Thursday, May 14, 2009

Combined Starter-Alternator Reduces Petrol Use

Dow Corning is helping auto manufacturers and suppliers boost gas mileage and reduce emissions. Dow Corning's technical centres are delivering process engineering, testing and even prototyping to help customers accelerate product development and reduce time to market. Among recent fuel-saving advances is a design for a combination starter-alternator that outperforms the conventional starter and alternator as separate components. The device is at the heart of the 'start-stop' system, which conserves fuel and reduces pollution by automatically shutting off the engine when a vehicle is stationary.

The system immediately and silently restarts the motor as soon as a gear is engaged; or in the case of a car with automatic transmission, whenever the driver releases the brake. In starter mode, the belt drive and low inertia of the device mean it starts three-times faster than a conventional starter and with a fraction of the noise. When the vehicle stops, noise and tailpipe emissions drop to zero. The approach has been estimated to reduce consumer fuel-consumption by up to 10 per cent in urban traffic conditions, operating discreetly and without disrupting normal driving habits.

Reduced weight from the dual-function unit contributes to the gain. Unfortunately, the under-hood environment presents some of the toughest operating conditions in any automotive application, where below-zero ambient temperatures can quickly rise to 150C (302F) or even higher. Fuel, oil, glycol coolant and solid contaminants can all interfere with sensitive electronics, forcing device manufacturers to protect circuits and components from harsh service conditions. To achieve the reliability and longevity needed in the starter/alternator combo, a French manufacturer has specified Dow Corning 7091 Adhesive/Sealant for critical lid-sealing of its designs.

The silicone formulation easily withstands the temperature range and repeated thermal shock, remaining virtually unaffected by automotive fluids and cleaners. Dow Corning's role wasn't limited to material supply in this application, however. Once the customer finalised and tested the design, the company faced significant costs and lead time to develop the 40 prototypes needed for customer trials. Instead, the Dow Corning Applications Center in Seneffe, Belgium fabricated the prototype units, helping to reduce expenses and shorten development time.

The lid-sealing operation was completed using automated dispensing-equipment with a six-axis robot, controlling the location, application speed and volume of the one-part silicone formulation. The non-slumping RTV material requires no energy to cure. Once the adhesive skins over, it cures from the surface inward, typically requiring about 24 hours (at room temperature and 50 per cent humidity) for sections 2mm thick. The material reacts with moisture in the ambient air, ultimately forming a tough, flexible elastomer that remains stable from -55 to +180C.

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