马自达的创新 Skyactiv X 火花控制压缩点火 (SpCCI) 燃烧系统预计将于2018年投产，这款发动机是数十年来内燃发动机领域发展的最重要成果之一。SpCCI发动机可以延长汽油发动机的实际使用寿命，这在全球排放规定愈发严格，且电动汽车加速发展壮大的大背景之下尤为重要。

马自达公司董事、高级管理执行官藤原清志表示，未来几十年内，SpCCI 技术将成为公司汽油发动机战略的基础。藤原告诉《汽车工程》，为了实现“零尾气排放”的目标，未来汽车的动力单元将最终采用以微藻类为基础的生物燃料，而 SpCCI 经过专门设计，可以支持更大排量的动力系统。

经过超过 8 年的集中研发工作，马自达的SpCCI 系统终于成型。该系统结合了柴油发动机的能效和扭矩输出优势，以及双凸轮汽油发动机的转速优势，还能同时保证每公里不超过60 克的 CO2 排放水平。除此之外，SpCCI 发动机的生产无需对原材料表单进行大规模变更，主要的改动仅集中在电子控制和气缸头部分。

目前，SAE 已经收到了大量有关奥托循环 (Otto) 柴油发动机的技术论文，各家汽车厂商也进行了大量相关工作，比如：梅赛德斯-奔驰一直在大力研发奥托循环柴油机DiesOtto（见 http://papers.sae.org/2009-01-2701/）；通用汽车在开发均质压燃 (HCCI) 发动机（见http://articles.sae.org/6635/）；本田和现代等厂商也在进行类似的研发（见http://books.sae.org/b-hon-016/）。然而，尽管这些资金充裕的大型汽车厂商一直在关注 HCCI 技术的研发，但截至目前尚没有公司公布任何有关这些新兴燃烧技术的量产计划。

据悉，马自达的最新技术合作伙伴丰田也对 SpCCI 系统兴趣满满，并考虑在公司的主力混合动力电动车产品中应用这项技术。

柴油机般的强劲扭矩输出

最近，在马自达德国 Oberursel 欧洲技术中心举办的一次媒体活动中，笔者有幸试驾了两款 2.0 L SpCCI Mazda 3 原型车（分别为手动挡和自动挡），完成了总长超过 30 英里（48 公里）的测试循环。真实体验证明，火花点火模式(SI) 和压缩点火模式 (CI) 之间难以顺畅切换的长期挑战，似乎已经得到克服。工程师表示，SpCCI原型发动机在低转速时的表现的确有些生硬，但现在距离发动机的最终校准和生产仍有大约 1年时间，还可以进行进一步优化。

可以确定的是：在日常驾驶工况中，SI 和CI 之间的切换已经几乎很难察觉了。本次测试循环的驾驶环境包括高速公路、乡村和城市道路。在整个测试循环中，这款全新SpCCI 发动机转速曾飙到接近6000 转，整体排放水平即使说不上完美，也算是相当不错了。这款发动机有 6挡转速，可轻松降至 1200 转，充分展示了自身如柴油机般强劲的扭矩输出能力。

由于扭矩输出的范围更加宽广，马自达得以更加高效地调整车辆的档位分布，从而进一步改善车辆的燃油经济性并降低排放。

遗憾的是，截止本文发表之时，马自达尚未公布 SpCCI 发动机的孔冲程、额定扭矩、功率等其他技术细节。马自达称，与现行 Skyactiv-G 汽油发动机相比，SpCCI 发动机的能效取得了 20% 到 30% 的提升。然而，从《汽车工程》得到的测试结果来看，情况可能并非如此喜人。在测试循环中，当 SpCCI 模式在整个循环测试中的运行时间超过95% 时，手动挡标配 Mazda 3SKY-G 取得了13.3% 的燃油效率提升，平均燃油经济性从此前的29.4 提升至 34.0 mpg （即 100 公里油耗从8.0 L 下降至6.9 L）。

自动挡汽车的情况也很类似，当 SpCCI 模式在整个循环测试中的运行时间超过 95% 时，自动挡测试车也取得了 14.75% 的燃油效率提升，平均燃油经济性从此前的 29.9 提升至 35.1 mpg（即 100 公里油耗从35.1 L 下降至29.9 L）。

然而，取得这样的数据结果，与该循环相对极端的测试方式也有关系。具体来说，马自达的循环测试主要是为了评估发动机的属性，比如高档位下的低速和正常加速、SI和 CI 模式之间的切换等，而这种情况在“正常驾驶环境”中并不常见。

压缩比：从18:1 到15:1

从 2011 年启动 Skyactiv 全面能效提升项目依以来，马自达已经陆续推出了大量压缩比达 14:1 的发动机。此外，SpCCI 也是 Skyactiv 项目的主要成果之一。根据马自达工程师的数据，全新 Skyactiv-X 发动机的压缩比可达 15.0:1。

“我们之所以将压缩比控制在 15:1 ，是因为这最接近正常环境温度下的压缩点火条件，”马自达动力系统执行官 Ichiro Hirose 解释说，“火花会形成一个膨胀的火球，而这个火球如同一个次级的‘空气弹簧’，可以为发动机提供额外的压缩压力。由于火球是火花塞产生的，因此可以有效控制SI 和 CI 之间的切换。”

