得益于博格華納 (BorgWarner) 在渦輪增壓技術(shù)方面的最新突破，通用汽車（GM）的全新 L3B 汽油四缸發(fā)動機獲得了比V6 發(fā)動機更優(yōu)越的功率和扭矩輸出，可為全尺寸皮卡提供與柴油發(fā)動機媲美的燃油效率。
通用汽車（General Motors）專為卡車設(shè)計的全新 2.7L L3B 汽油四缸發(fā)動機配備豐富的先進功能，其功率輸出比現(xiàn)行 4.3L LV3 六缸發(fā)動機高 9%、扭矩輸出高 14％?，F(xiàn)階段，這款 L3B四缸渦輪增壓發(fā)動機主要搭載 Hydra-Matic 8 速變速器 8L90，它也是 2019 款雪佛蘭Silverados 皮卡（LT 級和RST 級）和 2019 款 GMC Sierra 皮卡的基礎(chǔ)配置發(fā)動機。
簡單來說，卡車的占空比范圍更大，因此設(shè)計一款適合全尺寸皮卡的四缸增壓發(fā)動機并非易事。在車輛掛高擋的穩(wěn)定巡航狀態(tài)下，渦輪增壓發(fā)動機可以提供杰出的經(jīng)濟性，而一旦轉(zhuǎn)速和增壓升至一定水平即可提供充足的推力。但為卡車設(shè)計增壓發(fā)動機的挑戰(zhàn)在于不同操作模式之間的轉(zhuǎn)換，也就是說當(dāng)油門突然全開時，發(fā)動機如何才能立刻準(zhǔn)備就緒開始運行。
Silverado皮卡首席工程師 Tom Sutter 解釋說：“我們的目標(biāo)是在節(jié)流閥升至 1500 rpm 后，讓發(fā)動機在 2 秒內(nèi)就具備輸出 90% 峰值扭矩的能力，遠超其他競爭對手。”
博格華納和雙蝸殼發(fā)動機
博格華納（BorgWarner）的 B03 雙渦殼渦輪增壓器已經(jīng)有數(shù)十年歷史，但直到最近才應(yīng)用至輕型車輛中。“我們從 2012 年開始嘗試將這項技術(shù)從商用柴油發(fā)動機領(lǐng)域轉(zhuǎn)移到輕型汽油發(fā)動機領(lǐng)域，”博格華納全球工程和創(chuàng)新副總裁 Hermann Breitbach 博士指出，“雙蝸殼渦輪增壓器的優(yōu)勢在于節(jié)流閥響應(yīng)，這種增壓器的節(jié)流閥響應(yīng)性能遠超任何同類產(chǎn)品。”
簡單來說，各類渦輪增壓器的基本工作原理都一樣，即從發(fā)動機排氣口回收隨廢氣一起排出的能量。具體來說，渦輪增壓器利用這些熱氣推動渦輪葉輪旋轉(zhuǎn)，進而驅(qū)動離心式壓縮機為氣體加壓，最終即迫使這些高于大氣壓的高壓氣體流入進氣歧管。一個多世紀(jì)以來，這項由瑞士工程師 Alfred Buchi 推出的重大創(chuàng)新已經(jīng)幫助無數(shù)飛機、輪船、卡車和汽車發(fā)動機提高了輸出功率和工作效率。
正如上文的介紹，為了能夠使發(fā)動機排氣口排出的熱氣順利推動渦輪葉輪的旋轉(zhuǎn)，所有渦輪增壓器都必須讓這些熱氣在抵達渦輪葉輪前形成旋轉(zhuǎn)的渦流。通常來說，這個步驟是由一條彎曲的管道來實現(xiàn)的，也就是渦輪增壓器的蝸殼管道或渦流管道。根據(jù)蝸殼或渦輪的不同設(shè)計，渦輪增壓器對熱氣中的熱量和動量的利用效率也有不同。
當(dāng)發(fā)動機轉(zhuǎn)速較高時，渦輪增壓器將使用一根粗管道處理這些廢氣，而且?guī)缀醪粫怏w流動施加任何限制。然而，當(dāng)發(fā)動機轉(zhuǎn)速較低時，渦輪需要些許推力才能開始旋轉(zhuǎn)。為此，雙蝸殼增壓器在在渦輪機入口處設(shè)計了兩根纏繞其上的管道，連接廢氣出口，保證每次的排氣都能“盡全力”推動渦輪機葉輪轉(zhuǎn)動。
專為四缸發(fā)動機工況量身定制
考慮到全新 L3B 發(fā)動機采用了 1-3-4-2 的點火順序，最佳布置應(yīng)為：將發(fā)動機的氣缸 1 和氣缸 4 連接至渦輪增壓器的一根管道，并將氣缸 3 和氣缸 2 連接到另一根管道。這樣做能夠盡可能增加每股廢氣之間的間隔，并降低廢氣抵達葉輪前被中斷的可能性。
接著，下一個需要考慮的問題是管道與渦輪機的連接。通常來說，“雙渦流”渦輪機的設(shè)計更加常見，也就是說將兩條管道并排放置。遺憾的是，在這種設(shè)計下，廢氣流出管道的位置可能出現(xiàn)短路，也就是說，當(dāng)從兩根管道排出的氣流本應(yīng)共同推動渦輪葉輪轉(zhuǎn)動時，卻可能由于相互碰撞和沖擊而意外流回管道中。對于大中型發(fā)動機而言，這并不是什么大問題。但對于較小的四缸發(fā)動機而言，這種短路會增加渦輪延遲，也就是延緩渦輪增壓器增壓的速度。
對比之下，雙蝸殼發(fā)動機的兩根管道則呈同心圓排列，內(nèi)部管道在即將抵達葉輪時，會先在渦輪機外殼之外繞半圈，而外部管道還會再多繞半圈，從而避免了任何短路或相互干擾的可能性。
