Stop-start technology has been on the market for gasoline engine (and some diesel engine) passenger cars in Europe and Japan for years, but the OEMS have been reluctant to bring this technology to North America. One reason is that over 90 percent of new cars sold here have torque-converter automatic transmissions that lose line pressure with the engine off, thus delaying vehicle movement after engine restart. The addition of an electric-driven hydraulic pump is a costly but effective solution. Nonetheless, stop-start-stop technology for gasoline engines is coming to North America via Ford in 2012 equipped with an automated dual-clutch automatic transmission which should avoid restart issues.
The stop-start concept is to enable engine shut off at every stop--as at intersections, to save fuel, typically on the order of 5 to 10 percent, and cut exhaust emissions—particularly desirable in congested urban areas. Lifting off the accelerator pedal will shut off the fuel injectors, stopping the engine. As soon as the driver lifts a foot off the brake pedal, the engine automatically restarts within a fraction of a second.If engineered correctly, the stop-start process will not be especially intrusive, but mild vehicle shudder may be detected by cabin occupants. That is an annoyance uniquely associated with stop-start technology.
By some estimates, there could be as many as 37 million stop-start vehicles on the road in North America, and 186 million globally by 2020.That may be conservative. Cost estimates vary, but industry participants say that stop-start systems will add $200 to $300 to the OEM cost of a new vehicle, assuming high unit volumes (as seen in Europe). In North America, which will start out at lower unit volumes, the OEM costs will be somewhat higher.
Mild-type hybrid electrics (i.e.: Honda Civic) and full-type hybrid electrics (i.e.: Toyota Prius) have built in stop-start, but stop-start also includes the so-called micro hybrid, also known as engine stop-start vehicles, that have no electric propulsion assist. With micro hybrids, there may be some regenerative braking energy capture and storage, including a modest added battery upgrades to handle the increased electrical load during engine off operation. The system may include a robust belt-driven alternator/starter (BAS), or a beefed-up conventional starter motor engaged with the flywheel. All micro hybrids will require a more robust starter motor than those used in current conventional gasoline power trains to handle the frequent engine restart activity.Starter upgrades will likely include better heat rejection capability, stronger bearings, and perhaps higher power ratings.
Standard starter motors for automotive gas engines draw approximately 1.5kW at 12-14 VDC (or around 125 amps). Cold starts in the winter with viscous motor oil will draw many more amps, typically over 200.Hot restarts can sometimes be more challenging for engineers than cold starts as they require more cranking power. During each engine start, the severe current draw is such that the vehicle DC power bus voltage will sag and incandescent lamps will dim temporarily. That is a problem for some accessory loads, which will automatically shut down when faced with low voltage. For such sensitive accessory loads, stop-start micro hybrids will need an added voltage-regulated DC-DC power supply module, sized at around 500 to 1,000 watts. Stop-start hybrids also typically need to add an electric coolant pump to keep the cabin heater running when the engine is turned off. Engineers must build-in robust levels of reliability as owners will not tolerate any failed or delayed restarts. The delay can be dangerous (impatient drivers behind or failure to respond in impending collision situation) in heavy urban traffic.
Frequent start-stop activity is not kind to passenger car engines.The constant acceleration and deceleration of moving parts may increase bearing wear and decrease the interval between engine overhauls resulting in more frequent repair activity. Frequent stop-starts may be manageable and acceptable, if the severe duty cycle is considered up front in the design and construction of vulnerable mechanical and electrical components, as well as in the choice for engine lube oil quality and service intervals. Lube oil can break down prematurely in demanding engine stop-start service. That could dictate shorter drain intervals. Lube oil must flow quickly and easily to critical engine parts if your engine is to last. Well-designed additive packages and high performance lube oils could restore drain intervals, pleasing both drivers and OEM car builders.
