Engine Stop-Start Systems Save Fuel at Low Cost

The rumble of idling cars and trucks is a familiar noise at America's intersections. But it's a sound that's likely to begin fading away in coming years: More and more automakers are using engine stop-start systems to boost fuel efficiency at relatively low cost.

The system, which shuts down a vehicle's engine at idle and immediately restarts it when the driver presses the accelerator or lifts off the brake or clutch pedals, is a feature of every conventional hybrid vehicle in the market. But now stop-start systems are beginning to appear as stand-alone features on conventionally powered cars. Up to 40 percent of all new cars and light trucks sold in the U.S. in 2015 could be equipped with engine stop-start systems, according to Johnson Controls, a major global manufacturer and supplier of the advanced batteries used in stop-start systems.

Fuel Savers

Stop-start systems can cut combined city-highway fuel consumption and greenhouse gas emissions by 3-10 percent, and even more when they're combined with hybrid drive systems. By eliminating engine idling, stop-start systems also reduce toxic and smog-causing tailpipe emissions. The systems deliver their fuel economy improvements in city situations that involve lots of stop-and-go driving. And they can do it at relatively little cost to the consumer.

While stop-start systems have typically been included in the overall price premium for hybrids, many industry analysts say their actual cost is between $300 and $400.

Ford Motor Company recently announced a $295 price for its first stand-alone stop-start system, offered as an option on the automatic transmission version of the 1.6-liter EcoBoost model of the 2013 Ford Fusion SE sedan. By comparison, a full hybrid system can add $6,000 or more to the price of a car.

Consumers Gain

With gasoline at $4 a gallon, a $300 stop-start system could pay for itself in three years or less — a payback period most U.S. consumers find quite acceptable.

A 20-mpg car outfitted with a system that offers a 3.5 percent fuel economy improvement over 15,000 miles of annual driving would use almost 25.5 fewer gallons of gas each year. That's an annual savings of $102. A $300 system offering a 10 percent fuel economy boost would earn back its cost in just 13 months with gas at $4 a gallon.

How They Work

The brains behind engine stop-start systems are the computer programs that control them. But drivers don't interact with the engine computer, so for the person in the pilot's seat, it's the accelerator and brake pedal that make the magic happen.

Stop-start systems are designed so that depressing the brake pedal sends a "ready" signal to the engine controller. The clutch pedal is often the control device on vehicles with manual transmissions.

When the car comes to a complete stop, the controller shuts the engine down and — depending on the design of the system and type of transmission — pre-positions the starter motor, transmission and fuel injection system to provide instantaneous engine restart when the driver either depresses the accelerator pedal or releases the brake pedal.

Most systems aim for the restart to occur in half a second or less. Ford's stop-start reignites the Fusion's engine in just a quarter of a second.

What Else Stops?

Turn off the engine in your car or truck and both the air-conditioning and heating systems stop functioning. That's because they draw lots of electricity from the generator, which doesn't generate with the engine off. Most A/C systems also use power from the engine to run their compressors.

"Heating and cooling is one of the big challenges in stop-start development," says Stephen Poulos, GM's chief engineer for eAssist and battery-electric propulsion systems. GM models with the eAssist mild hybrid system include a stop-start system. "If you stop the engine, how do you keep the temperature in the cabin comfortable?"

It's not a huge problem on mild days or when the stop period is very short. But it is on very hot or very cold days. It's also an issue when there's a lengthy delay, such as at a railroad crossing or a major intersection with long intervals between red and green lights. In those cases, "you've got to do something," Poulos says.

Full hybrid systems, such as Toyota's Prius, use an electric air-conditioning and heating system that isn't dependent on the car's internal combustion engine.

For its eAssist system, GM developed control logic that maintains cabin temperatures for up to 2 minutes when the engine is stopped. After that, the system restarts the engine to keep the climate system working and cabin temperature comfortable.

Ford uses a similar system. Some stand-alone systems simply kick the engine back on whenever the cabin temperature falls above or below the driver's preset limits.

Battery Burden

Stop-start systems aren't terribly hard on the engines, but they do demand a lot of the vehicle batteries. While a conventional car or truck might call on the battery for engine ignition and starter-motor power three or four times a day, a vehicle equipped with a stop-start system might place that kind of drain on the battery several dozen times a day.

In addition to providing electrical power for climate control systems, the battery in a vehicle equipped with a stop-start system also has to maintain audio and lighting each time the engine shuts down.

It's not a big problem in conventional hybrids because their batteries are huge compared to standard 12-volt lead-acid batteries. But to keep costs down, conventional vehicles with stop-start systems don't use those powerful hybrid batteries. Instead, they use a type of 12-volt battery called an absorbed glass mat (AGM) battery. The electrolyte is contained in thin woven glass mats rather than flooding the cells as in a typical "wet" battery. It also is pressurized for a greater power density.

AGM batteries' advantage over conventional 12-volt lead-acid batteries is that they recharge up to five times faster and can be deeply discharged with no damage. Those are essential characteristics for a battery that's being called on to start and restart a vehicle's engine scores of times a day.

Some critics claim that the demands of stop-start systems can quickly degrade AGM batteries. But Milwaukee-based Johnson Controls, which has been selling its AGM batteries for use in automakers' stop-start systems in Europe since 2008, says it has not experienced any increase in warranty claims due to premature battery failures.

