DCT Solenoid Development

September 12, 2015

As CAFE sets strict standards to reduce emissions and improve fuel economy for passenger vehicles more major automotive companies will look to implement multi-speed dual-clutch transmissions. Dual-clutch transmissions are essentially two separate manual transmissions working as one unit where the gear shifting is carried out by the electrohydraulic components inside the transmission.

TLX Technologies has developed a proportional control solenoid for the electrohydraulic component in dual-clutch transmissions. These proportional control solenoids are used to operate spool valves in the hydraulics of dual-clutch transmissions during gear shifting. Proportional control of the spool valves is required for synchronization of gears during a shift to reduce shift shock. During the development of these proportional control solenoids TLX Technologies encountered multiple challenges which ultimately led to an innovative and efficient design.

Nine-Speed Solenoid

Solenoid Design

One of the major challenges encountered with the design of the proportional control solenoids for dual-clutch transmissions was the overall package size of the solenoid. As automotive companies develop dual-clutch transmissions with more gears, the package size of the control solenoid must shrink to fit in the same real estate previously occupied by fewer control solenoids while the operational requirements remain the same. For example, increasing the number of gears in a dual-clutch transmission from six gears to nine gears reduces the maximum allowable outside diameter of the control solenoid by 30%, which means all of the force producing elements within the solenoid are also reduced.

Using magnetic finite element analysis TLX Technologies has been able to model and achieve sufficient magnetic force to shift the hydraulics and maintain proportional control in a small package size by using a combination of radial and conical magnetic designs. The radial part of the magnetic design is used to achieve the high initial force requirement and the conical part of the magnetic design is used to keep the force consistent as the solenoid moves throughout the working stroke of the hydraulics. Using this combination of radial and conical magnetic designs TLX Technologies was able to achieve cycle-to-cycle repeatability of less than +/-0.25% and improve the controllability of the hydraulics in dual-clutch transmissions.

Figure 1

Magnetic Finite Element Analysis
Magnetic finite element analysis of proportional control solenoid using radial and conical magnetic design strategies to achieve force requirement in reduced package size.

Another challenge encountered during the design of the solenoid was producing enough magnetic force in the smaller package size while addressing the force losses due to hysteresis. Hysteresis loss seen in magnetic design is due to both the friction between sliding components and the hysteresis properties of the magnetic material being used. To limit the effects of material hysteresis loss and maximize the saturation flux density TLX Technologies used annealed silicon core iron.

Annealing silicon core iron produces a fine-grained microstructure material which possesses low residual magnetism and minimum magnetic hysteresis. To reduce the friction between the sliding components TLX Technologies used a combination of low-friction diamond-like carbon coatings and Polytetrafluoroethylene fabric. The diamond-like carbon coatings provide a low coefficient of friction and a high micro-hardness making it a perfect solution for reducing friction in tribological applications.

Another benefit to using diamond-like carbon coatings is increased wear resistance which is critical when using annealed soft magnetic materials. Since these proportional control solenoids are mounted in the transmission where there is not always lubrication available, the Polytetrafluoroethylene fabric has been incorporated to maintain lubrication between the sliding surfaces. The Polytetrafluoroethylene fabric also acts as a break between the sliding magnetic components reducing magnetic side loading seen in radial magnetic designs.

Through the design and development of these components the overall hysteresis was reduced to less than 2.5% of the maximum force output from the control solenoid.

This article was originally published by Transmission Technology International in September 2015

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