What’s Old Is New Again – The Return Of The Inline-Six

By and far, the inline-six cylinder has proven itself to be a robust engine configuration over the span of the last 50-plus years. However, in the more modern times, the configuration has fallen out of favor with automakers due to size and space constraints imposed by modern technology and packaging requirements, along with some manufacturing concerns.

With engine parts commonality on the assembly line being placed in ever-higher importance and previous generations of inline-six engines not really being common to any modern engine platform, it is relatively easy to see how they fell out of favor when manufacturers looked to design a new powerplant for a new vehicle.

However, Mercedes-Benz flipped the script, and decided to take all the inherent advantages of an inline-six cylinder engine, and reimagine the engine in a modern, 21st-century way. Addressing all the packaging concerns and engine manufacturing commonality issues and then adding in technologies which have been seen in their Formula One racing efforts, Mercedes has breathed new life into the inline-six cylinder engine and the industry as a whole has taken notice.

“I think [inline-six cylinders] are one of the coolest engine styles out there,” says Jason Fenske of Engineering Explained. “Part of that is the balance they have. Assuming your manufacturing tolerances are good, your primary and secondary forces are all balanced out, as are your primary and secondary moments. So you get a buttery smooth-running engine.”

Fenske continues, “In addition it’s so much simpler than a V6. You only have one cylinder head instead of two. Two camshafts instead of four; One exhaust manifold instead of two, one turbocharger instead of two, and you only have one catalytic converter hanging off that exhaust manifold instead of two. That alone can save you from manufacturing and maintenance costs.”

Everything Is Getting Smaller These Days

In his video, Fenske walks us through the Mercedes M256 engine, which entered production a little less than two years ago, and dives into how Mercedes addressed the shortcomings of previous inline-six engines, and made them the new hotness. Wading through his brutally honest opinions of the GLE 450 it resides in, we find a trove of information, as is usually the case in one of his videos.

“Downsizing of engines is becoming a very popular thing [for automakers] to do,” says Fenske. “Where it used to be that you had a popular V8 platform, you could just lop off two cylinders and have a V6 that fits in the same packaging, and can be made in a similar fashion, and makes a lot of sense from a production line.”

The inline-six-cylinder engine layout has some inherent benefits in both manufacturing and performance over its V6 cousin, with the downside of its extra three cylinders-worth of length. Mercedes essentially added two cylinders to their 2.0-liter four cylinder engine architecture for a 3.0-liter inline-six.

However, as Fenske points out, even the V6 engines are being downsized. “We are now in a world where inline four-cylinders are really taking over. These smaller engines are becoming more popular,” Fenske says. “If you have a really popular inline four-cylinder platform, why not just pop on two additional cylinders for the vehicles you want to have more power? That’s exactly what Mercedes did. They can make both the I4 and I6 on the same production line, saving on manufacturing costs.”

So with the manufacturing commonality issues addressed, you might be wondering how they addressed the packaging concerns which have always plagued the inline-six. After all, it’s two-cylinders longer than a V8 engine, and those are being clipped left and right under the guise of packaging concerns, right?

The Shortest Distance Is a Straight Line

The challenge of fitting an inline-six cylinder engine into an engine bay that was previously designed for a V6 was no small task. “Mercedes did two clever things here. The first, is they simply reduced the bore size,” explains Fenske. “They went from an 88mm bore down to a 83mm bore.” It makes sense, reducing bore size allows for closer bore centers. Closer bore centers in turn allow for a more compact engine block.

“They were able to take a few inches off the length of the engine simply by reducing the bore and increasing the stroke compared to their V6 engines,” Fenske explains. “The second thing they did was that they electrified the engine. Where you would normally see a torque converter, you now have an integrated starter and alternator. This little pancake motor at the rear, because it acts as a generator, allows them to get rid of the accessory belt and drive at the front of the engine.”

By eliminating the traditional accessory drive on the front of the engine, the overall length of the engine is further reduced, allowing for ample room in the space originally designed for a compact V6 powerplant. Remember when we mentioned that some of the technology from F1 was incorporated as well? That pancake starter alternator motor also happens to produce a whopping 160 lb-ft of torque, which is not only available on demand to boost the engine, but also power the 48-volt electrical system, that is also fueled by regenerative braking.

The first step Mercedes-Benz took in shortening the overall length of the M256 was to decrease the bore size from 88 to 83mm per cylinder, increase the stroke to 92.4mm to maintain the 0.5-liter-per-cylinder displacement, and reduce the bore spacing a commensurate amount. That alone dropped several inches from the block length.

“This tiny little pancake motor thrown on the end of the crankshaft on an engine that already makes quite a bit of power puts out a huge amount of torque. While it’s only a 22 horsepower motor, that 160 ft-lb of torque is huge,” says Fenkse. “That electric torque allows you to fill the gap where the turbocharger hasn’t fully spooled up, or for instant torque off the line as well as whenever you put your foot down.”

Fenske further explains that in the AMG performance version of the engine, an air compressor is incorporated to the system. “That’s essentially an electric supercharger that runs of the 48-volt system, which is able to provide boost to the engine in 0.3 seconds.” Fenske says. “So you have the electric boost from the pancake motor, you have boost from this electric air compressor, and then you have boost from the turbocharger.”

While it might be a stretch to say that Mercedes-Benz has single-handedly saved the inline-six in modern times, especially since BMW has been utilizing them continuously since, well, forever, hopefully the market takes notice of the Mercedes’ modernization of the platform and turbocharged inline-six-cylinder engines start to make a comeback in modern vehicles. Afterall, we can think of at least two handfuls of I6 engines that have made an indelible impact on automotive culture, and it would be nice to see the engine layout survive well into the 21st century.

Another compacting feature was the use of the ISG or Integrated Starter Alternator assembly. The 22-horsepower motor acts as both a starter and alternator, with the benefit of being able to add 160 lb-ft of torque directly to the crankshaft on demand. On the AMG version, an electric supercharger is incorporated to the 48-volt system to provide additional airflow boost to the engine while the turbocharger spools up.

About the author

Greg Acosta

Greg has spent over a decade in automotive publishing as Senior Editor of Race Pages magazine. In his free time, he is a firearms instructor and volunteer in the police armory.
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