發(fā)動機的輕量化有時會帶來一些問題，包括低速狀態(tài)下扭矩明顯不足, 以及瞬態(tài)響應速度較慢等等。即便應用了敏感度較高的渦輪增壓技術，仍然無法解決這些問題。目前Torotrak與巴斯大學正在研發(fā)一種可能成功的解決方案——可變驅動增壓技術。在英國政府的支持下，這個合作研究項目正在順利開展，目標是實現(xiàn)這一技術的商業(yè)化應用。
該項目正在研究業(yè)內頂尖水平的輕量化汽油發(fā)動機與可變增壓器相結合之后，所形成的系統(tǒng)運行的狀態(tài)。以及發(fā)動機與Torotrak的V-Charge可變驅動增壓器如何共同工作，以便在低速狀態(tài)下顯著提高扭矩，加快瞬態(tài)響應速度，并降低燃耗。
V-Charge的設計和研發(fā)目的，是利用一個增壓器和一個變速機械傳動裝置，在任何發(fā)動機轉速下實現(xiàn)Torotrak所稱的“接近即時”的快速反應狀態(tài)，并創(chuàng)造出尺寸較大的自然吸氣發(fā)動機能夠帶給用戶的感覺。
Torotrak的首席技術官Doug Cross表示：“為了實施更進一步的輕量化策略，我們需要一種既可以提高駕駛性能，又不會增加成本與復雜度的方案。而我們遇到的一個難題就是，牽涉到成本和復雜程度的不僅僅是增壓系統(tǒng)，還有越來越復雜的尾氣后處理系統(tǒng)，它必須保持較高且穩(wěn)定的溫度才能運作，而添加在尾氣流中的渦輪增壓器會減低其效果。”
最佳方案
除了上述問題之外，還有一個根本性的難題。在發(fā)動機轉速較慢、油門并未全開時，進氣系統(tǒng)中的氣流不夠多，就連一個二級式系統(tǒng)中的小渦輪都難以驅動。Cross表示，在一個輕量化的發(fā)動機中，如需在怠速狀態(tài)下瞬間喚起油門反應，需要一個增壓器。這一措施還能解決許多后處理難題，因為增壓器是在發(fā)動機溫度較低的那一面工作的，不會干擾尾氣流。
當然，傳統(tǒng)的機械式風扇肯定有自身局限。如果專為發(fā)動機低速響應而將風扇尺寸縮小，則無法在高速運作時提供所需的空氣量。而如果使用較大的風扇并用齒輪調整低速響應能力，那么則需要使用旁路裝置或離合器來避免高速時氣流過強（這樣會浪費能量）的問題。添加一個離合器可能對附件驅動造成負荷壓力，再添加一個子系統(tǒng)也會帶來同樣的問題。
某些汽車制造商，比如大眾公司，已經決定推行小型增壓器與渦輪增壓器結合的解決方案，前者可以增強發(fā)動機低轉速時提升扭矩和反應速度，而后者可以在發(fā)動機高轉速時提供較高功率。
另一種有潛力的技術是電動增壓，但Cross指出，與其能夠提供的空氣量相比，這一技術的功率受限程度很高，限制了它在發(fā)動機低速運轉時能夠發(fā)揮的作用。1個12伏的電動增壓系統(tǒng)可以多提供2-3kW (2.6-4 hp)的功率用以壓縮進氣，但即便是一個48伏的系統(tǒng)，也只能多提供6kW (8 hp)左右的功率。
“最好的解決方案是使增壓器即便在發(fā)動機低速運轉的狀態(tài)下也能迅速做出瞬態(tài)響應，并在發(fā)動機的整個轉速范圍內配合其需求，”Corss告訴《汽車工程雜志》的記者。它不僅不會在需求未達到最大水平的時候因為使用節(jié)流閥而降低效率，也不會浪費多余的能量，而且安裝簡便，能夠降低制造成本和復雜程度。
巴斯大學的研究對象中還包括了福特汽車。福特的1.0-L三缸Ecoboost系列發(fā)動機的轉速范圍可以滿足多種車型的轉速要求，其中包括C/D級的蒙迪歐（具體請閱讀http://articles.sae.org/14204/）。
V-Charge目前已經發(fā)展到了生產前的設計階段，下一步將與現(xiàn)有的增壓方案進行對照評估。第一步是集中模擬，第二步是將設備安裝在一個輕量化汽油發(fā)動機上進行測試。Torotrak已經借助一輛1.1-L的汽車向潛在客戶展示了這一設計理念的可行性。
“傳統(tǒng)的渦輪增壓器在經過優(yōu)化的穩(wěn)定狀態(tài)下，燃油效率提升非常理想，但當發(fā)動機的比輸出升至150kW/L或更高水平時，傳統(tǒng)的單階式渦輪增壓器可以提供低速所需的功率。” Cross解釋道。
他表示，一旦未來的排放法規(guī)生效，小型發(fā)動機將會用在與更接近真實情況的駕駛工況測試中，瞬態(tài)增壓的重要性就越來越顯著。這也將是適用于汽油機和柴油機的V-Charge發(fā)揮作用的時候，因為它可以解決目前限制發(fā)動機小型化發(fā)展的一個主要問題。
這個系統(tǒng)的運作方式是通過一個緊湊型的變速驅動裝置將傳統(tǒng)的離心增壓器與發(fā)動機連接起來。這個機制可以讓氣流調整不受發(fā)動機的速度或尾氣能量的影響，獨立進行，以更好地匹配發(fā)動機的要求。增壓器需要安裝在發(fā)動機前端的輔機傳動裝置（FEAD）上，它可提供顯著的增壓能力，持續(xù)功率為20kW (27hp)。
無齒輪機械牽引驅動裝置可提供10:1的比例范圍，因此轉速范圍很廣。這使得壓縮機的可以運作的范圍比定速驅動裝置理想得多。
冷卻進氣是V-Charge的另一項優(yōu)勢
V-Charge可以像電動增壓器一樣，在低速狀態(tài)下快速旋轉，然后在更高的轉速下提供所需的氣流，但不超過壓縮機的性能范圍。Cross稱，使用V-Charge后，發(fā)動機的扭矩輸出可在400毫秒內從0升至95%，與最先進的單階式渦輪增壓器相比，可將“到達所需扭矩的時間”最多降低70%。
Cross還表示，因為比率是用一只10W的作動器通過機電控制來調整的，而且不需要使用額外的功率保持其穩(wěn)定，因此該系統(tǒng)的效率比傳統(tǒng)的增壓器驅動系統(tǒng)要高得多。
