine was satisfactory. The 2.0-L has a classic wastegate design but the 2.5-L gets VTG (variable turbine geometry) technology a la the 911 Turbo. Porsche claims to be the only auto manufacturer using VTG in production gasoline-engined cars. It gets a second waste gate for optimal efficiency by targeted control of the exhaust gas stream, according to Porsche engineers.

The turbocharger also has a “pre-conditioning” mode at part load when the driver selects Sport or Sport Plus profiles. The bypass valve is closed, ignition timing retarded, and the throttle opened slightly. The effect is to boost air throughout and also boost charge pressure, so when the accelerator pedal is flattened again, the higher charge pressure immediately delivers higher torque.

Additionally, a Dynamic Boost function operates when the accelerator is briefly lifted, the throttle remaining open but with fuel injection cancelled. In this situation, charge pressure doesn’t drop completely and the engine will react immediately to another shove on the pedal by responding like a naturally-aspirated engine.

Getting the turbo to sound "right" to the driver's ears in all conditions was an added challenge, the engineers noted. And as with the 911, the Boxsters get a Sport response button for 20 s of what Porsche terms “spontaneous responsiveness.”

Like the new 3.0-L 911 engines, the Boxster’s use centrally positioned injectors. System pressure has been increased to 250 bar (3626 psi). Variable valve lift control is employed on the exhaust camshaft, while the inlet side uses Porsche's established VarioCam Plus system.

Porsche designers focused on reduction of friction losses as well as increased output. The B4’s cylinder liners feature an iron-plasma-coated surface, and a fully variable oil pump and switchable water pump are used.

Aural integrity maintained 

The new 718 may look externally similar to the outgoing model but in fact it is very extensively, if subtly, changed. All panels and exterior components except its folding fabric roof (operating time to open or shut: 9 s at speeds up to 50 km/h) and its windshield, are different, giving the car a tauter look and an enhanced road presence. The car’s nose is lower and very large air front intakes are used. Fenders and side sills are re-styled and wheels are 19-in standard, 20-in optional.

The rear wing is wider, its aerodynamic effect optimized according to roof up or down. The wing extends 45 mm when up and 55 mm down (1.77 and 2.16 in, respectively). The car's optimum aerodynamic figure is 0.31 Cd.

The Boxster chassis has been re-tuned and there is a new lateral member to strengthen the rear subframe and enhance rigidity. Shock absorbers get larger piston and cylinder tube diameters and additional rebound buffer springs are used. The 911 Turbo’s steering has been adapted and is claimed to be 10% more direct than the outgoing car's.

Porsche Active Suspension Management (PASM) with a 10mm (.39 in) lower ride height is available. The S offers an optional 20mm lower sport chassis.

How does the new 4-banger boxer go? Very well indeed. The author's brief experience with both versions at the Fontage Michelin Test Center near Marseilles, France, showed the 2.5-L S reaching more than an indicated 250 km/h (155 mph) on the long straight before a banked left hand corner.

Aficionados of the aural signature of all Porsches, who are fearful that something may have been lost in the translation to 4-cylinder power, need fear no more. Both 2.0-L and 2.5-L 718 Boxsters have very convincing “sound” engineering. The 2.0-L car has a single oval tailpipe, the S two round tailpipes. Both have an optional, driver-selected sport exhaust system that produces more than a hint of race car decibel levels.

Author: Stuart Birch
Source: SAE Automotive Engineering Magazine