Stop-start systems have been praised as a quick way to boost fuel economy.
But if the stop-start activation is balky or uncomfortable, the customer isn't going to care how much fuel it saves.
Case in point: The BMW 328i.
I own a 2012 model, and I love its dynamic response, snappy acceleration and taut handling. But the stop-start system is just awful.
With stop-start -- also known as idle-stop -- the car's engine is shut off when the vehicle is stopped and the driver applies the brakes. Releasing the brake pedal fires up the engine.
When my Bimmer's engine cuts out, everything is fine -- even the air conditioning continues to blow cold. But when I take my foot off the brake to restart the engine, the trouble starts.
Ever sit on an airplane with a bothersome child seated behind you, one who keeps kicking your seatback just a little too hard? That's the feeling of the BMW stop-start system re-engaging.
It's annoying, a real detriment to the driving experience. Checking BMW enthusiast sites on the Internet shows that I am far from alone in this feeling.
There's a button on the car's instrument panel that manually defeats stop-start, but a driver has to reset it every time he or she gets in the car. Or a BMW dealership can perform a software hack that defeats the system forever.
That shouldn't be the solution to the problem, though. That's just an admission that there is a problem.
I asked dealer service techs, BMW PR guys -- even Ludwig Willisch, the CEO of BMW of North America -- why the system is so harsh. The main explanation was that the BMW 2.0-liter turbo has more power than other four-cylinder engines with idle-stop, so I should expect a bit more choppiness and vibration at startup. In other words, if you want a powerful engine that still is fuel efficient, that is the trade-off.
I almost bought the explanation, until I drove a Porsche 911S. Its boxer-six engine generates 400 hp and 325 pounds-feet of torque -- far more than the BMW turbo-four's 240 hp and 255 pounds-feet of torque. Porsche's start-stop system was so seamless I almost couldn't notice when it re-engaged.
How much gasoline stop-start systems save is the subject of a raging debate. Some engineers claim it can increase fuel economy by as much as 10 percent over not using stop-start. Others say that is a wildly overinflated estimate, ascribing gains of only 0.1 or 0.2 mpg. That gap depends on which testing cycle is used or if calculating real-world gains.
BMW needs to send its engine team back to the drawing board to solve the problem, not just wait for aggravated customers to deactivate the system altogether. Hopefully BMW can create a software patch that will allow owners to save gasoline without getting kicked in the back every time they come to a red light.