In new litigation this week, three US states have accused Volkswagen of trying to cheat its way past diesel emissions testing since 1999.

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The attorney generals of New York, Maryland and Massachusetts have filed new lawsuits with evidence stating that Volkswagen has been working on emissions testing defeat devices since 1999.

According to these US states, the scheme began in 1999 when Audi started working on ways to calm down the distinctive clatter from a 3.0-litre V6 turbo-diesel it was developing. The luxury marque's preferred solution involved injecting extra fuel into the engine.

While this greatly subdued the trademark diesel sound, it also caused the engine to go beyond European emissions limits. The lawsuits allege that to get around this, Audi used a defeat device with the engine between 2004 and 2008.

The original defeat device detected an emissions test via the absence of steering wheel motion. In this situation, the noise-reducing fuel injection system was disabled.

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While trying to meet newer, stricter NOx (oxides of nitrogen) emissions regulations that were to come into effect in the USA in the late 2000s, Volkswagen was presented with a conundrum.

It could use selective catalytic reduction (SCR) technology that injects a liquid urea, commonly known as AdBlue, into the car's exhaust to convert NOx into nitrogen and water.

The SCR system would not only add to the cost of the vehicle, but require tanks capable of holding many litres of urea. To make matters worse, at the time the only commercially-viable system was being licensed out by arch-rival Mercedes-Benz.

Eventually the company settled on a less expensive system involving a lean NOx trap inside catalytic converter. Occasionally the engine will run in an oxygen-lean, fuel-rich mode to help the trap convert NOx particles into nitrogen and oxygen.

In order for the engine to be compliant with US emissions regulations, this mode would need to be engaged frequently. So frequently, in fact, that the soot filter would clog up after around 80,000 kilometres. That's less than half the 193,000km life required by US law.

According to this week's filings, thanks to internal project deadlines and, allegedly, "with the knowledge and approval of their managers", the engineers began working on a version of Audi's defeat device software.

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At least five incarnations of this defeat device were installed on various Volkswagen, Audi and Porsche vehicles sold in the US from 2008 onwards.

While some later America-bound TDI cars were fitted with the more effective urea system, they were still fitted with a defeat device as the company wanted to maintain a market advantage with a 16,000km service interval — double that of some of its competitors.

Instead of resorting to larger urea tank to achieve this target, the company decided instead to a new variant of the defeat device, which would only inject the correct amount of urea during an emissions testing.

In total, just under 600,000 defeat device-equipped diesel cars were sold in US, and around 11 million worldwide. The vast majority of the Dieselgate engines come from the EA189 four-cylinder turbo-diesel family, with a smaller number belonging to the Audi V6 TDI clan.

Bloomberg reports that emails presented by the New York attorney general at a press conference seem to confirm earlier reports that the board members and senior management already had in-depth knowledge about the company's emissions cheating practices.

One 2013 email from Frank Tuch, the Volkswagen Group's head of quality management, told then-CEO Martin Winterkorn, "A thorough explanation for [high] emissions cannot be given to authorities".

In another piece of correspondence, dated August 2014, Oliver Schmidt, then head of Volkswagen's environmental and engineering office told Mark Gillies, a Volkswagen US spokesman: "[Audi's] V6 has exactly the same issue [as Volkswagen's diesels], but not public yet. They have not been caught."

Other high level executives implicated in this week's filings, including Porsche's R&D boss Wolfgang Hatz, head of R&D at Audi Ulrich Hackenberg, and Volkswagen's powertain boss Heinz-Jakob Neusser, have already fallen on their swords and left the company.

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A study published in 2014 by West Virginia University, and funded by the International Council on Clean Transportation, discovered that while two Volkswagen models were able to pass bench testing, they emitted up to 40 times the permitted levels of NOx during real-world driving.

This triggered investigations by the United States' Environmental Protection Agency (EPA) and the California Air Resources Board (CARB).

The lawsuits allege that once investigators started asking questions, the Volkswagen Group tried to cover up its illegal behaviour by attempting "sham recalls" and knowingly lying to authorities.

It's also claimed that Volkswagen employees began destroying evidence after a tip-off from an in-house lawyer that official investigations were about to begin.

Eventually, Volkswagen capitulated and made a public admission of guilt in September 2015.

Eric Schneiderman, attorney general for New York state, told reporters this week: "This cover-up was deep, wide and long-lasting. It extended from front-line engineers throughout the corner offices ... and right into the CEO suites."

Although an internal investigation is still on-going, the Volkswagen Group has repeatedly claimed that the Dieselgate scheme didn't involve senior management, although it did admit that former CEO Winterkorn was informed of emissions testing manipulation back in 2014.

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In a statement to news agencies, including Bloomberg, Jeannine Ginivan, a spokeswoman for Volkswagen, said, "It is regrettable that some states have decided to sue for environmental claims now, notwithstanding their prior support of this ongoing federal-state collaborative process".

Late in June 2016, Volkswagen agreed to a deal with the US authorities regarding the EA189 defeat device, which would see it set aside around $20 billion for compensation, buy backs, fixes, fines, electric vehicle promotion, and environmental remediation. So far, the company has yet to have any fixes approved by the US EPA.

It has, however, been more successful in getting its proposed fixes approved in Europe, where NOx emissions regulations aren't as strict. No fines have been levied against Volkswagen in Europe.