"AGM batteries are designed for the kinds of loads" placed on them by stop-start systems, says Garth Cole, executive director of product development for Johnson Controls' Original Equipment business unit.

Ford expects — and warranties — the stop-start Fusion's AGM batteries to perform throughout their three-year life expectancy, says Birgit Sorgenfrei, Ford's stop-start systems program manager. AGM batteries, like most lead-acid batteries, typically come with multiyear warranties that provide for free replacement if the battery fails in the first three years, and then offers a prorated replacement schedule for the rest of the warranty period.

Stop-Start Downsides

There's a small learning curve involved in driving a vehicle with a stop-start system. Many drivers find it disconcerting when the engine shuts off each time they come to a stop. And some systems aren't as smooth as others, with drivers reporting unexpected jerks and jolts when the engine restarts. BMW's system, for example, has been widely criticized for this. Many owners report that they have disabled the system with the shutoff switch that's standard with the BMW system. Indeed, most stand-alone stop-start systems come with driver-selectable on-off switches.

"Education needs to accompany introduction of this technology because car buyers will have to learn how these systems function differently from what they are used to," says Edmunds.com Senior Analyst Jessica Caldwell. "The burden for this will fall on the dealerships, but smart dealers will see it as another way to build relationships with consumers."

Finally, if the special 12-volt batteries needed for stand-alone stop-start systems do need replacement, they are about twice as expensive as conventional 12-volt lead-acid batteries.

Nothing New for Hybrids

Stop-start systems are old hat for people who drive hybrids. These cars always have incorporated stop-start to help improve fuel economy. Unlike stand-alone stop-start systems, which rely on 12-volt batteries to help keep costs down, hybrids are able to draw on their electric-drive systems' robust but expensive nickel-metal and lithium-ion batteries to provide the power needed to operate the systems.

One recent example of the stop-start/hybrid marriage is General Motors Corporation's eAssist system. It doesn't identify itself as being a mild hybrid, but that's what it is. GM introduced eAssist last year on four-cylinder 2013 Buick LaCrosse models and it now also is available on the four-cylinder 2013 Buick Regal and the 2013 Chevrolet Malibu Eco. It's coming on the Chevrolet Impala in the spring of 2013. Reviewers have given the GM system high marks for seamlessness — there's no jerking or jolting when the engine starts up.

In addition to its stop-start function, the eAssist system provides an electric-drive boost to the gas engine when needed for passing or hill climbing. It uses a 115-volt lithium-ion battery and a regenerative braking system to help recharge it.

Poulos says the "bare-bones" eAssist system can deliver a 10-15 percent fuel-efficiency improvement. Embellishments such as low-rolling-resistance tires and aerodynamic improvements to the vehicle can boost savings to 25 percent, he says.

Vive la Difference

Unlike their operation in hybrids, stand-alone 12-volt systems don't provide additional electric-drive assist to the gas motor. All they do is shut down and restart the engine when the vehicle comes to a complete stop — a considerable chore in its own right.

But that relatively simple function appears to make sense when fuel efficiency is the goal. Stand-alone stop-start systems have been in widespread use in Europe for years, mostly on cars with manual transmissions. So why are they only now appearing here?

In part it's because the official fuel-efficiency test system used by the Environmental Protection Agency doesn't incorporate much idling time, so the fuel savings that stop-start systems can provide haven't been reflected in the official fuel economy ratings. That's changing with the new 2017-'25 fuel-efficiency standards, which provide extra credit for "off-cycle" systems such as engine stop-start.

Additionally, it has been tough to develop smooth, quick-acting stop-start systems that work well with the automatic transmissions that U.S. consumers overwhelmingly prefer.

But engineers have managed to develop a control system that can recognize a driver's intent to stop without putting the transmission into Neutral, as occurs when a manual transmission's clutch is depressed.

Rapid Growth Predicted

With the big stumbling blocks apparently out of the way, stand-alone stop-start systems may become ubiquitous on new cars and trucks in the U.S. in coming years.

The start has been slow, with a number of Audi, BMW, Mercedes-Benz and Porsche models debuting 12-volt stop-start in the U.S. in the 2012 model year. Jaguar added stop-start to its 2013 XF and XJ sedans and Kia has plans for stop-start systems in upcoming 2013 Soul and Rio models.

Ford's 2013 Fusion will be the first domestic car with a stand-alone stop-start system. But others are expected to follow. Chrysler Group includes stop-start in the V6 HFE (high-fuel-efficiency) model of the just-launched 2013 Ram 1500 pickup. It will add a V8-powered HFE model of the Ram 1500 with stop-start early in 2013. The company claims a 3.3 percent fuel economy increase with the system.

Several market research firms that follow the technology are predicting significant increases in stop-start systems globally in the next decade, with the U.S. accounting for a lot of the increase.

The number of vehicles in the U.S. market with engine stop-start systems would more than triple to 10 million in 2020 from 3 million in 2015, according to a prediction in a new report by advanced technologies specialist Pike Research. Johnson Controls says that global production of the systems is expected to rise from 3 million a year today to 35 million a year in 2025.

It's all part of the new automotive landscape, as governments in the U.S., Europe and Asia establish increasingly strict fuel economy standards. Anticipated gasoline price increases also may give rise to growing consumer demand for more miles per gallon from their cars and trucks.