What is Stop/Start? How does it save you fuel?

It seems it doesn’t matter what car you’re after these days, it’ll come with a stop/start system either as standard or as an optional extra. From Dacia to Rolls-Royce, car manufacturers seem to be obsessed with the tech.

But what on Earth is stop/start and why are the manufacturers so keen on fitting it to our cars? Let us explain…

There are a massive array of these systems out there and, as usual, each manufacturer has its own name for it. But whether it’s stop/start, start/stop, i-Stop, Stop & Go or another branding exercise, the broad function is the same – to turn off the engine automatically when the car is stationary and turn it back on again when you need it. Not only do they save fuel but these systems make waiting in traffic more peaceful because you don’t have the drone of an idling engine to deal with.

The purpose is environmental in nature, of course. When you’re stuck in queueing traffic and going nowhere, you’ll be burning a small but not inconsequential amount of fuel to no effect other than keeping the engine idling. Turn the engine off and this vanishes, along with the CO2 emitted from the tailpipe – these systems can save around 10 per cent on emissions, as time spent at idle is part of the official fuel economy test.

Starting an engine – especially when it’s cold – puts a lot of strain on various components so the fitment of stop/start systems could be a source of concern given how many extra starts it puts the engine through. Manufacturers are aware of this, however, and have beefed-up the parts in question – this includes stronger starter motors, more resilient batteries and new cylinder liners to resist the strain of repeated starts.

Is there a downside? Most of these systems only operate when conditions permit, due to the high electrical load required to get an engine spinning. Commonly, they won’t function if you have the air-con set too high or have too many cabin electronics going at the same time. Some systems won’t function if your steering wheel is turned too far from the centre. Other systems won’t allow the engine to be stopped for too long otherwise the charge needed to restart it will dissipate.

Fortunately, all of the stop/start systems we’ve tested so far have an off switch so, if the 10 per cent fuel saving doesn’t sound like sufficient trade-off for you, the option to disengage it still remains.