The evils of idling

Many people still believe that engines need to be ‘idled’ for half an hour before being put under load, or at the end of a driving stint. This practice is not only unnecessary, but wastes fuel, contaminates engine oil, clogs emissions systems and shortens engine life.

It is still a common belief that it’s vital to have the engine and gearbox up to operating temperature before putting them under load. There are two issues with that claim. Firstly, idling doesn’t raise engine and gearbox temperature to anything like operating condition and, secondly, prolonged idling causes long-lasting harm to the engine and exhaust after-treatment systems.

The term ‘idling’ refers to the continuous operation of a vehicle’s main propulsion engine while the vehicle is stopped. Idling is unavoidable in traffic conditions, especially during urban driving, but idling periods in traffic are usually short.

Most idling is unnecessary and can be eliminated through conscious effort. Some manufacturers do recommend a very short warm-up or cool-down period, but mainly if an engine is about to undergo or has just finished high-load or high-power operation.

Generally speaking, more than five to 15 minutes of idling is excessive and should be avoided, according to every engine maker on the planet.

The trucksales story on modern oils shows that they’re designed to cope with cold starts and flow freely right from initial startup. Modern oil doesn’t need specific ‘warming up’, unless ambient temperatures are below -15 degrees C, and then that warm-up is best done with an engine sump heater.

Vehicle makers are unified in their opposition to the practice of idling and we’re indebted to the USA’s Oak Ridge National Laboratory for its 2016 publication: Summary of OEM Idling Recommendations from Vehicle Owner’s Manuals.

In essence, they all advise against prolonged idling, particularly at ‘low-idle’, which is usually in the 600-750rpm range. If an engine must be idled, it needs to be done at ‘high-idle’ – normally around 1000rpm. When a truck engine is operating a power take off (PTO) the engine speed is always at least high-idle and often higher.

Low-idle operation can actually be damaging to an engine and vehicle components, because idling can produce sulphuric acid, which eats away at the engine and emissions-system components.

Additionally, idling results in lower in-cylinder-temperature combustion. That’s easy to verify if you have an exhaust gas pyrometer that measures combustion temperature.

Typical diesel exhaust gas temperature at low-idle can be below 250 degrees C and that’s around the minimum temperature at which a diesel particulate filter (DPF) must operate. Below that figure, you’re just clogging your DPF with soot.

Low-temperature-combustion soot and varnish coatings also build up in the engine, where they can cause unnecessary wear, clogging the piston rings and ‘bore glazing’ the oil-retaining hone marks in the cylinder liners, so they no longer retain an oil film.

In addition to increased engine wear and tear, idling should be avoided for other important reasons, including fuel waste. Check your fuel consumption meter when you drive off after an idling session and you’ll see a huge L/100km figure. That’s burnt fuel that did nothing. This wasted fuel can be costly over the course of a year, especially for fleets.

The larger the engine, the more fuel is wasted while idling.

Engine oil quality is badly affected by extended idling, because the oil becomes soot- and acid-contaminated, resulting in a decrease in oil life and the need for more frequent oil changes – another additional (and avoidable) expense. Extended oil drain operation is conditional on strict idling limits.

The harmful emissions that idling creates should also be considered. These emissions include nitrogen oxides, carbon monoxide, carbon dioxide and particulate matter. Some of these pollutants pose health threats on their own and some combine with heat and sunlight to form ground-level ozone, a potent pollutant that can worsen asthma and other respiratory problems.

In Europe and the USA, idling is illegal and heavy penalties apply. In Australia it’s not specifically illegal, but there are local noise abatement rules.

"Unfortunately, there are no laws in Australia at present to prevent people from idling," Claire Walter, PhD candidate at the University of Queensland, told ABC Radio Sydney's Cassie McCullagh. "There is no public awareness either.”

In summary, idling is occasionally necessary, as when bogged or stranded in extreme weather (keeping aircon or the heater running in very hot or very cold conditions), but most idling is unnecessary and can be eliminated through conscious effort. If you must idle, make sure that engine speed is high-idle.

All truck manufacturers recommend a very short warm-up or cool-down period, but generally speaking, more than five to 15 minutes of idling is excessive and should be avoided.

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