新型發(fā)動機總會或多或少地帶來新的設(shè)計和制造難題，保時捷(Porsche)最新718 Boxster(http://articles.sae.org/14582/)所采用的渦輪增壓平置4缸發(fā)動機，更是給冷卻帶來了一個巨大挑戰(zhàn)。為了解決這一問題，保時捷的工程師最終拿出了34個可能的解決方案。
這個難題是：如何才能在一款專為自然吸氣平置6缸發(fā)動機設(shè)計的發(fā)動機室內(nèi)，安裝車輛的中冷器和兩個橫向冷卻系統(tǒng)之間的管道？
高級工程師Michael Wessels在保時捷的職位非常特別，他是公司水平對置發(fā)動機(boxer engine)汽車周邊設(shè)計部研發(fā)經(jīng)理。Wessels表示，“我們必須確保所有元件都各司其職，而且不能影響效率。”
他解釋說，我們的工程師會不斷對比各種封裝解決方案，直至角逐出最后兩個“優(yōu)勝者”。保時捷對此稱得上不遺余力：“最終，我們確定了一個能夠滿足所有要求的方法。”
這款發(fā)動機從概念到實現(xiàn)真正量產(chǎn)總共花了4年時間，還有一款與其平行研發(fā)的發(fā)動機，即專為保時捷911（見http://articles.sae.org/14336/）設(shè)計的全新3.0L bi-turbo雙渦輪平置6缸發(fā)動機，代號B6。在整個研發(fā)過程中，Wessels和他的20人團隊一直將精力集中在車輛的中冷器、空氣控制和熱需求上。
他解釋說，為了滿足所需的設(shè)計和空氣動力學要求，我們采用了一種間接冷卻系統(tǒng)，會使用一段延長的循環(huán)回路。
車門后沿區(qū)域背后的橫向進氣口可用于車輛的間接冷卻系統(tǒng)。車輛可通過冷卻系統(tǒng)中的一段輔助冷卻循環(huán)，降低增壓空氣的溫度。車輛發(fā)動機上方安置了一個換熱器，來自渦輪增壓器的壓縮空氣可通過該裝置將部分熱量傳遞至冷卻劑，而后冷卻劑液體將繼續(xù)流過每個進氣口的散熱器。
車輛采用了兩套冷卻系統(tǒng)，Wessels表示這款中冷器完全可以應(yīng)付車輛的道路行駛和跑道行駛。
渦輪增壓器的“預(yù)處理”
新Boxster采用的發(fā)動機是自保時捷911E問世40年來，保時捷推出的首款量產(chǎn)4缸發(fā)動機，現(xiàn)已更名為B4。這款發(fā)動機使用的材料中，有40%與保時捷911所采用的B6發(fā)動機相同。新款Boxster發(fā)動機有兩個版本：一款2.0L版本，輸出功率為220 kW (295 hp) ；另一款2.5L版本，輸出功率為257 kW (345 hp)，用于S版Boxster。新款發(fā)動機的輸出功率較之前采用的6缸發(fā)動機提升了26-kW(35-hp)。
新B4發(fā)動機的扭矩輸出表現(xiàn)也相當不錯，其中2.5L版本在轉(zhuǎn)速1900-4500rpm之間時的扭矩輸出為420 N·m (310 lb·ft)，較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)的驚人提升，峰值扭矩輸出可達380 N·m (280 lb·ft)。這兩款發(fā)動機的沖程均較短，為76.4 mm (3.0 in)，最高安全轉(zhuǎn)速為7500rpm。
保時捷稱，如果搭配可選的PDK（雙離合）7速變速器和Sport Chrono運動套裝，2.0L版B4發(fā)動機可在4.7秒內(nèi)靜止加速至100 km/h (62 mph)，較現(xiàn)行版本快0.8秒。S版Boxster則能在4.2秒內(nèi)靜止加速至100 km/h (62 mph)，較上一代車型快0.67秒。保時捷稱，2.0L Boxster和2.5L S版Boxster的最高時速分別可達275 km/h (171 mph)和285 km/h (177 mph)。如果搭配PDK變速器，車輛的燃料經(jīng)濟性最高可提升13%。Boxster的標配為6速變速器。
為了配合增加的扭矩，Boxster的傳動軸比之前大約厚100 mm(3.9 in)。
保時捷認為，單渦輪增壓器的設(shè)計完全可以滿足發(fā)動機的需求。2.0L發(fā)動機采用了傳統(tǒng)的廢氣門設(shè)計，而2.5L發(fā)動機則采用了911 Turbo使用的VTG（可變幾何形狀渦輪機）技術(shù)。保時捷稱，公司是唯一一家為量產(chǎn)汽油車型采用VTG技術(shù)的汽車廠商。保時捷的工程師表示，為了對廢氣流進行定向控制，從而取得最佳性能，這款VTG還搭配了第二個尾氣排氣口。
此外，當駕駛員選擇Sport或Sport Plus，發(fā)動機處于部分負載狀態(tài)時，這款渦輪增壓器還擁有一種“預(yù)處理”模式。此時，發(fā)動機的旁通氣門閉合、點火正時延遲，節(jié)氣門略微打開。這種模式可以提升氣流通量，并提高增壓。因此，一旦駕駛員踩下油門，已經(jīng)過增壓的氣體就可以立刻提供更高的扭矩，從而減少渦輪延遲。
