最近，美國諾德士(Nautilus Engineering)宣布，該公司的創(chuàng)新基礎(chǔ)發(fā)動機設(shè)計，能夠解決大多數(shù)長期困擾均質(zhì)壓燃(HCCI)發(fā)動機在汽車行業(yè)發(fā)展的固有缺陷問題，并通過SAE 2016全球汽車年會平臺發(fā)布了旗下諾德士循環(huán)(Nautilus Cycle)發(fā)動機，并利用這個機會向多個行業(yè)介紹高效率、低排放的HCCI技術(shù)。
雖然諾德士曾在最近的媒體發(fā)布中表示，公司“相信目前擁有的專利技術(shù)足以將可控HCCI燃燒技術(shù)推廣至多種發(fā)動機和應(yīng)用之中，”但公司CEO兼首席研發(fā)科學(xué)家Matthew Riley表示，他在剛開始進(jìn)行諾德士發(fā)動機研發(fā)時，并沒有制定這么高的目標(biāo)：“當(dāng)時，我只是想改善一下割草機的發(fā)動機而已。”
據(jù)悉，低溫HCCI燃燒技術(shù)可以保證為汽油發(fā)動機帶來柴油機級別的高能效，而且無需擔(dān)心柴油機的排放和成本問題，這仿佛是一座圣杯，不斷激發(fā)著汽車動力系統(tǒng)研發(fā)人員的征服欲。但是經(jīng)過十多年的研發(fā)，HCCI技術(shù)似乎是個無法實現(xiàn)的目標(biāo)。研發(fā)人員不僅需要采用大量燃燒控制策略，有效解決冷啟動的問題，還要保證發(fā)動機在整個轉(zhuǎn)速范圍內(nèi)的正常運轉(zhuǎn)，這些問題都阻礙了HCCI發(fā)動機在量產(chǎn)車型中的應(yīng)用。
然而，諾德士表示，公司解決問題的途徑并非不斷疊加成本高昂的發(fā)動機軟件控制。事實上，這款發(fā)動機采用了一款創(chuàng)新的活塞設(shè)計，這種活塞可以在工作時分隔出一個小型的“初級”燃燒室，在空氣和燃油進(jìn)入大型二次燃燒氣缸進(jìn)行可控的完全燃燒膨脹之前，先進(jìn)行小規(guī)模的預(yù)混和“引燃”。此外，諾德士還在二沖程循環(huán)的進(jìn)氣口和出氣口處應(yīng)用了類似的措施，通過這一架構(gòu)，實現(xiàn)對HCCI整個運行過程的簡單控制。該技術(shù)的模擬示意視頻如下。
“大家都想直接利用火花點火發(fā)動機，然后將其改造成HCCI發(fā)動機，” Riley在 SAE 2016 全球汽車年會上表示，“但這并不是一個好主意。”
諾德士在公司官網(wǎng)上表示，“由于采用了這種全新技術(shù)，我們得以在發(fā)動機的所有常規(guī)轉(zhuǎn)速、負(fù)載和工作溫度范圍內(nèi)，實現(xiàn)對HCCI循環(huán)的全面控制。這種技術(shù)可以快速應(yīng)用至現(xiàn)有平臺，達(dá)到甚至超過美國環(huán)保署的2025年排放目標(biāo)的要求。”
關(guān)鍵在于小型燃燒室
在諾德士的設(shè)計中，空氣和燃油可通過一個理想工作壓強在160到200 psi之間的燃油噴嘴，在氣缸外部的進(jìn)氣歧管中得到充分混合。諾德士發(fā)動機的小型初級燃燒室有幾個可選的設(shè)計方案，可以實現(xiàn)（小型單氣缸發(fā)動機的）固定壓縮比，也可以將大型二次燃燒室的固定壓縮比（大約10.5:1）提升1.5至2.5倍。發(fā)動機的進(jìn)氣口和出氣口均采用了傳統(tǒng)的提升閥，主要是通過數(shù)字液壓（甚至電力）促動器驅(qū)動。這對閥門也能通過更加傳統(tǒng)的凸輪軸組合進(jìn)行控制。
諾德士提到，這款發(fā)動機的設(shè)計采用了單個止回閥，可以盡量減少典型二沖程發(fā)動機暴露在進(jìn)氣口和出氣口蒸汽之中的情況。
根據(jù)設(shè)計，發(fā)動機的空氣/燃油比完全是由發(fā)動機負(fù)載決定的，諾德士循環(huán)發(fā)動機運轉(zhuǎn)時的空氣/燃油比大約在24:1到31:1之間。至于這款發(fā)動機的NVH性能（噪聲、振動和不平順性），Riley坦誠地表示，“我們并不確定”。Riley似乎在示意，諾德士發(fā)動機的理想配置方式，可能會是一種水平對置氣缸的布局。Riley認(rèn)為，這種發(fā)動機在汽車應(yīng)用中也有可能需要配置某種增壓功能，以輔助氣缸的填充和凈化，特別是在諾德士循環(huán)發(fā)動機需要在米勒或阿特金森循環(huán)下工作的情況下。
Riley介紹道，這款原型發(fā)動機目前僅存在于計算機之中，尚未真正能夠“跑”起來，其燃燒溫度預(yù)計在1200華氏度左右。他表示，公司計劃在未來8到10周內(nèi)讓發(fā)動機真正“跑”起來，并在未來6個月內(nèi)打造一款“可以運轉(zhuǎn)并且在功能上達(dá)到預(yù)期”的原型發(fā)動機。Riley認(rèn)為，在大約為期1年的原型研發(fā)階段之后，要繼續(xù)將這種技術(shù)應(yīng)用至“主流汽車應(yīng)用”之中，還需要2到3年。
Drone Duty?
