Stop-start technology automatically shifts the car into neutral to turn off the engine. Then when the driver lifts off the brake pedal, the starter motor re-engages the engine and the drivetrain shifts back into drive. “The whole process can take nearly a second,” according to the Automotive News’ story, which quoted Kentaro Yokoo, chief engineer of the hybrid drivetrain in Honda’s Fit.
It’s important to note, however, that hybrid/electric assist vehicles experience almost no delay in power from a stop, due to the instant availability of power from the traction battery to the electric motors.
On the other hand, non-hybrids must rely on a run-of-the-mill starter motor, which is usually too weak for a seamless reboot. That results in that slight partial-second delay in gasoline cars. One solution is to increase the size of the starter motor. But this would add weight and cost.
Whether a split second is a lag that can be eventually tolerated by drivers might all depend on consumer education and a better man-machine interface that informs the driver of stop-start activities.
Either way, to keep drivers happy with stop-start vehicle, “the process must be seamless,” reported Navigant Research, while “it must not have any measurable effect on performance.”
TI’s Smith pointed out, “When the system cuts off the engine, it needs to be smart enough to keep on the things that need to stay on,” instead of shutting down everything.
Previously, in some stop-start vehicles, consumers complained the loss of air conditioning during engine-off time. “It’s important to keep things like radio, heater, lighting system, power window on, during engine-off time,” Smith said. In sum, “automakers need to think differently -- in terms of how they power their entire automobile system,” she added.
Chip vendors also face an extra challenge in the stop-start application. Every time the system turns the engine on, “there is a large power surge,” explained Smith, “generating a lot of voltage noise.” That’s when the ability to protect the rest of the power components in a car becomes crucial, she noted.
Regardless of the challenges, stop-start vehicles are something the automotive industry can’t afford to ignore, as carmakers face aggressive fuel economy and emissions regulations in North America, Western Europe, and Asia Pacific.
Looming large is the CAFE standard of 54.5mpg by 2025. In 2011, President Obama announced an agreement with thirteen large automakers to increase fuel economy to 54.5 miles per gallon for cars and light-duty trucks by model year 2025.
So, you might ask how big a fuel saving benefit the various stop-start systems bring.
According to the SAE report, a belt starter generator version provides a fuel savings today “between five percent and six percent on the NEDC (New European Driving Cycle).” The NEDC is a driving cycle designed to assess the emission levels of car engines and fuel economy in passenger cars (excluding light trucks and commercial vehicles).
If five percent fuel saving in every stop-start vehicle is true, it’s nothing to sneer at. I’ll look for it in my next car (and skip the undercoating). What about you?