发动机的轻量化有时会带来一些问题，包括低速状态下扭矩明显不足, 以及瞬态响应速度较慢等等。即便应用了敏感度较高的涡轮增压技术，仍然无法解决这些问题。目前Torotrak与巴斯大学正在研发一种可能成功的解决方案——可变驱动增压技术。在英国政府的支持下，这个合作研究项目正在顺利开展，目标是实现这一技术的商业化应用。

该项目正在研究业内顶尖水平的轻量化汽油发动机与可变增压器相结合之后，所形成的系统运行的状态。以及发动机与Torotrak的V-Charge可变驱动增压器如何共同工作，以便在低速状态下显著提高扭矩，加快瞬态响应速度，并降低燃耗。

V-Charge的设计和研发目的，是利用一个增压器和一个变速机械传动装置，在任何发动机转速下实现Torotrak所称的“接近即时”的快速反应状态，并创造出尺寸较大的自然吸气发动机能够带给用户的感觉。

Torotrak的首席技术官Doug Cross表示：“为了实施更进一步的轻量化策略，我们需要一种既可以提高驾驶性能，又不会增加成本与复杂度的方案。而我们遇到的一个难题就是，牵涉到成本和复杂程度的不仅仅是增压系统，还有越来越复杂的尾气后处理系统，它必须保持较高且稳定的温度才能运作，而添加在尾气流中的涡轮增压器会减低其效果。”

最佳方案

除了上述问题之外，还有一个根本性的难题。在发动机转速较慢、油门并未全开时，进气系统中的气流不够多，就连一个二级式系统中的小涡轮都难以驱动。Cross表示，在一个轻量化的发动机中，如需在怠速状态下瞬间唤起油门反应，需要一个增压器。这一措施还能解决许多后处理难题，因为增压器是在发动机温度较低的那一面工作的，不会干扰尾气流。

当然，传统的机械式风扇肯定有自身局限。如果专为发动机低速响应而将风扇尺寸缩小，则无法在高速运作时提供所需的空气量。而如果使用较大的风扇并用齿轮调整低速响应能力，那么则需要使用旁路装置或离合器来避免高速时气流过强（这样会浪费能量）的问题。添加一个离合器可能对附件驱动造成负荷压力，再添加一个子系统也会带来同样的问题。

某些汽车制造商，比如大众公司，已经决定推行小型增压器与涡轮增压器结合的解决方案，前者可以增强发动机低转速时提升扭矩和反应速度，而后者可以在发动机高转速时提供较高功率。

另一种有潜力的技术是电动增压，但Cross指出，与其能够提供的空气量相比，这一技术的功率受限程度很高，限制了它在发动机低速运转时能够发挥的作用。1个12伏的电动增压系统可以多提供2-3kW (2.6-4 hp)的功率用以压缩进气，但即便是一个48伏的系统，也只能多提供6kW (8 hp)左右的功率。

 “最好的解决方案是使增压器即便在发动机低速运转的状态下也能迅速做出瞬态响应，并在发动机的整个转速范围内配合其需求，”Corss告诉《汽车工程杂志》的记者。它不仅不会在需求未达到最大水平的时候因为使用节流阀而降低效率，也不会浪费多余的能量，而且安装简便，能够降低制造成本和复杂程度。

巴斯大学的研究对象中还包括了福特汽车。福特的1.0-L三缸Ecoboost系列发动机的转速范围可以满足多种车型的转速要求，其中包括C/D级的蒙迪欧（具体请阅读http://articles.sae.org/14204/）。

V-Charge目前已经发展到了生产前的设计阶段，下一步将与现有的增压方案进行对照评估。第一步是集中模拟，第二步是将设备安装在一个轻量化汽油发动机上进行测试。Torotrak已经借助一辆1.1-L的汽车向潜在客户展示了这一设计理念的可行性。

 “传统的涡轮增压器在经过优化的稳定状态下，燃油效率提升非常理想，但当发动机的比输出升至150kW/L或更高水平时，传统的单阶式涡轮增压器可以提供低速所需的功率。” Cross解释道。

他表示，一旦未来的排放法规生效，小型发动机将会用在与更接近真实情况的驾驶工况测试中，瞬态增压的重要性就越来越显著。这也将是适用于汽油机和柴油机的V-Charge发挥作用的时候，因为它可以解决目前限制发动机小型化发展的一个主要问题。

这个系统的运作方式是通过一个紧凑型的变速驱动装置将传统的离心增压器与发动机连接起来。这个机制可以让气流调整不受发动机的速度或尾气能量的影响，独立进行，以更好地匹配发动机的要求。增压器需要安装在发动机前端的辅机传动装置（FEAD）上，它可提供显著的增压能力，持续功率为20kW (27hp)。

无齿轮机械牵引驱动装置可提供10:1的比例范围，因此转速范围很广。这使得压缩机的可以运作的范围比定速驱动装置理想得多。

冷却进气是V-Charge的另一项优势

V-Charge可以像电动增压器一样，在低速状态下快速旋转，然后在更高的转速下提供所需的气流，但不超过压缩机的性能范围。Cross称，使用V-Charge后，发动机的扭矩输出可在400毫秒内从0升至95%，与最先进的单阶式涡轮增压器相比，可将“到达所需扭矩的时间”最多降低70%。

Cross还表示，因为比率是用一只10W的作动器通过机电控制来调整的，而且不需要使用额外的功率保持其稳定，因此该系统的效率比传统的增压器驱动系统要高得多。

 “无论时增压期间还是非增压期间，我们都已将V-Charge的寄生损失降到了最低水平，”他指出。“今后，它还有可能在油门开口较小的时候切断驱动。这将带来一个巨大的优势，因为增压器在重新接合的时候往往会在FEAD区域产生巨大的惯性冲击。但我们的可变驱动系统则可以在重新接合的时候降低比率和参照惯量。”

除了上述优势之外，离心式风扇还能带来其他效率上的好处，因为它可以使V-Charge的进气温度比螺旋式增压器更低。这有助于解决轻量化发动机面对的一个根本难题——燃烧温度过高。Cross认为，在柴油排放的控制方面，SCR（选择性催化还原）后处理将成为降低氮氧化物含量的最佳方案，因为，将增压器移至发动机温度较低的一侧“可以减小SCR系统的尺寸和成本。”

温度更低的进气流还有助于提升燃油经济性的其他策略的实现，如米勒循环发动机（丰田最近在其非混动汽车上采用了这种发动机，而奥迪则在A4上采用），这种发动机需要在整个转速范围内使用温度较低的进气温度，并获得较高的进气压力。

作者：Stuart Birch

来源：SAE 《汽车工程杂志》

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