RESEARCH SNAPSHOT: 'Stop-start' systems

By John Voelcker | 25 October 2011

In this extract from just-auto's QUBE research on electric vehicle technologies, we take a quick look at so-called stop-start systems. Do you know, for example, that Mazda's solution does not involve the starter motor or that stop-start has been a non-starter in North America?

Also known as idle-stop systems, these are sometimes erroneously referred to as hybrids. There’s even a made-up category name for them: micro-hybrid.

But in fact they’re not hybrids, in the sense that they have neither a secondary battery pack nor an extra electric drive motor. The sole factor that sets apart a vehicle fitted with a stop-start system from a standard petrol or diesel car is that the engine switches off when it decelerates or comes to a stop.

As soon as the driver puts in the clutch or moves the shift lever (in a manual-gearbox car), or lifts off the brake and accelerates (in cars with automatic transmission), the stop-start system uses a high-output version of a standard 12V battery to start up the engine, and the car moves away as usual.

Start-stop systems are being fitted to a number of new models in Europe and Asia, from small (various Mini models) to large (the Porsche Panamera). Among others, certain models from BMW, Citroën, Fiat, Mini, Peugeot and a handful of Volkswagen Group brands are offered with a start-stop option.

A variation on conventional start-stop systems is the clever Mazda i-Stop system, which does not involve the starter motor at all. Instead, the engine is stopped when one piston is just a few degrees past the top of its compression stroke. As the brake pedal is lifted, the system’s control software sends power to that cylinder’s spark plug to fire the engine, and it resumes operation as standard – with no involvement from the starter motor.

No vehicle is sold in North America with a stop-start system, due to two factors:

Robert Davis, who is senior vice-president for quality, research and development at Mazda North America, said the company’s tests show the i-Stop system would increase gas mileage on the city test cycle by only 0.1-0.2mpg. In traffic like that of London, Tokyo, or New York, on the other hand, he said the reduction in fuel usage could add as much as 3mpg to the mileage figure.

That list is expected to grow rapidly, especially in those European and Asian countries with either explicit limits on vehicular CO2 emissions or zero-/low-emissions cordons in central city areas. Thus far, the total number of start-stop systems installed globally is less than 1m (versus more than 2m full hybrids manufactured since 1997).

But start-stop technology, of any design, may grow over the next decade to volumes of several million a year in Europe and Asia. Parts maker Valeo, which produces start-stop components and systems that are fitted by several European makers, estimates that in the densest urban traffic, vehicles can idle fully one-third of the time. Those emissions are entirely eliminated by start-stop systems.

Indeed, the greatest gains of such systems might be to the foul urban air quality of China’s growing cities, leading some analysts to suggest that China could benefit the most from universal fitment of start-stop systems. That is unlikely to happen without government mandate, however.

It is also unclear whether consumers in very hot or very cold climates throughout the world will willingly accept the loss of air-conditioning or heating while the engine remains off.

We do not define start-stop systems as hybrids, and they are far from ‘electric-drive vehicles’ because the starter motor does not provide motive power. Hence they are not included in this series of market analyses.