英國(guó)的Aeristech公司高調(diào)宣稱已研制出“世界上功率密度最大的可變速電機(jī)”，有助于解決小尺寸發(fā)動(dòng)機(jī)的技術(shù)難題，更好地實(shí)現(xiàn)二氧化碳減排目標(biāo)。

公司CEO Bryn Richards表示：“對(duì)于極小尺寸發(fā)動(dòng)機(jī)而言，多級(jí)或機(jī)械增壓均不能解決問題，因?yàn)檫@些增壓設(shè)備不能保證在低速時(shí)提供充足動(dòng)力。下一代的小尺寸發(fā)動(dòng)機(jī)需要電動(dòng)增壓器，功率充足，可以滿足低轉(zhuǎn)速時(shí)的發(fā)動(dòng)機(jī)增壓需求，并且冷卻性能優(yōu)良，可以實(shí)現(xiàn)持續(xù)運(yùn)轉(zhuǎn)。”

因此，Aeristech研發(fā)出了eSupercharger，旨在提供低速扭矩和即時(shí)節(jié)氣門響應(yīng)性能，消費(fèi)者期望獲得這些優(yōu)良性能，但在小尺寸和“正常尺寸”的渦輪增壓發(fā)動(dòng)機(jī)中卻難以實(shí)現(xiàn)。

Richards解釋說，多級(jí)渦輪增壓整合了不同尺寸的渦輪，有利于小尺寸發(fā)動(dòng)機(jī)在更大的速度范圍內(nèi)驅(qū)動(dòng)，但也帶來了諸如成本、復(fù)雜度、熱管理和催化性能方面的其他問題。他也強(qiáng)調(diào)，隨著尾氣排放限制日益嚴(yán)苛，催化性能帶來的問題很可能會(huì)越來越嚴(yán)重。他表示：“對(duì)于一部安裝在發(fā)動(dòng)機(jī)和催化裝置間的兩級(jí)渦輪增壓系統(tǒng)來說，其熱質(zhì)量會(huì)增加車輛在輕負(fù)載或發(fā)動(dòng)機(jī)低速運(yùn)行時(shí)的熄火概率。”

顯然，替代方案是使用增壓器，但常規(guī)的機(jī)械增壓機(jī)無法在發(fā)動(dòng)機(jī)低速運(yùn)行時(shí)提供充足助力。Richards認(rèn)為，電驅(qū)動(dòng)單元“目前僅能提供最短暫的瞬時(shí)助力”。

依據(jù)目前的發(fā)展情形，Richards認(rèn)為，汽車行業(yè)很有可能會(huì)大力發(fā)展48V系統(tǒng)，并打造出在低速時(shí)也能提供充足助力的電渦輪增壓器。

“要實(shí)現(xiàn)持續(xù)工作，首先必須要滿足冷卻性能的要求，”他告訴SAE《汽車工程雜志》，并指出Aeristech所有的電機(jī)技術(shù)可在1秒內(nèi)充分滿足發(fā)動(dòng)機(jī)的氣動(dòng)力負(fù)荷需求，而不會(huì)引起常規(guī)發(fā)動(dòng)機(jī)經(jīng)常出現(xiàn)的冷卻問題。

新電機(jī)控制策略

目前，整個(gè)行業(yè)正在大力發(fā)展備受期待的48 V輕混合系統(tǒng)，可以為電動(dòng)增壓器提供所需功率，以保證小尺寸發(fā)動(dòng)機(jī)在低速運(yùn)行時(shí)提供充足的助力需求。同時(shí)，機(jī)械渦輪性能也得以優(yōu)化，可以只在發(fā)動(dòng)機(jī)高速運(yùn)行時(shí)提供助力，可以實(shí)現(xiàn)更大的比功率和更低的排氣阻力。

Richards解釋道，為了充分發(fā)揮48 V系統(tǒng)的優(yōu)勢(shì)，提供電動(dòng)增壓，Aeristech公司采用了永磁技術(shù)。這種技術(shù)的響應(yīng)速度更快，并且不會(huì)引起傳統(tǒng)開關(guān)磁阻（SR）電機(jī)經(jīng)常出現(xiàn)的冷卻問題。目前，Aeristech公司已獲得永磁技術(shù)專利。Richards認(rèn)為，永磁技術(shù)是SR電機(jī)唯一的可選擇的解決方案。他預(yù)計(jì)，一旦SR電機(jī)實(shí)現(xiàn)大規(guī)模量產(chǎn)，這兩套系統(tǒng)的成本將基本持平。

“我們的電機(jī)控制策略可以分隔換流和功率控制，也就是說，電轉(zhuǎn)換頻率無需高于運(yùn)轉(zhuǎn)速度，”他說。“這種全新的方法可以大幅削減許多關(guān)鍵的開關(guān)元件的成本，并保證精準(zhǔn)的高瞬時(shí)速度控制。”

這種方法也將賦予永磁技術(shù)很高的性價(jià)比，他表示，“這也是eSupercharger首次能夠?qū)崿F(xiàn)持續(xù)提供2.5bar及更高的助力”

