Start / Stop systems work by turning off the engine when the car is stationary, automatically. This is a feature commonly noticed in city driving and stop-start traffic. Every time the car is stationary the engine will stop turning over requiring no fuel to power the engine mechanics, therefore, prevent unnecessary idling.
Using the internal computer, the car detects if it is out of gear or stationary, fuel delivery is halted and not activated again until the car is moving or clutch pressed. Although automatic, it can usually be turned off with the capital A button on the dash somewhere.
Conventional starter motors engage a pinion gear to a larger 'ring' gear fitted around the outside of the engine flywheel.
Modern start-stop technology is similar but as expected, more powerful, durable and faster. Some use a 'TS' stamp to denote a tandem solenoid able to deal with more unexpected scenarios of rapid decision changes, making sudden acceleration more smooth. These are likely in traffic when sudden movement can occur requiring a stamp of the pedal. At this crucial stage, the engine might be in the process of shutting off as the clutch depresses and the car halts, but it is still spinning. Here one solenoid will fire up the starter motor to sync its speed with the engine rpm before the second engages the gear.
Although Start-Stop systems are relatively new, it has been common knowledge for a while in the motor industry that idling a car is one of the most inefficient uses of an engine, wasting fuel to produce no progress on your journey. Unless you're revving at 9'000rpm or tearing around racetracks, you're unlikely to use more fuel any other way.
As the video explains, @6:24 under testing the start-stop is an instant success after 7 seconds. So the hear-say you may have heard that starting an engine and turning it off in quick succession uses more fuel (like halogen light bulbs used to use more power) is true, but in real-world testing becomes insignificant to the savings after a full journey in all types of driving.
Whether it wears out the engine, all this constant halting and restarting, rubbing the mechanics is also negligible. Although based on some kind of reasoning, in real life situations it is less apparent, especially with new batteries designed to handle the extra cycles used to power the car, you can trust manufacturers have accounted for these factors before releasing new cars.
Increased Stop-Start cycles lead to increased engine wear, or do they? If the necessary precautions are taken, even with increased power cycles both battery and engine can last, in some cases longer than similar engines.
"A normal car without auto stop-start can expect 50'000 stop-start cycles in its lifetime", says Gerhard Arnold - Design at Federal Mogul.
"But with auto start-stop activated, every standstill, this figure can see as many as 500'000 cycles"
This level of increase poses a threat to the durability of the engine components and bearings. For example, the crankshaft is a heavy component that will affect the start motor after multiple cycles. In motion, a thin oil film will create hydrodynamic lubrication, but when stopped the metal surfaces will come into contact.
Stop-Start means the boundary condition (metal-to-metal contact) could happen 500'000 times rather than 50'000, where normal bearings would wear out far before this amount. To prevent this, manufacturers develop new bearing materials with self-lubricating properties to resist wear. Federal-Mogul has developed a new material called Irox with a polymer coating containing rust to be slippery, 50% lower friction than conventional aluminium that can still last the lifetime of a start-stop engine.
So really although the theory is correct that more cycles do still equal more wear, with better batteries equipped and better components fitted, you can be sure your new car is prepared for anything.