Stefan Knirsch (Photo: Audi)

Audi's head of R&D, Stefan Knirsch, is stepping down from his post with immediate effect, as investigations into Volkswagen's emissions-test cheating continue.

The engineer, who took over from Ulrich Hackenberg at Audi in January, is leaving the company, Audi said.

Germany's Bild am Sonntag newspaper reported earlier this month that Knirsch was suspended amid investigations of his role in Volkswagen emissions cheating.

It remains unclear which Volkswagen Group executives sanctioned the use of software - so-called defeat devices - to make VW Group vehicles perform better from an environmental point of view during tests. Roughly 11 million VW Group vehicles equipped with diesel engines passed emissions tests this way.

German press reports this week also say that former VW Group CEO Martin Winterkorn was aware of the "dieselgate" actions long before the company was forced to admit to the deceit in September last year.

Knirsch joined Audi in 1990, left the company for senior positions elsewhere in the industry in 1996, and returned to the Volkswagen Group's premium brand in May 2013 as head of powertrain development.