對比以往的情況，諾德士給出的時間表可以說是一反常態(tài)地謹(jǐn)慎，但公司CEO卻非常信任這種發(fā)動機設(shè)計本身的吸引力：“可能這周就有人會來找我們簽研發(fā)合同。”
Riley表示，對于立足于堪薩斯州的諾德士而言，公司目前的目標(biāo)是與美國堪薩斯州立大學(xué)(Kansas State University)建立合作，共同研發(fā)一款適合無人機(UAV)使用的發(fā)動機。
Riley還表示，諾德士發(fā)動機也很容易進(jìn)行傳統(tǒng)四沖程循環(huán)。他堅信，這種設(shè)計的主要優(yōu)勢之一是尺寸靈活可控，因此可以輕松適應(yīng)一系列的不同的交通運輸設(shè)備的要求。
作者：Bill Visnic
來源：SAE《汽車工程》雜志
翻譯：SAE 上海辦公室
Nautilus claims radical advance in HCCI engine development
Claiming its novel base-engine design solves most of the intrinsic drawbacks that have slowed auto-industry development of homogenous-charge compression-ignition (HCCI) engines, Nautilus Engineering used the 2016 SAE World Congress to introduce its Nautilus Cycle engine, saying the unique platform offers an opportunity to introduce efficient, low-emissions HCCI technology to a variety of industries.
Although Nautilus said in a news release the company “believes it now holds patents to make controlled HCCI combustion possible for a wide range of engines and applications” company CEO and chief research scientist Matthew Riley said he started the Nautilus effort with a somewhat humbler horizon: “I just wanted to clean up lawn mower engines.”
Low-temperature HCCI combustion has been a lingering Holy Grail of automotive powertrain development, promising to impart on gasoline engines the efficiency of diesel—without diesel’s emissions and cost baggage. But a decade or more of serious development has proved HCCI an elusive target, its need for intensive combustion control, issues with cold-start operation and difficulty in addressing the entire automotive rpm range suppressing introduction for production-vehicle application.
Nautilus, however, said its approach to the issue was not to layer on yet more levels of costly engine software control. Instead the engine’s defining feature—a novel piston design that creates a small “primary” combustion chamber that propagates the air/fuel detonation to the larger secondary cylinder for full and controlled expansion—and an equally unique approach to dealing with the intake and exhaust paths of the two-stroke cycle create an architecture conducive to relatively simple control of full-range HCCI operation. Nautilus' video of the operation can be seen here.
“Everybody’s always tried to take a spark-ignition engine and make it HCCI,” Riley said at the 2016 SAE World Congress. “Bad idea.”
On its website, Nautilus said, “With this new technology, we've achieved full control over the HCCI cycle in all conventional rpm ranges, loads and temperatures. This may be rapidly implemented into existing platforms and will meet and even exceed 2025 EPA (emissions) regs.”
It's all about that small combustion chamber
Air and fuel are mixed outside the cylinder in the intake manifold via a fuel injector ideally operating at something approaching 160-200 psi. A couple of potential design options for the Nautilus engine’s small primary combustion chamber mean compression ratio could be fixed (say for small single-cylinder engines) or enable a variable compression ratio of perhaps 1.5 to 2.5 times the large secondary combustion chamber’s fixed compression ratio of somewhere around 10.5:1. Twin intake and exhaust ports are valved with conventional-looking poppets envisioned to be driven by digital hydraulic (or perhaps even electric) actuators. The valves also could be operated by a more conventional camshaft arrangement.
The engine design uses a single check valve, the company said, to minimize the typical two-stroke engine’s exposure of the intake and exhaust streams to engine lubricating oil.
The Nautilus Cycle engine will run air/fuel ratios approximately ranging from 24:1 to 31:1, as the engine design means the air/fuel ratio is determined strictly by engine load. As for a projected noise, vibration and harshness signature for the engine, “We’re not sure yet,” Riley admits. He seemed to indicate an ideal configuration for the Nautilus engine could be an opposed-cylinder “boxer” layout. And he would not rule out the likelihood that an automotive application might require some kind of supercharging to aid cylinder filling and scavenging—particularly if the Nautilus Cycle engine is adapted to operate under Miller or Atkinson cycles.
Riley said combustion temperatures are expected to be around 1,200º F for an engine that for the moment exists only in computer design: no running prototype yet exists. He said the company plans to have a running version in eight to ten weeks and a prototype engine “up and functional” within six months. After a year of prototype development, he sees a window of two to three years “for major streamlining of automotive applications.”
Drone Duty?
Given some past experiences, the timeline seems uncharacteristically cautious for a new-engine promoter—but the Nautilus Engineering CEO is openly bullish about the engine design’s intrinsic appeal: Somebody’s probably going to come to us by the end of the week with a development contract,” Riley predicted.
He said the immediate goal for Kansas-based Nautilus Engineering is to work with academic partner Kansas State university to develop a version of the engine suitable for an unmanned aerial vehicle (UAV) drone.
Riley also said the Nautilus engine also could easily run a conventional four-stroke cycle. One of its chief attributes, he insisted, is scalability to accompany its easy adaptability for a range of mobility applications.
Author: Bill Visnic
Source: SAE Automotive Engineering Magazine
諾德士表示，HCCI發(fā)動機的制造成本低于火花點火汽油機，公司無需對制造設(shè)施進(jìn)行特別投資，就能實現(xiàn)最高30%的燃料經(jīng)濟(jì)性提升，以及接近于零的NOx排放。
這張剖面圖展示了諾德士循環(huán)發(fā)動機的特征：一個頂部突起的活塞可以在向頂部靠近時，分隔出發(fā)動機的“初級”燃燒室。