該技術(shù)同時(shí)適用于柴油和汽油發(fā)動(dòng)機(jī)。“在日益嚴(yán)苛的法規(guī)面前，現(xiàn)在柴油發(fā)動(dòng)機(jī)依賴于過濾系統(tǒng)來處理廢氣中的顆粒物，但卻增加了發(fā)動(dòng)機(jī)的壓力，”Richards指出。“如果能將這些顆粒物在內(nèi)燃艙中轉(zhuǎn)化為能量，而不是直接過濾出去，肯定更有意義。通過eSupercharger的運(yùn)用，可以在發(fā)動(dòng)機(jī)低速運(yùn)行時(shí)及時(shí)響應(yīng)瞬時(shí)變化，供應(yīng)燃燒所需當(dāng)量的氣體，這樣即可以解決排放問題，也一并提高燃油經(jīng)濟(jì)性。”

BMEP的顯著收益

Aeristech表示，eSupercharger具備行業(yè)領(lǐng)先的功率密度和低慣性水平，可以在0.4秒內(nèi)，以怠速到目標(biāo)速度（idle-to-target speed）的瞬時(shí)響應(yīng)性能，加速至150,000 rpm。該技術(shù)已成功通過里卡多公司（Ricardo）和英國(guó)馬勒動(dòng)力總成公司（Mahle Powertrain）的獨(dú)立評(píng)估。

Richards表示，Ricardo曾使用其公司的WAVE模擬軟件，為一款采用單極渦輪增壓機(jī)、輸出功率為221 kW的2.0 L汽油發(fā)動(dòng)機(jī)建立詳細(xì)的模型。他說，加裝48 V的Aeristech eSupercharger能將渦輪尺寸提高80%，“可以在發(fā)動(dòng)機(jī)的整個(gè)速度范圍內(nèi)提升BSFC（汽車制動(dòng)油耗率），扭矩響應(yīng)和瞬時(shí)響應(yīng)。”

Ricardo最后表示，如果依賴于常規(guī)的單級(jí)渦輪增壓機(jī)，而不使用eSupercharger，那么僅有大型發(fā)動(dòng)機(jī)才能滿足需求。Mahle用一輛D級(jí)別評(píng)估用車展示了48 V的eSupercharger，該車配置了公司生產(chǎn)的小尺寸1.2 L三缸汽油發(fā)動(dòng)機(jī)。這款車原本配置的是兩級(jí)渦輪增壓機(jī)，發(fā)動(dòng)機(jī)輸出功率為144 kW（193 hp），BMEP（制動(dòng)平均有效壓力）剛剛超過30 bar。

Richards表示，“如果用eSupercharger替代小渦輪，大渦輪就可以實(shí)現(xiàn)再次優(yōu)化，發(fā)動(dòng)機(jī)的最大輸出功率可達(dá)193 kW，2000 rpm時(shí)的峰值BMEP可達(dá)33 bar，這是非常了不起的。”他表示，在發(fā)動(dòng)機(jī)的整個(gè)速度范圍內(nèi)，扭矩都有所提升，最大可達(dá)313 N·m （231 lb·ft）；發(fā)動(dòng)機(jī)在1200 rpm時(shí)的BMEP從20 bar提升至接近29 bar。

“Mahle表示，采用了eSupercharger后，單位氣缸容量扭矩要高于市面上任一量產(chǎn)發(fā)動(dòng)機(jī)，”Richards說。eSupercharger可以直接提高扭矩，并間接提高輸出功率。通過用eSupercharger替換兩級(jí)渦輪增壓機(jī)中的小渦輪，Mahle的工程師可以加大主渦輪的尺寸，實(shí)現(xiàn)優(yōu)良的操控性能和瞬態(tài)反應(yīng)性能。

Richards預(yù)測(cè)，在未來，對(duì)于旨在達(dá)到95g/km的二氧化碳排放要求的C級(jí)車而言，其發(fā)動(dòng)機(jī)將進(jìn)一步減小尺寸，甚至?xí)∮诂F(xiàn)有產(chǎn)品的最小尺寸。他表示，采用eSupercharger的高增壓800cc的發(fā)動(dòng)機(jī)可以提供與常規(guī)的2.2 L自然吸氣發(fā)動(dòng)機(jī)相當(dāng)?shù)墓β屎团ぞ兀?ldquo;并提供同樣高效的油門響應(yīng)，而油耗卻更低。”

作者：Stuart Birch
來源：SAE《汽車工程雜志》
翻譯：SAE上海辦公室

Power struggle produces eSupercharger

With the bold claim of inventing the “world’s most power-dense variable speed electric motor,” U.K. specialist company Aeristech believes it can help solve a significant challenge facing the use of downsized engines in pursuit of low CO₂ emissions.

Said CEO Bryn Richards: “For extreme engine downsizing neither multi-stage nor mechanical superchargers are the answer, as an engine using them would lack power at low speeds. To support the next generation of downsized engines requires an electric supercharger with enough power to supply all the low speed boosting needs of the engine combined with sufficiently effective cooling to enable continuous operation."