當駕駛員松開油門時，車輛的Dynamic Boost功能還可以保證節(jié)氣門仍處于打開狀態(tài)，但此時并不會繼續(xù)給油。增壓在這種情況下并不會立刻完全消失，此時的渦輪增壓發(fā)動機可以像自然吸氣發(fā)動機一樣，對油門的操作做出立刻反應(yīng)。
工程師指出，為了保證渦輪增壓器在各種條件下的表現(xiàn)都能使駕駛員滿意，這給設(shè)計帶來了額外挑戰(zhàn)。與保時捷911一樣，Boxster也有20秒的Sport響應(yīng)按鈕，可以實現(xiàn)保時捷所說的“自發(fā)響應(yīng)(spontaneous responsiveness)”
與新3.0L保時捷911一樣， Boxster的發(fā)動機也采用了置于中心位置的噴油器。系統(tǒng)壓強可提升至250 bar (3626 psi)。廢氣凸輪軸采用了可變氣門提升控制，進氣一側(cè)采用了保時捷現(xiàn)有的VarioCam Plus系統(tǒng)。
保時捷的設(shè)計師非常注重減少摩擦損失和提高功率輸出。B4發(fā)動機的氣缸套特別采用了鐵等離子涂層表面，而且還搭配了完全可變機油泵(fully variable oil pump)和可切換水泵。
繼承了標志性發(fā)動機聲音
新保時捷718的外形看起來與目前的版本比較相似，但事實上新一代車型進行了大量改變。除了可折疊式織物材質(zhì)車頂（車頂打開/關(guān)閉所需時長：9秒，僅可在車速50 km/h以下時進行操作）和擋風窗外，新保時捷718的所有面板和外部元件均有所不同，車輛因此看起來更加整潔，在路上也更加顯眼。新車型的前臉位置更低，前方采用了非常大的進氣口設(shè)計。擋泥板和側(cè)檻均經(jīng)過重新造型。車輛標配為19吋車輪，也可升級為可選20吋車輪。
后穩(wěn)流板比之前版本寬，可根據(jù)車頂?shù)拈_啟或折疊狀態(tài)進行調(diào)整，優(yōu)化車輛的空氣動力學性能。當車頂開啟時，穩(wěn)流板延長45mm；當車頂折疊時，則延長55mm（分別折合1.77和2.16英尺）。該車的風阻系數(shù)為0.31 Cd。
Boxster的底盤經(jīng)過重新調(diào)試，新增了一個橫向部件，用以鞏固后副車架并提升硬度。車輛減震器采用了更大的活塞和氣缸管徑，還搭配了額外的反彈緩沖彈簧。保時捷911 Turbo的轉(zhuǎn)向系統(tǒng)經(jīng)過調(diào)整，據(jù)稱轉(zhuǎn)向性能比現(xiàn)行版本高10%。
保時捷還可提供一款保時捷主動懸掛管理（Porsche Active Suspension Management，下簡稱PASM）系統(tǒng)，懸架高度可調(diào)低10mm(39 in)。S版（運動版）Boxster還可提供一款可選運動底盤，比標配低20mm。
全新4缸boxer的表現(xiàn)怎么樣？的確非常棒。本文作者曾在法國馬賽附近的Fontage米其林測試中心(Fontage Michelin Test Center)短暫接觸過兩個版本的保時捷Boxster，其中2.5L S版本在長直車道上的時速甚至可以超過官方公布的250 km/h (155 mph)。
保時捷的狂熱粉絲曾擔心4缸保時捷是否會丟失品牌的標志性發(fā)動機聲響，結(jié)論是他們無需再擔心了。2.0L和2.5L 718 Boxster的發(fā)動機“聲音”均非常有說服力。2.0L版本采用了一個橢圓排氣管，2.5L的S版本則采用了兩個圓型排氣管。這兩個版本均可提供一款可選的駕駛員選擇運動排氣系統(tǒng)(driver-selected sport exhaust system)，這個系統(tǒng)在發(fā)動機運轉(zhuǎn)時帶來如同賽車般的轟隆聲。
作者：Stuart Birch
來源：SAE《汽車工程雜志》
翻譯：SAE上海辦公室
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
保時捷S版718 Boxster的增壓空冷系統(tǒng)，工程師稱，該設(shè)備的封裝設(shè)計是個挑戰(zhàn)。
新保時捷718 Boxster的渲染效果圖。
新保時捷718 Boxster的滾動底盤。工程師在設(shè)計時特別關(guān)注了車輛的尾氣系統(tǒng)以及該系統(tǒng)發(fā)出的聲音。
2.5L保時捷718 Boxster發(fā)動機（如圖）的功率輸出可達257 kW。
2.5L保時捷718 Boxster采用的可變幾何形狀渦輪增壓器。
保時捷718 Boxster的可選升級配置包括一款LED頭燈，帶有4點整合式LED日行燈。
保時捷718 Boxster的儀表盤設(shè)計也有一些變化，其中包括減少了通氣孔的數(shù)量。
新保時捷718 Boxster的前方進氣口更大。
更寬的尾部設(shè)計讓保時捷718 Boxster看起來非常特別。