我们在美国高速公路上常看到的重型牵引卡车，其燃油经济性非常低，油耗通常高达5.8mpg(40.5 L/100km)。一项由美国能源部支持的先进技术——康明斯-彼德比尔特超级卡车项目（Cummins – Peterbilt SuperTruck）却在去年的测试中其油耗数值仅为10.7mpg (21.98L/100km)为常规车的一半。如果所有的重型卡车的油耗能向超级卡车靠拢，那全美每年可以节省近3亿桶燃油，相当于节约150亿美元，每辆8级货卡每年可以节省1万美元的燃油开支。

尽管半拖挂卡车仅占全美汽车总量的4%，但却烧掉了大约20%的燃油，因此提卡车的燃油经济性能够显著减少CO2的排放量。

该原型车是由康明斯-彼德比尔特公司为SuperTruck项目的开始，该项目为政府与行业间的合作项目，耗资7800万美元研发周期长达4年，旨在研发下一代低油耗高效的柴油半拖挂卡车。美国能源部希望能进一步提升发动机系统的效能正着力推进后续项目。

Wayne Eckerle，自称“燃烧小子”，康明斯集团研发科技副总裁表示，“SuperTruck项目融汇了我们将过去研究的数项先进发动机技术并得了可行性验证，让我们受益良多。”

SuperTruck的动力系统达到了美国能源部所设定的目标，发动机系统峰值制动热效率达到50%。Eckerle表示：“达到50%相当不容易，现在的柴油机大约只能达到43%。”

作为全美唯一的独立柴油发动机制造商，能源部将在2年内拨款450万美元资助康明斯，希望将热效率在现在基础上再提升5个百分点至55%。Eckerle表示，“我们的目标证明是在真实的工作环境下，效率还有进一步提升的空间，我们会借助SuperTruck项目的成果作进一步的研究工作。”

根据美国能源部的资料显示，该《重型发动机使能技术项目》是由康明斯与政府间平摊经费的研究项目，旨在“通过SuperTruck项目投资，促进柴油发动机的设计、分析、研发工作，实现发动机系统峰值热效率提升至55%制动热效率（BTE）高效柴油发动机的研发工作，并高度整合燃烧/后处理系统。”

“要想实现55%制动热效率（BTE）并没有什么特效药，”Lyle Kocher，康明斯先进系统集成技术顾问，兼柴油机55BTE项目首席研究员提醒道，“既要提高燃油经济性，但又要符合排放的相关法规，我们必须多管齐下。”

康明斯团队计划采用新方法来微调燃油燃烧过程、优化燃油与空气处理系统、改进排放系统、减少发动机主体与涡轮增压器的寄生损失、并增加底循环来回收废热。

改善燃烧

Kocher通过观察后认为，改善燃油过程或将是提高整体效率的关键之所在。燃料的燃烧过程非常复杂，燃料在柴油发动机内的燃烧过程中，伴随着2,000次同时或连续发生的化学反应。据报道，为了优化柴油机的燃烧模型，康明斯每年都在商用计算机代码的授权许可上就要花掉100多万美元。

Kocher表示，研发团队使用了多种先进燃烧策略来优化热释放率，在降低氮氧化碳物的排放量的同时保持低温燃烧，但在效率与排放之间我们必须要有所取舍。

“我们希望尽可能的缩短燃烧时间来减少传热损失”，Kocher说，“特别是控制燃烧曲线的形态，我们希望延长燃烧进程的前半段，缩短后半程。此外，我们还希望通过隔热涂层和其他方式来降低在气缸内的热损耗。”

与其他的柴油发动机一样，55%BTE发动机从少油燃烧点火循环开始提升燃油经济性，并通过低速运行降低机械损耗，提高压缩比、在稀释燃油条件下的燃烧率，摒弃节气门，通过优化空气-燃油比控制系统进气避免泵送损失，以此来提柴油的燃烧效率。

Echerle补充道，“在达成这一目标的同时，工程师还必须注意峰值缸压，并避免产生噪音。”

除了采用更高的喷射压力、更精准的雾化控制及多次喷射等最新燃油喷射技术外，更灵活的阀门控制、优化发动机的换气也是提高动力系统效率的关键之所在。“气缸无论是在进气还是排气时效能都会有所损失，”Eckerle强调到，“我们必须尽可能的减少此类泵损失。”