Hirose 补充说，“首先，取得超高压缩比是使用稀薄空气燃料混合燃烧的关键突破点；其次，发动机空燃比越低，相应的热比则越高。为了实现这一重大突破，我们至少需要将空燃比提高一倍，从 14.7:1 提高至 30.0:1。”

正如《汽车工程》此前的报道，在 Skyactiv-X 项目的 G1、G2和 G3 阶段，工程师通过将均质压燃系统的压力和温度调整至理想状态，进而将 λ为 2.5 时的发动机压缩比调整至 18:1，取得了 40% 的热效率提升。

马自达 SpCCI 量产发动机取得的一大关键突破是对燃烧过程的精准控制（见 SAE 技术论文 http://papers.sae.org/2015-01-1803/ 点击最左下方阅读原文直达）。SpCCI 发动机的每个气缸均安装了独立压力传感器，可以对温度、压力等发动机参数进行实时监测。发动机管理系统则会控制发动机的双电动可变凸轮轴、空气泵，以及将在500 bar (5252 psi) 时启动的分离喷射系统。《汽车工程》了解到，马自达的空气泵采用了罗茨(Roots) 循环原理，由伊顿公司(Eaton Corp.) 专为马自达SpCCI 发动机研制提供。

事实证明，对于处于 CI 模式下高负荷运转的HCCI发动机，增压系统和废气再循环系统的作用非常显著。但马自达研发管理执行官Takahisa Sori 也表示，在2013 年时，马自达的目标是在更多不同负荷级别的自然吸气发动机中成功实现稀燃HCCI 技术。Sori 的工程师担心，气泵的使用会影响发动机在真实世界中的燃油经济性。

在与《汽车工程》记者的讨论中，Sori表示非常看好HCCI 汽油发动机与混合动力系统的结合，认为在电机的协助下，可以扩大发动机的理想运行范围。发动机完全可以通过小型化电机和电池的运用来降低成本。在此背景下，马自达选择与丰田的混动系统工程师展开合作也就不足为奇了。

SpCCI 发动机将分2 个步骤实现空气和燃料的混合，分别在进气冲程和压缩冲程中进行燃油喷射。此时，燃烧室内将形成一个强劲的漩涡，进而产生燃料密度梯度，即CI 外围的空燃混合物相对稀薄，而火花塞中心处的空燃混合物则较为丰富（有利于火球的形成）。

火花点火可以在高负荷条件下启动发动机，但并不会在任何预设的点切换至压燃模式。Hirose 解释说，当发动机到达进气边界条件时，燃烧室内将产生一个不断膨胀的火球。此时，由于点燃模式提供的额外压缩力，发动机的实际压缩比会高于约15-16:1 的几何压缩比，进而触发CI 模式。

Hirose 同时提到，SpCCI 发动机的制造成本将介于柴油和汽油发动机之间。

One of the most significant developments in internal combustion engine (ICE) technology for decades, Mazda’s innovative Skyactiv-X Spark Controlled Compression Ignition (SpCCI) combustion system is slated for production in 2018. It has the potential to extend the practical life of gasoline engines, which are increasingly under threat from both global emissions legislation and the accelerating development of electric vehicles (EVs).

According to Kiyoshi Fujiwara, a company director and Senior Managing Executive Officer, SpCCI will form the foundation of Mazda’s gasoline-engine strategy until mid-century. Fujiwara told Automotive Engineering that SpCCI is designed to embrace larger-displacement power units that eventually will run on micro-algae bio-fuels to deliver “zero tailpipe emissions” (see sidebar).

The SpCCI system is the culmination of more than eight years of intensive development by Mazda to design a gasoline engine that embraces the frugality and torque of a diesel with the high-revving capacity of a twin-cam gasoline unit, while delivering sub-60 g CO2/km emissions. Adding to the attraction, SpCCI requires relatively minimal investment in the engine bill of materials—electronic controls and a revised cylinder head comprise the major changes.

Creating an ICE with Otto and Diesel attributes is an engineering target discussed in scores of SAE Technical papers and vigorously pursued by Mercedes-Benz with its DiesOtto (see http://papers.sae.org/2009-01-2701/), General Motors with its Homogenous Charge Compression Ignition (HCCI; http://articles.sae.org/6635/) Honda with its similar investigations (http://books.sae.org/b-hon-016/) and Hyundai, among others. But while these larger, better-financed OEMs have focused significant R&D on HCCI, they have thus far stopped short of committing the combustion regime to production.

Mazda’s new technology partner Toyota also is said to be interested in SpCCI, including potential applications for hybrid-electric vehicles (HEVs), long a Toyota specialty.

Diesel-like torque

At a recent media technical background event at Mazda’s European technical center in Oberursel, Germany, the author test-drove two prototype Mazda 3s powered by a 2.0-L SpCCI engine—one fitted with a manual transmission, the other automatic—over a 30-mile (48-km) test loop. The experience seemed to confirm that the longstanding challenge of smooth transition from spark-ignition mode to compression-ignition had been overcome. The prototype SpCCI engines did display some low-rpm harshness, but final calibrations and engine production still are about a year away, engineers said.

