Depending on driving conditions, the fuel economy improvements of the start/stop function were between 7.27 and 26.4 percent in fuel use during testing, according to the Society of Automotive Engineers (SAE).
“If you’re constantly on the highway and the engine never shuts off, the fuel savings are going to be much lower,” says Alex Knizek, director of auto test development at Consumer Reports. “But if you’re doing a lot of city driving with frequent idling, there is a legitimate reduction in fuel use with start/stop technology.”
Not all systems are the same. Most vehicles use a single absorbed glass mat (AGM) battery for all of the vehicle’s 12-volt electrical systems. An AGM battery is designed to discharge more slowly so the battery will have a longer lifespan than more traditional lead-acid car batteries. Some automotive manufacturers, such as BMW, Jeep, and Mercedes-Benz, offer models often referred to as “mild hybrids.” These vehicles have a small electric motor that assists the gas engine, which allows the car to coast with the engine off, and the engine can remain off longer at stoplights. These kinds of vehicles use a separate battery to power the start/stop function.
Regardless of whether your vehicle has one or two batteries for the start/stop system, or if it’s a mild hybrid or not, the technology is designed to handle the repeated starting and stopping of the engine.
“There’s sometimes a misconception that these systems are bad for the engine or starter, so some drivers disable the functionality,” Knizek says. They’re designed for this function, but like any added technology, there’s the possibility that they’ll need maintenance or repair down the road.
A common complaint is the inability to permanently turn off the start/stop function. Vehicles have a button that can deactivate it while driving, but the next time you restart the car, the start/stop function will be activated again.
Knizek adds, “Some people just think these systems are annoying, and reach for the system’s off button as soon as they start the car.” This can be helpful in instances where the stops are brief and there’s a need for quick acceleration to join traffic, because the start/stop systems can cause a slight pause as they restart the engine. This change in regulation would allow automakers to include the function, but it could remain disabled between ignition cycles instead of needing to be deactivated with each drive.
Full hybrids, such as a Prius or Camry, don’t use a traditional start/stop system. By design, hybrid systems seamlessly start and stop the engine as needed, often so smoothly that most drivers barely notice. The result is the best of both worlds: a refined powertrain in stop-and-go traffic and improved fuel economy.
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