It’s not unusual for auto makers to roll out a new technology and play up its attributes, especially when it comes to improving fuel economy.

What is unusual is that the technology actually delivers. Such is the case with stop/start systems, which were featured on two of the engines on this year’s Ward’s 10 Best Engines list. A number of other engines featuring the technology were tested, but did not make the list.

The concept is simple. Shut down the engine when at a complete stop to conserve fuel and automatically restart it when the accelerator is depressed or the brake is released. Auto makers claim real-world fuel savings of up to 10%, and during this year’s testing we find those numbers aren’t farfetched, particularly in city driving.

But the devil is in the details. When not properly executed, stop/start can become an annoyance, even if it ultimately saves fuel.

Among this year’s winners, there were some systems that performed flawlessly, switching the engines off and on seamlessly, with little feedback to the driver. The 2.0L turbocharged DOHC I-4 tested in the BMW Z4 and 528i springs to mind. The system jumped into action frequently and with little fanfare.

The Kia Soul was a different story. The system did shut down the 1.6L I-4 when expected, but it restarted with a loud whirring sound that filled the cabin. The noise was similar to when the car is first started by the driver, and after a while it became grating to some testers.

Yes, the 1.6L direct-injection I-4 won a Ward’s 10 Best Engines award this year for its application in the Hyundai Accent, but a number of judges found the stop/start version of the engine in the Kia Soul less appealing because of the system’s NVH issues, despite its fuel-saving benefits.