这台原型发动机主要利用废气再循环装置（EGR）来降低燃烧温度，Kocher表示“通过EGR来降低温度控制NOx的排放是其最重要的方法”，此外它还能用来辅助控制泵损失。

提升涡轮增压效率

“我们使用涡轮增压器来将效率最大化，” Kocher表示。“相较于低压，高压发动机的运行更高效。”涡轮增压不仅能提升功率密度，而且还能回收部分废气余热。

为了使涡轮增压器更高效，康明斯正打算缩小其叶片和外罩间的缝，“我利用完整的计算流体力学与反应分析以及仿真技术优势，来针对涡轮在循环的瞬态条件下气流的脉动情况进行建模。”

“在过去，其实也不过就是3年前，我们还做不到这一点”，Kocher说。了解压力系数和其他细节信息后，“我们可以更好地设计涡轮增压器的架构。我们对整体设计进行了优化，充分利用每个循环过程中的气流脉动，在此之前都些都被忽略了。”

为了尽可能降低热能损失并提升整体功率，发动机系统设计中还应考虑相应的冷却方案，可变气流冷却泵可以解决寄生泵损失的问题。同时，能过采用涂层及其他技术来减少能量输送过程中所产生的不可避免的摩擦损耗。

余热回收

柴油机55BTE团队计划采用有机朗肯循环(ORC)系统，回收EGR系统、尾气中余热，并将其转换成有用的能量。该系统包括热交换器、载热冷媒、扩展器、泵以及冷凝器，将直接通过涡轮扩展器与发动机进行机械耦合。

余热回收系统将作为发动机的底循环。Eckerle表示，“康明斯针对这个课题已经研究了一段时间,”在早期的实验报告中，在理想环境下EPA 2010发动机采用EGR系统后燃油经济性得到了7.4%以上的提升，“但仍有上升空间。”

最后，研究团队还希望将尾气后处理系统与发动机紧密耦合，尽可能避免热量损失。Kocher表示他很期待迎接这一挑战。

作者：Steven Ashley
来源：SAE 《非公路工程杂志》
翻译：SAE 上海办公室

Cummins aims to boost heavy-duty diesel efficiency to 55%

The big-rig tractor-trailer trucks that we see on the highway get only about 5.8 mpg of diesel fuel. In tests, the Cummins-Peterbilt SuperTruck—a U.S. Department of Energy-supported advanced technology demonstrator unveiled last year—achieved nearly double that number: 10.7 mpg. If all the heavy-duty trucks in the U.S. were as efficient as the SuperTruck, domestic consumption of oil would fall almost 300 million barrels, a potential $15-billion savings that would reduce the annual fuel outlay of the average Class 8 operator by perhaps $10,000.

And although semitrailer trucks comprise only 4% of the vehicles on America roads, they consume about 20% of the fuel, so improved fuel economy would also cut emissions of CO2 significantly.

The Cummins-Peterbilt prototype was developed and built as part of the SuperTruck Initiative, a four-year, $78-million government-industry collaboration to develop a next-generation diesel semi-truck with greatly improved fuel consumption. Now the DOE is back with a follow-on project that aims for even better engine system efficiency.

“We learned a significant amount in the SuperTruck program,” said Wayne Eckerle, Vice President of Corporate Research & Technology for Cummins, a self-described “combustion guy.” “It gave us the chance to demonstrate the feasibility of several advanced engine technologies that we’d been working on previously and integrate them into an operating system.”

The resulting SuperTruck powertrain achieved the DOE’s target goal of a peak diesel engine system brake thermal efficiency of 50%. “That wasn’t at all easy,” he stressed, noting that “diesels today are probably 43% efficient.”

The Energy Department recently awarded Cummins, the nation’s only independent diesel engine maker, a two-year, $4.5-million grant to boost its previous mark by 5 percentage points to 55% brake thermal efficiency, Eckerle said. “Now we’re aiming to demonstrate another substantial increase in efficiency in a real-world duty cycle, an effort that leverages and carries forward what we were doing on the SuperTruck project.”

The Heavy Duty Engine Enabling Technologies Project, a 50-50 cost-shared R&D endeavor, aims to “leverage the design, analysis and development work that has been invested through the Cummins SuperTruck program to demonstrate a peak diesel engine system efficiency of 55% Brake Thermal Efficiency (BTE) while also implementing an advanced, highly integrated combustion/after-treatment system,” states DOE documents.

