当大众汽车（Volkswagen）埋头专注于满足美国与欧洲的燃油排放标准要求时，通用汽车在就已经将发展重心转移到了小型涡轮增压柴油机乘用车市场。当马自达（Mazda）正在验证其2.2升Skyactiv-D发动机，为CX-5（预计2018年春发布的新款）做准备时，通用汽车（GM）也同样希望能够填补这一市场空白。事实上，作为2018款雪佛兰Cruze and Equinox所选用的发动机，这款造价2,200美元的1.6升涡轮增压柴油机4年前就在意大利完成了设计，之前还曾在底特律进行校准，而目前则正在匈牙利进行制造。

低油耗是柴油动力系统的一大优势，这在公路行驶工况下尤为明显。根据美国环保署（EPA）公路测试标准，在搭载通用与福特合资的9T50 9级自动变速箱时，Cruze柴油机的油耗为52mpg(4.52L/100km)。两轮驱动的6级自动变速Equinox油耗为39mpg(6.03L/100km)，而该车型的四轮驱动版油耗则为38mpg(6.19L/100km)。由于Equinox整备质量优势明显，在真实驾驶工况下，9级变速相对于6级变速的燃油经济性优势并不明显。

获得SAE认证的“flüsterdiesel 安静发动机”

这台SAE认证的功率为102千瓦（合137马力）的发动机，是通用全球MDE家族的一员。发动机转速在每分钟1500至3250转之间时，可达到理论峰值扭矩的90%，即325牛·米（合240磅·英尺）。在《汽车工程》杂志（Automotive Engineering）的道路测试中发现，这台发动机的噪音非常小。德国汽车行业的撰稿人率先给它起了个别名叫做“der flüsterdiesel”，翻译过来就是“安静的发动机”。当你站在怠速状态的发动机旁时，几乎听不到任何的震颤声。

这款发动机的汽缸盖、汽缸体和汽缸座都是铝制的，压缩比为16:1，采用固定气门正时技术。发动机的性能和排放表现都有明显提升，这主要归功于位于发动机轴上的旋转蝶阀。通过从这一装置，每个汽缸都能增加一次进气。博格华纳的可变形涡轮增压量最高可达290千帕（合42 psi），采用空气对空气的冷却方式，可让加速更顺畅。同大多数柴油机一样，这款发动机在快速空转时可以实现最大扭矩，使汽车启动时体验良好。

尽管顶部盖有一层吸音棉，喷油器和共轨装置也覆有吸音棉，并采用了翻盖式歧管，但要最终要成就这样一款低噪音发动机，还是要依靠优秀的工程设计。这台1.6升的发动机里，装有10步螺线管喷油器（每个喷油器有8个孔），定时齿轮和定时链的位置都靠后，这样钟型外壳和传动轴就能够吸收定时齿轮或定时链带来的噪音。

通用全球推进系统区域总工程师Mike Siegrist表示，该款多重喷油器更趋近于压电式喷油器。不过，他也透露，如果真的采用压电式喷油器的话，那么在1.5升基本款的燃油发动机里，每个孔的造价都要再贵上30美元。这款新一代螺线管喷油器的脉动频率可以达到之前的2倍，当系统在2,000 bar（合29,000 psi）的压强下工作时，性能可以接近压电式喷油器。

螺线管喷油器的多次脉冲喷油方式

螺线管喷油器起初进行预燃烧喷油，接下来会进行很多次高频预点火尝试，以缩短延迟时间，之后才是燃烧喷油。这些措施可减少氮氧化物的生成，并降低噪音或振动。接下来则是燃烧—点火脉冲，之后是燃烧后脉冲，用以控制蒸汽温度，尽可能多地充分燃烧微粒，并进一步减少尾气排放。最多可达十次的脉冲过程，最终可以实现以上所说的结果。

Siegrist解释说，废气再循环（EGR）系统包含液体冷却，尽管柴油发动机一般是在非节流状态下运转的，1.6升的Ecotec发动机里却装有一个电子节流阀以此来提升歧管的压力，以增加EGR空气流动，成为发动机控制系统的一部分。该系统将后燃烧选择性催化还原（SCR）与尿素喷射合在一起，进一步控制氮氧化物的排放。此外，这一系统里还包括了一种氧化催化剂和带有再生功能的颗粒捕集器。很明显，通用正在不遗余力地进行研发，以期达到美国对于该类发动机的认证标准要求，确保低排放。