Thus the development of the Aeristech eSupercharger, designed to provide the low-speed torque and instant throttle response that customers expect but can be challenging to achieve in turbocharged downsized and "right-sized" engines.

Richards explains that multi-stage turbocharging, combining large and small turbos, makes downsized engines easier to drive across a wider speed range, but introduces other issues such as cost, complexity, thermal management, and catalyst performance. The latter, he noted, is likely to be of growing significance as tailpipe emissions are further restricted: "The thermal mass of a two-stage turbocharger system located between the engine and the catalyst makes light-off more difficult to maintain during periods of light load or low engine speed,” he explained.

The obvious alternative is to use a supercharger, but conventional mechanical superchargers don’t deliver at very low engine speeds. And Richards believes that electrically-driven units “have so far only been able to provide the briefest transient boost."

He regards the achievement of a satisfactory electric supercharger with sufficient power to meet, convincingly, all low speed boosting needs could be achieved by the industry’s present likely move towards 48-V systems.

“As for meeting cooling capability requirements, that would be a must for continuous operation," he told Automotive Engineering, pointing out that Aeristech's proprietary electric motor technology delivers the full load air requirements of the engine in under 1 s without the cooling issues associated with conventional motors.

New motor-control strategy

The much-anticipated move towards mild hybrids with 48-V architecture would provide the power necessary for an electric supercharger to deliver the boost levels required by a downsized engine in the lower half of its speed range. With this arrangement, the mechanical turbo could be optimized purely for higher speeds, allowing greater specific power and less exhaust restriction.

Richards explained that to unlock the potential of 48-V systems to provide electric supercharging, Aeristech uses permanent magnet technology, providing a faster response without the cooling challenges arising from the alternative switched reluctance (SR) motors. He believes that SR motors are the only alternative to Aeristech’s patented technology and he expects that when mass produced, the cost of the two systems would be similar.

“Our motor control strategy separates commutation and power control, which means the electrical switching frequency need be no higher than running speed," he said. This is a fundamentally new approach that dramatically reduces the cost of many key switching components while ensuring exceptionally accurate high-transient speed control."

The strategy would make permanent magnet technology cost-effective, he claimed, "meaning that for the first time an eSupercharger can run continuously at boost levels of 2.5 bar or more.”

The technology is applicable to both diesel and gasoline engines. “In the face of increasingly restrictive legislation, diesels are relying on filter systems to remove particulates from the exhaust, increasing the back pressure," Richards noted. "It makes more sense to convert the particulates to useful energy in the combustion chamber, rather than filtering them out of the exhaust. The ability to match the air supply to the fuel input at low engine speeds by using an eSupercharger with lag-free response to transient changes would both cure the emission problem and improve fuel economy.”

Significant BMEP gains

Aeristech claims industry-leading power density and low inertia for the motor in its eSupercharger, which accelerates to 150,000 rpm with a transient response of idle-to-target speed in under 0.4 s. The company has successfully subjected its technology to independent evaluation by Ricardo and Mahle Powertrain U.K.

Richards said that Ricardo had carried out detailed modeling of a 221-kW 2.0-L gasoline engine with a single-stage turbocharger using Ricardo’s WAVE simulation software. Adding a 48-V Aeristech eSupercharger enabled the turbine to be increased in size by 80%, "improving BSFC [Brake Specific Fuel Consumption], torque and transient response throughout the engine speed range," he said.

Ricardo concluded that without the eSupercharger only a larger engine could have met the requirements, if relying on a conventional single-stage turbocharger, according to Richards. Mahle demonstrated the 48-V eSupercharger in a D-segment appraisal vehicle using its downsized 1.2-L 3-cylinder gasoline engine. Originally configured with 2-stage turbocharging, the engine achieved 144 kW (193 hp) and just over 30 bar BMEP.

"When the smaller turbo was replaced by the eSupercharger, allowing the larger turbo to be re-optimized, the engine achieved a maximum power output of 193 kW and a remarkable 33-bar peak BMEP at 2000 rpm," Richards reported. He said torque was increased over the entire speed range with a maximum value of 313 N·m (231 lb·ft); BMEP at 1200 rpm was increased from 20 bar to almost 29 bar.

"Mahle Powertrain U.K. has said that this is a higher torque per unit of cylinder capacity than any series production engine,” he noted. The eSupercharger directly influences the high torque figure, Richards claimed, and is also indirectly responsible for the increased power output. By replacing the smaller turbo in a two-stage turbocharging arrangement with the eSupercharger, the Mahle engineers were able to increase the size of the main turbo with satisfactory drivability and transient response.

For a future C-segment vehicle aiming at CO2 emissions of 95 g/km, Richards predicts greater levels of downsizing than even the most ambitious of today’s engines. He said a highly boosted 800cm³ engine with an eSupercharger can develop the power and torque of a conventional 2.2-L naturally aspirated unit, "providing equally satisfying throttle response but with much lower fuel consumption."

Author: Stuart Birch
Source: SAE Automotive Engineering Magazine