“無論時增壓期間還是非增壓期間，我們都已將V-Charge的寄生損失降到了最低水平，”他指出。“今后，它還有可能在油門開口較小的時候切斷驅動。這將帶來一個巨大的優(yōu)勢，因為增壓器在重新接合的時候往往會在FEAD區(qū)域產生巨大的慣性沖擊。但我們的可變驅動系統(tǒng)則可以在重新接合的時候降低比率和參照慣量。”
除了上述優(yōu)勢之外，離心式風扇還能帶來其他效率上的好處，因為它可以使V-Charge的進氣溫度比螺旋式增壓器更低。這有助于解決輕量化發(fā)動機面對的一個根本難題——燃燒溫度過高。Cross認為，在柴油排放的控制方面，SCR（選擇性催化還原）后處理將成為降低氮氧化物含量的最佳方案，因為，將增壓器移至發(fā)動機溫度較低的一側“可以減小SCR系統(tǒng)的尺寸和成本。”
溫度更低的進氣流還有助于提升燃油經濟性的其他策略的實現(xiàn)，如米勒循環(huán)發(fā)動機（豐田最近在其非混動汽車上采用了這種發(fā)動機，而奧迪則在A4上采用），這種發(fā)動機需要在整個轉速范圍內使用溫度較低的進氣溫度，并獲得較高的進氣壓力。
作者：Stuart Birch
來源：SAE 《汽車工程雜志》
V-Charging aims to add muscle to downsized engines
Engine downsizing can sometimes include a distinct lack of torque at low revs and slow transient response, even when subtle turbocharging techniques are applied. A potential solution to these issues is variable-drive supercharging, currently under investigation by Torotrak and the University of Bath. The joint research project, supported by the U.K. government, aims for productionization of the technology.
The project is examining how a state-of-the-art downsized gasoline engine combined with a variable supercharger performs at a system level. The research is studying interactions with Torotrak’s V-Charge variable-drive supercharger unit to deliver far higher levels of low-end torque, fast transient response, and reduced fuel consumption.
V-Charge has been designed and developed to provide what the company describes as “near instant” response at any engine speed through the use of a supercharger with a mechanical variable speed drive, and to create the performance “feel” of a larger, naturally-aspirated unit.
Torotrak’s Chief Technical Officer, Doug Cross, said: “For more aggressive downsizing strategies to be implemented, we need solutions that will improve drivability without adding cost and complexity. And part of the challenge is that this cost and complexity is not just in the pressure charging system. The growing sophistication of exhaust aftertreatment, with its need for high and stable temperatures, is also compromised by having turbochargers in the exhaust stream.”
The optimum solution
There's also the fundamental challenge of insufficient airflow in the intake system at low engine speeds and throttle positions, making it difficult to drive even the smaller turbo in a two-stage system. To provide instant throttle response from idle on a downsized engine requires a supercharger, Cross maintains. This also mitigates many of the aftertreatment challenges as superchargers operate on the cold side of the engine without interrupting the exhaust stream.
