馬自達的創(chuàng)新 Skyactiv X 火花控制壓縮點火 (SpCCI) 燃燒系統(tǒng)預計將于2018年投產(chǎn)，這款發(fā)動機是數(shù)十年來內燃發(fā)動機領域發(fā)展的最重要成果之一。SpCCI發(fā)動機可以延長汽油發(fā)動機的實際使用壽命，這在全球排放規(guī)定愈發(fā)嚴格，且電動汽車加速發(fā)展壯大的大背景之下尤為重要。
馬自達公司董事、高級管理執(zhí)行官藤原清志表示，未來幾十年內，SpCCI 技術將成為公司汽油發(fā)動機戰(zhàn)略的基礎。藤原告訴《汽車工程》，為了實現(xiàn)“零尾氣排放”的目標，未來汽車的動力單元將最終采用以微藻類為基礎的生物燃料，而 SpCCI 經(jīng)過專門設計，可以支持更大排量的動力系統(tǒng)。
經(jīng)過超過 8 年的集中研發(fā)工作，馬自達的SpCCI 系統(tǒng)終于成型。該系統(tǒng)結合了柴油發(fā)動機的能效和扭矩輸出優(yōu)勢，以及雙凸輪汽油發(fā)動機的轉速優(yōu)勢，還能同時保證每公里不超過60 克的 CO2 排放水平。除此之外，SpCCI 發(fā)動機的生產(chǎn)無需對原材料表單進行大規(guī)模變更，主要的改動僅集中在電子控制和氣缸頭部分。
目前，SAE 已經(jīng)收到了大量有關奧托循環(huán) (Otto) 柴油發(fā)動機的技術論文，各家汽車廠商也進行了大量相關工作，比如：梅賽德斯-奔馳一直在大力研發(fā)奧托循環(huán)柴油機DiesOtto（見 http://papers.sae.org/2009-01-2701/）；通用汽車在開發(fā)均質壓燃 (HCCI) 發(fā)動機（見http://articles.sae.org/6635/）；本田和現(xiàn)代等廠商也在進行類似的研發(fā)（見http://books.sae.org/b-hon-016/）。然而，盡管這些資金充裕的大型汽車廠商一直在關注 HCCI 技術的研發(fā)，但截至目前尚沒有公司公布任何有關這些新興燃燒技術的量產(chǎn)計劃。
據(jù)悉，馬自達的最新技術合作伙伴豐田也對 SpCCI 系統(tǒng)興趣滿滿，并考慮在公司的主力混合動力電動車產(chǎn)品中應用這項技術。
柴油機般的強勁扭矩輸出
最近，在馬自達德國 Oberursel 歐洲技術中心舉辦的一次媒體活動中，筆者有幸試駕了兩款 2.0 L SpCCI Mazda 3 原型車（分別為手動擋和自動擋），完成了總長超過 30 英里（48 公里）的測試循環(huán)。真實體驗證明，火花點火模式(SI) 和壓縮點火模式 (CI) 之間難以順暢切換的長期挑戰(zhàn)，似乎已經(jīng)得到克服。工程師表示，SpCCI原型發(fā)動機在低轉速時的表現(xiàn)的確有些生硬，但現(xiàn)在距離發(fā)動機的最終校準和生產(chǎn)仍有大約 1年時間，還可以進行進一步優(yōu)化。
可以確定的是：在日常駕駛工況中，SI 和CI 之間的切換已經(jīng)幾乎很難察覺了。本次測試循環(huán)的駕駛環(huán)境包括高速公路、鄉(xiāng)村和城市道路。在整個測試循環(huán)中，這款全新SpCCI 發(fā)動機轉速曾飆到接近6000 轉，整體排放水平即使說不上完美，也算是相當不錯了。這款發(fā)動機有 6擋轉速，可輕松降至 1200 轉，充分展示了自身如柴油機般強勁的扭矩輸出能力。
由于扭矩輸出的范圍更加寬廣，馬自達得以更加高效地調整車輛的檔位分布，從而進一步改善車輛的燃油經(jīng)濟性并降低排放。
遺憾的是，截止本文發(fā)表之時，馬自達尚未公布 SpCCI 發(fā)動機的孔沖程、額定扭矩、功率等其他技術細節(jié)。馬自達稱，與現(xiàn)行 Skyactiv-G 汽油發(fā)動機相比，SpCCI 發(fā)動機的能效取得了 20% 到 30% 的提升。然而，從《汽車工程》得到的測試結果來看，情況可能并非如此喜人。在測試循環(huán)中，當 SpCCI 模式在整個循環(huán)測試中的運行時間超過95% 時，手動擋標配 Mazda 3SKY-G 取得了13.3% 的燃油效率提升，平均燃油經(jīng)濟性從此前的29.4 提升至 34.0 mpg （即 100 公里油耗從8.0 L 下降至6.9 L）。
自動擋汽車的情況也很類似，當 SpCCI 模式在整個循環(huán)測試中的運行時間超過 95% 時，自動擋測試車也取得了 14.75% 的燃油效率提升，平均燃油經(jīng)濟性從此前的 29.9 提升至 35.1 mpg（即 100 公里油耗從35.1 L 下降至29.9 L）。
然而，取得這樣的數(shù)據(jù)結果，與該循環(huán)相對極端的測試方式也有關系。具體來說，馬自達的循環(huán)測試主要是為了評估發(fā)動機的屬性，比如高檔位下的低速和正常加速、SI和 CI 模式之間的切換等，而這種情況在“正常駕駛環(huán)境”中并不常見。
