Stop-start systems | where and when they save fuel and money

10 February 2016
 by 
, CarsGuide

The world’s desperate need to use less oil and produce less stinky, choky emissions has led to inventions large and small.

Sure, wind turbines are probably annoying if you live right next to them, or if you’re a bird, but they’re not forcing their way into our lives the same way stop/start systems in cars are.

Designed to save your wallet, and your lungs, by turning your engine off at every possible occasion, these tricky, techy little systems have gone from hateful and borderline dangerous to at best mildly annoying. So are they worth the bother?

Not off to a good start

Stop-start certainly entered the public domain fairly poorly; the early systems cut and restarted the engine about as intrusively as a prostate exam.

Worse still, with some early efforts they would scare you witless by taking just that millisecond too long to fire back up, just as you were trying to accelerate into a gap in a roundabout. Sometimes resulting in a stop-start experience for your heart.

Even though manufacturers tried to pretty them up with a series of names such as ‘start-stop’, ‘stop/start’ or ‘idle-stop’, to anyone behind the wheel, they were usually referred to as ‘that stupid engine cut-off thing’.

Luckily, over the years, they’ve mostly improved to the point where drivers aren’t confronted with a dreadful, shuddering mess every time they kick in on petrol engines; it’s become more of a polite cough.

Unfortunately for diesel drivers, there’s no real way to stop and restart an oil-burner without an almost consumptive level of shakes, rattles and barks. The very best systems, in high-end diesel Audis, are getting close to seamless, while a diesel VW Golf is still some distance away from that description.

So, why do we bother with the recalcitrant engine nannies at all?

It’s to do with saving fuel, of course; if an engine isn’t running, it’s not using any fuel. And, perhaps more importantly to manufacturers, it’s not pumping out any horrid, toxic fumes like NOX and CO2.

Emitting too much noxious gas means serious tax implications for manufacturers and, if the levels are bad enough, they won’t be allowed to sell the car at all, thanks to Euro and EPA regulations.

So, it all sounds like a win-win, at least in theory. It’s a wonderfully simple solution to cut the engine when we’re stopped and idling, saving fuel that would otherwise be burned for nothing.

But how much of a benefit does start-stop offer in the real world? That’s the sticky issue.

Passing the test

The stop-start system is actually at its best during the Euro-backed official emissions test cycle and, as we’re beginning to find out, that test can at times be about as accurate as a shotgun with the barrel removed. Or, as VW has shown, it can be fooled.

The New European Driving Cycle (NEDC) is a method used across Europe, the UK and Australia. Devised in the 1990s, the NEDC seeks to assess the environmental impact of a car by putting it in a lab and running it on a rolling road in a way that purportedly simulates real-world driving.

Some would argue that it imitates the real world about as well as punching yourself in the face simulates being a UFC fighter, but car companies insist that it provides a fair and level playing field, and that actual testing in the actual world has too many variables to be fair.

There’s no denying the test itself lacks some rigour, with tepid acceleration, low-speed cruising at constant rates and placid coasting the norm for the entire cycle.

A decent chunk, however, is the amount of time cars spend at a standstill and idling during the test; in fact, out of the 13 minutes of the urban section of the NEDC, a total of a minute is spent at a dead stop.

It’s the time spent idling that really helps stop-start systems; spending almost eight per cent of the test with the engine off means an instant eight per cent improvement in the official fuel use and emissions figures.

Spending time idling at traffic lights can really feel like a waste of fuel and, with an ever-increasing focus on economy and emissions, it makes sense to switch your engine off.

To achieve the same level of benefits from stop-start in your car use, you’ll need to really be in the thick of some serious traffic and – crucially – sitting completely still in it. Very rarely, you can find a car that will shut everything off as you coast but, for the most part, you’ll have to come to a full stop.

The slow, rolling traffic that so typifies much of Australia’s peak hours obviously doesn’t work with start-stop.

However, for lengthy periods at traffic lights, stop-start does offer real benefits.

For those with older cars – and those that remember them – spending time idling at traffic lights can really feel like a waste of fuel and, with an ever-increasing focus on economy and emissions, it makes sense to switch your engine off when you don’t need it.

And any gains, even if it’s not the eight per cent on offer in the official test, are worth having, right?

Something else to consider is that cars running the NEDC test also have their climate control and ancillary systems turned off.

In the real world, you’ll likely have your air conditioning on, which will drain the battery. Modern stop-start systems will run accessories in a low-power mode to conserve power but in our hot summers, the drain of aircon means there won’t be any juice left and the onboard computer will be forced to kick the engine back into life to recharge the battery.

This will usually happen after repeated stop-start interventions, when the system detects that the battery level is too low to sustain auxiliaries and ancillaries without restarting the engine.

Turned off

So, it all sounds pretty grim. If you have stop-start, chances are you turn it off as soon as it cuts in, muttering something mostly unpublishable as you do so. For those that leave it on, the benefits don’t really add up to an enormous hill of beans.

However, stop-start finds its niche in vehicles that seem to spend half their lives idling. Think about taxis, garbage trucks and buses. If stop-start has a place, it’s there.

Or, if you live in Perth, which seems to have the longest and most tortuous red lights in the country, you might actually get some benefit.

Overall, though, you have to think there’s a good reason all stop-start systems come with an off switch. For now.

Do you love your stop/start button or do you switch it off at the first chance? Tell us in the comments below.

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