Traditional mechanical blowers bring their own limitations, of course. A unit sized for low speed engine response is unable to deliver the volume of air required at higher speeds, while a larger unit — if suitably geared for low speed response — either requires a bypass to avoid over-delivery at higher speeds (thus wasting energy), or must be clutched. Adding a clutch can create loading challenges for the accessory drive as well as introducing an additional subsystem.
Some vehicle manufacturers, notably VW, have elected to use a small supercharger to enhance low speed torque and response, combined with a turbocharger to provide high power at the upper end of the engine range.
Another technology demonstrated as a potential solution is electric supercharging, but this is acutely power-limited in terms of the air it can deliver, restricting its contribution at low engine speeds, Cross noted. A 12-v system provides an extra 2-3 kW (2.6 to 4 hp) to compress the intake air, while even a 48-v system only produces some 6 kW (8 hp).
“The optimum solution is a means of boosting that responds quickly to transient changes, even at low engine speeds, and keeps pace with engine demand throughout the speed range,” he told Automotive Engineering. It would not introduce inefficiencies through throttling at times of partial demand or wasting surplus energy, and would minimize cost and complexity through simplicity of installation.
The Bath research project also involves Ford, whose 1.0-L 3-cylinder Ecoboost range of engines is available across several model ranges including the C/D segment Mondeo (see http://articles.sae.org/14204/).
Having evolved to a pre-production design level, V-Charge will be evaluated against current boosting solutions, initially through extensive simulation, then via a downsized gasoline engine. The concept has already been demonstrated by Torotrak fitted in in a 1.1-L car to potential customers.
“A conventional turbocharger is a highly effective device for optimizing steady-state fuel economy but as the specific output of engines climbs to 150 kW/L and beyond, no conventional single-stage solution can deliver the required low-speed drivability,” Cross explained.
He said that as future emissions regulations take effect, the combination of smaller engines and drive cycles closer to real-world use patterns will make engine operation under transient boost conditions increasingly important. This is the region where V-Charge (applicable to both gasoline and diesel engines) is particularly effective, addressing one of the major constraints that presently limits engine downsizing.
The system operates by connecting a conventional centrifugal supercharger to the engine through a compact variable-speed drive. This allows air delivery to be altered independently of engine speed or exhaust energy to match the engine’s requirement. It is designed to be installed on the front end accessory drive (FEAD) of an engine and provides a significant boost capacity, with a continuous rating of 20 kW (27 hp).
The gearless mechanical traction drive provides a 10:1 ratio spread, giving a wide speed range that allows a much greater compressor operating envelope than would be possible with a fixed speed drive.
Cooler intake air is added benefit
The unit is able to spin up quickly like an e-supercharger at low speed, then carry on to deliver the required air mass flow at higher engine rpm, within the limits of compressor performance. Using V-Charge, engine torque output can increase from 0-95% in less than 400 ms, cutting the time-to-torque by up to 70% compared with the latest state-of-the-art single turbocharger technologies, claimed Cross.
Because ratios are changed by a 10W actuator using electro-mechanical control, and no power is required to hold the unit at a given ratio, the system offers much higher efficiency than a conventional supercharger drive, according to Cross.
“We have minimized parasitic losses, not just when the unit is boosting, but also when off-boost," he reported. "We also have the potential to disconnect the drive at small throttle openings. This provides a big advantage because superchargers normally generate a huge inertial shock on the FEAD when re-engaging. But our variable drive can reduce the ratio and the referred inertia from the supercharger at the moment of re-clutching.”
A further efficiency gain, via the centrifugal blower, allows the V-Charge to deliver cooler intake air than a screw-type supercharger. This helps to overcome a further fundamental constraint on downsized engines: high combustion temperatures. His prediction for diesel emissions control is that SCR (selective catalytic reduction) aftertreatment will become the preferred option for reducing NOx, because moving the pressure charger to the cold side "will allow smaller, lower cost SCR systems.”
Cooler intake air also benefits other strategies for improving fuel economy, such as Miller cycle operation (recently adopted by Toyota for non-hybrids as well as by Audion its A4), which relies on cooler induction temperatures and higher induction pressures throughout the rev range.
Author: Stuart Birch
Source: SAE Automotive Engineering Magazine
Torotrak的V-Charge增壓器由一只傳統(tǒng)的離心壓縮器與該公司的緊湊型環(huán)形（Torodia）變速器組成，以提供所需的壓縮空氣。
這是Torotrak的環(huán)形變速器的組成部件。
Torotrak的V-Charge系統(tǒng)是專門針對輕量化渦輪增壓汽油機開發(fā)的，旨在提高低轉速下的扭矩與瞬態(tài)響應性能。
Torotrak的Doug Cross 表示，V-Charge可以實現(xiàn)“接近瞬時的”油門反應速度。