Stop-start engine systems are one of those technological features that are becoming more common in cars to meet stronger emissions regulations rather than any strong customer demand.
Also known as idle-stop, they cut the engine when the car is not moving, such as at a red light, to save fuel that would otherwise be wasted with the engine running while waiting for the light to turn green.
Auto stop-start systems are proven to save fuel by up to 10 percent in heavy traffic conditions, and have their pros and cons, but how do they work?
As above video from tech website CNET shows, auto-stop start does a lot more than simply turn the ignition off for you. It has to stop the engine without cutting power to other systems and then has to restart quickly and smoothly allowing you to drive off without delay.
This is an electronic process, which turns off the spark to the ignition and the fuel flow to the cylinders, moments after you press the brake to come to a stop. When you take your foot off the brake the power is restored and the engine is restarted by the starter motor.
All this is done without cutting power to the rest of the car, meaning lights, infotainment and air-conditioning will continue to operate.
For auto stop-start to work properly, it needs systems such as power steering and pumps to be electric driven, not via belts which only work when the engine is turning.
The system also requires a tougher starter motor, which will be operating a lot more than usual, and a more high-tech battery, designed to run down and recharge. Some cars even have a second battery for this purpose.
And the engine itself needs harder and have smoother workings to reduce friction when starting and increase durability.
What to look for
As the video shows, not all Auto Stop-Start systems are created equally so make sure you assess its operation as part of your test drive.
The best Auto-Stop start system is the one you barely notice, so you want a fast restart, with the engine back up and ready to go in the time it takes you to move your foot from the brake pedal to the accelerator.
This means the engine needs to restart with minimal turning over. As well as starting quicker this reduces engine vibration making restart seamless. Well-designed engine mounts will also help reduce vibration.
And finally, you should be able to override Auto Start Stop, preferably with the push of a button and not hidden somewhere in the car’s setup menu.