Researchers at Leeds University who monitored the exhaust emissions of thousands of cars found that certain Vauxhall Corsa, Astra and Vectra models typically produced twice the level of toxic nitrogen oxides (NOx) as rival manufacturers’ cars.
The worst offender was the Vectra. Some versions produced up to 10 times the levels permitted by EU law.
James Tate, who led the study, said his team’s findings showed that while models produced by eight of the manufacturers it tested showed either no change or reductions in their NOx emissions since the introduction of new, stricter Euro 6 limits, levels of NOx emitted by some Vauxhall models had increased.
For example, the team found that Corsas produced between 2009 and 2014 under the new regulations emitted 50% more NOx than their predecessors; Astras’ emissions had more than doubled.
Tate said he thought Vauxhall’s pollution reduction systems were to blame.
“I suspect they work well at low speeds but pour out NOx in normal driving,” he said.
Vauxhall agreed that the system that feeds exhaust gas back into the combustion chamber to be burnt — a process called exhaust gas recirculation (EGR) — has its limitations.
“The EGR system functionality is physically limited,” it said. “Additionally, during certain combustion chamber conditions, the use of EGR causes unwanted emission trade-offs.”
However, the company also said that all Vauxhalls had passed the official emissions tests. Its engineers are seeking a meeting with Tate to discuss his findings.
Tate has contacted the Vehicle Certification Agency, the UK government body that enforces EU emissions rules, asking for further checks.