壓縮比：從18:1 到15:1
從 2011 年啟動 Skyactiv 全面能效提升項目依以來，馬自達已經(jīng)陸續(xù)推出了大量壓縮比達 14:1 的發(fā)動機。此外，SpCCI 也是 Skyactiv 項目的主要成果之一。根據(jù)馬自達工程師的數(shù)據(jù)，全新 Skyactiv-X 發(fā)動機的壓縮比可達 15.0:1。
“我們之所以將壓縮比控制在 15:1 ，是因為這最接近正常環(huán)境溫度下的壓縮點火條件，”馬自達動力系統(tǒng)執(zhí)行官 Ichiro Hirose 解釋說，“火花會形成一個膨脹的火球，而這個火球如同一個次級的‘空氣彈簧’，可以為發(fā)動機提供額外的壓縮壓力。由于火球是火花塞產(chǎn)生的，因此可以有效控制SI 和 CI 之間的切換。”
Hirose 補充說，“首先，取得超高壓縮比是使用稀薄空氣燃料混合燃燒的關鍵突破點；其次，發(fā)動機空燃比越低，相應的熱比則越高。為了實現(xiàn)這一重大突破，我們至少需要將空燃比提高一倍，從 14.7:1 提高至 30.0:1。”
正如《汽車工程》此前的報道，在 Skyactiv-X 項目的 G1、G2和 G3 階段，工程師通過將均質壓燃系統(tǒng)的壓力和溫度調整至理想狀態(tài)，進而將 λ為 2.5 時的發(fā)動機壓縮比調整至 18:1，取得了 40% 的熱效率提升。
馬自達 SpCCI 量產(chǎn)發(fā)動機取得的一大關鍵突破是對燃燒過程的精準控制（見 SAE 技術論文 http://papers.sae.org/2015-01-1803/ 點擊最左下方閱讀原文直達）。SpCCI 發(fā)動機的每個氣缸均安裝了獨立壓力傳感器，可以對溫度、壓力等發(fā)動機參數(shù)進行實時監(jiān)測。發(fā)動機管理系統(tǒng)則會控制發(fā)動機的雙電動可變凸輪軸、空氣泵，以及將在500 bar (5252 psi) 時啟動的分離噴射系統(tǒng)。《汽車工程》了解到，馬自達的空氣泵采用了羅茨(Roots) 循環(huán)原理，由伊頓公司(Eaton Corp.) 專為馬自達SpCCI 發(fā)動機研制提供。
事實證明，對于處于 CI 模式下高負荷運轉的HCCI發(fā)動機，增壓系統(tǒng)和廢氣再循環(huán)系統(tǒng)的作用非常顯著。但馬自達研發(fā)管理執(zhí)行官Takahisa Sori 也表示，在2013 年時，馬自達的目標是在更多不同負荷級別的自然吸氣發(fā)動機中成功實現(xiàn)稀燃HCCI 技術。Sori 的工程師擔心，氣泵的使用會影響發(fā)動機在真實世界中的燃油經(jīng)濟性。
在與《汽車工程》記者的討論中，Sori表示非?？春肏CCI 汽油發(fā)動機與混合動力系統(tǒng)的結合，認為在電機的協(xié)助下，可以擴大發(fā)動機的理想運行范圍。發(fā)動機完全可以通過小型化電機和電池的運用來降低成本。在此背景下，馬自達選擇與豐田的混動系統(tǒng)工程師展開合作也就不足為奇了。
SpCCI 發(fā)動機將分2 個步驟實現(xiàn)空氣和燃料的混合，分別在進氣沖程和壓縮沖程中進行燃油噴射。此時，燃燒室內將形成一個強勁的漩渦，進而產(chǎn)生燃料密度梯度，即CI 外圍的空燃混合物相對稀薄，而火花塞中心處的空燃混合物則較為豐富（有利于火球的形成）。
火花點火可以在高負荷條件下啟動發(fā)動機，但并不會在任何預設的點切換至壓燃模式。Hirose 解釋說，當發(fā)動機到達進氣邊界條件時，燃燒室內將產(chǎn)生一個不斷膨脹的火球。此時，由于點燃模式提供的額外壓縮力，發(fā)動機的實際壓縮比會高于約15-16:1 的幾何壓縮比，進而觸發(fā)CI 模式。
Hirose 同時提到，SpCCI 發(fā)動機的制造成本將介于柴油和汽油發(fā)動機之間。
One of the most significant developments in internal combustion engine (ICE) technology for decades, Mazda’s innovative Skyactiv-X Spark Controlled Compression Ignition (SpCCI) combustion system is slated for production in 2018. It has the potential to extend the practical life of gasoline engines, which are increasingly under threat from both global emissions legislation and the accelerating development of electric vehicles (EVs).
