十年前，全球对未来车辆的二氧化碳减排预期为：美国至2025年实现93g/km的标准、欧洲至2020年实现95kg/m标准（2025年可能达到70g/km标准）、中国至2020年实现117g/km标准、而日本至2020年实现105g/km标准。这些目标在当时看来简直就像天方夜谭。此外，由于官方的燃耗数据经常不具备代表性，因此在政府要求实现这些目标时，经常听到另一种呼声，要求放弃这些不切实际的空想。

预计于2017年开始实施的全球轻型车测试规程（WLTP）旨在向汽车买家们提供可靠的燃耗数据，帮助他们在进行购车选择时能更有信心地进行预算规划。WLTP创立的目标是改变OEM实现排放/燃耗合规的方法，他们将以现实评估代以“循环测试”，还将引入全新的测试规程。

Jon Hilton是英国传动系统公司Torotrak的产品研发主管。他认为，仅仅跟随不断更新的法规要求来提升减排水平是不够的，必须在汽车现实能效上实现根本性的变革，才能满足二氧化碳和颗粒物等其他污染物的减排要求。

他表示，最大的挑战在于重新定义内燃机（ICE）的角色。“目前，较温和的混合动力装置是由一个120kW的燃烧发动机和一个15kW的混合动力电源组成的，因为这样搭配成本最合理。但是，要实现未来的目标必须采用一种与之相反的搭配方式，比如说一个30kW的发动机搭配一个100kW的电机。但这不意味着内燃机会降级成为纯粹的增程器。”

Hilton指出，生产这样一种混合动力装置所面临的重量、成本和封装难题，给基于高速飞轮技术的机械能回收系统带来了一线生机。Hilton已参与过大约20个相关的研发项目。Torotrak公司的Flybrid能量回收系统使用一台机械驱动的飞轮来收集制动时所产生的动能，并将其以高效的方式重新传输至车轮。（http://articles.sae.org/12401/）。

发动机减速能手

Hilton解释道，一般来说，飞轮混动装置的重量相当于一台同等性能的电驱动系统的三分之一，生产成本为四分之一左右。这是因为它使用的是常见且容易制造的传统部件，此外还应归功于其功率密度的优势。

Hilton认为大型飞轮能量供应装置与内燃发动机可以产生协同效应。为了研究如何实现该效应，他正与几家欧洲汽车制造商合作。“传统上，飞轮的功能是加速，而发动机的功能则是保持匀速前进。但飞轮也能给车辆提供动力、启动发动机、或是为发动机补充动力。为了确保这种加速持续可行，在再生能量不够的时候，必须使用特定装置对飞轮的加速进行高效的控制。”

Hilton称，Torotrak在该领域中的研究已经推翻了混合动力装置只适用于城市用车的理论。该研究完成了多次场地实车测试，并记录了多种类型的驾驶员、汽车和道路数据。由该数据得出的精确模拟结果显示，飞轮混合动力装置具备非常明显的优势，即便在野外驾驶中也毫不逊色。

发动机在稳定巡航状态下的负荷较轻，因而制动燃耗（BSFC）很难达到最佳值。但在为飞轮储能的时候，发动机的负荷比巡航状态稍微高一些，因此运行效率也更高，Hilton解释说。一旦飞轮完成蓄能，便可关闭发动机，利用飞轮储蓄的能量来驱动汽车。随后不断循环重复这一过程。

沃尔沃的Flybrid展示车辆，便使用了这一“增速后巡航”的方法，在真实驾驶测试中，该车型的燃油效率比只使用内燃机的同车型车辆高出了25%。Hilton打了个比方，同样一辆2.0L的轿车在24km（15英里）的郊外道路上行驶时，搭载飞轮混动装置的那一辆的制动燃耗可以从470g/kW·h降至280g/kW·h。

在美国FTP75驾驶循环测试中使用了一辆1000kg（2200磅）的B级车，该车搭载了0.9L30kW（40hp）发动机，并配有机械飞轮系统。燃耗结果显示，其二氧化碳排放量为58g/km，相当于2.5L/100km。Hilton表示，该结果符合2030年的欧盟减排目标与2035年的美国目标要求，而厂商无需为任何新技术投入成本并承担相应的风险。他还补充，使用Flybrid技术，现在就能实现这种汽车的量产。

他同时表示，机械飞轮混动装置还有其他潜在优势，包括使发动机的自动减速更容易实现。这会使得各种重要技术突破变得更容易，其中最显著的优势为：一台转速减半的发动机因摩擦而造成的能源损失，仅为原来的四分之一。

但是自动减速会降低尾气中释放的能量，而该能量本来应该用于为涡轮增压机增压。但是，飞轮所释放的能量将超越涡轮增压机的损失，足以实现标准扭矩，因此无需考虑昂贵的双涡轮增压器方案。

