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Emissions: the importance of being idle

By / 9 years ago / Features / No Comments

Claiming pollution from idling contributes to the 4,300 premature deaths in London each year, the capital’s Mayor Boris Johnson has revealed plans to increase the penalties levied against drivers who leave their engines ticking over in traffic. Should these come into effect, London will become a no-idling zone, with offenders hit by a £120 fine for leaving the engine running for more than 60 seconds.

But if doing so both reduces harmful emissions and cuts costs from wasted fuel, is avoiding idling an example set by London that fleets UK-wide should be following?

The No Idling campaign itself is nothing new – the City of London Corporation made the announcement last March that it was planning to make the capital a no-idling zone, and was working with the Mayor to do so. 

But while Boris Johnson said 2002 road traffic regulations allowed local authorities to issue fines to drivers who allow their cars to idle unnecessarily, he added that the ruling needed to be tightened up.

In his Clean Air Strategy, issued last month, he commented: ‘It is problematic to define “unnecessarily” and the penalty charge is fixed at £20, which is too low to be a powerful disincentive. The £20 penalty for idling compares unfavourably with the £120 penalty charge issued for parking offences. The Mayor urges the Government to increase the penalty charge for idling offences, so that it is in line with that for parking offences.’

But while the Mayor demonises idling, even manufacturer-fitted stop/start systems do not always switch off the engine while stopped in traffic. Furthermore, the majority of cars on the roads are fitted with conventional starter motor systems which are not designed to be used repeatedly. So is it always a good idea to switch off when stationary?

Jo Hudson, corporate communications director at Bosch, which makes the majority of factory-fit stop/start systems, explained that the way its technology works was not directly comparable to switching off manually. Such systems are fitted with an uprated starter motor and a battery designed to cope both with the load of repeatedly cranking the engine and to operate at lower states of charge.  Battery charge, cabin and oil temperatures are also monitored and factored into the car’s ‘decision’ to stop or not.

‘If a driver stopped and started the engine manually, these checks would not take place and they would not know whether the battery had sufficient charge to restart the engine. In addition, there is the consideration of ancillaries. In turning the engine off, a driver may also end up turning off electrical devices such as headlights, posing a safety risk,’ she added.

There’s a further question too. Just how much fuel would you save by repeatedly switching the engine off? Getting definitive fuel consumption figures for idling and restarting is difficult, and hard to test scientifically. As well as environmental factors, fuel consumption at idle depends on the engine and the alternator load from auxiliary systems. Cranking an engine also places a heavy load on the battery – visible on conventional starter motors because lights tend to dim on startup – which varies depending on the vehicle and the number of on-board systems running.

So while Bosch claims a 5% improvement in fuel economy on the NEDC fuel efficiency test cycle, it’s difficult to work out how this would compare to switching the engine on and off using traditional starter motor technology. If there are any results for the real world difference between stop/start fitted cars and those without it, they are a closely guarded secret by the industry. It makes you wonder why.

One engineer, who has worked for motor manufacturers but didn’t want to be named, told Fleet World: ‘It’s not a simple problem. You could get to work with a cold engine still on warm-up dependent on your drive cycle, so switching on and off repeatedly would use more fuel and your emissions would be much higher. The idea that it’s beneficial for an idling period of more than 60 seconds is purely circumstantial and can’t be a cross-generalisation. 

‘Plus in a non stop/start car without direct injection it takes longer to restart, which is potentially a problem for emergency vehicles moving through traffic all with their engines off. And for sure component life will be significantly shorter.’

Repeatedly switching the engine off can also play havoc with tracking systems. Clive Girling, marketing director at Tracker, said modern telematics systems pick up electrical noise from the alternator to detect when the engine is running. This allows data to be gathered only when the car is using fuel, showing accurate idling data and ignoring times when the ignition is running to keep on-board systems running – such as when the driver is stopped for lunch and listening to the radio.

However, if drivers are turning their engines on and off repeatedly in traffic, the data can be affected.

 ‘It has an impact and as is always the case there is no simple answer,’ Girling explained. ‘It means for us we have to provide the option to set up a minute delay before it records it as a stop, and anything under a minute won’t be recorded. Because it’s fairly new we don’t have a lot of customers with the issue. We give customers the ability to decide for themselves.’

Although idling can be a needless waste of fuel, it’s difficult to give a definitive answer as to when it’s more efficient for the engine to be switched off. Cars with stop/start systems make a complex set of decisions and use uprated components to cope with repeated cranking – factors which would be impossible for a driver to judge. As such, fleets should consider their advice to drivers carefully, rather than taking such a broad-brush approach to idling.


The Mayor’s argument against idling

As part of its No Idling campaign, the City of London Corporation published the results of research which it said support switching off in traffic. The findings argued: 

Modern batteries require less running time than their predecessors, and aren’t reliant on the engine running to maintain charge

It takes up to an hour for engines to cool down, so turning off won’t cause cabin temperatures to drop if the fans are left running.

Idling does not maintain catalytic converter temperatures, and though these are most efficient when hot, they retain heat for 25 minutes after the engine is turned off

If all drivers in Central London switched off when stopped for over a minute, it would reduce annual PM10 particulate emissions – which can cause heart disease, asthma and lung cancer – equivalent to the volume emitted by driving the equivalent of the distance a return journey to the moon.


The electronic decision-making process

Stop/start systems don’t simply turn off whenever the engine is left to idle. Instead, the process is based on a number of factors. Audi’s system, for example, requires the following conditions to be satisfied:

•  The engine has reached a minimum operating temperature.

•  The climate control system has reached the set cabin temperature or the windscreen is not being defrosted. If the interior temperature rises or falls after the engine has switched off, it will restart automatically.

•  The exterior temperature isn’t too hot or too cold.

•  The battery charge is sufficient – if it drops below a set level while switched off or on-board systems require increased power, the engine will restart automatically.

•  The steering wheel isn’t close to full lock and the car isn’t in reverse gear.

•  The driver’s door must be closed, with driver’s seatbelt buckled.

•  The car must have travelled 4mph+ since it last stopped.

•  The car must not be towing a trailer.

•  The car must not be on a steep gradient.

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Alex Grant

Trained on Cardiff University’s renowned Postgraduate Diploma in Motor Magazine Journalism, Alex is an award-winning motoring journalist with ten years’ experience across B2B and consumer titles. A life-long car enthusiast with a fascination for new technology and future drivetrains, he joined Fleet World in April 2011, contributing across the magazine and website portfolio and editing the EV Fleet World Website.