Lately we've registered some complaints about the start-stop system in our new BMW 328i. It mimics a hybrid car by shutting down the engine at stops, rather than wasting fuel idling. It sounds like a great idea in theory.
One of the latest in a suite of fuel-saving technologies to aid manufacturers in reaching ambitious new federal mpg targets, a start-stop system shuts down the engine when the car stops and automatically restarts it as soon as you take your foot off the brake pedal. Many hybrid cars have done this for years as part of their overall strategy to save fuel. But with their expensive batteries, hybrid systems are costly, whereas start-stop systems are relatively cheap to implement.
Start-stop systems have been popular in Europe since the 1980s, but it's a foreign concept here in the good-old U.S of A. If each car in the United States idles just six minutes per day, about 3 billion gallons of fuel are wasted annually, according to Argonne National Labs (pdf). That adds up to more than $10 billion a year spent going nowhere and much of that money leaves the country. Put another way, automakers say start-stop can achieve a real-world fuel economy boost of between 3 and 7 percent, depending on idle time.
We've tested several cars with start-stop systems recently: the BMW 328i and Porsche Cayenne with simple start-stop systems, as well as the Chevrolet Malibu Eco, Honda Civic Hybrid, Toyota Camry Hybrid, and a seemingly constant string of Prius variations. And we've found the key to success is in the execution.
In driving the 328i, as well as the Cayenne last year, several of our testers complained that the engine was rough when restarting—a particular concern in a high-priced luxury vehicle. Most of our 328i drivers said they used the defeat switch next to the ignition to keep the engine running.
In two days of driving the 328i recently, including some New York City stop-and-go, I found system didn't bother me—or at least I thought the trade off was worth it not to waste the gas. That is, so long as the air conditioning wasn't affected and it wasn't because if the cabin gets too hot, the car is smart enough to restart even if your foot is firmly on the brake. Nonetheless, the car would shudder with a jolt when the engine suddenly restarted. Clearly not a $43,000 experience. After just getting out of a long weekend with the Chevrolet Malibu Eco, the contrast was startling. The redesigned Chevy delivered silky-smooth restarts every time.
Our experience shows that start-stop systems work as intended. In our own city course that we use for fuel economy testing, the 328i showed a 1-mpg improvement which accounted for a 5.3 percent improvement compared to running the test with the feature defeated.
Unfortunately, stop-start systems are not rewarded under U.S. fuel economy regulations, as they are elsewhere in the world. Under the driving cycle the EPA uses to test fuel economy, cars spend almost no time idling. This doesn't comport with the modern reality in most, urban and suburban areas in the United States today. Nevertheless, as our tests show, this feature has its merits as long as it works unobtrusively.
Everybody wants a car that's more sparing with fuel so let's pick the low-hanging fruit in improving fuel economy: Stop idling!