“There is no magic bullet to get to 55% BTE,” warned Lyle Kocher, technical advisor for advanced system integration at Cummins and principal investigator on its Diesel 55BTE project on a team that includes 20 dedicated engineers. “Reaching new fuel efficiency levels while complying with all the emission limits means that we’ll have to use multiple strategies.”

The Cummins team plans to apply new ways to fine-tune the fuel combustion process, optimize both the fuel and air handling systems, modify the emissions system, reduce parasitic losses in the base engine and the turbocharger, as well as to add a bottoming cycle to recover waste heat.

Better burning

Probably the largest contribution to any overall efficiency gains will derive from improving the combustion process, Kocher observed. Various fuel combustion tweaks can better the complex fuel-burning process, which in a diesel engine entails some 2000 simultaneous and sequential chemical reactions. To improve its combustion modeling, the diesel maker reportedly licenses commercial computerized code at a cost of more than $1 million a year.

The team, the mechanical engineer said, will implement several advanced combustion strategies that will optimize heat-release rates, but still retain burning at reduced temperatures for low nitrogen oxide (NOx) emissions. “There can be a trade-off between efficiency and NOx emissions,” Kocher acknowledged.

“We want to minimize the duration of the burn to reduce heat transfer loss,” he said. “In particular, we want to control the rate shape; that is, we want to slow the front end of the combustion process and speed up the back. We also want to minimize heat losses to the cylinder by using insulating coatings and other approaches.”

Like other diesels, the 55% BTE engine will derive its basic efficiency from its fuel-stingy combustion-ignition cycle and fewer mechanical losses due to lower-speed operation. The cycle’s efficacy in burning diesel arises from high compression ratios, high combustion rates under lean conditions, and the use of air-fuel ratios to control system loading rather than throttling to avoid part-load pumping losses.

Added into all the other considerations, Eckerle said, the engineers must accomplish this goal without “exceeding peak cylinder pressures or producing noise.”

Besides implementing the latest fuel-injection techniques—which might involve higher injection pressures, finer spray control, and multiple injection events—flexible valve control and enhanced engine breathing are also key to boosting powertrain efficiency. “We need to minimize pumping losses,” Eckerle emphasized. “Whenever we have to push gases back into the cylinder or draw them in that costs us work.”

The prototype engine will rely primarily on exhaust gas recirculation (EGR) to reduce combustion temperatures. “EGR is critical way to control NOx by keeping the temperature low,” Kocher said. It also aids in controlling the pumping losses.

Turbo boosts efficiency

“We use turbochargers to maximize efficiency,” he said. “High-pressure engines run more efficiently than low pressure.” Turbocharging raises power density and recovers some of the wasted exhaust heat.

Cummins is engineering more effective turbochargers with smaller gaps between the blades and the housing, Eckerle said. “We’re taking advantage of full CFD and reaction analysis and simulation techniques to model the turbo down to brief in-cycle transient conditions—essentially, pulsations in the flow.”

“In the old days—really, only 3 years ago—you couldn’t do that,” he said. Knowing the pressure coefficients and other fine details “allows us to do a much better job of designing the turbo architecture. We optimize the general design to take advantage of the pulses within each cycle, something that we kind of ignored before.”

They said that the engine system design should also feature strategic cooling to minimize thermal energy losses and augment overall power. Parasitic pump losses will be addressed in part with variable-flow cooling pumps. Likewise, friction losses inherent in the power transfer process will be mitigated with sliding friction-reducing coatings and other techniques.

Waste heat recovery

The Diesel 55BTE team is employing an organic Rankine cycle (ORC) to capture waste heat from the engine EGR system as well as the charge air and exhaust streams, and convert it into useful work. The system, which will include heat exchangers, a heat-carrying working fluid/refrigerant, expanders, pumps and condensers, will be coupled to the engine mechanically via the turbine-expanders.

The waste heat recovery system will serve as a bottoming cycle for the engine. “It’s been a subject of research here at Cummins for quite some time,” Eckerle said. In earlier reported tests, fuel-economy benefits greater than 7.4% were demonstrated when coupled with EPA 2010 engine system under ideal conditions, “but there’s considerable room for improvement.”

Finally, the team wants the exhaust aftertreatment system to be close-coupled to the engine to avoid undue heat losses, said Kocher, who concluded by saying that he was looking forward to the challenge.

Author: Steven Ashley
Source: SAE Off-Highway Engineering Magazine