The C3's Stop & Start system makes light work of gridlock, says Tom Stewart

Back in the days before fuel injection systems had more computing power than Nasa, it was considered imprudent to switch off your engine while driving unless you were stuck in the mother of all traffic jams.

Back in the days before fuel injection systems had more computing power than Nasa, it was considered imprudent to switch off your engine while driving unless you were stuck in the mother of all traffic jams.

As carburettors sluiced petrol like it was going out of style and ignition systems were slow to respond to the actions of their bronchial starter motors, it was generally thought that more fuel was consumed by restarting the engine than by leaving it idle for a while.

The correctness of this wisdom would have depended on many things, but if you drive Citroën's latest C3, the £11,500 Stop & Start with SensoDrive, then technology takes such decisions for you. From the driver's perspective it's simple; when the Stop & Start stops, the engine stops too. And when you choose to go again, the engine's running and ready. Why? Because, when stationary, not having the engine on saves fuel - up to 10 to 15 per cent less in heavy traffic - and it cuts CO2 (similar figures) and noise emissions.

The principles have been tried before; VW fitted Ecodrive to a Golf a decade or so ago, but partly because that system could cut the engine while at speed, thereby allowing the car to "coast", it didn't catch on. It also required a large, expensive and heavy battery and employed a traditional starter motor, the latter not being ideally suited to constant Stop & Start use. More recently, the tiny Lupo and Arosa 3L (94mpg) employ a similar system to this Citroën, but they are for mainland Europe only.

Here's how Stop & Start works in a little more detail. When the brakes are applied to stop the car, the engine cuts out at 4mph as it's slowing to a complete standstill. A green "eco" light comes on, the rev counter needle disappears and the automated clutch in the SensoDrive transmission stays disengaged. The engine then remains on standby until the brake is released, whereupon the engine automatically restarts. When the throttle is applied the clutch automatically re-engages and, with the engine already on, you're off. The system can easily be deactivated and it won't operate in certain situations, such as when the engine temp is too low, a low battery charge or while in reverse.

The key component of Stop & Start is a reversible alternator, which performs the functions of both alternator and starter motor. This reversible alternator is inaudible and fires the engine twice as quickly as a conventional starter.

In practice, the system operates so seamlessly that driving it seems like a non-event - you barely know it's happening. The SensoDrive clutchless transmission is already available on a couple of 1.6-litre C3s, but it's compulsory on the 1.4-litre Stop & Start. It offers the choice of sequential manual gear-changing or a fully automatic mode which, if you're not too heavy on the throttle, provides smooth upshifts with perfect downshifts.

Though at 50mpg (combined) it's not hard to appreciate a significant saving on your fuel bill, or indeed the decreased emissions, you'll be pleasantly surprised by how much more relaxing it is to be going nowhere in a car without its engine running.

But lest you think that the C3 S&S is some kind of overly green eco-machine, uncomfortable anywhere other than in a dense urban environment, think again. With a 1.4-litre four-cylinder 16V 90bhp petrol unit, it's perfectly capable of tackling distance. With a top speed of 112mph and a 0-62mph time of 13 seconds it won't embarrass hot-hatch pilots, but the Stop & Start's performance is sufficient for most situations.

You may be thinking that Stop & Start technology could also be usefully employed on diesels, and it could, but for the time being it's petrol engines that need help where CO2 emissions are concerned. Citroën's 1.4HDi C3 for example has a CO2 figure of just 110g/km, (25g/km fewer than the 135g/km Stop & Start), but Citroën reckons to first give help where it's needed most.

As for congestion charges, cars in the capital are granted a concession for their electric or hybrid engine technology and/or their alternative fuel type rather than the actual quantity of CO2 emitted. Any car running solely on diesel or petrol, no matter how clean or frugal, still faces the £5 charge. Still, at least you should feel better about being stuck in traffic than before.

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