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Ford Wrestles With ‘Torsional Excitations’

Engine downsizing is a compelling powertrain strategy, but one that in some ways creates as many challenges as it solves.

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SAE World Congress

DETROIT – Downsizing an engine and integrating forced induction is a popular powertrain strategy these days as auto makers devise ways to meet stricter fuel-economy mandates worldwide.

With turbocharging, six cylinders can do the work of eight, four cylinders can do the work of six and three cylinders can do the work of four, while reducing carbon-dioxide emissions and maintaining or improving performance.

It’s a compelling theory, but one that in some ways creates as many challenges as it solves.

For instance, a boosted engine increases inertia, which means higher “torsional excitations,” says Steven Thomas, manager-global transmission and driveline, research and advanced engineering at Ford. The task of addressing these vibrations often falls to transmission and driveline engineers.

“As we reduce the engine torque, particularly just off idle prior to the boost coming on, we’re going to adversely impact the ability to accelerate the vehicle,” Thomas says at a Transmission Technologies forum at the SAE World Congress here.

“I would challenge you all to think about new ways of dealing with this,” he says to SAE attendees.

Last year, Ford launched its Fiesta small car in the U.S. with a 120-hp 1.6L DOHC gasoline 4-cyl. engine mated to either a 5-speed manual gearbox or 6-speed dry dual-clutch PowerShift transmission.

Thomas says Ford’s new DCT, which also appears this year in the ’12 Focus, is “great for CO2 reductions and fuel economy, but I have to tell you one of its challenges is the amount of inertia in a dual dry-clutch assembly.”

A dual-mass flywheel is one potential way to offset torsional excitations, but the Fiesta’s current powertrain doesn’t have one because the vehicle was developed before forced induction was being considered, Thomas says. “However, now that the boosted engines are on the horizon and coming, we’re looking at this issue.”

Ford engineers are considering a dual-mass flywheel for a turbocharged version of the Fiesta, but they are proceeding cautiously because initial tests suggest a DMF actually could exacerbate the problem by increasing inertia up to 15%, Thomas says.

Pendulum dampers are being considered to address the problem. And with automatic transmissions, torque converters incorporating improved dampers can quell some of the vibrations, but more work is necessary.

The key goal is to find a solution that does not hinder launch performance as 3- and 4-cyl. engines proliferate, Thomas says. “So these challenges will be with us for some time, and they’re actually going to increase.

“We could really use new designs to deal with these challenges to optimize the fuel economy, but at the same time deal with (noise, vibration and harshness) and performance issues presented by these new engines.”

Research into torsional excitations will accelerate in the U.S. if turbocharged 3-cyl. engines become popular, partly because the odd number of cylinders creates severe NVH problems.

“Actually, it becomes the worst torsional problem compared to the others (4-cyl. engines),” Thomas says of the 3-banger.

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