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From the article John : If you've ever tried to start a car that's already running you'll have discovered that it makes a hideous crunching sound as the starter pinion tries to engage with a rapidly spinning flywheel! As a result single solenoid Stop-Start systems can pause the starting process and prevent the engine from starting until it's fully stopped – this isn't ideal as the car doesn't start 'on demand' but a moment later. A tandem solenoid works by spinning up the starter pinion to synchronise it with the flywheel, then the second solenoid seamlessly engages the gear. Highlighted and underlined the relevant bit - surely a contradiction in terms? The idea is to prevent the graunching of the starter pinion on a turning flywheel/ring gear so by spinning a starter pinion up and engaging with a ring gear (moving or not) the effect would be the same as trying to engage a stationary pinion with a spinning ring gear. Maybe it's just that haynes haven't explained it all that well but either way, a solenoid has a hefty current draw and i'm struggling to reconcile the concept of an excessive current draw with the need to start 50uS quicker than the conventional arrangement. More research is needed i think! *** EDIT *** Having done a bit more digging, it appears that is exactly how the sytem works. I'm much more used to starters that turn the engine at 250-300rpm to start them, it seems the newer motors are capable of spinning much faster to enable meshing with the ring gear at the same speed but it must be a very critical line of getting the two speeds matched! 😮 Also discovered the Permanently Engaged starter which presumably has a starter clutch inside to connect the starter pinion to the motor. Definitely Marmite!

Hi Dave In answer to your first point I agree, that is what I said 🙂 , I woould never use it if my destination was an Airport car park! However proponents would argue that is why the cars use AGM batteries that are designed for repeated deep discharge (I don't see myself approaching the expected service specification of over 300000 starts in the next five years). On the second point, again I sort of agree. However, as I said on wear and tear, this is accounted for by the design to minimse wear. A nice little article here: https://haynes.com/en-gb/tips-tutorials/stop-start-technology-explained As I said it is a marmite issue 🙂 J

Errr, i beg to differ - pre-engaged starters have been around for donkeys years and holding the pinion in so it was "ready" to restart would cause a considerable battery drain (~30A usually for hold-in current) which would flatten the battery fairly quickly. Also because of how the starter solenoid operates, it pulls the pinon into engagement and then makes the circuit but there are no half-measures - it pulls in and then the contacts are automatically closed so the motor operates.