博格華納北美工程總監(jiān) Douglas Erber 表示，“這種雙蝸殼分離設(shè)計可以增加高壓氣體脈沖之間的間隔，確保讓更多能量抵達渦輪，總體可以減少渦輪延遲，并顯著改善節(jié)流閥響應(yīng)性能。”
當(dāng) L3B發(fā)動機轉(zhuǎn)速在 1500 rpm 時，增壓廢氣將以 40 毫秒的周期輪番沖擊增壓器葉輪的不同側(cè)面，使其快速加速旋轉(zhuǎn)。蝸殼管道出口與渦輪葉輪之間的間隙只有 1 毫米（0.039 英寸），而雙渦流渦輪通常則有 5 毫米（0.197英寸）的間隙。
細心的觀察者可能會注意到，在雙蝸殼增壓器中，外蝸殼的橫截面積要比內(nèi)蝸殼更大。這是由于渦輪增壓器的氣流主要由A/R 比控制，即渦輪機殼體導(dǎo)管的橫截面積除以導(dǎo)管圓心與渦輪機葉輪軸之間的距離得到的比值。因此，為了使雙蝸殼設(shè)計的兩個蝸殼都具備相同的 A/R 和相似的流動特性，外蝸殼的面積必須更大。
當(dāng)然了，在一些發(fā)動機設(shè)計中，雙蝸殼增壓器內(nèi)外蝸殼的A/R 可能會特意略有不同。然而，經(jīng)過工程師的驗證，對于 L3B 發(fā)動機來說，相同的A/R 比更加適合。
多種配置
L3B發(fā)動機助理總工程師 Craig Marriott 解釋道：“我們在早期研發(fā)中評估對比了多種不同的渦輪配置，最終確定雙蝸殼增壓器的架構(gòu)最適合我們的卡車應(yīng)用。這種渦輪增壓器不僅可以顯著優(yōu)化發(fā)動機的瞬時響應(yīng)性能，甚至還稍稍提高了發(fā)動機的峰值功率輸出。”
在L3B 卡車發(fā)動機中，B03 渦輪增壓器可以提供 22 psi（1.5 bar）的增壓。
當(dāng)被問及這項技術(shù)是否可能擴展到其他通用渦輪增壓發(fā)動機時，Marriott 補充道，“目前，我還無法透露未來計劃，但請保持期待！”
在重量、體積和成本方面，通用汽車的渦輪增壓器工程師Alec Peeples 透露，雙蝸殼設(shè)計“比雙渦流設(shè)計略輕，因為雙蝸殼增壓器中的內(nèi)蝸殼僅繞了一個半圈，此外同心圓的布局也稍微更緊湊一些，盡管確實從中心軸線向外延伸得更遠。由于渦輪機殼體材料相對較重且成本較高，因此雙蝸殼設(shè)計可能比雙渦流設(shè)計更便宜。”
博格華納 Erber 表示，這兩種設(shè)計的成本非常相似。他補充說，事實上，推進系統(tǒng)工程師在選擇增壓器時，不僅可以考慮雙蝸殼設(shè)計、雙渦流設(shè)計、單蝸殼設(shè)計，而且還可以考慮一些可變?nèi)~輪設(shè)計，重點是了解每個產(chǎn)品都有自己獨特的需求。
不過，無論具體選擇哪種類型的渦輪增壓器，毫無疑問，這項由瑞典工程師Alfred Buchi 在 1905 年創(chuàng)造的發(fā)明將繼續(xù)協(xié)助更多發(fā)動機產(chǎn)品不斷提高性能。
The first four-cylinder gasoline engine ever offered in a full-sized pickup truck brings V6-beating power and torque, with fuel efficiency that is expected to rival a diesel. And key to its performance is a new twist in turbocharging.
GM’s all-new 2.7-L L3B was designed for truck use, with a wealth of advanced features (see https://www.sae.org/news/2018/05/gm-2.7-l-i-4-revealed). The result is 9% more power and 14% more torque than GM’s incumbent LV3 4.3-L V6. Teamed with the 8L90 8-speed Hydra-Matic transmission, the L3B turbo four is the base engine for 2019 Chevy Silverados in LT and RST trim and also is offered for the 2019 GMC Sierra, which shares the Silverado’s all-new architecture.
