Bosch’s second-generation stop-start system under development for 2015-16 will add a stop-restart capability during vehicle coasting, as well as being more robust than the current system that activates only during engine-idle conditions.
“The advanced system will require a more robust and reliable starter for a higher number of starts and an advanced power net architecture—for example, electric power steering or two electrical power storage units,” explained Scott Dahl, Regional President of Starter Motors and Generators North America at Robert Bosch LLC.
With the new system, when the vehicle is coasting and the driver is not depressing the accelerator, the engine shuts off then restarts when the accelerator is depressed. (It will also switch off the engine in stationary situations.) The system is capable of functioning at any speed greater than zero mph through highway speeds.
Bosch's current system only switches off the vehicle's engine in stationary situations, such as at a standstill in a traffic jam, or at a stop sign. The current system doesn't have the ability to stop, then restart the engine during vehicle coasting.
“Predictive algorithms apply to situations where the driver changes his mind about slowing down and needs to suddenly re-accelerate, requiring the engine to immediately restart. This requires an effective synchronization between the starter and engine, enabling the starter to restart the engine quickly even as the engine continues to rotate,” Dahl noted.
He added that Bosch’s systems-engineering competence helps ensure that “this takes place as effectively and reliably as possible.”
Critical operating parameters including engine temperature, ambient temperature, and battery state-of-charge play a role in the new system's operation similar to that which they play in the current stop-start system.
“The software logic basically makes sure that the engine is never shut down unless engine and cabin temperature conditions are met and the battery charge is sufficient to guarantee an immediate restart,” noted Dahl. Additional software program parameters are needed for change-of-mind events—such as a slow-down followed by an immediate acceleration—that require the synchronization of engine and starter, he said.
In addition to reducing tailpipe emissions, Bosch’s stop-start system currently in production can improve fuel economy by 15% in city-only driving, according to Dahl. However, in the 2012-2016 U.S. CAFE regulation, stop-start technology only counts for a 2-3% fuel economy improvement.
With stop-start coasting, fuel economy improvements in real-world driving can feasibly achieve 20-25% while the CAFE benefits under the current regulation could reach about 9-10%, he said.
Bosch’s stop-start coasting system will reach the European marketplace in CY2015 or 2016 with the U.S. version likely in the 2016-2017 timeframe. From an operational standpoint, the European and U.S. versions will be essentially the same.
“In terms of functionality, North American automakers focus more attention on maintaining driver comfort, including reduced noise, vibration and harshness; shorter engine restart time; and maintaining climate control,” Dahl noted.
Stop-start technology is increasingly becoming commonplace in many OEMs' offerings.
According to Christian Bokich, Dept. Manager, Product & Technology Communications for Mercedes-Benz USA, “By the end of calendar year 2015, all Mercedes-Benz models in the USA will offer one or more engine variant with Eco start-stop technology."