Phrase Of The Day: "Torsional Excitations"

What keeps powertrain engineers up at night? C’mon, get your mind out of the gutter. The move towards downsized, turbocharged engines is creating a number of new engineering challenges, and “torsional excitations” grabbed the spotlight at this year’s Society of Automotive Engineers Congress. Steven Thomas, manager of Ford’s global transmission and driveline, research and advanced engineering, illuminated the issue [via Wards].

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. I would challenge you all to think about new ways of dealing with this. 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.

The problem: the increased inertia of forced-induction engines. The practical example: a turbocharged Fiesta. A worthy adversary, a worthy cause. Let’s do this.

Inertia is already a challenge for the Fiesta, as Thomas reveals that

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.”

Add a downsized, forced induction engine, which the Fiesta was not designed for, and the potential for “torsional excitation” rises. One possible solution, the use of dual-mass flywheels, is being tested by Ford for use in a possible turbocharged Fiesta, but initial results show it could actually increase engine friction by as much as 15%. If “DMF”s don’t work, the options become somewhat more limited:

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.

And, says Thomas, three-cylinders are even tougher to keep smooth, as their uneven firing pattern works with torsional excitations to create severe NVH conditions. The future of engines may be downsized and turbocharged, but it’s still got a few bad vibrations to work out.