Engineering a boosted I4 for fullsize pickups’ broad duty cycle was no mean feat. Such engines provide impressive economy during steady-state cruising in top gear and generate ample thrust under load once the revs and boost are up. The challenge is in the transition between those operating modes—when the engine must leap into action as the throttle is quickly moved from barely open to WOT.
“Our goal was to top every competitor by providing 90% of peak torque less than two seconds after the throttle is floored at 1500 rpm,” explained Silverado chief engineer Tom Sutter.
BorgWarner and dual-volute
Enter BorgWarner’s B03 dual-volute (DV) turbocharger—a design that’s existed for decades but is new to light-duty vehicle applications. “We began transferring this technology from commercial diesels to light-duty gasoline engines in 2012,” noted Dr. Hermann Breitbach, BW’s VP of global engineering and innovation. “The advantage a DV turbo offers is throttle response that’s superior to any alternative.”
All turbos recycle energy that exits the engine’s exhaust ports. The hot gas spins a turbine wheel which in turn drives a centrifugal compressor that forces intake air—above atmospheric pressure—through the intake manifold. Swiss engineer Alfred Buchi’s invention has boosted aircraft, ship, truck and automobile engine power and efficiency for more than a century.
In every turbocharger application, the linear stream of hot gas exiting the exhaust manifold must be twisted into a spiral to spin the turbine wheel. This is achieved by means of a curved duct inside the turbine housing called a scroll or volute. The configuration of these ducts determines how efficiently they exploit the exhaust stream’s heat and momentum.
A single large duct handles the exhaust flow with minimal restriction at high rpm. At low rpm, however, the turbine needs a nudge to start spinning. In the dual-volute design, that’s achieved by using two ducts wrapped around the turbine inlet, with exhaust port connections that encourage every burst of exhaust gas to strike the turbine wheel with maximum force.