According to Kiyoshi Fujiwara, a company director and Senior Managing Executive Officer, SpCCI will form the foundation of Mazda’s gasoline-engine strategy until mid-century. Fujiwara told Automotive Engineering that SpCCI is designed to embrace larger-displacement power units that eventually will run on micro-algae bio-fuels to deliver “zero tailpipe emissions” (see sidebar).
The SpCCI system is the culmination of more than eight years of intensive development by Mazda to design a gasoline engine that embraces the frugality and torque of a diesel with the high-revving capacity of a twin-cam gasoline unit, while delivering sub-60 g CO2/km emissions. Adding to the attraction, SpCCI requires relatively minimal investment in the engine bill of materials—electronic controls and a revised cylinder head comprise the major changes.
Creating an ICE with Otto and Diesel attributes is an engineering target discussed in scores of SAE Technical papers and vigorously pursued by Mercedes-Benz with its DiesOtto (see http://papers.sae.org/2009-01-2701/), General Motors with its Homogenous Charge Compression Ignition (HCCI; http://articles.sae.org/6635/) Honda with its similar investigations (http://books.sae.org/b-hon-016/) and Hyundai, among others. But while these larger, better-financed OEMs have focused significant R&D on HCCI, they have thus far stopped short of committing the combustion regime to production.
Mazda’s new technology partner Toyota also is said to be interested in SpCCI, including potential applications for hybrid-electric vehicles (HEVs), long a Toyota specialty.
Diesel-like torque
At a recent media technical background event at Mazda’s European technical center in Oberursel, Germany, the author test-drove two prototype Mazda 3s powered by a 2.0-L SpCCI engine—one fitted with a manual transmission, the other automatic—over a 30-mile (48-km) test loop. The experience seemed to confirm that the longstanding challenge of smooth transition from spark-ignition mode to compression-ignition had been overcome. The prototype SpCCI engines did display some low-rpm harshness, but final calibrations and engine production still are about a year away, engineers said.
What can be confirmed: in everyday driving, the transitions from SI to CI are barely noticeable. On a route that included high-speed autobahn and country and urban roads, the all-new SpCCI unit pulled strongly to its approximately 6000-rpm redline, accompanied by a healthy—if not outright sporty—exhaust note. The engine’s diesel-like torque curve was amply demonstrated by its willingness to pull without fuss from as low as 1200 rpm in sixth gear.
This wider spread of torque has allowed Mazda to revise the development cars’ gearing to further improve fuel economy and reduce emissions.
Frustratingly, specifications such as bore and stroke, rated torque, power and other technical details remain under wraps as this article was published. Although Mazda is claiming a 20-to-30% efficiency gain over its current Skyactiv-G gasoline engine, the test results seen by Automotive Engineering were less demonstrative. They included a 13.3% improvement in fuel efficiency for the manual over the standard Mazda 3 SKY-G—34.0 U.S. mpg vs. 29.4 (6.9 L/100 km vs. 8.0 L/100 km)—with the engine operating in SpCCI mode more than 95% of the time.
It was a similar tale for the automatic-transmission car: a 14.75% improvement (29.9 U.S. mpg vs. 35.1; 29.9 L/100 km vs. 35.1 L/100 km), while the automatic-backed SpCCI engine spent even more time operating in SpCCI mode.
However, this can be attributed to the exaggerated testing regime, aimed more at assessing the engine’s attributes such as low-speed and in-gear acceleration in high(er) ratios and trying to detect the SI-to-CI switchover points than would typically occur in “normal” driving.