Torotrak于2007年开始进行Flybrid系统的研发工作，Hilton说，他们的研究中涵盖了先进飞轮技术。先进的碳复合材料结构，使得飞轮可以在高达60,000rpm的转速下安全旋转。当速度以平方数级方式增加的时候，相应储能将高达4倍。有趣的是，如果使用质量更大的飞轮材料（如钢材料）反而会降低安全运行的速度，而此时储存的能量也会降低。

为了满足SAE J1240标准的安全要求，一台钢制飞轮的最低起动转速必须是最高运行时的2.6倍。由于不能超过安全工作压力限值，这一要求将使一台与Torotrak飞轮尺寸相当的钢制飞轮的转速只能限制在20,000rpm左右。

他还指出，碳结构的安全性能从根本上优于钢制结构。因为它是一种单纤维的散绕结构，每一分层都能产生又长又轻的纤维，既容易控制，也能更高效的传递能量。

Torotrak机械混合动力科技的另一法宝为离合飞轮变速箱（CFT），该变速箱将飞轮内置于传动系统中，同时使飞轮转速可以独立控制 不受发动机转速影响。因此，飞轮可以在不影响到发动机速度的前提下，在制动时通过能量传递实现增速。此外，飞轮还可以在（定速）巡航与加速过程中释放能量来驱动汽车。

下一步：变速箱整合

CFT 技术本来是为Flybrid动能回收系统的竞赛项目而研发的，但它现在同样可以提供Hilton称之为“杰出的”反应时间。只需轻踩制动踏板，飞轮便能迅速储能。在不需损耗电池储能的前提下（通常在快速储能时，电池的储能能力都会受到损耗），能量传输率可以达到非常高的水平。

此外，传动系统的结构也能在瞬间提供扭矩，从而实现即时的加速器反应，其中包括突然降速——纯电动系统的强项之一。

无论具有多么诱人的潜力，所有先进的汽车系统技术都必须具备合理的成本，这是OEM和最终用户对混动系统的一大期望。目前为一辆普通的混合动力汽车配备高压系统、电池组和控制系统的成本会比一辆只配备内燃机的相同汽车高出20%左右。而Hilton 相信，为一辆高容量(high volume)汽车配备Flybrid系统的成本还将“大大”减少。

这一点非常重要，因为Torotrak的下一个研发阶段是将该系统整合进变速箱，使结构变得更紧凑，最终能够实现在高容量（High Volume ）乘用车中的应用。如果外罩、冷却系统和润滑系统的部件可以共享，那么便能极大地缩小尺寸（这意味着大约减少30%的部件）、降低重量（减重30%左右）以及单位成本。

Hilton非常看好飞轮技术的未来，他表示：“实现发动机小型化后，可以留出足够的空间，使飞轮系统整合进目前的传动系统。让OEM可以更方便地应用该技术。”

A decade ago, the current global requirements for future new-car CO₂ emissions—USA 2025, 93g/km; EU 2020 95g/km (possibly 70g/km by 2025); China 2020, 117g/km; and Japan 2020, 105g/km—would have seemed like numbers from fantasyland. What's more, the challenges that these figures herald are compounded by the “get real” demands to move away from “official” but often unrepresentative published fuel consumption figures.

The Worldwide harmonized Light Vehicle Test Procedure (WLTP) scheduled for introduction in 2017, is aimed at providing car buyers with fuel consumption figures that will allow them to budget with confidence when choosing a vehicle. The WLTP is set to change OEMs’ approach to achieving emissions/fuel consumption compliance. “Cycle beating” strategies will be out, reality will be in, and new test procedures will be a must.

Jon Hilton, Product Development Director of Torotrak, a U.K-based specialist powertrain technology company, believes that keeping pace with the new developments in legislation will require not just incremental advances but a step-change in real-world vehicle efficiency performance to deliver required lower CO₂and other pollutant (such as particulates) emission levels.

The challenges are most likely to be met by redefining the role of the ICE (internal combustion engine), he said. “Today’s mild hybrids may typically consist of a 120-kW combustion engine with a 15-kW hybrid power source because this is the most affordable combination, but reaching future targets will require an opposite approach, with something like a 30-kW engine and a 100-kW electric drive. But that does not mean demoting the ICE to become a mere range extender.”

The weight, cost and packaging challenges of producing such an arrangement with electric hybrid power will provide an opportunity for a mechanical kinetic energy recovery system based on high speed flywheel technology, noted Hilton who has been involved in some 20 projects that have used the technology. Torotrak’s Flybrid energy recovery system uses a mechanically-driven flywheel to capture kinetic energy during braking and efficiently return it to the wheels (http://articles.sae.org/12401/).

Enabler for engine downspeeding

Flywheel hybrids are typically around a third of the weight and a quarter of the cost of an equivalent electric system thanks to their use of familiar and easily manufactured components, and to their inherent power density, he explained.

