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Return of the inline-six: Why the classic engine layout is making a comeback

Nearly 70 years since it was blown away by the high-compression Oldsmobile Rocket V8, the inline-six engine is poised to make a comeback.

I’m not the first to connect these dots, but the reason for the return of the inline-six has everything to do with manufacturing efficiency and not because of an inherent fault of the V-6. As V-8s fade away, the V-6s they spawned will also diminish in numbers.

BMW, which never abandoned the inline-six, has created the template for the modern modular inline engine family that other automakers are adopting. At BMW, each cylinder is 500cc and the engines are modular, meaning that they use the same basic menu of internal parts, such as valves, pistons, bearings and pumps.

“The advent of the modular 500cc cylinder has brought us a flurry of 2.0-liter I-4 engines, 3.0-liter V-6 engines and 4.0-liter V-8s. A lot of this was due to cost,” says AutoPacific analyst Dave Sullivan. “Being able to use the same hardware in multiple applications helps to bring costs down. The inline-six is naturally harmonically balanced, reducing the need for balancers or any expensive treatment systems,” he added.

When Jaguar Land Rover developed its acclaimed Ingenium gasoline and diesel engine family, it used BMW’s 500cc per-cylinder and modular component strategy, but with engineering twists of its own. JLR’s announcement last month that it plans to stop buying gasoline engines from Ford’s Welsh engine plant in 2020, opens the door to larger Ingenium engines. JLR officials won’t confirm six-cylinder Ingeniums are on the way, but it is a sure bet they are.

JLR buys its V-8 and V-6 engines from Ford, and without those engines, it would have only turbo four and hybrid vehicles — which would not develop enough torque to provide the kind of performance a Range Rover needs to take on Bentley and others. But a supercharged 3.0-liter Ingenium six could easily replace the 5.0-liter V-8.

JLR’s plan to halt the Ford V-6 and V-8 gasoline engine purchases in 2020 indicates the six-cylinder Ingenium engine will likely be ready for the 2021 model year — or sooner if JLR discontinues the V-6 before the V-8.

As three- and four-cylinder engines continue to deliver more power and efficiency, it’s far less expensive and disruptive to add another pair of cylinders for a bigger engine with more torque than to build a V-6 that doesn’t share its parts with a V-8.

Mercedes’ new inline-six, a 3.0-liter, comes in two flavors, Automotive News affiliate Autoweek reports, including one that cranks out 435 hp.

But there are problems related to inline-sixes. Most are longer than the V-6s they will replace, making it tough to mount the engine in front-wheel-drive cars because the engine’s length leaves little room for the transmission. That length also can be problematic for rear-wheel-drive cars, which may need longer hoods to accommodate the engine.

And then there’s safety.

Engineers appear close to solving a few lingering safety problems that they didn’t have to deal with when a V-6 was installed. “One of the long-standing issues [for the inline-six] has involved the length of the engine and crash standards. It appears that manufacturers are confident they can have an engine ‘deform’ and not penetrate the cabin,” says Sullivan.

But, if you’ve driven a BMW six lately, you know how smooth and silky an inline-six can be. Now, with direct injection, variable valve timing, electric superchargers and electrification, the inline-six just may be the configuration that propels the internal combustion engine to the finish line.

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