Engine idle-stop has been used to improve fuel economy and lower emissions in Europe, where high fuel prices, emissions regulations, and regulatory drive cycles favored the technology. However, for conventional gasoline engines on U.S. models, idle-stop (also called stop-start) still is a year from any introduction, and even by then, issues will remain.
What Is Start Stop Technology
At the recent North American International Auto Show, however, Ford announced its system will debut in MY2012. Denso also displayed its latest idle-stop technology which includes three different starter types.
Idle-stop seems simple, so why the delay? Actually it’s complex, costly, and requires trade-offs. However, U.S. EPA mandates five-cycle testing for fuel economy starting in 2012, and fuel economy standards are tightening. And unlike the previous city/highway testing, with few city stops and none in the highway cycle, five-cycle has enough idle-stops for the technology to boost fuel economy numbers by 3-8%, perhaps more.
Some basic systems started in Europe over 30 years ago. Their engagement was often abrupt and slow, and so the early systems were not overly popular. However, Tier 1 suppliers, particularly Bosch, Valeo, and Denso, have been working with OEMs ( Original Equipment Manufacturer), and today idle-stop is approaching a seamless feel for some applications.
Seamless restart is easier with a high-voltage hybrid, which can get the car rolling in electric drive and then start the engine with the integrated starter-generator. Idle-stop in a conventional gasoline engine car, often called a “microhybrid,” requires the engine to be running almost the instant the driver’s foot presses the accelerator. Early European systems had more time to operate because they were on manual transmission cars and could perform the restart sequence while the driver lifted his foot off the brake pedal, depressed the clutch, and shifted into gear.
Start Stop Technology Engineering challenges
Idle-stop must deal with a number of system issues. First, in heavy traffic with repeated idle-stops, the battery must be able to handle the cycling loads, a difficult task. So-called "smart" charging, perhaps incorporating regenerative braking for greater efficiency, is among methods available to maintain the battery level.
An ordinary battery cannot take idle-stop punishment for an acceptable life span, and many European vehicles have used two batteries or one special design, such as lead-carbon electrode or absorbed glass mat (AGM) types. Supercapacitor modules are an alternative as well. Ford has disclosed its system will have an AGM.
If the ambient temperatures require air-conditioning or heat, motorists don’t want to lose performance during an idle-stop. An electric-drive compressor can provide idle-stop cooling, but such units are costly and typically limited to premium cars in Europe and some full-function hybrids.
Denso and Delphi executives have disclosed separately their companies are developing evaporator-mounted chambers with thermal salts that solidify during A/C operation and dissolve to provide cooling with A/C off. However, neither system is ready for production due to insuffient performance life and other issues.