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.
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, 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.
The restart must be faster with an automatic transmission. When Ford introduces idle-stop on an unnamed car next year, it promises 0.3 s restarts vs. perhaps 1.5 s or longer for a conventional system.
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.
Many batteries do the job when new but soon lose full capability, forcing the control system to default to strategies with fewer idle-stops.
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.
Ford’s 2012 system, like those on many European cars, will have an electric water pump to circulate heated coolant during an idle-stop. However, Ford's system will be calibrated to simply shut the A/C; if cooling performance degrades below a programmed level, the engine will restart.
Another issue is related to engines with port fuel injection, which require a time delay for the fuel-air charge to travel from the port into the cylinder. Direct injection eliminates this delay, so only engines with this fuel system will be the first in the U.S. to use idle-stop, including the forthcoming Ford.
In fact, Mazda has developed a system that controls the engine stop point precisely, so spark and direct fuel injection in the right cylinder are enough to produce a restart—no special starter is required.
Starter pinion and ring gears have to be capable of many times more engagement and cranking events. At NAIAS, Denso showed one starter with brushes that engineers said have six to 10 times the life of those in a conventional starter. The company also displayed a pinion gear spring that reduces engagement friction by 90%.
There is an opportunity for better fuel economy if the engine can be restarted before it comes to a complete stop, as it permits a more aggressive idle-stop strategy. Ford said the starter pinion gear in its system would be able to engage the flywheel ring gear before the engine is stopped.
Denso showed two alternatives. One, a twin-solenoid starter, is the same size as a conventional starter. Denso engineers said it can smoothly engage a slowly rotating flywheel. One solenoid moves the pinion gear into engagement with the flywheel ring gear; the other energizes the starter motor. The engagement function requires synchronization software, possibly also a more sophisticated crankshaft position sensor. This is because as the flywheel is coming to a stop, its clockwise rotation may be combined with counterclockwise oscillation, complicating the engagement strategy.
Denso also exhibited a sensor that can identify these oscillations and more precisely pinpoint crankshaft position.
An even faster system, developed by Denso in conjunction with Toyota, has been used since 2008 on the European market Yaris and Auris. The pinion gear is in permanent engagement with the flywheel ring gear. The flywheel is on a one-way clutch that normally keeps it freewheeling when the engine is running. But as the engine comes to a stop, the clutch engages, so flywheel and crankshaft are locked together.
Because the pinion and ring gear are permanently engaged, merely energizing the starter will crank and restart the engine more quickly than having to wait for pinion and ring gears to engage at zero rpm. Permanent engagement also means that pinion and ring gear teeth can be optimized for quiet operation and have assured durability.
The design also permits what Denso engineers call “change of mind.” In this scenario, a car is coasting to a stop, perhaps with light braking, and the engine is at zero rpm. A traffic light turns green or a traffic blockage clears; the motorist steps on the accelerator and there’s virtually instant restart.
However, if the engine is allowed to idle as part of any operating strategy, including battery state-of-charge, the engine must be optimized for smoothness, as the freewheeling flywheel does not dampen cylinder firing pulses.