Synergy can be defined as a whole greater than the sum of the parts, but achieving it depends on more than simply pulling together disparate elements. When GM R&D brought together a team to develop the new Magnetorheological Fluid (MRF) Fan Clutch, the researchers and engineers in mechanical, electrical, and fluids engineering realized that achieving all of their sometimes conflicting design objectives could be a daunting task. They found success by developing a way to focus on all the different objectives at once.

"The key was to integrate specialized technical skills: in the chemistry and formulation of the MRF; in unique mechanical and electromagnetic systems engineering; and in system dynamics analysis and control," says Patrick Usoro, team leader for the MRF Fan Drive program. "We tried to develop an optimal solution up front instead of fixing problems at the end."

The MRF fan clutch relies on a GM proprietary MRF (Figure 1), a fluid that consists of a synthetic hydrocarbon or silicone base coupled with a suspension of magnetically soft particles. In the Off state, the particles disperse randomly and the fluid exhibits Newtonian behavior. But in the On state - when an applied magnetic field aligns the metal particles into fibrous structures - the rheological behavior changes from Newtonian to Bingham plastic, which makes the fluid act as though it's more viscous.

"The iron particles form links that resist being broken," explains Usoro. "By changing the magnetic field, we can control the amount of torque transmitted through the device."


Figure 1. In the Off state, an MRF is a random dispersion of magnetizable particles. In the On state, the applied magnetic field aligns the metal particles into fibrous structures.

The MRF fan drive (Figure 2) relies on a controller that measures the cooling needs of the engine and directly commands the MRF clutch to produce the fan speeds needed for optimal cooling. The MRF responds to the input in milliseconds - almost instantaneously. Although conventional fan clutches typically are either On or Off, the MRF offers the benefit of variable speed. A higher maximum speed enhances the MRF fan drive's cooling performance; a lower disengaged speed improves fuel economy by 1-3%. Other benefits include an inherently simpler design, excellent controllability, and reduced noise.


Figure 2. The MRF fan clutch consists of an input and output shaft, with the MRF sealed between the two clutch plates. An electromagnetic coil produces a flux through the fluid. Changing the current through the coil changes the magnetic flux density, varying the torque transmitted from the input shaft to the output shaft.

Of course, the team faced many challenges before achieving these results. Critical challenges for the MRF included viscosity, volatility, stability, and temperature sensitivity. The fluid had to be compatible with other clutch materials and sealed to prevent leakage. For the clutch, reducing slip-heat generation, torque capacity at high internal temperatures, durability, and nonlinear dynamics and controllability were all significant technical issues that the team addressed.

Ultimately, the team's synergy and diligent efforts translated into a clutch that will go into production on a number of GM trucks and rear-wheel-drive cars, offering significant cost savings and performance improvements. The knowledge gained will spread still further, however, as the team develops MRF clutches for other vehicle applications

By Diane Kightlinger