今年，伊顿 (Eaton) 在德国 (汉诺威) IAA商用车展 (IAA Commercial Vehicles show) 期间展示了一系列适用于重型柴油发动机的余热回收及可变气门正时技术，这反映了伊顿在节能减排方面的强大实力。

在今年的车展上，伊顿同时展示了间接和直接两个类型的余热回收系统，即基于有机郎肯循环 (ORC) 的间接回收系统和基于“电气化”设计的直接回收系统。其中，间接系统是利用热交换器搭配废气系统的组合，间接回收余热；“电气化”系统则通过由废气驱动的罗茨压缩机搭配电机的组合，直接回收能量。

ORC余热回收系统可以获得5%左右的燃料经济性提升，但需要配置一个小型外燃活塞发动机 (ECPE)，系统成本较高。伊顿公司则基于车辆现有流体着手设计ORC系统，从而避免引入新的流体循环系统，增加复杂度。公司第一个考虑的就是发动机冷却系统中的乙二醇流体。

正如伊顿车辆集团高级工程总监Larry Bennett所言，冷却液已经从发动机的冷却循环中吸取了相当多的热量，那还有潜力从废气中吸取更多热量，并最终为我们的回收贡献更多能量吗？“现在，我们绝大多数的ORC余热回收系统均是基于车辆现有零部件设计的。”Bennett表示，“我们已经拿到了美国能源部的经费。另外，伊顿还有很多重量级合作伙伴，共同完成真正的测试工作，比如帕卡公司 (Paccar) 、壳牌 (Shell Oil) 和密西西比州立大学 ( Mississippi State University ) 等。”

 “壳牌正在帮我们研发一款符合要求的流体。我们要先看该流体能否收集足够的余热，再看能否实现转化，最后再加以应用，判断到底能否真正行得通。”

伊顿的一名研究科学家表示，柴油废气液 (DEF) 搭配AdBlue尿素氮氧化物尾气后处理系统的组合可能是一种非常理想的ORC余热回收解决方案。

 “这里的思路完全一样，也就是通过沸腾DEF流体开始进行朗肯循环。”Bennett解释道，“这个过程中会产生一个有趣的副产物--氨气。当你进行加热时，氨气就会出现，而当你再次冷却时，氨气却不会立刻回到液态，我们就可以把这些氨气储存起来。”

一旦发动机冷却下来又重新启动时，氮氧化物的产生和处理就成了一个问题。“如果你立刻重新启动发动机，则可能产生1吨的氮氧化物气体，但却没有足够氨气进行处理。”Bennett表示，“你必须要让废气系统的温度达到250°C，这样才能让液氮汽化，从而开始处理氮氧化物。”

系统中存储的氨气必须足够在发动机启动15分钟内处理氮氧化物，直至发动机上升至正常工作温度，即让液氮开始汽化为气体。Bennett表示，“我们还需要进行大量的研究、模拟和测试，但这看起来似乎可行。”

直接余热回收系统需要在紧邻排气歧管的位置安装一个罗茨压缩系统，并利用废气气流驱动压缩机转子转动。短期测试结果显示，这种方式可以重新回收能量。

Bennett表示，“最初的概念是为系统连接一个电机以回收能量，而后再将回收的能量存储在电池系统中。”这的确不是最有效的方式，但的确可行。伊顿的研究科学家认为，这种系统可以加快废气循环的速度，并不会受到发动机转速的限制，因此可以在需要高速废气循环时起到泵的作用。

伊顿之前曾开发过一款电子辅助可变速率超级增压器，适用于汽油发动机。“通过这种设计，我们可以在不受发动机转速限制的情况下，获得不同程度的发动机增压。”Bennett解释说，“对于柴油发动机而言，这种设计在控制尺寸和提供瞬时扭矩方面有很多优势。最关键的就是可以不受发动机转速限制，控制气流和废气流速。”对发动机厂商来说，这种功能非常具有吸引力。

伊顿声称，在我们的模拟测试中，直接余热回收系统已经展示了22%的燃料经济性提升，且同时还能减少氮氧化物的排放。

现阶段，可变气门驱动技术尚未在柴油发动机领域得到广泛应用，但的确具有多种优势。可变气门驱动可以起到发动机压缩制动的作用，提前或延后关闭进气阀均可以降低燃烧温度和氮氧化物的排放，也就是说，可以提高效率。

 “你可以在某个瞬间提前关闭排气阀，从而加速涡轮增加器的增压。”伊顿高级气门工程经理Majo Cecur表示，“你也可以在轻载状态下进行气缸钝化，从而提高燃料经济性。”

伊顿正在研究多项气门操作设计，并寻求通过这些设计使能更多功能的可能性。

Eaton demonstrated a range of waste heat recovery (WHR) technologies for heavy-duty diesel engines at the IAA Commercial Vehicles show in Hanover, Germany, as well as variable valve timing systems, highlighting their potential to reduce fuel consumption and help reduce emissions.

