Why Do Diesel Cars Emit More Pollution During Cold Weather?

As if the diesel automotive industry didn’t have enough to deal with in the wake of the VW scandal at the end of last year, new research has now cast further aspersions on the integrity of car manufacturers.

All vehicles in Europe are required to meet the Euro 6 emissions standards, which regulate how much nitrogen oxides (NOx) and carbon dioxide (CO2) the cars can emit into the atmosphere and were introduced as a method of improving air quality and reducing transport-related pollution. However, in cold weather, the engines of these cars can be severely inhibited; to offset that detriment, the legal limits can be waived if temperatures fall below a certain threshold.

It has now come to light that certain manufacturers have been manipulating this loophole in the legislation by intentionally setting the threshold too high. By disregarding the safety checks whenever the external temperature falls below 18°C, the cars could be emitting illegal levels of pollution for much of their operational time.

Almost five times the amount of legal NOx

A study conducted by Emissions Analytics tested 213 models of vehicle from 31 different manufacturers and found that when the thermostat fell below 18°C, the cars emitted as much as 4.6 times the legal limit for NOx. Indeed, even when temperatures were elevated, things were not much better as cars on average still emitted 3.6 times the legal level above 18°C.

“I would say from the Euro 5 generation of cars, it’s very widespread, from our data. Below that 18°C many have higher emissions... the suspicion is, to give the car better fuel economy,” explained CEO of the company Nick Molden. “If we were talking about higher emissions below zero, that would be more understandable and there are reasons why the engine needs to be protected. But what we’ve got is this odd situation where the threshold has been set far too high, and that is a surprise.”

Many of the models in question are expected to still have another five to ten years of road life in them, meaning the problem could continue for many years to come. Fortunately, newer models which must comply with Euro 6 standards fared better in the testing, although on average they were still guilty of exceeding legal limits by a significant percentage. This is thought to be due to the fact that many test scenarios for new cars are not representative of real world conditions, though mobile monitoring stations to assess diesel emissions is being touted as one way round this problem.

“Not doing anything wrong”

When contacted for comment, the major manufacturers involved stressed the fact that they were not breaking the law in and that in fact, the cars were intentionally designed this way to improve performance and reduce engine damage.

“The temporary reduction or switching off of some emissions control systems under certain temperatures is allowed by law and necessary to avoid damage to vehicles’ engines,” said a statement from the Society of Motor Manufacturers and Traders (SMMT).

“Without it, there could be a significant cost to the consumer for major repair work. In its recent report, government recognised the need for such technology and was satisfied with how vehicle manufacturers were using it. Manufacturers are investing billions of pounds in developing ever-more advanced technology, and this along with new Real Driving Emissions regulation from next year will see significant lowering of emissions across a full range of driving conditions and temperatures.”

While this may be true, it’s almost certain that 18°C is far too high a threshold, especially in the UK where temperatures regularly stay below that level.