What can be confirmed: in everyday driving, the transitions from SI to CI are barely noticeable. On a route that included high-speed autobahn and country and urban roads, the all-new SpCCI unit pulled strongly to its approximately 6000-rpm redline, accompanied by a healthy—if not outright sporty—exhaust note. The engine’s diesel-like torque curve was amply demonstrated by its willingness to pull without fuss from as low as 1200 rpm in sixth gear.

This wider spread of torque has allowed Mazda to revise the development cars’ gearing to further improve fuel economy and reduce emissions.

Frustratingly, specifications such as bore and stroke, rated torque, power and other technical details remain under wraps as this article was published. Although Mazda is claiming a 20-to-30% efficiency gain over its current Skyactiv-G gasoline engine, the test results seen by Automotive Engineering were less demonstrative. They included a 13.3% improvement in fuel efficiency for the manual over the standard Mazda 3 SKY-G—34.0 U.S. mpg vs. 29.4 (6.9 L/100 km vs. 8.0 L/100 km)—with the engine operating in SpCCI mode more than 95% of the time.

It was a similar tale for the automatic-transmission car: a 14.75% improvement (29.9 U.S. mpg vs. 35.1; 29.9 L/100 km vs. 35.1 L/100 km), while the automatic-backed SpCCI engine spent even more time operating in SpCCI mode.

However, this can be attributed to the exaggerated testing regime, aimed more at assessing the engine’s attributes such as low-speed and in-gear acceleration in high(er) ratios and trying to detect the SI-to-CI switchover points than would typically occur in “normal” driving.

From 18:1 to 15:1 CR

SpCCI is a progression of Mazda’s comprehensive Skyactiv efficiency-improvement initiative unveiled in 2011, which debuted gasoline and diesel engines with a common 14:1 compression ratio (CR). The new Skyactiv-X engine operates at a 15.0:1 CR, according to company engineers.

“We selected 15:1 compression ratio as it is close to compression-ignition conditions in normal ambient temperatures,” explained Powertrain Executive Officer Ichiro Hirose. “The spark creates an expanding fireball that acts like a secondary 'air spring' to create additional compression. Because the spark plug creates this fireball, it effectively controls the switch between spark ignition and compression ignition,” he noted.

Hirose added that achieving a “super-high compression ratio was a key breakthrough in realizing combustion with a lean fuel-air mix. Secondly, the leaner you make the air-fuel ratio, the more the specific heat ratio increases. To make the big step forward we needed to double stoichiometric levels from 14.7:1 to 30.0:1, at the very minimum.”

As AE reported previously, during Skyactiv-X’s development through G1, G2, and G3 program stages, engineers targeted an 18:1 compression ratio at lambda 2.5—a 40% improvement in thermal efficiency by setting the ideal pressure and temperature for homogeneous-charge compression ignition.

A key to Mazda’s achievement for the production engine is precise control of the combustion process (see SAE Technical Paper http://papers.sae.org/2015-01-1803/). The SpCCI engine uses pressure sensors in each cylinder to enable real-time temperature and pressure monitoring, in addition to other engine parameters. The engine-management system controls the twin electrically-variable camshafts, the new split-injection strategy that operates at 500 bar (7252 psi) and the air pump. The latter is a unique Roots-type device engineered by Eaton Corp. for the Mazda SpCCI application, Automotive Engineering has learned.

Supercharging and exhaust gas recirculation are known to be effective for operating an HCCI engine in CI mode at high loads. But as of 2013, Mazda R&D was aiming to achieve successful lean-burn HCCI within a broad load range using normal aspiration, according to Takahisa Sori, then Managing Executive Officer for R&D. Sori's engineers were concerned about an air pump compromising real-world fuel economy.

Speaking with Automotive Engineering, Sori also was bullish on the potential of mating HCCI gas engines with hybrid-electric drivetrains that let the engine run in its most efficient operating range, with e-motor assist as needed. In this de-emphasized role, the electric motor and battery can be downsized, reducing their cost. Such an arrangement would seem to be ideal for collaborative work with Toyota hybrid systems engineers.

SpCCI’s air-fuel mixture is created by two-phase, split injection on the intake and compression strokes. A strong swirl is created in the combustion chamber to create an intentionally uneven distribution of fuel density, with a lean mixture around the periphery for CI and a relatively rich air-fuel mixture around the spark plug in the center—conducive to creating the fireball.

Spark ignition is used to start the engine and under heavy-load conditions, but the switchover to CI is not at any predetermined point. When the right intake-charge boundary conditions are achieved, the expanding fireball in the combustion chamber is created, with SI providing additional compression to the geometric compression ratio of approximately 15-16:1. This reaction induces CI, Hirose explained.

He noted that manufacturing costs for the SpCCI engine fall between those of a diesel and a gasoline engine.

Author: Ian Adcock
Source: SAE Automotive Engineering Magazine