Hilton has identified the synergies that can exist between a large flywheel energy source and a combustion engine. To explore how they could be harnessed, he is working with several European vehicle manufacturers. “Fundamentally, you use the flywheel for acceleration and the engine for cruising," he said. "The flywheel can also power the car, start the engine or supplement engine power for additional performance. To ensure that the expected acceleration is consistently available, the control system uses the engine to efficiently spin-up the flywheel when there is insufficient regenerated energy.”

He claims that Torotrak’s research in this field has disproved the theory that hybrids are only effective in urban driving. Extensive field trials with data logging covering many types of driver, vehicle and route, had enabled accurate simulation to be achieved with the data showing a clear benefit with flywheel hybrids, even when driving on the open road, he said.

Under steady cruise conditions, when the engine is lightly loaded, BSFC (Brake Specific Fuel Consumption) is rarely at its optimum value. While charging the flywheel, the engine is placed under slightly greater load and therefore operates more efficiently, Hilton explained. Once the flywheel is energized, the engine is switched off and the stored kinetic energy released to power the vehicle, then the cycle is repeated.

This ‘boost and cruise’ approach has contributed to Volvo’s Flybrid demonstrator achieving a 25% fuel efficiency improvement in real world driving, compared to an equivalent pure ICE powertrain. Hilton instanced one case, using a 2.0-L sedan on a 24-km (15-mi) cross-country route, when the predicted BSFC improved from 470 g/kW·h to 280 g/kW·h when charging the flywheel.

Simulation of a 1000-kg (2200-lb) B-segment car with a 0.9-L 30-kW (40-hp) engine mated to a mechanical flywheel system showed 58 g/km CO₂, equivalent to 2.5L/100 km fuel consumption, on the U.S. FTP75 drive cycle. This would satisfy the proposed 2030 EU targets and 2035 U.S. targets without requiring the cost and risk of any new technology, Hilton claimed, adding that such a car could be built for production today using the Flybrid technology.

Potential collateral benefits of mechanical flywheel hybrids will bring various additional advantages including making engine downspeeding easier to achieve, he said. This would facilitate significant further improvements; notably an engine running at half its original speed suffers only a quarter of the original friction losses.

But downspeeding reduces the energy in the exhaust available to spool-up a turbocharger. However, releasing energy from a flywheel would overcome this turbo lag, providing the necessary in-fill torque without recourse to a probably costly bi-turbo solution.

Torotrak’s Flybrid system, on which R&D began in 2007, is described by Hilton as incorporating advanced flywheel technology. Its advanced carbon composite construction allows the flywheel to spin safely at speeds up to 60,000 rpm. As energy increases with speed squared, so doubling the speed stores four times the energy within the same package. Ironically, using a flywheel material with greater mass, such as steel, would actually reduce the safe operating speed to a level where the stored energy would be lower, he explained.

To meet the safety requirements of the SAE J1240 standard, the minimum burst speed of a steel flywheel must be 2.6 times the maximum operating speed. To keep within safe working stresses would limit a steel design similar in size to that of the Torotrak flywheel, to around 20,000 rpm.

Carbon construction has a fundamental safety advantage over steel, he noted. Because it is filament wound, any delamination generating long, lightweight fibers that can be easily contained, and which dissipate energy more effectively.

The other key element in Torotrak’s mechanical hybrid technology is the clutched flywheel transmission (CFT) that integrates the flywheel into the powertrain while allowing flywheel speed to remain independent of engine speed. So the flywheel can increase in speed through energy transfer under braking without influencing engine speed. Energy can also be released to the vehicle during the (constant-speed) cruise as well as during acceleration.

Next phase: Transmission integration

After being developed for Flybrid Kinetic Energy Recovery System racing projects, the CFT also offers what Hilton termed “exceptional” response time. This allows the flywheel to be rapidly charged from even a brief touch of the brake pedal. Energy transfer rates can be very high without the degradation of storage capacity suffered by batteries that are subjected to rapid charging.

The powertrain architecture also provides virtually instant torque for immediate accelerator response including rapid step-off, a strong point of pure electric power systems.

All advanced technology automotive systems, however impressive their potential, must be totally cost effective—a major aspect of hybrid credibility in both the OEM’s and the end-user’s analysis. A regular electric hybrid, with its use of a high voltage system and battery pack and controls, may add some 20% to the cost of an equivalent ICE only model. The Flybrid system applied to a high volume vehicle would be “significantly” less, believes Hilton.

This is particularly significant, as Torotrak’s planned next development phase is to see its system integrated into a gearbox, giving it a compact architecture that could eventually lead to high volume passenger car applications. Sharing components such as casings, and cooling and lubrication systems, there would be a substantial reduction in size (~30% fewer parts), weight (also ~30%) and unit cost.

Hilton is very bullish about flywheel technology’s future: “Engine downsizing potentially releases enough space to integrate the flywheel system within the existing package of today’s typical powertrains," he argues. "That makes it much easier for OEMs to introduce the technology.”