Think beyond stop-start in automobiles

June 26, 2016 // By Shaw Lynds
Shaw Lynds
Shaw Lynds of Maxwell Technologies discusses what must come next to improve fuel economy without down-grading the driving experience.

Hybrid vehicles are old hat, and nearly always relegated to a niche market – currently about 3 percent of US car sales. Electric vehicles are sexy, but it’ll be a long wait until they’re an economically sound choice for the average Joe. As the country’s corporate average fuel economy (CAFE) standards rise, what are car manufacturers to do?

The industry’s latest answer – and hot topic – is the stop-start method. Basically, this involves turning off an engine when the car isn’t moving. This approach saves fuel, but it doesn’t add the peppy responsiveness to cars that hybrids and electrics enjoy from the massive low-end torque of their electric motors. Nevertheless, most major auto OEMs (original equipment manufacturers) have several start-stop vehicles in the works, if not already in their fleets, as it is a relatively inexpensive way to improve fuel economy by a few percentage points.

The costs to add stop-start to a vehicle are relatively minor: an increase in battery size to handle peripheral loads while the engine is off (lights, radio, airconditioning and power steering), an increase in alternator size to charge larger batteries faster once the engine is back on, and improvements to the starter system to make the restart and acceleration of the vehicle as seamless as possible. Auto OEMs may love stop-start for its relatively inexpensive boost to their CAFE numbers, but how do they get consumers excited enough for broad market adoption of a system that, at best, has no noticeable impact on the driving performance of a car, and at worst, leaves the car feeling sluggish or non-responsive when the driver accelerates from a stoplight?

The next logical step is to add intelligence to a stop-start’s already upsized alternator and make the vehicle a true micro-hybrid. Typical car alternators are rated for about 80 amps or about 1 kilowatt (kW) of power-run peripheral loads and recharge batteries. In stop-start systems, it is