From 18:1 to 15:1 CR
SpCCI is a progression of Mazda’s comprehensive Skyactiv efficiency-improvement initiative unveiled in 2011, which debuted gasoline and diesel engines with a common 14:1 compression ratio (CR). The new Skyactiv-X engine operates at a 15.0:1 CR, according to company engineers.
“We selected 15:1 compression ratio as it is close to compression-ignition conditions in normal ambient temperatures,” explained Powertrain Executive Officer Ichiro Hirose. “The spark creates an expanding fireball that acts like a secondary 'air spring' to create additional compression. Because the spark plug creates this fireball, it effectively controls the switch between spark ignition and compression ignition,” he noted.
Hirose added that achieving a “super-high compression ratio was a key breakthrough in realizing combustion with a lean fuel-air mix. Secondly, the leaner you make the air-fuel ratio, the more the specific heat ratio increases. To make the big step forward we needed to double stoichiometric levels from 14.7:1 to 30.0:1, at the very minimum.”
As AE reported previously, during Skyactiv-X’s development through G1, G2, and G3 program stages, engineers targeted an 18:1 compression ratio at lambda 2.5—a 40% improvement in thermal efficiency by setting the ideal pressure and temperature for homogeneous-charge compression ignition.
A key to Mazda’s achievement for the production engine is precise control of the combustion process (see SAE Technical Paper http://papers.sae.org/2015-01-1803/). The SpCCI engine uses pressure sensors in each cylinder to enable real-time temperature and pressure monitoring, in addition to other engine parameters. The engine-management system controls the twin electrically-variable camshafts, the new split-injection strategy that operates at 500 bar (7252 psi) and the air pump. The latter is a unique Roots-type device engineered by Eaton Corp. for the Mazda SpCCI application, Automotive Engineering has learned.
Supercharging and exhaust gas recirculation are known to be effective for operating an HCCI engine in CI mode at high loads. But as of 2013, Mazda R&D was aiming to achieve successful lean-burn HCCI within a broad load range using normal aspiration, according to Takahisa Sori, then Managing Executive Officer for R&D. Sori's engineers were concerned about an air pump compromising real-world fuel economy.
Speaking with Automotive Engineering, Sori also was bullish on the potential of mating HCCI gas engines with hybrid-electric drivetrains that let the engine run in its most efficient operating range, with e-motor assist as needed. In this de-emphasized role, the electric motor and battery can be downsized, reducing their cost. Such an arrangement would seem to be ideal for collaborative work with Toyota hybrid systems engineers.
SpCCI’s air-fuel mixture is created by two-phase, split injection on the intake and compression strokes. A strong swirl is created in the combustion chamber to create an intentionally uneven distribution of fuel density, with a lean mixture around the periphery for CI and a relatively rich air-fuel mixture around the spark plug in the center—conducive to creating the fireball.
Spark ignition is used to start the engine and under heavy-load conditions, but the switchover to CI is not at any predetermined point. When the right intake-charge boundary conditions are achieved, the expanding fireball in the combustion chamber is created, with SI providing additional compression to the geometric compression ratio of approximately 15-16:1. This reaction induces CI, Hirose explained.
He noted that manufacturing costs for the SpCCI engine fall between those of a diesel and a gasoline engine.
Author: Ian Adcock
Source: SAE Automotive Engineering Magazine
馬自達將在 2018 年推出 Skyactiv-X發(fā)動機，打造世界首款壓燃汽油機。這款發(fā)動機采用了伊頓專為馬自達設計的增壓系統(tǒng)，具體請見圖片中央位置（圖片來源：馬自達）。
作為一種新興的燃料原材料，藻類將逐漸展現(xiàn)更大的潛力（圖像來源：NOAA 五大湖環(huán)境研究實驗室）。
與現(xiàn)行發(fā)動機架構相比，SpCCI 發(fā)動機的主要變化體現(xiàn)在汽缸頭、電子控制系統(tǒng)和氣泵增壓器方面。
馬自達集合其汽油機和柴油機設計之精華，打造了新款 Skyactiv-X 發(fā)動機（圖片來源：馬自達）。
馬自達的數(shù)據(jù)表明，與公司當下的 2.0 L 汽油發(fā)動機相比，全新 Skyactiv-X 發(fā)動機不僅燃耗明顯更低，而且性能也更加強大。
馬自達 SpCCI 系統(tǒng)的燃燒策略圍繞火花塞展開，進而增強發(fā)動機的有效壓縮比，并“促進”壓縮點火過程（圖片來源：馬自達）。
在 SpCCI 發(fā)動機中，火花塞產(chǎn)生的火球可提高發(fā)動機的有效壓縮比，優(yōu)化整個燃燒室內的燃燒環(huán)境（圖片來源：馬自達）。