几年前，当梅赛德斯-奔驰（Mercedes-Benz）确认将推出一款新的直列六缸发动机架构时，除了公司高层领导和一些真正的技术“信徒”之外，我想也许没人比我更高兴了。多少年来，这种发动机架构一直都是梅赛德斯品牌出色性能与优化水平的基石。

自上世纪50年代进入“现代”以来，梅赛德斯-奔驰几乎成了直列六缸发动机的“代言人”，并在接下来的近半个世纪内陆续推出了多款直列发动机车型，其中奔驰的传奇300 SL运动赛车就采用了博世的3.0L直喷M198直列发动机。1998年，梅赛德斯推出公司历史上最后一款I6车型，之后直列发动机逐渐成为了这家德国厂商一段尘封的记忆。

今年10月，梅塞德斯-奔驰公布了旗下新直列六缸发动机系列的基本信息。该系列基于模块化的500cc气缸打造，大量采用了各种新一代电气化功能。

毫无疑问，我本人一直非常怀念I6发动机。众所周知，随着前驱架构的流行，其实也就是全球正向碰撞安全法规的收紧，直列六缸发动机的研发开始逐渐成为汽车厂商的负担。通常来说，直列六缸发动机的长度太长，很难达到横置发动机设计的尺寸要求（虽然也有一些成功案例）。同样由于长度问题，这种发动机无法预留出足够的缓冲区域，在车辆发生正向碰撞时吸收能量，因此甚至很难用于纵置发动机设计。

这些年来，当直列发动机在全球市场节节败退时，只有BMW坚持了下来。如今，BMW在直列发动机方面的声誉甚至超过了梅赛德斯-奔驰。我曾多次咨询BMW的资深工程师、设计师和高管，当全世界都说直列发动机没有未来时，BMW到底是怎样坚持下来的。这里我引用他们最常见的回答：“可能因为我们总是把直列六缸发动机放在第一位。”

直列发动机的内在平衡性和扭矩表现都非常出色，最终决定放弃这种发动机的人也一定经历了一番困难的抉择。正如通用汽车首席工程师Ron Kociba所言，与V6发动机相比，I6发动机在设计和制造方面也拥有很多优势。这位工程师在V型发动机盛行之时，“反其道而行”地设计了Vortec4200直列六缸发动机，Kociba也总是乐此不疲地介绍直列发动机的优势。讽刺的是，现在I6发动机的这些优势又被作为“成本优势”重新提起，而当时梅赛德斯等一众公司抛弃I6时给出的理由也正是“成本考虑”，他们认为V6与V8发动机的兼容性更强，可以共用更多模块。天啊，世界变得真快——现在，通过模块化设计分担成本又成了被大家“抛弃”的直列I4发动机，而不是V8发动机的关键优势。

I6并不是德国厂商的专利。举例来说，日本丰田公司就曾推出过几款非常出色的直列六缸发动机，其中包括我个人的最爱：末代Supra采用的双涡轮增压3.0L直列2JZ-GTE发动机。但无论如何，直列发动机的名字将永远与梅赛德斯-奔驰和BMW的牌子连在一起。

更让直列发动机爱好者高兴的消息是：正如上世纪50年代的M198一样，梅赛德斯-奔驰推出的新一代I6发动机也采用了当下最为尖端的技术：48V系统。这种设计极大地推动了多项节能优化技术的诞生，其中包括有助于消除涡轮“延迟”的电加速涡轮增压器。

如今，经典I6发动机“重出江湖”。在梅赛德斯-奔驰的直列发动机复兴中，“老”可能代表“古老”，但更多意味着“经典”。无论如何，只要记住一点，梅赛德斯-奔驰的最新I6发动机绝对不是“老古董”。世界真奇妙啊！

Apart from top brass and various other engineering true believers at Mercedes-Benz, perhaps no one was more delighted than me when it was confirmed a couple of years ago that Mercedes was introducing a new architecture for inline 6-cylinder engines, an engine layout that long served as a cornerstone for the brand’s reputation for consummate powertrain performance and refinement.

The inline six became synonymous with Mercedes in its “modern” era that started roughly in the 1950s and appeared in some form or other for the next half-century (including the seminal 3.0-L Bosch direct-injected M198 for the legendary 300 SL), with the company cutting ties with its last I-6 in 1998.

In October, Mercedes made known the basic details of the new inline 6-cylinder family, the foundation of which is a modular 500-cc cylinder—not to mention the incorporation of new-age electrification features; Automotive Engineering European correspondent Stuart Birch delivers the initial rundown at: http://articles.sae.org/15093 .

I’ve certainly missed the I-6. As many know, inline sixes gradually became a vehicle-development liability as front-drive vehicle architectures began to dominate and, more directly, as global frontal-crash regulations tightened. Inline sixes typically were too long to be effectively packaged in transverse-engine platforms (although it was done)—and the straight six’s physical length similarly made it difficult to fit one even in a platform designed for longitudinal placement, because the unyielding engine didn’t permit enough energy-absorbing crumple zone in frontal impacts.

Only BMW—which over the years became more famous for the straight six than even Mercedes—somehow kept the faith. When inline sixes started to bite the dust all over the globe, I repeatedly asked senior engineers, designers and executives how BMW could somehow get on the right side of crash physics when everyone else claimed it couldn’t be done. The usual answer, I’ll paraphrase, tended to: “Perhaps it’s because we make the inline six-cylinder a priority.”

It surely must have been difficult for those concerned with engine superiority to turn their backs on the I-6 layout’s inherent balance and outstanding torque characteristics. Then there are the considerable design and manufacturing advantages compared with a V-6, as Ron Kociba, former General Motors chief engineer of the "zig-when-everyone-else-is-zagging" Vortec 4200 I-6 never tired of explaining. It’s ironic those benefits are being revisited as cost advantages when compared with a V-6, given that one justification Mercedes and others cited for moving to vee-arranged 6-cylinder engines was modular compatibility with V-8s. My, how the world has changed: now the critical cost-sharing modularity metric is with inline 4-cylinder engines, not V-8s.

The German luxury-car makers didn’t have an exclusive on the I-6 secret—Toyota, for one, authored some magnificent inline sixes, including my personal craving, the ballistic twin-turbocharged 2JZ-GTE 3.0-L used in the last-generation Supra—but for Mercedes and BMW, there’s no doubt the format is indelibly tied to those brands.

What makes it all the better for straight-six disciples: as with the M198 from the 1950s, Mercedes is launching its new-generation I-6s with the era’s most bleeding-edge technology: integration with a 48-volt electrical system that facilitates several nifty efficiency and performance game-changes, included an electrically-accelerated turbocharger as the last word in eradication of turbo “lag.”

What’s old really is new again. In Mercedes’ revival of the inline-six, “old” may equate to vintage attributes, but these new engines promise to be anything but vintage. What a world.

Author: Bill Visnic
Source: SAE Automotive Engineering Magazine