搭载这款发动机的汽车都有一个18.5升（合4.9加仑）的容器来存放尿素溶液（也称为“柴油发动机排放处理液”）。尽管制造商试图将处理液更换的里程数调整到12,000公里以上（合7,500英里），使之接近发动机油的换油期，然而Siegrist还是略显保守地表示，根据最终校准结果来看，1.6升的Ecotec发动机更有可能在行驶8,000至9,600公里（5000~6000英里）左右后就需要补充尿素溶液，因为在这个里程数下，尿素箱检测系统曾多次通过仪表盘发出补充处理液的警告。

怠速启停也是这款发动机的一大优势，而根据《汽车工程》的道路测试显示，发动机的再次启动迅捷而轻巧，甚至让人几乎感觉不到。

When Volkswagen fell afoul of U.S. and European emissions rules, it effectively stripped the U.S. car market of small turbodiesels for passenger vehicles. While Mazda is still certifying its 2.2-L Skyactiv-D for the CX-5 (now expected on sale in spring 2018), General Motors is also eager to fill the void. Its new 1.6-L turbodiesel, a $2,200 option for the 2018 Chevrolet Cruze and Equinox, was engineered four years ago in Italy, is built in Hungary, and was calibrated in Detroit for the hitherto-abandoned market.

Fuel economy, with highway numbers particularly impressive, are diesel strong points. The Cruze diesel is EPA highway-rated at 52 mpg with the GM-Ford joint venture 9T50 9-speed automatic. The Equinox is rated at 39 mpg for 2WD, 38 mpg for 4WD with the 6-speed automatic. Equinox’ greater curb weight precludes any real-world economy advantage for the 9-speed vs. the 6-speed.

SAE-certified "flüsterdiesel"

The engine, part of GM's global MDE family, SAE-certified at 137 hp (102 kW). GM claims 90% of the 240 lb·ft (325 N·m) peak torque is available from 1500 to 3250 rpm. The engine proved extremely quiet during an Automotive Engineering road test. German autowriters first dubbed it der flüsterdiesel, or "whisper diesel." We heard no audible chatter while standing next to the car with the engine idling.

Featuring an aluminum cylinder head, block and bedplate, the engine has a 16:1 compression ratio and fixed valve timing. Performance and emissions are improved by variable intake air swirl from a shaft-mounted lineup of pivoting butterfly valves for one of each cylinder’s two intakes. The BorgWarner variable-geometry turbo machine, with peak boost of 42 psi (290 kPA), uses air-to-air intercooling which smoothes acceleration. Like most diesels, peak torque is at fast idle, providing pleasing launch.

Although there’s an acoustic foam top cover, acoustic foam-covered injectors and common rail; and clamshell manifold cover, quiet design also is engineered-in. The 1.6-L incorporates 10-step solenoid fuel injectors (each with eight holes), and rear location of timing gears and chains, where the bell housing and transaxle absorb gear/chain noise.

That level of multi-injection would be more typical of piezo injectors, admitted Mike Siegrist, GM Global Propulsion's Regional Chief Engineer. But piezo units would add an intolerable “$30 per hole” over the 1.5-L base gasoline engine, he revealed. This new generation of solenoid injectors, with twice the previous pulsation rate, he said, provides near-piezo performance with the system operating at 29,000 psi (2000 bar).

Multi-pulse solenoid strategy

Solenoids start with pre-combustion pilot injection, and follow with a varying number of rapid pre-ignition “shots” for a shortened delay before the combustion injection. These reduce NOx generation and lower noise/vibration. The combustion-ignition pulse follows, and then post combustion pulses to control exhaust gas temperature, burn off maximum amount of particulate and further lower emissions. It’s the up-to-10 pulses that produce the total results.

The EGR (exhaust gas recirculation) system incorporates liquid cooling and although a diesel normally runs unthrottled, the Ecotec 1.6 L has an electronic throttle to raise manifold pressure, to increase EGR flow as part of the engine control system, Siegrist explained.

The system incorporates post-combustion SCR (selective catalyst reduction) with urea injection for further NOx control. There is also an oxidation catalyst and particulate filter with regeneration. GM clearly approached U.S. certification of this engine aggressively to ensure low emissions.

The vehicles have an 18.5-L (4.9 gal) tank of  “diesel emission fluid” urea solution. Although manufacturers try to calibrate for a 7500-mi-plus (12,000 km) range to coincide with the oil change interval, Siegrist conceded that with the final calibrations the 1.6-L Ecotec is more likely to need a tank refill in the 5000-6000 mi (8000-9600 km) range. He explained that the urea tank monitoring system provides continuous instrument panel warnings in plenty of time for a refill.

Idle stop-start is incorporated with both automatics, and AE's road testing showed it’s barely perceptible, with restarts quick and gentle.

Author: Paul Weissler
Source: SAE Automotive Engineering Magazine