2014 BMW 328i xDrive Gran Turismo: Start-Stop Fuel Savings Test
August 28, 2014
Get used to automatic start-stop systems like the one on our 2014 BMW 328i xDrive Gran Turismo because they're only going to become more widespread in years to come. Such systems are quickly gaining favor as a fuel-saving and pollution-reducing measure because they temporarily shut down the engine when a vehicle is going nowhere at stoplights.
The basic logic of the concept is hard to argue with, but anything that's this much of a departure from the norm is bound to earn itself a heaping helping of detractors. Some can't get past the feeling that their engine just stalled. Others can't stand it when their car does something they didn't initiate. And there will always be those that give a thumbs-down to anything that stands in the way of a quick getaway.
For them there's an off button.
But what are the disablers missing out on? Are the fuel savings big enough to be noticeable or is this just another one of those theoretical decimal-place increases that automakers chase after because they're desperate for anything that can nudge their CAFE score up a peg?
I cooked up a test to see if I could find out.
We already have an established One Lap of Orange County hybrid fuel economy loop, but it's a bit longer than necessary for this test. A Prius is rated at 50 mpg in the city, but our BMW has a city rating of just 22 mpg. It'll burn enough fuel for a good measurement even after I prune about 30 miles and one hour out of the loop.
This gave me the opportunity to chop out some of the One Lap route's longer cruising sections and increase the signal density. The result is a 76.9-mile course that takes about three hours to complete.
I drove our BMW 328i GT around this modified loop on successive days, starting at 7:45 a.m. each time so the traffic pattern would be similar. I kept the start-stop system fully operational for the first circuit and disabled it for the second one. I used the same pump at the same station at the beginning and end of each loop.
Here's what I found.
Start-Stop Disabled: 25.1 mpg (4.0 gallons per 100 miles)
Start-Stop Enabled: 27.5 mpg (3.6 gallons per 100 miles)
Our BMW delivered 2.4 more mpg when the start-stop system was allowed to do its thing, a 9.4-percent improvement. Presuming you do a lot of city driving, that's about a gallon per fill-up. I don't know about you, but that's more significant than I expected.
The system did have one thing going for it in this test. My early morning start made it unnecessary to run the air conditioning, which ensured the climate control system would never force a premature engine restart at the longer red lights in the name of comfort.
As for technique, once I stopped I never released the brake to "pack up" behind the cars ahead if their drivers decided to creep forward. Such a move would have restarted the engine and disabled the system until the next cycle. I made sure to stand my ground until it was time to go.
So, yeah, hot weather and heavy A/C use will trim the benefit somewhat. And anxious types that can't bear to let a small gap develop in front of them as they wait for the light can nullify the system almost entirely with packing behavior.
This test is only the beginning. I need to repeat this test a couple more times, of course, and expand it to include other vehicles. And the fact that I beat the BMW's city mpg rating in both cases suggests I need to add some more stops to my route.
But I don't expect any of that to negate the basic result. The start-stop system in our 2014 BMW 328i GT appears to deliver much more than a theoretical benefit. The potential savings seem to be big enough and real enough to leave the system on and make peace with it.
Dan Edmunds, Director of Vehicle Testing @ 15,301 miles