The diesel scandal surrounding Audi, BMW, Mercedes-Benz, Porsche and Volkswagen has taken a further twist with news that the EU has opened an investigation into claims the five German car makers worked together to set standards and collaborate on software manipulation methods for diesel engine development in secret meetings. One of these meetings allegedly took place on the sidelines of the 2010 Paris motor show, Autocar understands.

Acting on evidence provided by a former Volkswagen employee, the EU has opened the cartel investigation following claims that up to 200 employees from Audi, BMW, Mercedes-Benz, Porsche and Volkswagen were involved in the secret, closed-door meetings. During these encounters, the German car makers are said to have swapped vital information on methods to circumnavigate test procedures for CO2 and particulate emissions as well as SCR (selective catalytic reduction) thermo switching, among other manipulations.

Said to have taken place since the 1990s, the talks are also claimed to have involved a consensus on the size of AdBlue urea tanks, with Audi, BMW, Mercedes-Benz, Porsche and Volkswagen all agreeing to keep the tanks under a certain capacity to aid packaging compared with other car makers' diesel systems.

The decision to limit the size of AdBlue tanks, which are used to inject urea in exhaust after-treatment systems to lower tailpipe emissions, is said to be behind moves first introduced by the German car makers towards so-called thermo switching. In theory, this process would involve the SCR (selective catalytic reduction) tank being switched off both below and above certain temperatures to ensure the AdBlue mixture is not depleted, albeit at the expense of CO2 and particulate emissions.

Audi said it won't be "commenting further until after the Diesel Summit involving the German government on August 2" while Mercedes-Benz said "please understand that we do not comment on speculation," and Volkswagen said "we have no comments to make on the speculation and conjecture".

A statement released by BMW said: "As a matter of principle: BMW Group vehicles are not manipulated and comply with respective legal requirements. Of course this also applies to diesel vehicles. Confirmation of this is provided by the results of relevant official investigations at the national and international level."

"From a BMW Group perspective, the objective of discussions with other manufacturers concerning AdBlue tanks was the installation of the required tanking infrastructure in Europe."

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