US authorities accuse the four men of creating the defeat device so cars could have a long AdBlue refill interval, and 'a large trunk and a high-end sound system'.
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A US federal grand jury last week indicted four senior managers — Richard Bauder, Axel Eiser, Stefan Knirsch and Carsten Nagel — for their role in developing and covering up the emissions testing defeat device fitted to the Audi-designed 3.0-litre turbo-diesel V6 engine.

All four held senior management positions at Audi's diesel design centres in Neckarsulm and Ingolstadt, Germany. All four are believed to still reside in Germany.

Bauder and his team were responsible for the development of the Audi turbo-diesel V6 engine. The motor injects AdBlue, a urea mixture, into its exhaust to reduce oxides of nitrogen (NOx) emissions.

According to the indictment, Bauder's team told him the engine would need an enlarged AdBlue tank in order to meet US standards, and Audi's requirement for a 16,000km refilling interval.

This larger tank, though, would make it difficult or impossible to meet the company's requirement for "a large trunk and a high-end sound system".

The managers came up with a "dosing strategy", which would use software to detect when an emissions bench test was taking place. In these situations, suitable quantities of AdBlue would be injected into the exhaust to make the car compliant with US regulations.

In all other situations the engine emitted significantly more NOx than permitted by law.

From 2008 to 2013 the managers received protests from some in the engineering team, and warnings from the testing and compliance department about the illegality of this scheme. The men were even warned it would be "simple" for regulators to discover their cheating.

The 3.0-litre turbo-diesel V6 was fitted to various Audi and Volkswagen models sold in the US from the 2009 to 2015 model years.

When West Virginia University's Centre for Alternative Fuels conducted some real-world emissions tests in 2014, it unearthed a similar cheat device on Volkswagen's 2.0-litre turbo-diesel engine. That motor emitted up to 35 times the permitted amount of NOx.

Upon hearing about this, one of the indictees ran some real-world tests on the 3.0-litre V6 and discovered it emitted up to 22 times more NOx than allowed.

As government questioning increased in 2015, Oliver Schmidt head of the engineering and environmental office for the Volkswagen Group of America, emailed one of the managers stating "our worst fears have come true" and "we need help with arguments".

Despite increasing scrutiny, the indictees allegedly kept on lying to authorities, and insisted the V6 engine didn't have the same issues as Volkswagen's 2.0-litre motor.

Schmidt is one of the few involved in the Dieselgate saga to feel the full force of the US justice system as the German government has a long-standing policy of handing over its own nationals to the US.

The former Volkswagen executive was arrested after he re-entered the US. He agreed to a plea deal and is currently serving seven years in jail.