Both indirect and direct WHR systems were on display. The organic Rankine cycle (ORC) design recovers waste heat indirectly using a heat exchanger with the exhaust system. Alternatively, Eaton’s “electrified” system recovers energy directly using an exhaust-driven Roots compressor in conjunction with a motor/generator.

ORC WHR systems can yield fuel-economy improvements of around 5%, but system cost is high, involving a small external combustion piston engine. Eaton looked at what fluids were already carried on board the vehicle to avoid adding another for the ORC WHR system. The first fluid the company considered was ethylene glycol, already used in engine coolant systems.

As Larry Bennett, director of advanced engineering, Eaton Vehicle Group, observes, the fluid would already be hot from use as an engine coolant, but would it have the potential to add more exhaust heat to it and extract more energy? “Now we can create most of the ORC system with existing componentry,” said Bennett. “We now have a U.S. Department of Energy (DOE) grant. We’re working with Paccar, ShellOil and Mississippi State University, which is where all the actual testing will take place.

“Shell is working on a fluid that has the capability to do this for us. The idea is to see if we can achieve enough waste heat and form this face transition, then utilize it and see if it’s going to work.”

One of Eaton’s research scientists suggested that diesel exhaust fluid (DEF)/AdBlue urea solution for exhaust aftertreatment of oxides of nitrogen (NOx) would be an ideal ORC fluid.

“The idea is exactly the same—you perform the Rankine cycle by boiling the DEF fluid,” explained Bennett. “One of the interesting byproducts is that after you heat it up, ammonia gas comes off and when you cool it down, it doesn’t want to readily return to the liquid state. We can store that ammonia gas.”

Once the engine has cooled down and is restarted, NOx output and treatment is an issue. “If you start it back up, you’re producing a ton of NOx because you don’t have ammonia gas to be able to treat it,” said Bennett. “You need the temperature in the exhaust system to get up to 250°C in order to take the liquid ammonia you’re injecting to get it to vaporize so that you can treat the NOx.”

The ammonia stored from the evaporated DEF should be enough to treat NOx for the first 15 minutes as the engine comes up to operating temperature. “It’s all research, all modeling simulation, but it appears feasible,” he said.

The direct WHR system involves fitting a Roots compressor system right next to the exhaust manifold and using the exhaust gas flow to drive the compressor rotors. Short-term testing shows that the energy can be recaptured.

“The initial concept is to have a motor/generator hooked up to it, then basically take the energy and put it in a battery,” said Bennett. This is not the most efficient way to recapture energy, but it offers another possibility. Eaton’s research scientists believe that this system could be used as a pump when there is a need for high rates of exhaust gas recirculation (EGR). The system could potentially deliver high rates of EGR independent of engine speed.

Eaton had previously developed an electronically assisted variable speed supercharger for use with gasoline engines. “In this application, we can now vary, independent of engine speed, the amount of boost that the engine can get,” Bennett explained. “On a diesel, that has a lot of advantages in the form of downsizing and instant torque. The big thing would be to manage airflow and exhaust flow independent of engine speed.” That capability would be highly attractive to engine manufacturers.

The direct WHR system has shown through simulation a 22% improvement in fuel economy while reducing NOx, Eaton claims.

Variable valve actuation has not been widely used so far with diesel engines, but there are a number of potential advantages. The first is a compression engine brake. Early intake valve closing and late intake valve closing could reduce combustion temperatures and NOx, or improve efficiency.

“You could have early exhaust valve closing for a transient to give faster boost in a turbocharger,” said Majo Cecur, engineering manager, advanced valvetrain, Eaton Vehicle Group. “You could de-activate cylinders in light load conditions so that you could have better fuel efficiency.”

Eaton is investigating valve operation designs that would enable a range of such functions.

Author: John Kendall
Source: SAE Truck & Off-highway Engineering Magazine