Specifically tailored for 4-cylinder duty
Given the new L3B’s 1-3-4-2 firing order, connecting cylinders 1 and 4 to one of the turbocharger’s ducts and cylinders 3 and 2 to the other duct is the optimum arrangement. This maximizes the separation between bursts of exhaust gas and minimizes the flow disruption before the gasses reach the turbine wheel.
The next consideration is how the ducts meet the turbine. The more common configuration, typically called “twin scroll,” has the two ducts positioned side-by-side. Unfortunately, this allows short circuiting where the exhaust gas leaves the ducts: as the two streams mix just before they strike the turbine wheel, a small portion of the exhaust gas runs the wrong way back up a duct. That’s not an issue in medium to large engines, but it can increase turbo lag—a delayed arrival of boost pressure—in small four-cylinder engines.
A DV design locates one duct inside the other in a concentric arrangement. The inner channel wraps half way around the turbine housing before its exhaust gas impinges against the wheel. The outer volute carries on an additional 180 degrees to guard against short circuiting and any chance of unproductive flow interference.
BorgWarner’s North American engineering director, Douglas Erber, said, “DV segregation and the resulting greater spacing between exhaust pressure pulses assures that more energy reaches the turbine wheel. This diminishes lag and significantly improves throttle response.”
At 1500 rpm, each side of the L3B engine’s turbine wheel is struck by an exhaust pulse every 40 milliseconds, quickly accelerating its rotation. The gap between each volute’s exit and the turbine wheel is only 1 mm (.039 in.) compared to the 5-mm gap that’s common for a twin scroll turbo.
Close observers will note that the outer volute has a larger cross-sectional area than the inner volute. This is because a turbocharger’s flow is governed by its A/R ratio—the cross-sectional area of the turbine housing’s duct divided by the distance between the duct’s centroid and the turbine wheel’s axis; for both volutes of a dual-volute design to share a common A/R and similar flow characteristics, the area of the outer volute must be larger. While there are situations when slightly different A/R volutes are desirable, engineers determined they should be identical in the L3B application.
Multiple configurations tested
Craig Marriott, assistant chief engineer for the L3B engine explained: “While evaluating several turbo configurations during the alpha [early] phase of development, we determined that the DV architecture demonstrated the best performance for our truck application. In addition to its significant improvement in transient response, we found a small horsepower gain at the top end.”
The B03 is capable of generating 22-psi (1.5-bar) boost in the L3B truck engine application.
Asked if this technology is likely to extend to other GM turbo engines, he added, “I’m not at liberty to speak of future plans except to recommend staying tuned!”
Regarding weight, bulk and cost considerations, GM’s turbocharger engineer Alec Peeples revealed that the DV design “is slightly lighter than the twin-scroll alternative because one of the two volutes wraps only half way around the wheel. The concentric layout is also somewhat narrower, though it does extend further outboard from the center axis. Since the turbine housing material is relatively heavy and costly, the DV design is potentially less-expensive than twin scroll.”
According BW’s Erber, the costs are very similar for both designs. He added that propulsion-system engineers will be wise to consider not only DV, twin-scroll and single-scroll turbos but also variable-vane designs, as each product application has specific needs.
Whatever the case, Alfred Buchi’s 1905 invention is the engine power-enhancing technology gift that keeps on giving.
Author: Don Sherman
Source: SAE Automotive Engineering Magazine
通用汽車的全新 L3B 汽油四缸發(fā)動機專為輕質(zhì)卡車設(shè)計，這也是通用首次在全尺寸皮卡中使用的四缸發(fā)動機。
通用汽車推進系統(tǒng)工程師表示，雙蝸殼（左）和雙渦流（右）渦輪增壓器設(shè)計。其中，雙蝸殼增壓器的重量更輕，成本也有可能低于雙渦流發(fā)動機。
博格華納 B03 雙蝸殼渦輪增壓器的內(nèi)蝸殼和外蝸殼呈同心圓排列（CAD 效果圖）。
博格華納 B03 渦輪增壓器的橫截面。渦輪增壓器的氣流主要由A/R 比控制，即渦輪機殼體導(dǎo)管的橫截面積除以導(dǎo)管圓心與渦輪機葉輪軸之間的距離后得到的值。