"By knowing the crankshaft position from the start, we can eliminate the time that's required to crank [the engine] for one whole revolution," says Frister of Robert Bosch LLC.
Advanced solenoid designs are also helping to reduce start times. Denso has rolled out its TS (tandem solenoid) starter, which inserts the pinion into the ring gear before the vehicle comes to a full stop. This means the vehicle is ready to restart almost immediately.
"Start-stop sounds very easy," says Guenther Raab, director of systems engineering for Continental Automotive in North America. "Anybody can stop an engine and restart it again. But when you have to do it automatically, there are a lot of matters to consider."
Coasting and comfort
As start-stop technology has evolved, suppliers have begun to consider some of those other technical matters, including noise, comfort, and turning off the engine while the vehicle coasts.
In the interest of being "unnoticeable," noise has become a growing concern. Suppliers want to minimize the sound of a cranking starter, especially since it could be cranking dozens of times a day. Engineers are addressing that by adding rubber isolators and noise absorbers in or near the starter. To further reduce cranking noise, they are also working with automakers to insulate noise in the ring gear, which sits outside the starter.
"When you're talking about 10 times as many starts, you want to do your best to help lower that cranking noise," says Martin of Denso.
Driver comfort is an even bigger concern. In today's vehicles, engine shutdown means no air conditioning (AC) or cabin heat. To combat that, suppliers are looking at ways to keep the AC and heat going, if only for a minute. The simplest way is to use an ambient temperature sensor that would help the vehicle decide if the car needed to be restarted for comfort reasons. A more advanced solution involves the use of a cooling storage evaporator piggybacked atop the AC evaporator. Such evaporators would incorporate a phase change material that would allow drivers to continue cooling the vehicle for 50 seconds or maybe even a minute.
"That would cover 90 percent of the stops," says Frister. "So you'd get your air conditioning effect and your full fuel economy improvement at the same time."